Chapter 10 Breathing and Exchange of Gases by TEACHING CARE online tuition and coaching classes

Chapter 10 Breathing and Exchange of Gases by TEACHING CARE online tuition and coaching classes



Cells continually use oxygen (O2) for the metabolic reactions that release energy from nutrient molecules and produce ATP. At the same time, these reactions release carbon dioxide. Since an excessive amount of CO2 produces acidity that is toxic to cells, the excess CO2 must be eliminated quickly and efficiently. The two systems that cooperate to supply O2 and eliminate CO2 are the cardiovascular system and the respiratory system. The respiratory system provides for gas exchange, intake of O2 and elimination of CO2, whereas the cardiovascular system transports the gases in the blood between the lungs and body cells. Failure of either system has the same effect on the body: disruption of homeostasis and rapid death of cells from oxygen starvation and buildup of waste products. In addition to functioning in gas exchange, the respiratory system also contains receptors for the sense of smell, filters inspired air, produces sounds, and helps eliminate wastes.

Respiration : Respiration is the exchange of gases between the atmosphere, blood and cells. It takes place in three basic steps :

  • Pulmonary ventilation : The first process, pulmonary (pulmo = lung) ventilation, or breathing, is the inspiration (inflow) and expiration (outflow) of air between the atmosphere and the
  • External (pulmonary) respiration : This is the exchange of gases between the air spaces of the lungs and blood in pulmonary The blood gains O2 and loses CO2.
  • Internal (tissue) respiration : The exchange of gases between blood in systemic capillaries and tissue cells is known as internal (tissue) The blood loses O2 and gains CO2. Within cells, the metabolic reactions that consume O2 and give off CO2 and give off CO2 during production of ATP are termed cellular respiration.


Respiration is a process which involves intake of oxygen from environment and to deliver it to the cells. It


include stepwise oxidation of food in cells with incoming oxygen, elimination of

release of energy during oxidation and storing it in the form of ATP.


produced in oxidation,


  • Respiratory surface : The surface at which extend of gases (CO2 and O2) takes place is called respiratory Respiratory surface must be vascular and have enough area for gas exchange. For example – plasma membrane in protozoa, body wall (skin) in annelids, alveocapillary membrane in men.
  • Respiratory medium : Oxygen is dissolved in air and Thus water and air are source of oxygen for animals and called respiratory medium. Water and air are external respiratory medium. Respiratory medium comes in contact with respiratory surface and gaseous exchange takes place between respiratory medium and blood or any other transport medium through respiratory surface by simple diffusion. Inside the body an internal respiratory medium is also found. This internal respiratory medium is tissue fluid. Cells exchange their gases with tissue fluid through plasma membrane.
  • Types of respiration : It is of two types
  • Aerobic respiration : It occurs in the presence of molecular oxygen. The oxygen completely oxidises the food to carbon dioxide and water, releasing large amount of The organisms showing aerobic respiration, are called aerobes. It is found in most of animals and plants. Aerobic respiration is of two main types direct and indirect.






C6 H12 O6 + 6O2 ®


+ 6H 2O+ 2830 kJ




Carbon dioxide




  • Direct respiration : It is the exchange of environmental oxygen with the carbon dioxide of the body cells without special respiratory organs and without the aid of blood. It is found in aerobic bacteria, protists, plants, sponges, coelenterates, flatworms, roundworms and most

Protists : Amoeba proteus is about 0.25 mm. Wide and has a large surface area to volume ratio. Diffusion of gases occurs over the entire surface via cell membrane, and is enough to fulfill its metabolic requirements.

Coelenterates : In Hydra and Obelia, practically all cells are in contact with the surrounding water. Each cell can exchange gases sufficient for its own needs through the cell membrane adjacent to water.

Flatworms : Planaria can also exchange gases sufficient for its needs by diffusion over its body surface. This is facilitated by its very thin body which increases the surface area to volume ratio.

  • Indirect respiration : It involves special respiratory organs, such as skin, buccopharyngeal lining, gills and lungs, and needs the help of blood. The respiration in the skin, buccopharyngeal lining, gills and lungs is respectively called cutaneous buccopharyngeal, bronchial and pulmonary Cutaneous respiration takes place in annelids, some crustaceans, eel fish, amphibians and marine snakes. It occurs both in water and in air. Buccopharyngeal respiration is found in certain amphibians such as frog and toad. It occurs in the air. Branchial respiration is found in many annelids, most crustaceans and mollusks, some insect larvae, echinoderms, all fishes and some amphibians. It occurs in water only. Pulmonary respiration is found in snails, pila, some amphibians and in all reptiles, birds and mammals. It takes place in air only.
  • Anaerobic respiration : It occurs in the absence of molecular oxygen and is also called fermentation. In this, the food is only partially oxidised so only a part of energy (5%) is released and of energy remains trapped in the intermediate compounds. It is found in lower organisms like bacteria and yeast. It is also found in certain parasitic worms (Ascaris, Taenia) which live in deficient The organism showing anaerobic respiration, are called anaerobes. These involve one of following reactions.


C6 H12O6


¾¾In y¾eas¾ts  ®

(Fermentation of sugars)

2C2H5OH+ 2CO2 + 118 kJ



C6 H12 O6  ¾¾In in¾test¾inal¾wor¾m¾s ® 2CH3 CHOHCOOH+ Energy

Glucose                                                        Lactic acid

Certain body tissues of even aerobes also show anaerobic metabolism e.g., during the vigorous contraction of skeletal muscle fibres. In this, the glucose is metabolised into the lactic acid in anaerobic conditions. The rapid formation and accumulation of lactic acid are responsible for muscle-fatigue. The mammalian RBCs shows anaerobic respiration as these lack the mitochondria. In lens of eye and cornea of eye respiration is anaerobic because these structures are a non vascular. Anaerobic respiration appeared first in primitive organisms because there was absence of O2 in primitive atmosphere.







Difference between aerobic and anaerobic respiration

Aerobic respiration / Metabolism Anaerobic respiration / Metabolism
It uses molecular oxygen. It does not use molecular oxygen.
Always release CO2. May or may not release CO2.
It produces water. It does not produce water.
It produce much more energy (whole energy present in glucose). It produce less energy (only 5% of that available in glucose).
It yields inorganic end products only. It yields organic end products with or without inorganic product.
It is found in majority of animals. It found in some parasitic worms. (Ascaris, Taenia).


 Respiratory organs.

  • Skin : Respiration by skin is called cutaneous respiration. Skin is the only respiratory organ in most annelids (earthworm and leeches) and an additional respiratory organ in amphibians (Toads and frogs). Skin should be thin, moist, naked, permeable and well vascular for For cutaneous respiration animal should have large surface area then its volume and should have relatively inactive life to minimize the use of oxygen. In earthworms, epidermis has rich network of blood capillaries and their body surface has a moist film containing secretions of epidermal mucous glands, excretory wastes and coelomic fluids. The epidermal capillaries that in turn


release the

CO2 , take up oxygen dissolved in film of surface moisture. Some marine annelids such as sandworms


(nereis) have parapodia (locomotory appendages) for respiration. In frog 100% cutaneous respiration during hibernatin. In all marine snakes 20% respiration by skin.

  • Tracheae : In insects, peripatus centipedes and millipedes tracheae are found for Tracheae are complex system of whitish, shining, intercommunicating air tubules. Tracheae are ectodermal air tubes. In cockroaches, three pairs of longitudinal tracheal trunks are present all along the length of body which are further connected with each other with the help of transverse branches. The main tracheae give off smaller tracheae whose branch repeatedly form a network of trachioles throughout the body. Trachioles internally lined by chitinous cuticle called intima, which spirally thickened to form taenidae. Tracheae without taenidae, tracheae lined by trachein protein. From each tracheal trunk three branches come out. The dorsal branch is supplied to the dorsal muscles where as ventral one to nerve cord and ventral muscles and middle one to the alimentary canal.

Tracheae open out on body wall through ten paired lateral apertures called stigmata or spiracles or stigmatum. Stigmata are two pairs thoracic and eight pairs abdominal. Each spiracle is surrounded by an annular sclerite (peritreme) which opens into air filled cavity called atrium or tracheal chamber. Expansion of abdominal cavity allows the space inside the tracheal trunk to expand. As a result air enters through the spiracles and distributed in body cavity through tracheal system. When abdominal cavity contracts the tracheal system also contracts the pressure of air inside the tracheal systems increases causing the release of air to the outside. Most of CO2 diffuse out by chitin. At rest, the tracheoles are filled with watery fluid, oxygen is dissolved in this fluid and diffuses to cells. During activity the fluid in the tracheoles is drawn osmotically into the tissues. Consequently more air rushes into the tracheoles. Similarity between the trachea of cockroach and rabbit is that, wall of both are non collapsible.









































Fig. – Trachea of cockroach                                                                       Fig. – Show respiratory system of cockroach.

  • Book lungs and book gills : Spiders ticks, mites and scorpion (belongs to class arachnida) have book lungs for In scorpion 4 pairs of book lungs are present. A book lung is a chamber containing a series of thin vascular, parallel lamellae arranged like the pages of book. Book gills are found in marine king crab or horse shoe crab.
  • Gills : Aquatic animals such as prawn, unio, fishes, sea stars and tadpoles respire by Respiration by gills called bronchial respiration. Gills are of two types :
  • External gills : External gills are found in arenicola (lug worm), larvae of certain insects g. damsel fly and some amphibians e.g. necturus, siren, proteus, frog tadpole first develop external gills which are replaced by internal gills later.
  • Internal gills : The internal gills may be phyllobranch (prawn), monopectionata (pila) eulamellibrach (unio), lameellibranch, fillibranch (pisces). In all fishes, gills are hemibranch or demibranch and holobranch. In gills, gill lamellae are found which have capillary Water is drawn into gills ® blood flowing in the capillaries of


gill lamellae absorb oxygen from water and release


® water containing


is thrown out from gills. The


80% of O2 of incoming water is absorbed. Water breathing causes some problems such as

  • For indrawing the water inside the gills, animals have to make great muscular effort because water is about 800 time more denser than


  • Water has less uncombined the gills to fulfill the oxygen

O2 than air. Therefore large quantity of water is required to be passed over


  • As the temperature rises the O2 content of water falls and animals face problem.


Oxygen content of respiratory media
Respiratory media Oxygen content
Air 209.5 ml./l.
Fresh water at 25ºC 5.8 ml./l.
Fresh water at 5ºC 9.0 ml./l.
Sea water at 5ºC 6.4 ml./l.





  • Buccopharyngeal lining : Frog breathes by buccopharyngeal lining of buccopharyngeal cavity. This is called buccopharyngeal


Animals Respiratory organs
Protists, Bacteria Direct respiration through plasma membrane
Porifera and Coelenterates Direct respiration by each cells through plasma membrane also by canal system in porifera.
Platyhelminthes (Fasciola hepatica, tapeworm) Anaerobic
Nematodes (Ascaris) Anaerobic
Annelids (Earthworm and Leeches) Skin
Nereis Parapodia
Insects Trachea
Centipedes Trachea
Millipedes Trachea
Spider and Scorpion, ticks, mites Book lungs
Marine king crab Book gills
Prawns, Unio and Pila Gills
Echinodermata Bronchiole, Tube feet, Respiratory tree, Bursae and water lungs.
Fishes, Sea star, Tadpoles Gills
Frogs, Toads Buccopharyngeal, Lungs, Skin
Reptiles, Birds, Mammals Lungs
Aves, chemeleon, house fly, locust Air sacs.
Bony fish Air bladder.
Urochordata Test
Marine turtle Clocal respiration
Mollusca, Herdmania Mental


 Respiratory system of human.

Human respiratory system is derived from endoderm. Human respiratory system may be divided into two components.

(i) Respiratory tract or conducting portion               (ii) Respiratory organs

(i) Respiratory tract or conducting portion : It is the passage for the air. In this part gaseous exchange does not takes place. It is also called dead air space. It is divided in following parts :

  • Nose : (Latin-Nasa) (Greek-Rhine) cavity of nose is called nasal cavity. Nasal cavity is divided into two parts by nasal septum called Each part is called nasal chamber. Each nasal chamber opens out side by external nares. Nasal septum has two part. First part is small and is made of cartilage (hyaline). Second part is major and it is bony. Vomer is the main bone. Each nasal chamber has three region.





  • Vestibular region : Vestibular region also known as vestibule, lined by non keratinized squamous epithelium, it is ectodermal in origin and have sebaceous gland, sweat gland and Vestibule is also found in inner air larynx, mouth and vagina. It acts like a seive to check the entry of large dust particles and other things.
  • Respiratory region : Middle region lined by respiratory epithelium which is ciliated pseudostratified columnar epithelium. It contains mucus and serous cells. Mucus cells produce mucus and serous cells produce watery Respiratory epithelium is highly vascular and appears pink or reddish. Respiratory region acts as a air conditioner and makes the temperature of in going air nearly equal to body. It also acts as a filter not give entry to dust particles, flies or mosquitoes.
  • Olfactory region : It is upper region. It is lined by olfactory epithelium. This is also called Schneiderian Olfactory region is the organ of smell and detect the odour of inspired air. Inspiration is stopped if odour of air is foul or offensive. According to new researches pheromone receptors are found in nasal cavities.
  • Nasal conchae : Lateral wall of nasal cavity have three shelves like structures called conchae or 3 pairs of nasal conchae are found. Nasal conchae are covered with mucus membrane. They increase the surface of nasal chamber. Both the chambers of nasal cavity open into nasopharynx by their apertures called internal nostrils or conchae. Adjacent to internal nostril there are opening of eustachian tube. Names of these three conchae and names of the bones that form them are given below.
    • Superior conchae : The dorsal most chochae is supported mainly by nasal bone called nasoturbinate. It is the smallest
    • Middle conchae : Ethmoid bone called
    • Inferior conchae : The ventral most conchae supported by maxilla bone called maxilloturbinate. It is a separate bone
  • Pharynx : It is the short vertical about 12 cm long tube. The food and air passages cross here. It can be divided in 3 parts –
    • Nasopharynx : Nasopharynx is only respiratory upper part in which internal nares open. There are 5 opening in its wall; two internal nares, two eustachian tube opening and opening into
    • Oropharynx : Middle part is called In this part oral cavity open known as fauces. Two pair tonsils the palatine and lingual tonsils are found in the oropharynx.
    • Laryngopharynx or hypopharynx : Lowest part is called laryngopharynx. It leads into two tubes. One at the front is wind pipe or trachea and one at the back is food pipe or Both oro and laryngo pharynx is both a respiratory and a digestive pathway.

Nasopharynx lined by ciliated pseudostratified epithelia, oropharynx and laryngopharynx lined by non keratinized epithelium. Mouth serves as an alternate route for air when nasal chambers are blocked. Foramen by which pharynx opens into larynx called glottis. In general it remains open. During swallowing it is closed. It provides passage for air. Pharyns leads into the oesophagus through an aperture called gullet. In general condition it remains closed and opens at the time of swallowing. During swallowing epiglottis closes the glottis.

  • Larynx or Voice box : It is found both in frogs and rabbits. Larynx does not help in respiration. It is present on tip of trachea and is made up of 9 cartilages such as thyroid (single) has a prominence called pomum admi or adam’s apple, cricoid (single), arytenoid (paired) are piece of hyaline cartilage. While epiglottis (single), carniculate (paired) cuniform (paired), santorini are piece of elastic Clinically, the cricoid cartilage is the landmark for making an emergency air way.






Larynx is a short tubular chamber and opens into the laryngopharynx by a slit like aperture called glottis. Glottis always remains open except during swallowing. Larynx is more prominent in men than women due to male harmone. Before puberty, the larynx is inconspicuous and similar in both sexes. Larynx is a voice producing instrument. For this purpose larynx have two types of vocal cord. In birds voice producing organ is syrinx, found at lower end of tracheae.

  • False vocal cord or vibrating fold or anterior vocal cord : These are folds of mucus membrane. Gap between them is called rema vestibuli. These are not responsible for sound production. In elephants only true vocal cords are present and are responsible for this trumpet
  • True vocal cord or posterior vocal cords : They are made up of yellow elastic fibres. Gap between them is called rema glottides or peep hole. In males the length of true vocal cord is 2.25 cm and in female is 1.75 The free inner rim of each vocal cord is set into vibrations as air is expelled from the lungs and the sound is produced when the cords vibrate. Pitch of the sound can be altered by contracting or relaxing the vocal cords to varying the degrees. Sound produced by rabbit is called quaking. Hippopotamus lacks true vocal cords. Vocal cords or folds in lined by non keratinized stratified squamous epithelium. Pitch is controlled by the tension of vocal folds. Lower sound produced due to decreasing muscle tension on vocal cords. Due to influence of androgens, vocal cords are usually thicker and longer in males than in females, therefore they vibrate more slowly. Thus men generally have a lower range of pitch then women. The pharynx, mouth nasal cavity act as resonating chamber. Muscle are face, tongue and lips help us enunciates words.
  • Trachea : It is a tubular structure of about 12 in length and 2.5 cm in diameter. The wall of trachea is made of fibres, cartilage muscles and the mucus membrane. In middle of thorax at the level of 4th and 5th thoracic vertebra it divides into two branches called right and left primary bronchi. Right primary bronchus is short and broad and divides into three branches called lobes or secondary bronchi which extend separately into the three lobes of right lung. Left primary bronchus divides into two lobes or secondary bronchi that pass into two lobes of left lung. At the point of bifurcation trachea has projection of cartilage called carina. Further division of secondary bronchi is given in form of arrow diagram.




No exchange of gases (from nose to terminal bronchiole passage is a called conducting passage or dead space)






Respiratory zone

Exchange of gases takes place



Major or primary bronchi


Secondary bronchi


Tertiary or segmental bronchi


Terminal bronchiole


Respiratory bronchiole


Alveolar duct


Alveolar sac or atrium


Air sac or alveoli (gaseous exchange)






Bronchioles are narrowest and most numerous tubes of lungs. Alveoli are not tube they are sacs like structures. Into alveolar sac 3 to 6 air sacs or alveoli open. There are 300 millions of alveoli in the two lungs. Air capillaries

replace alveoli in birds. O2 carried in inhalation ultimately reaches in alveoli. Area of internal surface of both lungs

(alveoli) is about  70m2 (750 ft 2 ), about the size of a handball court. Thus provide large surface for gaseous exchange. Area of inner surface of bronchiole is 10 m2 . Trachea and its branches up to alveoli are called bronchial tree.

The alveolar walls consists of two types of alveolar epithelial cells or pneumocytes. Type I alveolar (squamous pulmonary epithelial) cells are simple squamous epithelial cells that forms a continuous lining of the alveolar wall, interrupted by occasional, type II alveolar (septal) cells. Type I alveolar cells are the main site where gas exchange, takes place. Type II cuboidal alveolar cells secrete alveolar fluid. Associated with the alveolar wall are alveolar macrophages (dust cell). The thickness of alveolar-capillary membrane is 0.5mm (about 1/16 the diameter of RBC).

Pulmonary blood circulation differes from systemic circulation in two ways –

  • Pulmonary blood vessels provide less resistance to blood
  • Less pressure is required to remove blood through pulmonary

In trachea about 16 – 20 c- shaped cartilagenous (hyaline) rings are found. These rings are incomplete posteriorely or incomplete dorsally. Cartilagenous rings are also found in the bronchi. In bronchioles these rings are absent. In insects trachea also find supporting rings cartilagenous rings keep trachea and bronchi open permanently even during negative pressure created by expiration. Larger bronchioles are supported by connective tissue alone which extend from the intertubular septa. Muscles of human tracheo bronchial tree are smooth and are supplied by sympathetic and parasympathetic


nerves. Contraction of these muscles

leads to narrowing of the bronchus. It is

Fig. Bronchial tree


called bronchiospasm. Effect of bronchiospasm is remarkable on fine bronchioles where muscles are present but cartilagenous rings for support are absent. Bronchiospasm below tertiary bronchi clinically called bronchial asthma. Sympathetic nerves stimulation causes relaxation of bronchial muscles and hence drugs which causes stimulation of sympathetic nerves called sympathomimetic drugs, are given in treatment of bronchial asthma.






Wall of trachea,upper bronchi is lined by pseudostratified ciliated columnar epithelium rich in mucus secreting cells. Mucus holds the dust and bacteria which are swept by cilia toward the pharynx from where they are swallowed or thrown out. Tobacco smoke contains ciliotoxius which damages the cilia. Terminal bronchioles and beginning of respiratory bronchiole are lined by simple ciliated columnar epithelium without mucus cells. The mucus if present may block the these narrow tubules. Rest of respiratory bronchiole and alveolar duct have non ciliated cuboidal epithelium. There are 10 bronchioles in right lung and 8 bronchioles in left lung. The bronchioles contain 3 special types of cells along with normal epithelium.


Different epithelium living in respiratory tract
Vestibular region of nose Skin having hair
Respiratory region of nose Ciliated pseudostratified
Olfactory region of nose Olfactory (Schneiderian) epithelium
Pharynx (Oropharynx, Laryngopharynx) Non-keratinised stratified squamous
Trachea and bronchi (Upper) Pseudostratified ciliated columnar epithelium with mucus cells
Lower bronchi (Secondary / Tertiary) Lined by simple ciliated columnar epithelia
Terminal bronchioles and beginning of respiratory bronchiole Simple ciliated columnar epithelium without mucus cells
Rest of respiratory bronchioles, alveolar duct Non ciliated cuboidal epithelium
Alveoli Non ciliated squamous
Alveoli of frog’s lungs Columnar ciliated epithelium


  • Kultchitsky cells or argentaffin cells : They secrete serotonin and histamine. Serotonin dilate while histamine constrict the
  • Clara cells : They secrete a phospholipid named diapalmityl lecithin which acts as a surfactant. This surfactant prevents the collapse of bronchioles lacking cartilagenous rings. Collapsing of lungs is called atelectesis. Pottle in 1956 proved the existence of surfactant. Surfactant is formed by clara cells only at later stage of foetal life. Some times at birth some infants are devoid of surfactant so there is


great respiratory difficulty because lungs refuse to expand. In this condition death may occur. This is called respiratory distress syndrome (RDS) or hyaline membrane disease (HMD) or glassy lung disease.

  • Dust cells : They are phagocytes which eat foreign particles (dust).

(ii) Respiratory organs : In men the respiratory organ are a pair of lung. Some snakes have unpaired lungs. Respiration by lungs is called pulmonary respiration. Lungs are found in all vertebrates except fishes. In Lung fishes such as protopterus, neoceratodus and lepidosiren air bladder is found, which is modified lung. Respiration in men and rabbit is pulmonary.




























(a) Lungs : Lungs lie in thoracic cavity on both side of heart in





mediasternum space. Base of lung is attached to diaphragm. Right lung

is divided into 3 lobes viz. Superior, Middle, Inferior and left lung is

Fig. –  Lungs  of  man


divided into two lobes Superior and Inferior. In rabbit, the left lung is divided into two lobes left anterior and left posterior where as the right lung has four lobes anterior azygous, right anterior, right posterior and posterior





azygous. Lungs of reptiles are more complex than those of amphibians. In birds lungs are supplemented by elastic air sacs which increase respiratory efficiency. The narrow superior partion of lung is termed the apex or cupula.

Each lung is enclosed in two membrane called pleura. Pleura are layers of peritonium of thorax. Inner membrane is called the visceral pleuron. It is firmly bound to surface of lungs. The outer membrane is called parietal pleuron. It is attacked to chest wall or wall of thoracic cavity. A narrow space exists between the two pleura. It is called pleural cavity. In pleural cavity a watery fluid is found called pleural fluid. Pleural fluid is glycoprotein in nature and secreted by pleura. Pleural fluid lubricate the pleura so that they may slide over each other without friction. This fluids reduces friction bewteen the membrane. When the lungs expand and contract in respiration. Pressure inside pleural cavity is negative – 5 mm Hg. Plurisy is inflamation of pleura and cause collection of fluid in pleural cavity. It results painful breathing (dyspnea). The surface of lung lying against the ribs, known as coastal surface. The mediastinal (medial) surface of each lung contains a region – the hilus, through which bronchi, pulmonary blood vessels, lymphatic vessels and nerve enter and exit.

 Pulmonary volumes and capacities.

In clinical practice, the word respiration (ventillation) means on inspiration plus one expiration. The healthy adult averages 12 respiration’s a minute and moves above 6 litres of air into and out of the lungs while at rest. A lower-than-normal volume of air exchange is usually a sign of pulmonary mal-function. The apparatus commonly used to measure the volume of air exchanged during breathing and the rate of ventilation is a spirometer (spiro=breathe) or respirometer. The record is called a spirogram. Inspiration is recorded as an upward deflection and expiration is recorded as a downward deflection, and the recording pen usually moves from right to left.

There are 4 respiratory volumes and capacity.

Respiratory volumes :

  • Tidal volume (TV) : It is volume of air normally inspired or expired in one breath (i.e. inspiration and expiration) without any extra effort. It is about 500 ml. in normal healthy adult. In infants it is 15 ml and in fetus it is 0 Tidal volume varies considerably from one person to another and in same person at different times. In an average adult, about 70% (350 ml) of tidal volumes reaches respiratory bronchioles, alveolar duct, sacs and alveoli (respiratory portion). The other 30% (150ml) remains in air spaces of nose, pharynx, larynx, trachea, bronchi, bronchioles and terminal bronchioles (conducting portion). These areas are known as anatomic dead space.

The total volume of air taken in during 1 minute is called the minute volume of respiration (MVR) or minute ventilation, It is calculated by multiplying the tidal volume by the normal breathing rate per minute. An average MVR would be 500 ml times 12 respirations per minute of 6000 ml/min. Not all of the MVR can be used in gas exchange, however, because some of it remains in the anatomic dead space. The alveolar ventilation rate (AVR) is the volume of air per minute that reaches the alveoli. In the example just given, AVR would be 350 ml times 12 respirations per minute or 4200 ml/min. Remains 3 air volumes result when one engages in strenous breathing.

  • Inspiratory reserve volume (IRV) : By taking a very deep breath, you can inspire a good deal more than 500 This additional inhaled air, called IRV is about 3000 ml.
  • Expiratory reserve volume (ERV) : If you inhale normally & then exhale as forcibly as possible, you should be able to push out 1200 of air in addition to 500ml. of T.V. The extra 1200 ml. is called ERV.
  • Residual volume (RV) : Even after expiratory reserve volume is expelled, considerable air remains in the lung, this volume, which can not be measured by spirometry, it is called residual volume is about 1200






  • Dead space : Portion of tracheobronchial tree where gaseous exchange does not occur called dead

It is also called conductive zone. Dead space is 150 ml.

  • Functional residual capacity (FRC) : It is the amount of air that remains in the lungs after a normal It is about 2300 ml.


= 1100 + 1200 = 2300 ml.

  • Vital capacity (VC) : This is the maximum amount of air that can be expired forcefully from his lungs after first filling these with a maximum


deep inspiration. It is about 4600 ml.


= 3000+500+1100 = 4600 ml.

6, 000 ml


5, 000 ml

















4, 000 ml







Tidal volume

500 ml

Reserve END OF  

1,200 ml

RECORD Functional Residual
Residual Volume Capacity 2,400 ml
1,200 ml  


  • Total lung capacity (TLC) : TLC is the sum of vital capacity (VC) and residual volume (RV). It is about


= 4600 + 1200 = 5800 ml.

  • Inspiratory capacity (IC) : It is the total amount of air a person can


3, 000 ml



2, 000 ml



1, 000 ml

3, 100 ML




3,600 ml

4,800 ml






6,000 ml









inspire by maximum distension of his





I.C. = TV + IRV

= 500 + 3000 = 3500 ml.

Fig. – Spirogram of lung volumes and capacities (average values for a healthy adult)


 Process of Respiration.

The process of respiration is completed in 4 steps :

(i) Breathing or ventilation                    (ii) Exchange of gases or External respiration

(iii) Transport of gases                                (iv) Cellular respiration

  • Ventilation or breathing : Movement of thorax, expansion (inflation) and deflation of lungs and flow of air into the lungs and from the lungs. It is extracellular, energy consuming and physical process. Sum of inspiration and expiration is called respiratory There are two steps of breathing :

Difference between breathing and respiration


Breathing (Ventilation) Respiration
It is a physical process. It is a biochemical process.
It is simply an intake of fresh air and removal of foul air. It involves exchange of gases and oxidation of food.
No energy is released rather used. Energy is released that is stored in ATP.
It occurs outside the cells, hence it is an extra-cellular process. It occurs inside the cells, hence it is an intra-cellular process.






No enzymes are involved in the process. A large number of enzymes are involved in the process.
Breathing mechanism varies in different animals. Respiratory mechanism is similar in all animals.
It is confined to certain organs only. It occurs in all living cells of the body.



  • Inspiration : Intake of fresh air in lungs from It is an active process. Blood pressure increases during later part of respiration. Following muscles are involve in inspiration.
    • Diaphragm : Principle muscles of inspiration. Its skeletal muscles attached to sternum, vertebral column and ribs. It is formed by radial muscles fibres. The last end of radial muscles fibres form flat sheet called aponeurosis which encircle the aperture present at the centre of diaphragm through which oesophagus Muscular diaphragm is present only in mammals and its primary function is to divide body cavity in two parts upper thoracic and lower abdominal.







































In relaxed condition it is dome shaped. Convex towards thoracic cavity and concave towards abdominal

Fig. – Mechanism of breathing. A – Inspiration (Chest cavity

enlarged) B – Expiration (Chest cavity reduced) C – Intercostal muscles


cavity. During inspiration it contract and become straight and descends down. This cause an increase in vertical diameter of thoracic cavity. In quiet (i.e. tidal air volume breathing) breathing it descends about 1.5 cms. In very deep inspiration it may descends for about 10 cms. Descent of diaphragm can explain about 75% of tidal air volume. 70% muscles fibres of diaphragm have some resistance to fatigue. Nerve which supply to diaphragm is phrenic nerve. Contribution of diaphragm in breathing of full term pregnant lady is 0%. Most important function of diaphragm of mammals is to aid in inspiration. If diaphragm is punctured, respiration will stop and patient will die.

  • External intercostal muscles : Gaps between the ribs are called intercostal spaces. They are filled by intercostal Intercostal muscles are of two types external intercostal muscles and internal intercostal muscles.

External intercostal muscles are related to inspiration and internal intercostal muscles are related to expiration. Here we are concerned with external intercostal muscles. External intercostal muscles start from lower border of upper rib and comes to end outer lip of upper border of lower rib. Thus direction of external intercostal muscles fibres is downward forward. Contraction of external intercostal muscles causes increase in anteroposterior diameter of thoracic cavity and transverse diameter of thoracic cavity.

This two dimensional increase in diameter (i.e. anteroposterior and

transverse) of thoracic cavity is due to special arrangement of ribs. This increase of thoracic cavity assist by diaphragm the most important muscle of


inspiration, it is dome-shaped skeletal muscle. Contraction of diaphargm           POSITION AFTER EXPIRATION


causes it to flatten lowering its dome. For simplification we can assume that each rib attach anteriorly to sternum by its anterior end and posteriorly to

Fig. – Thoracic rib cage in side view, showing movements of ribs and sternum during breathing.






vertebral column by its posterior end. Note these two points carefully –

  • Anterior end of rib is lower than the posterior
  • Middle portion of rib which is called shaft lies at lower level than the two end of rib (e. anterior and posterior)

Contraction of external intercostal muscles cause elevation of ribs. Anterior end of ribs which lies lower than the posterior end. Elevates and moves forward and causes an increase in anteroposterior diameter of thoracic cavity. This movement of ribs is called pump handle movement. This movement mostly occur in vertebrosternal rib

i.e. 2nd to 6th ribs. Elevation of shaft of rib due to contraction of external intercostal muscles causes outward movement of rib. This causes an increase in transverse diameter (side by side) of thoracic cavity. This movement of ribs called bucket handle movement. This movement occurs in vertebrochondral ribs i.e. 8th, 9th and 10th ribs. In infants the ribs are approximately horizontal (i.e. shaft does not lie at lower level than the two ends of rib and anterior end of ribs is not lower than posterior end) thus respiration is mainly diaphragmatic.

  • Accessory muscles of inspiration : These muscles normally are not called into action but in forced inspiration they come into Accessory muscles are scaleni, sternomastoid and alae nasi.
  • Expiration : Out flow of the air from the lungs is called expiration. When the inspiratory muscles relax. As the external intercostal relax, ribs move inferiorly and as the diaphragm relaxes, its dome moves superiorly owing to its elasticity. These movements decrese vertical and anterior-posterior dimentions of thoracic cavity. Normally it is a passive process and occurs when the muscles of inspiration stop to contract. Lungs are made of elastic fibres. These elastic fibres behave as rubber band. These elongate when stretched but recoil back to its original length when stretch is Thus lungs shrink due to elastic recoil of elastic fibres and air is expelled out from the lungs. Thus expiration takes place. In forced expiration (i.e. asthma, coughing) the muscles of expiration contract and help to reduce the volume of thoracic cavity. In this conditions expiration is active process. Following muscles are involve in expiration.





















Fig. – Show how ‘pump handle’ movements of the sternum bring about an increase in the anteroposterior diameter of the thorax

Fig. – Scheme to show how ‘bucket handle’ movements of the

vertebrochondral ribs bring about an increase in the transverse diameter of the thorax.






  • Internal intercostal muscles : Direction of fibres is backward and downward. Action is just opposite to external intercostal muscles. These muscles by their action reduce antero-posterior and transverse diameter of thoracic
  • Abdominal muscles : Muscles of anterior abdominal These muscles push the diaphragm up.



(c)  Mechanism of ventilation/breathing :

Enlargement of thoracic cavity due to action of muscles of inspiration.


Lungs follow the enlargement of thoracic cavity and expand.


Expansion of lungs leads to reduction of air pressure within the lungs than the atmospheric pressure.


Air from the outside, where pressure of air is high, comes inside the lungs.


At the top point of inspiration contraction


of inspiratory muscles suddenly stops.


Impulses from CNS


Inspiratory muscles relax and also due to elastic recoil, lungs shrink.


Air pressure within lungs exceeds atmospheric air pressure.


Air is expelled out into the atmosphere.


Pressure within pleural cavity is called intrapleural pressure. It is normally negative (–5 cm.). if atmospheric pressure (760 mm Hg) is considered to be zero (or base) than the 755 mm Hg intrapleural pressure is said to be negative i.e. – 5 mm. Hg. During inspiration, negativety increases and during expiration it decreases. In quiet respiration intrapleural pressure varies between – 3.5 mm Hg (at the end point of inspiration) to +0.7 mm Hg (at then end of expiration) In very deep inspiration intrapleural pressure may be – 30 mm Hg. In violent expiration, especially with glottis closed the intrapleural pressure may be +20 mm Hg. In clinical practice, the intrapleural


pressure is measured in cm

HO . (1 cm

H 2 O = .7 mm Hg). Pressure within lungs is assumed to be equal


intrapleural pressure. In diseases in which breathing is difficult (i.e. asthma, emphysema) patients are most comfortable on sitting up, leaving forwards and fixing the arms.

During inspiration fresh air follows following path external nares ® nasal chambers ® internal nares ®

pharynx ® glottis ® larynx ® trachea ® bronchi ® bronchioles ® alveolar ducts ® alveoli.

Advantage of negative pressure breathing : Mammals have negative pressure breathing, i.e. the lungs draw air due to reduction in pressure within them. This allows them to eat and breathe at the same time. If air were to be forced into the lungs, it might carry food particles into the trachea and block it. Negative pressure breathing gently moves air which is less likely to carry food particles into the wind pipe.

Positive pressure breathing : Frog closes the mouth, opens the nares and lowers the throat. This enlarges the buccopharyngeal cavity, where reduced pressure draws fresh air via nares. This part of breathing occurs on the negative pressure principle. The frog then closes the nares and raises the throat, forcing the air into the lungs. This is positive pressure breathing. After exchange of gases in the lungs, frog opens the nares and expels foul air by contracting abdominal muscles.






(ii)  Exchange of gases :

  • Exchange of gases in lungs : It is also called external In this gaseous exchange oxygen


passes from alveoli to pulmonary capillary blood and

CO2 . Comes to alveoli from pulmonary capillary. In order to


be exchange the gases have to pass through alveolocapillary membrane or respiratory membrane. Composition of alveolocapillary membrane is epithelium lining of alveolar wall, epithelial basement membrane, a thin interstitial space, capillary basement membrane and capillary endothelial membrane.

Thickness of respiratory membrane is 0.5 mm. Respiratory membrane has a limit of gaseous exchange between alveoli and pulmonary blood. It is called diffusion capacity. Diffusion capacity is defined as volume of gas that diffuse through membrane per minute for a pressure difference of 1 mm Hg. Exchange of gases through alveolocapillary membrane is a purely physical diffusion phenomenon. No chemical reaction is involved. Diffusion of a gas depends upon pressure gradient across the membrane and solubility of gas.














Fig. – Exchange of O2 and CO2 in the lungs (external respiration)



More pressure gradient ® quickly diffusion. Diffusion of

CO2 is 20 times faster than oxygen. Diffusion is also


directly proportional to thickness of membrane, surface area of membrane, permeability of membrane. As already mentioned, we inhale 500 ml. of air (tidal volume) in each breath, i.e., 6000 ml. each minute (at a normal rate of 12 breaths per minute). About 150 ml. of inspired or expired air in each breath is retained in the respiratory passageways. Since this air is not involved in gaseous exchange, the space enclosed by respiratory passageways is called “dead space”. Obviously, only 350 ml of inhaled air in each breath (4200 ml per minute) reaches into the lung alveoli, mixes with the functional residual air of alveoli and, thus, takes part in gaseous exchange. It brings with it about 69 ml of O2 . When it is expired, it takes back only about 48 ml of O2 . With each breath, thus, about 21 ml

of O2 becomes available to pulmonary blood for absorption from alveolar air. Thus, our normal intake of O2


amounts to about 250 ml per minute. Similarly, the inspired air in each breath brings only about 0.14 ml of



into alveolar air, but when expired, it contains about 18.55 ml of of about 18.4 ml of CO2 (about 220 ml each minute).

CO2 . Thus, with each breath, our blood gets rid



Since gaseous exchange occurs continuously between the 2300 ml of alveolar air and pulmonary blood, the 350 ml of atmospheric air, reaching into and leaving lung alveoli in each breath, in effect, merely serves to slowly



renew the alveolar air. This slow renewal of alveolar air prevents sharp and sudden changes in P                  ,


concentrations in blood.


and pH






Partial pressure : Partial pressure of a gas is the pressure it exerts in a mixture of gases, and is equal to the total pressure of the mixture divided by percentage of that gas in the mixture. For instance, if the pressure of atmospheric air at sea level is 760 mm. Of mercury (Hg) and oxygen forms 21% of the air, the partial pressure of oxygen will be 21% of 760, or 159 mm. Hg. In other words, the partial pressure of a gas is proportional to its


concentration in the mixture. Only about 0.3 ml. of

O2 can dissolve in 100 ml. of plasma, about 20 ml. of

O2  is


carried by haemoglobin in 100 ml. of blood. In atmospheric air except these gases some traces of helium, argon and neon are also found.

Partial pressures of respiratory gases in mm. Hg
Gas Inspired air Alveolar air Venous blood Arterial blood Expired air Tissue cells
Oxygen 158 100 – 105 40 95 – 100 116 20 – 40
Carbon dioxide 0.3 40 46 40 32 45 – 52
Nitrogen 596 573 573 573 565

Composition of three samples of air

Gases Inspired air Expired air Alveolar air Gain / loss %
Oxygen 20.84% 15.70% 13.6% Gain 5.14%
Carbon dioxide 0.04% 4.00% 5.3% Loss 3.96%
Nitrogen 78.62% 74.50% 74.9% Gain 4.12%
Water 0.5% 6.2% 6.2% Loss 5.7%

A part of the inspired air is left in the respiratory tract, the so called “dead space”, where no gaseous exchange occurs. This “dead space” air is expelled at the next expiration. The expired air, thus, contains fresh air from the “dead space” and foul air from the lungs. Therefore, the alveolar air has less oxygen and more carbon dioxide than the expired air. A part of the expired air is also left in the dead space. This air enters the lungs at the next inspiration. Some air is also left in the lungs after expiration as residual air. The fresh inspired air is mixed up in the


lungs with the foul air from the “dead space” and the stale residual air. Therefore, the alveolar air has less O2





than the inspired air also. The inspired air has the composition of the atmospheric air. Exchange of


gases in lungs can be divided into two steps :



Uptake of O by blood in lung : The P


(partial pressure of oxygen) of the alveolar air is higher than



the P


of blood in alveolar capillaries. Due to a P



difference between air and blood, oxygen diffuses rapidly


from the alveolar air into the blood of alveolar capillaries. It may be remembered that gases always diffuse from a region of higher partial pressure (concentration) to a region of lower partial pressure or concentration. At rest RBC stay for only one second in a pulmonary capillary. Hb becomes saturated within about 0.3 sec. During exercise, circulation speed is high RBC stays for 0.3 sec in a capillary, again it sufficient duration for oxygenation.


(2)   Release of CO2 by the blood : The


(partial pressure of carbon dioxide) of blood reaching the



alveolar capillaries is higher than the


of alveolar air. Therefore, carbon dioxide diffuses from the blood of



alveolar capillaries into the alveolar air. The exchange of gases in the alveoli that raises the P


of blood and lowers




is the external respiration. The blood oxygenated by this respiration is returned from the lungs by



pulmonary veins to the left side of the heart. The heart supplies the oxygenated blood to the body tissues. In


alveolar air partial pressure of

O2 is 100 mm Hg and in pulmonary capillary blood is 40 mm Hg. Thus

O2  from







alveolar air transfer to blood.


in alveolar air is 40mm Hg and is pulmonary capillary blood CO2

is 46mm Hg.



Thus CO2 flows from blood to alveolar air.

  • Exchange of gases in tissues : In the tissues, exchange of gases occurs between the blood and the tissue This exchange occurs via tissue fluid that bathes the tissue cells. The blood reaching the tissue capillaries



has P


higher than that in the tissue cells and


lower than that in the tissue cells. The tissue cells constantly



use oxygen in oxidation that produces carbon dioxide. Therefore, they always have lower P


and higher

PCO than



the blood coming to them. Because of

P     and



PCO differences between blood and tissue cells, oxygen separates



from oxyhaemoglobin and diffuses from the blood into the tissue fluid and thence in to the tissue cells; and carbon dioxide diffuses from the tissue cells in to the tissue fluid and thence in to the blood in the tissue capillaries. Gases mostly diffuse through the tissue fluid as such, only small amounts dissolve in it.



Fig. – Exchange of O2 and CO2 in the tissue (internal respiration)



Exchange of gases in the tissues that lowers the

P     of the blood and raises its



PCO   is called internal



respiration. The blood deoxygenated by this respiration returns to the right side of the heart that sends it to the lungs for reoxygenation.


  • Transport of gases : Blood carries

O2 from respiratory organs to the tissue cells for oxidation and CO2


from tissue cells to respiratory organs for elimination. Blood should be slightly alkaline to help the transport of O2

and CO2 properly.

  • Transport of oxygen : Lung contains atmospheric From the lung O2 diffuses into the blood. The


blood transport O2 from the lung to the cells. This is called oxygen transport. forms –

O2 is carried in the blood in three


  • In physical solution : 100 oxygenated blood contains 20 ml(0.3 ml dissolved in plasma and 19.7 ml


bound to Hb) of oxygen. 2-3% oxygen is transported in form of physical solution. Thus 0.3 ml to 0.6 ml.

O2  is in


physical solution form in 100 ml of blood. Pressure of a gas in blood is produced by only that portion of

O2 which


is in physical solution.

HbO2 (oxyhaemoglobin) is not directly responsible for pressure produced by gas.

O2 first


dissolves in plasma and forms physical solution. As soon as the conc. of

O2 in physical solution in plasma exceeds


to 0.3 ml to 0.6 ml/100 ml of blood, O2 goes to combine with Hb as HbO2 thus Hb acts as a sink.





  • As oxyhaemoglobin (HbO2) : Most of O2 is transported in form of 98.5% in the form


of HbO2, 1.5% is carried in the dissolved state in watery blood plasma. oxyhaemoglobin. Following events are happened in this connection.

O2 is transported in form of







O2 flows from alveolus to capillary blood


O2 form physical solution in plasma


Tension of O2 in plasma increase


Hb exposed to higher O2 tension


Hb form oxyhaemoglobin with O2 (Hb4 + 4O2 ® Hb4 O8 )

One gram haemoglobin binds about 1.34 ml of oxygen. 100 ml. of blood contain 15 gm. haemoglobin. Thus


100 ml. blood can carry 20 ml. (19.4 ml exactly)

O2  as

HbO2 . The iron of haemoglobin normally remains in


reduced (ferrous Fe ++ ) state. If made to react with ozone or some other oxidizing agent, it loses an electron and becomes oxidized (ferric – Fe +++ ). Haemoglobin, thus oxidized, is called methaemoglobin. The latter cannot deliver


oxygen. Pure oxygen does not oxidize the iron of haemoglobin; a molecule of

O2 binds reversibly with each iron


atom one after the other in such a way that the iron does not lose an electron and, hence, remains in ferrous condition. That is why, this reaction is called the process of oxygenation, rather than oxidation of haemoglobin. Oxygenated haemoglobin is called oxyhaemoglobin : It is bright red, while haemoglobin has a slight tinge of violet. That is why, oxygenated or purified blood appears bright red and deoxygenated or impure blood bluish.

  • By cell membrane of RBCs : Traces of O2 is presumably transported as bound with the cell membrane of

Oxygen content : Total volume of O2 in 100 ml. of whole blood i.e. volume of O2 in physical solution form and oxyhaemoglobin form. It is equal to 19.7 + 0.3 = 20 ml of oxygen.


Oxygen capacity : Maximal amount of

37o C . Oxygen capacity is about 20 ml/100 ml.

O2 that can be held by the blood at 760 mm Hg pressure and



Percentage of saturation of haemoglobin :

 oxygen content ´ 100 . In healthy men 5% saturation of Hb

oxygen capacity


in arterial blood is

19 ´ 100 = 95% . In mixed venous blood at rest it is about 75%.







Oxygen-haemoglobin dissociation  curve  :  When a graph is plotted between % saturation of


haemoglobin and oxygen tension, a curve is obtained termed as

O2 – Hb dissociation  curve. Oxygen-Hb


dissociation curve is sigmoid shaped-or S shaped. This sigmoid shaped curve is characteristic for Hb.

Biological advantage of the sigmoid shape (curve)


  • Due to sigmoid shape


value of Hb is high. It is 25 mm Hg. So in the tissues

O2 unloading is


  • Even if there is moderate low atmospheric pressure sufficient amount of Hb becomes







(mm Hg)

% Sat. of Hb
10 13.5
20 35
30 57
40 75
50 83.5
60 89
70 92.7
80 94.5
90 96.5
100 97.5



















Partial pressure of oxygen (Po2) in mm Hg

Fig. – Oxygen-haemoglobin dissociation curve at pH 7.4 and temperature 38º


Dissociation of oxygen from oxyhaemoglobin

50                      Fetal hemoglobin has a higher affinity

40                        for O2 than does adult hemoglobin





10 20 30 40 50 60 70 80 90 100

pO2 (mmHg)

Fig. – Oxygen-hemoglobin dissociation curves comparing fetal and maternal of hemoglobin


Arterial blood reaches to the tissue level


It is exposed to low tension of O2 in tissue


O2 flows from plasma to the tissue


Concentration of O2 in physical solution decrease


HbO2 dissociated to make up the loss of O2 from physical solution in plasma



If O2

concentration in tissues is equal to the respiratory surface,


dissociation   of   Hb- O2 cannot   be   possible   into   Hb   and

O2 .       The


pH (7.6)


percentage of haemoglobin that is bound with O2 is called percent saturation of haemoglobin. It depends upon the partial pressure of


pH (7.4)





(P   )



in blood. As already noted, the partial pressure of oxygen

pH (7.2)



(P ) in the oxygenated blood leaving the lungs is about 95 to 97 mm Hg



and, at this

P     , haemoglobin is about 97% saturated with



O2 .



Conversely, the P


in deoxygenated blood returning from body tissues is

pO2 (mm Hg)


only 40 mm Hg and, at this

P     , haemoglobin is only about 70% to 75%



Fig. – Effect if pH on affinity of hemoglobin for oxygen







saturated with

O2 . Thus about 25% Hb gives up its

O2 during one circulation of blood. Ratio of oxyhaemoglobin


and haemoglobin in the blood is based upon CO2

Shift of Hb-O2 curve

tension in blood. (See Bohr’s effect)


  • Right shift means : Low affinity of Hb for O2 as a result release of O2 from HbO2



Thus it is clear that right shift is required when supply of

O2 is needed. At tissue level blood loses its

O2  and


curve shifts to right. During active exercise muscles require more O2

than normal condition. This supply of extra


O2 comes from shifting of curve to right.

Factors which shift curve to right

  • High CO2 tension in blood. This is called Bohr’s effect (PCO2 60 mm Hg).



  • Low pH of blood or increase concentration of H +
  • High body temperature (43ºC or 110ºF).

ions (pH 7.2).


  • Increase level of 3 DPG (Diphosphorylglycerate) now called BPG (Biphospho glycerate) in RBC’s. BPG

– previously called diphosphoglycerate (DPG), decreases the affinity of Hb for O2 and thus helps to release O2 from Hb. BPG formed in RBC certain hormones such as thyroxine, human growth hormone, epinephrine, norepinephrine and in people increases the formation of BPG. The level of BPG also higher in people living at high attitude. It forms during glycolysis in RBCs. At tissue level right shift of curve is favourable.

Bohr’s effect : Hb- O2 dissociation curve shifts to right when CO2 tension in blood is high. Bohr discovered

this effect in 1904. Bohr effect is the effect of CO2 on oxyhaemoglobin. Deoxygenation of oxygaemoglobin is


directly proportional to blood pCO2. Extent of Bohr’s effect depends upon the tension of

CO2 in blood only.



of tissue fluid and alveoli does not exert Bohr’s effect. During exercise muscles need more

O2 and want to remove



which has high production. Because of pressure gradient


moves from tissues fluid to capillary blood ®


Exert Bohr’s effect ® O2 release is hastened from Hb- O2 i.e. Hb- O2 curve shifts to right.

  • Left shift means : Increase affinity of Hb for O2 . Hb- O2 formation is hastened.


In lungs CO2 is released from capillary blood.


Affinity of Hb for O2 increases i.e. curve shift to left.


Hb- O2 formation is hastened. Which is strongly required for proper oxygenation of blood. At lung level left


shift is favourable.

PCO2 20 mm Hg

Increase pH of blood – 7.6

Low body temperature 20ºC or 68°F

Concept of P50 : Partial pressure of




O2 at which 50% haemoglobin of blood becomes saturated called




P50 .



value of Hb is 25 mm Hg.


indicates the affinity for Hb toward oxygen. High value of


indicates low







affinity for Hb toward

O2 thus

O2 release hastened. Low value of



indicates high affinity for Hb toward

O2 .


Thus formation of

HbO2 hastened. High P50

value is same as shift to right and low P50

value is same as shift to left.


Foetal haemoglobin and myoglobin have low


value. Presence of


acidity (low pH), temperature rise, 2, 3


DPG cause rise of


value. If


value of Hb rises to 100 mm Hg person will die of

O2 deficiency because


loading and unloading of Hb will not occur.

Haemoglobin : Oxygen carrier or respiratory pigment in vertebrates blood is haemoglobin. Hb molecule is made of two components haem and globin. Globin part is globulin protein which is made of four polypeptide chain, two a chains (141 amino acid) and two b chains (146 amino acid). Thus total no. of amino acid in Hb 574. Haem is iron containing compound and belongs to the class of compound called protoporphyrins. The plasma membrane of RBC encloses 280 millions Hb molecules i.e. 33% of cell weight. Hb in RBC synthesized before loss of nucleus.

Hb also transport about 23% of total CO2. Hb is conjugate or chromo protein. Iron of Hb is in ferrous state (Fe ++ )


and even after the combination with O2

it remains ferrous. Iron of Hb remains in

Fe ++

state due to presence of


methaemoglobin reductase enzyme. One Hb molecule has 4 haem molecules. Each haem is associated without polypeptide chain. Each Hb molecule can combine with one molecule (2 atoms) of oxygen. Thus each molecule of


Hb combines 4 molecules of

O2 . Haem molecule is made up by 4 pyrrole structures. Iron is found in centre of 4


pyrrole rings. Hb is synthesized by cells of erythroid series in the red bone marrow. Affinity of Hb for oxygen is ideal


neither excessive nor little. Affinity of Hb for

O2 is more than the

CO2 . Haemoglobin act as a buffer. Addition of



hydrogen ions would make the blood very acidic. However, most of the hydrogen ions are neutralized by combination with haemoglobin, which is negatively charged, forming acid haemoglobin. This reduces the acidity of


the blood, and also releases additional oxygen.

HbO + H +

HHb + O2


If the blood becomes too basic, acid haemoglobin dissociates, releasing hydrogen ions.

Thus, the haemoglobin also acts as a buffer, a substance that keeps the pH from fluctuating.

HHb ® H + + Hb .


Myoglobin : It is chemically and functionally similar to Hb. It is made up of one polypeptide chain (153


amino acids) attached with on haem group.


value for myoglobin is 5 mm Hg. This indicates that myoglobin


release oxygen less readily than Hb. It is found in muscles. It acts as a store house for

store about 1.5 litre oxygen in myoglobin. Hb- O2 dissociation

O2 . An average man can


curve for myoglobin is hyperbola.

(ii) Transport of  CO2  : Transportation of     CO2  by






blood is mush easier due to its high (20 times that of

O2 )



solubility in water. Blood can carry upto 50% or 60% of CO2


by volume, but normally about 4 ml of CO2 on an average is

CO2         CO2


transported from tissue to the lungs in each 100 ml of blood in man. With 5 litres of cardiac output per minute, the blood thus

transports about 200 to 220 ml of CO2 each minute.                                           O2

Obviously, this is the rate at which CO2 is produced and released into tissue fluids by cells, and at which it diffuses out


Fig. – Transportation of CO2 by blood






into alveolar air from pulmonary arterial blood. The blood transports this CO2 in three ways.

  • In dissolved state : Deoxygenated ( PCO2 is 45 to 46 mm Hg) and oxygenated (PCO2 is 40 mm Hg)

bloods respectively carry about 2.7 and 2.4 ml of CO2 per 100 ml of blood in dissolved state in plasma (= in solution with plasma). Thus, about 0.3 (2.7 minus 2.4) ml of CO2 is transported by each 100 ml. of blood in dissolved state in plasma. This is about 7% of all the CO2 transported by blood from tissues to the lungs.

  • In the form of bicarbonate ions : Most of the CO2 that dissolved in blood plasma reacts with water, forming carbonic acid – CO2 + H2O H2CO3 (carbonic acid)

This reaction is very slow in plasma, but occurs very rapidly inside RBCs, because an enzyme, carbonic

anhydrase, present in RBCs, accelerates its rate about 5000 times. That is why, about 70% of the CO2 (about 2.5 ml per 100 ml of blood), received by blood from the tissues, immediately enters into RBCs and hydrated to carbonic acid. Almost as rapidly as formed, all carbonic acid of RBCs dissociates into hydrogen and bicarbonate ions (H+ and HCO3). The hydrogen ions mostly combine with heamoglobin for keeping the pH of blood. (7.4) in steady state, because haemoglobin is a powerful acid base buffer. Being quite diffusible, the bicarbonate ions, on the other hand, diffuse from RBCs into the plasma. To maintain electrostatic neutrality of plasma, many chloride ions, in turn, diffuse from plasma into the RBCs. Obviously, the chloride contents of RBCs increase when oxygenated blood becomes deoxygenated. This is termed “chloride or Hamburger shift”.


Sequence of events : From tissues

CO2 enters in plasma ® a small fraction of CO2 is dissolved in plasma


® rest of


enters into the RBC ® within RBC


combines with


in presence of enzyme carbonic


anhydrase and forms H 2 CO3 ® H 2 CO3

splits into H + and HCO ® most of the HCO

comes out of RBC and



enters in plasma and form


NaHCO3 , small fraction stays back within the RBC to form


KHCO3 and H + combine


with Hb to form reduced haemoglobin H.Hb.


Transformation forms of CO2 Transported quantity
CO2 7 % (0.3 ml/100ml of blood)
HHbCO2 23% (2.5 ml/100 ml of blood)
HCO2 70% (1ml/100 ml of blood)


  • In the form of carbamino compounds : In addition to reacting with water, CO2 also directly and reversibly reacts with haemoglobin, loosely binding with it and forming an unstable compound, called carbaminohaemoglobin (CO2HHb). It also similarly forms loose bonds with some plasma It is estimated that about 23% of the CO2 (1 ml per 100 ml of blood), collected from cells through tissue fluids, is transported by blood in this form.

Haldane effect and CO2 diffusion into the alveoli : Whereas the Bohr effect promotes O2 transport, the Haldane effect is important in promoting CO2 transport. The Haldane effect results from the simple fact that oxyhaemoglobin behaves as a strong acid. This in turn, displaces CO2 from the blood in two ways.

  • Due to its increased acidity, the haemoglobin loses its capacity to combine with CO2. Hence all carbamino haemoglobin dissociates to release its CO2.





  • Secondly, the highly acidic oxyhaemoglobin releases an excess of H+ which bind with bicarbonate ions (HCO3), forming carbonic The latter soon dissociates into H2O and CO2. This CO2 diffuses into the alveoli.

Thus, in the lung, the haldane effect, increases release of CO2 because of O2 uptake by haemoglobin. In the tissues a reverse process occurs. The Haldane effect increases CO2 uptake because of removal of O2 from haemoglobin.

 Control of breathing.

Respiratory rhythm is controlled by nervous system. Inspiratory and expiratory centres are jointly called rhythmicity centres. Inspiratory centre is dominant over expiratory centre. When pneumotaxic is stimulated respiration rate increases inspiration as well as expiration is shortened. Respiratory movements are under control of medulla oblongata.












For the control of respiration following respiratory centres are found in hind brain


Type of centre Location Function
Inspiratory centre Medulla oblongata Inspiration (2 second active condition).
Respiratory centre Medulla oblongata Expiration (3 second inactive condition)
Apneustic centre Pons Slow and deep inspiration
Pneumotaxic centre Pons Control other centres and produce normal quite breathing
Gasping centre Pons Sudden and shallow respiration



  • Chemical control : This includes

CO2 , O2 and H +

conc. of blood for detection of concentration of O2 ,


CO2 and H +

ions in blood two types of receptors are found. These receptors are called chemoreceptor.


  • Peripheral chemoreceptor : These include two sets – Carotid body is present in the wall of the left and the right common carotid arteries and aortic bodies is present in the arch of They are placed in the vascular endothelium and come in contact with the blood. When PO2 decreases or increases in arterial blood, these receptors are stimulated and send impulses to respiratory centre to respectively increases or decreases the rate intensity of inspiratory signals.







Low partial pressure (concentration) of

O2 in blood increases respiratory rate. High partial pressure


(concentration) of


in blood increases respiratory rate. High concentration of H +

ion (low pH) increases


respiration rate. Carotid body cells are affected only by dissolved

O2 . For correction of


concentration central


chemoreceptors play the dominant role. Role of peripheral centre is minor. For correction of there are no central chemoreceptors. Only peripheral receptors are found.

O2 concentration,


  • Central chemoreceptors : These are present on ventral surface of Cells of these centres are


bathed in brain tissue fluid. They are in close vicinity of CSF. In brain tissue fluid as well as in CSF the

CO2  is


converted into

H 2 CO3  ® H 2 CO3


H + + HCO ® H +

ions are liberated ® H +

ions stimulate the central



chemoreceptor (C.C) cell ® stimulation from C.C goes to respiratory centre ® Respiration stimulate.

(ii)  Effect of different gases


  • Effect of CO2 : Rise in tension of arterial


or alveolar


causes stimulation of respiration. Both


the rate and depth of respiration increased. This leads to washing out of CO2 from body.




pO2 (mm Hg)


Fig. – Effect of pCO2 on affinity of hemoglobin for oxygen

pO2 (mm Hg)

Fig. – Oxygen-hemoglobin dissociation curve showing the relationship between temperature and hemoblobin saturation with O2


  • Effect of O2 : Fall of O2 concentration in inspired air causes stimulation of peripheral chemoreceptors

neural impulse arise from peripheral chemoreceptors. These impulse gots respiratory centre and cause respiratory stimulation. Some factor which increase the respiratory rate.

(iii) Factors increases respiratory rate

  • Sympathetic stimulation causes reduction of blood supply to carotid body by vasoconstriction ®
  • Excitement (c) Muscular exercise                        (d) Rise in body temperature


(e) Rise in H +

ion conc. or low pH.     (f) Renal failure                              (g) Diabetes acidosis


(h) Pain                                         (i) Blood pressure                           (j) Lymphatic system

  • Adaptations of diving mammals : Marine mammals (seals, whales) can make long underwater dives as they have more blood per kilogram of body weight, can store more oxygen in blood and muscles, have a large spleen with a considerable stockpile of blood, and can reduce O2 consumption rate when under

 Respiration in frog.






Frog is an amphibious animal i.e. they are live in water as well as on land hence according to their adaptations they process different modes of respiration, which are as follows –

  • Cutaneous respiration : By the skin. Under water, during hibernation frog respires by only skin. On land cutaneous respiration continues as usual. Thus cutaneous respiration take place always. By cutaneous respiration frog fulfill its 30% need of
  • Buccopharyngeal respiration : Like the skin, the mucosa of buccopharyngeal cavity in frog is also ideally adapted for gaseous Hence, while quietly floating upon water surface, and even when resting upon land, frogs respire by their buccopharyngeal cavity also. During this process, the mouth, gullet and glottis remain closed, but nares remain open. The floor of the cavity is alternately lowered and raised (oscillatory movements). Atmospheric air is sucked into the cavity through nares when the floor lowers, and it is forced out when the floor rises. Lowering of the floor is brought about by contraction of a pair of sternohyal muscles whose one end is inserted upon ventral surface of hyoid and the other upon dorsal surfaces of clavicle and coracoid bones of pectoral girdle. Similarly, raising of the floor is brought about by contraction of a pair of petrohyal muscles whose one end is inserted upon the sides of hyoid and the other upon the auditory capsules of respective sides near


squamosals. Intake of

O2 by buccopharyngeal respiration approximately accounts for only about 5% of total

O2 –


intake. It stops when mouth is opened.

  • Pulmonary respiration : In frog, pulmonary respiration accounts for about 65% of the total

O2 -intake.


It particularly occurs when frogs lead an active life during rains and spring; either hopping upon land in search of food, or actively breeding in water.

  • Inspiration : The floor of buccopharyngeal cavity is lowered by contraction of sternohyal muscles, so that air fills in the cavity. Next the submental muscles of lower jaw contract, raising the mentomeckelian bones located at the front end of lower jaw rami. These bones, in turn, raise the premaxillae which close the external nares. Now, the petrohyal muscles contract and raise the floor of the Due to the resultant pressure, the air of the cavity forces its way into the lungs through glottis, because the external nares are closed. This is, thus, inspiration.
  • Expiration : The floor of the cavity is first raised, and external nares are closed. Next, the floor of the cavity is lowered, so that air fills in the cavity from the lungs, because external nares are This expulsion of air from lungs into buccopharyngeal cavity is helped by contraction of abdominal and lung muscles. Soon, the submental muscles relax to open the external nares and the floor of the cavity is raised, so that the air is forced out of the cavity.

 Important concept of respiration.

  • Respiratory quotient (R.Q.) : Respiratory quotient is the ratio of carbon dioxide output to oxygen usage during It is measured by Ganong’s respirometer.

R.Q. = Volume of CO2 formed Volume of O2 utilized






High RQ Low RQ
Due to fat deposition

Due to fever

Due to muscle exercise

When CO2 is fixed

When CO2 retain in tissue In hibernating mammals

During glycolysis Due to acidosis
In low O2 environment

Due to oxidation of pyruvic acid.

Due to alkalosis

Due to diabetes

  In starvation
  During gluconeogenesis
  During glyconeogenesis


The volume of RQ depends upon the type of fuel substance being utilized for energy production.



Respiratory substrate Respiratory quotient
Carbohydrate 1.00
Proteins 0.5 – 0.9 Slightly less than 1 (0.9)
Fats 0.7
Organic acid 1.33


In an organism utilizing carbohydrates as source of energy anaerobically, the RQ is likely to be infinity. When carbohydrates are substrates for respiration, it is called ‘floating respiration’. Diabetic patient shows low R.Q. due to increased dissimilation of fats and the decreased dissimilation of carbohydrate.

  • Effect of CO : Carbon monoxide is a poisnous Hb has maximum affinity for CO. Carbon monoxide


binds with haemoglobin at the same place where

O2 binds, but about 250 times more readily than

O2 . Hence, it


readily displaces

O2 from haemoglobin and even a 0.4 mm Hg partial pressure of CO in alveolar air is enough to



occupy about half of the haemoglobin of pulmonary blood rendering it useless for    transport. A CO pressure of

about 0.7 mm Hg (concentration of about 1%) in alveolar air can be lethal. That is why, the atmosphere of industrial areas, being loaded with chimney smoke, is regarded harmful to health. It forms carboxyhaemoglobin


with Hb which is most stable. Sudden deep inspiration is due to either increase in concentration of

CO2   or


decrease in concentration of

O2 . Forced deep breathing for a few minutes by a person sitting at rest may be


followed by a temporary cessation of breathing. This is influenced by too much

O2 and least


in blood.


Sudden deep inspiration is due to either increase in concentration of CO2 or decrease in concentration of O2.

  • Regulation at high altitudes : At high altitudes, the composition of air remains almost the same as at sea-level, but the density (barometric pressure) of air gradually While ascending up a mountain, one inspires thin air, getting less oxygen. Less O2 level in the blood results in hypoxia. The chemoreceptor simulatory mechanism progressively increases the rate of ventilation. Ventilation ordinarily does not increase significantly until


one has ascended to about 2500 metres, because the


and pH remain almost normal. At an altitude of about



3500 to 4000 metres, increasing hypoxia causes drowsiness, lasstitude mental fatigue, headache and nausea. By the time one ascends upto about 5000 to 6000 metres, the rate of ventilation reaches about 65% above normal,






because a large amount of CO2 is breathed out, reducing


and increasing pH. A continuous exposure of a few



days to this height, however, increases ventilation to about 3 to 7 times normal, because


becomes normal and



pH decreases. Thereafter, a person gradually starts becoming acclimatized to high altitude conditions due to a significant increase in RBC-count and haemoglobin content in blood, in diffusion capacity of lungs, in vascularity of


tissues and in ability of cells to utilize oxygen despite low

P     . When acclimatized, the breathing in concerned




person becomes normal. Above on altitude of 6000 metres person becomes unconscious. At an altitude of 11000 metres, the air is so thin that a person can not remain alive even with the help of oxygen cylinder.

(iv)  Disorders of Respiratory system

  • Hypoxia : Hypoxia is a condition of oxygen shortage in the It is of two types :
    • Artificial Hypoxia : It results from shortage of oxygen in the air as at high (over 2400 m.) altitudes. It causes mountain sickness characterised by breathlessness, headache, dizziness, nausea, vomiting, mental fatigue and bluish tinge on the skin and mucous
    • Anaemic Hypoxia : It results from the reduced oxygen-carrying capacity of the blood due to anaemia (decreased haemoglobin content in blood) or carbon monoxide poisoning (some haemoglobin occupied by CO). in both cases, less haemoglobin is available for carrying O2 .


(b)   Asphyxia (Suffocation) : The

O2 content of blood falls and the


content rises and paralyses the


respiratory centre. Breathing stops and death occurs.

  • Bad cold : Disease-causing microbes present in the air attack respiratory tract, producing inflammation of the mucous membrane and caused increased secretion :
    • Rhinitis in the nasal
    • Sinusitis in the
    • Pharyngitis in the pharynx, often called sore throat, and is usually accompanied by tonsillitis (enlargement of tonsils).
    • Laryngitis in the larynx, causing hoarse voice and difficulty in
    • Bronchitis in the
  • Emphysema : The air-pollutants that cause chronic bronchitis, may breakdown the alveoli of the lungs, reducing the surface area for gas The victim becomes permanently short of breath.
  • Bronchial asthma : It is an allergic attack of breathlessness associated with bronchial obstruction or spasm of smooth muscle (contraction), characterized by coughing difficult breathing and wheezing patient has trouble
  • Bronchitis is caused by the permanent swelling in As a result of bronchitis cough is caused and thick mucus with pus cells is spitted out. Dyspnea fever develops. Dyspnea means hunger of air or deficiency of


oxygen in the blood or development of hypercapnia i.e., increase of


concentration in blood. This disease is


accelerated by fatigue, malnutrition, cold etc. the patient experiences difficulty in breathing. Here hypertrophy and hyperplasia of bronchi takes place.





  • Pneumonia : Oxygen has difficulty diffusing through the inflammed alveoli and the blood PO2 may be drastically Blood PCO2 usually remain normal because CO2 diffuses through the alveoli more easily than O2. In chronic patients of common cold and influenza, the lining epithelium of bronchi and lungs is inflammated. This disease is caused by streptococus pneumoniae, other bacteria, fungi, protozoans, viruses and the patient feels difficulty in breathing. Its prominent symptoms are trembling, pain in chest, fever, cough delirium etc. This disease is prevalent in either children or elderly persons in old age.
  • Lung cancer : It is believed that by excess smoking, lung cancer (carcinoma of lungs) is caused. The tissue increases limitlessly, which is called This disease is fatal. The frequency of occurrence of this disease in smokers is 20% more. Malignancy of tissues (neoplasia) causes pressure on the cells of other tissues and destroys them. The blood capillaries are ruptured, blood starts flowing and death is caused by excessive bleeding.
  • Tuberculosis : This disease is also called T.B. and was considered fatal, but these days its full cure is Thus, disease is called curable, these days. It is caused by bacteria Mycobacterium tuberculosis. These bacteria settle in lungs at different places and convert normal tissue into fibrous tissue. Since the respiratory surface is decreased, the difficulty in breathing is also experienced. If the patients start taking medical advice and the medicines right from the initial stage regularly, the patients can be fully cured of the disease. Now a days a new therapy DOT (Direct observed treatment) is used for tuberculosis treatment, recently launched by Indian Government. Many other drugs like rifampin and isoniazid are successful for the treatment of tuberculosis. Tuberculosis bacteria are spread by inhalation and exhalation.
  • Coryza : Common cold, due to rhinoviruses in
  • Influenza :
  • Occupational lung disease : It is caused because of the exposure of potentially harmful substances. Such as gas, fumes or dusts, present in the environment where a person Silicosis and asbestosis are the common examples, which occur due to chronic exposure of silica and asbestos dust in the mining industry. It is characterised by fibrosis (proliferation of fibrous connective tissue) of upper part of lung, causing inflammation.
  • Prevention and cure : Almost all the occupational lung diseases, express symptoms after chronic exposure, e., 10-15 years or even more. Not only this, diseases like silicosis and asbestosis are incurable. Hence, the person likely to be exposed to such irritants should adopt all possible preventive measures. These measures include :
    • Minimizing the exposure of harmful dust at the work
    • Workers should be well informed about the harm of the exposure to such
    • Use of protective gears and clothing by the workers at the work
    • Regular health check
    • Holiday from duty at short intervals for the workers in such
    • The patient may be provided with symptomatic treatment, like bronchodilators and antibiotics, to remove underlying secondary

(vi)  Special respiratory movements Cough






  • It is reflex action stimulation takes place from trachea and
  • Centre is medulla
  • Cough is a forcible expiration usually produced after a prolonged
  • When some food particle enters the windpipe instead of oesophagus, it is expelled by a process of
  • Air exploded through the


  • Reflex action stimulated by olfactory epithelium of nasal
  • Sneezing is a forcible expiration, air explodes out through nose and


  • Hiccuping is a noisy inspiration caused by muscular spasm of diaphragm at irregular
  • Noise is due to sudden sucking of air through vocal
  • Stimulation of hiccuping is usually irritation of the sensory nerve endings of the digestive tract.

Yawning : Yawning is a prolonged inspiration. Low oxygen tension in the blood causes yawning.



Apnea Absence of breathing
Eupnea Normal breathing
Hypopnea Decreased breathing rate
Hyperpnea Increased breathing rate
Dyspnea Painful breathing
Orthopnea Inability to breathe in a horizontal position
Acapnoea Absence of CO2 in blood
Hypocapnea Deficiency of CO2 in blood
Hypercapnea Excess of CO2 in blood
Hypoxaemia Lack of O2 in arterial blood
Anoxia Absence of O2 in tissues
Hypoxia Lack of O2 in tissues
Tachypnea Rapid breathing
Costal breathing Shallow (Chest) breathing


Respiratory pigments


Name of pigment Colour (oxidised) Metal Place Example
Haemoglobin Red Fe RBC Chordata (Vertebrate)
Haemocyanin Blue Cu Plasma Mollusca and arthropoda
Chlorocruorin Green Fe Plasma Annelida, sabella, serpulids
Haemoerythrin Red Fe Corpuscle Annelida, Sipunculoidea, lingula





Venadium Green Va Vanadocytes in Plasma Urochordata
Echinochrome Red Fe Coelomic fluid Echinodermata
Pinnoglobin Brown Mn Coelomic fluid Pinna
Molpedin Brown Mo Coelomic fluid Holothuria
Heamoglobin Red Fe Plasma Earthworm, nereis, arenicola, chironomas insect, planorbis.
Erythrocruonin Red Fe Plasma Leech


Important Tips

  • Protoplasmic respiration refers to the respiration of
  • Polarography is employed to measure the concentration of oxygen in
  • Accumulation of blood in pleural cavity is called
  • All pulmonary volume and capacities are about 20-25% less in females than
  • Accumulation of water is called
  • Accumulation of pus is called
  • Accumulation of air is called
  • If chest wall is punctured, then pressure inside the pleural cavity become equal to atmospheric pressure so breathing
  • Besides lungs, the term alveolus is associated with bony socket for tooth, and in mammary glands
  • Vital capacity represents the maximum amount of air one can renewed in respiratory system in a single
  • Values of vital capacity is higher in athletes, sportsmen, mountain dwellers, males than females, young’s than
  • Pregnancy and some diseases like emphysema, pleural effusion,  ascites (collection of water in abdominal cavity) reduce the vital VC decreases as much as 35% in age 70.
  • Measurement of expansion of chest during recruitment of police is done in the hope of getting an idea about vital capacity (greater expansion = greater vital capacity).
  • In general, a man respires about 16 – 18 time in a
  • A new born child respires 32/min.
  • A five year old child respires 26/min.
  • A fifty year old man respires 18/min.
  • Respiratory rate is lowest while sleeping (10 minute in human), respiratory rate during sitting (12 minute in human).
  • No respiratory pigment in
  • In all vertebrate respiratory pigment is Hb, except ishfish and angula fish
  • Orthinolarynology – The branch of medicine deals with the diagnosis and treatment of diseases of the ears, nose and
  • Tertiary bronchi also known as segmental
  • Located in the walls of bronchi and bronchioles within the lungs are receptor sensitive to stretch called bororeceptor or stretch
  • Rhinoplasty – or ‘Nose job’ surgically change in shape of external
  • Smaller the animal higher the respiratory
  • Rate of respiration is directly proportional to concentration of CO2 in
  • Inspired air has 48 ml. O2 in its 100 ml.
  • Expired air has 70 ml. O2 in its 100 ml.
  • Metabolic rate of body is directly proportional to the total pulmonary
  • Intra aortic balloon pump is inflated by
  • In pregnant woman diaphragm does not take part in
  • Respiratory tree is present in Holothurea (Echinodermata) .
  • In living fishes (Protopterus, Lepidosiren and Neocaratodus) lungs are





In frog larynx and trachea fused together to for larynngo tracheal chamber. Air sacs are presentin birds.

A copressure of about 0.7 mm mm Hg concentration of 1% in alveolar air can be lethal. Medullary respiratory centre is constantly under direct chemical control.

Impulse for voluntary forced breathing starts in cerebral hemisphere.

Lungs of frog acts as negative pressure pump, while lungs of mammal acts as positive pressure pump. Spirometer also known as respirometer.

Disorder such as asthma and emphysema can greatly reduce the expiratory reserve volume. Fetal lungs contain no air, and so the lung of a still born baby will not float in water.

At birth, as soon as lungs fills with air, O2 starts to diffuse from the alveoli into blood, through the interstitial fluid, finally into the cells. In general, lung volumes are larger in males, taller persons, younger adults and smaller in females, shorter persons, and the elderly. Carbonic, anhydrase is the fastest enzyme.

Carbon monoxide combines with Hb more rapidly than O2 to form carboxyhaemoglobin. Carbon monoxide has 200-250 times more affinity of Hb as compared to O2.

At about 4 weeks fetal development, the respiratory system begins as an outgrowth of endoderm of foregut, known as laryngotracheal bud.

After 6 months, formation of alveoli of lungs.

The medullary rhythmicity area in medulla oblongata. The pneumotoxic and the apneustic area in pons.

The function of the medullary rhythmicity area is to control the basic rhytum of respiration. If arterial PCO2 is more than 40 mm Hg, a condition called hypercapnia.

If arterial PCO2 is lower than 40 mm Hg, a condition called hypocapnia.

Double Bohr effect refers to the situation in the placenta where the Bohr effect is operative in both the maternal and foetal circulation. Man uses only 25% of the O2 of inhaled air, where as fishes use 80% O2 of water.

Ozone, a strong oxidizing agent, oxidises iron of Hb and forms a stable compound methaemoglobin which can not release O2. The exchange of gases in gills is called bronchial respiration.

External gills are present in some annelids (e.g. arenicola, amphitrite) young ones of certain insect (e.g., dragonflies, damsel flies) some tailed amphibians (e.g., necturus, siren, proteus) axolotal larva of tiger salamander and tadpole of frog.

Internal gills are found in prawn, unio, pila, fish and tadpole of frog.

Counter flow system, is a system for maximum gaseous exchange where blood and water flow in opposite direction (present in gills). Cutaneous respiration can occur both in air and water.

Air bladder, also known as swim bladder found in all bony fishes except lophius and cyanoglossus. Air bladder perform the functions of hydrostatic organ, sound production, audition and respiration.

Air bladder two types i.e. physostomus air bladder (associate with oesophagus by pneumatic duct) e.g., Lepidostelus, lepidosiren, arnia and physoclistous air bladder (without pneumatic duct) e.g., Anabas, cod, toadfish and hadhock.

Foetal Hb takes O2 from mother haemoglobin across the placenta due to double Bohr effect.

The foetal Hb has a sigmoid dissociation curve which is shifted to left relative to adult Hb dissociation curve because they have lower P50 (18 to 20 mm Hg) than adult (26.5 mmHg). This means fetal Hb has a higher oxygen affinity.

In embroys of mammals, respiration takes place by chorion. In lungs of birds are capillaries are present in place of alveoli.

Exchange of O2 takes place twice in lungs of birds. It is called double respiration. Aquatic salamander in lungless amphibians.

In snakes, only right lung in functional, left lung is reduced. In penguins double trachea is present.

Lungs of frog are air filled chambers and lungs of mammals are spongy.

Rate of breathing of a normal man during heavy exercise is 4050 times/minute. Diaphragm plays 75% part in breathing (abdominal breathing).

Ribs and sternum plays 25% part in breathing (thoracic breathing).





  • In pregnant females most part during breathing is played by intercostal
  • Whales and other aquactic mammals suffocate on land because their intercostal muscles can not expand their chest due to their massive body
  • In elephant, diaphragm plays important role during
  • In monkeys, kangaroo and other jumping animals, intercostal muscles play important role in
  • In hibernating animals breathing rate decreases to a lowest
  • At any given pressure the diffusion of CO2 is 20 times faster than O2.
  • If P50 value of Hb rises to 100 mm Hg, a person will die of O2 deficiency because now Hb will not be able to bind or release O2.
  • Aging and respiratory system, with advancing age –

Alveoli, respiratory tract less elastic. Lungs become less elastic.

Decreases in lung capacity. Decreases in 35% vital capacity. Decreases level of O2 in blood.

Elderly persons are more susceptible to pneumonia, bronchitis, emyphysema.

  • Smoke inhalation injury – Has three components that occur in sequence Inhibition of O2 delivery and

Upper airway injury from heat.

Lung damage from acid and aldehyde in smoke. Why smokers have lowered respiratory efficiency.

Nicotine constricts terminal bronchioles and this decreases air flow into and out of lungs. Carbon monoxide in smoke birds to haemoglobin and reduces its oxygen carrying capacity.

Irritants in smoke cause increased fluid secretion by mucosa of bronchial tree, inhibits the movement of cilia in lining of respiratory system.

Destruction of elastic fibres in lungs.

  • SARS – Severe Acute Respiratory Syndrome –

SARS is a highly infectious disease caused by corona virus.

Corona virus in RNA virus, its genome was sequenced with in 15 days.

The origin of SARC is from South China, from South China this disease spread to Hongkong. Bird sellers and presous in contact with birds to suffer from SARS.

Symptoms of infections are flue like symptoms. Fever occurs with dry cough. There is difficult in breathing. Fluid filled in lungs and death occurs with in one week of infection from respiratory failure.

Rate of death was initially 4% but now death rate has increased to 10%. Line of treatment is quarnatine and ribovinin durgs.

The causative agent of SARS was identified by Dr. Malik Peiris of Microbiology Department of Honkong University.