Chapter 12 Excretory Products and their Elimination by TEACHING CARE online tuition and coaching classes

Chapter 12 Excretory Products and their Elimination by TEACHING CARE online tuition and coaching classes

File name : Chapter-12-Excretory-Products-and-their-Elimination.pdf


Introduction : The component structural and functional units of the bodies of all organism are cells which have been looked as “miniature chemical factories” because of continuous metabolism taking place in these. It yields certain waste products which are, not only useless, but harmful to the cells and the body. Cells, therefore, throw out these wates, by diffusion, into their surrounding medium. Finally, these wastes are eliminated by the body into its external environment. This is, thus an important vital activity of all organism. It is called excretion.

Besides removing the metabolic wastes and impurities from the blood, the kidney also perform the important function of osmoregulation by regulating the amount of water in body fluids. The normally functioning kidneys produce a large volume of dilute urine when more water is taken, and a small volume of concentrated urine when water intake by the body is poor.

 Excretory organs of different organism.

  • Protozoans : In protozoans like Amoeba


and Paramecium carbon dioxide and ammonia are mostly excreted out by diffusion through general body surface. It is considered that the contractile vacuoles also play some role in the removal of excretory products.

  • Sponges : In sponges, the nitrogenous metabolic waste (ammonia) leaves the body in the outgoing water current by diffusion.

Most of the sponges are marine and have no problem of surplus water in their cells. A few sponges lie in hypotonic fresh water and have contractile vacuoles in most of their cells.




























  • Coelenterates : Hydra also lacks special excretory organs. The nitrogenous waste products like ammonia are removed through the general surface of the body by diffusion. Some nitrogenous waste products are also thrown along with indigestible matter through the
  • Platyhelminthes : Planaria, liverfluke and tapeworm possess a large number of excretory cells called the flame cells (solenocytes) and long excretory ducts (also called canals of vessels). The flame cells open into the ductules which in turn open into the excretory

Excretory canals are present on each lateral side or the collecting tubules of which one is dorsal and the other ventral. In the last proglottid, they join to form a pulsatile caudal vesicle, which is open to a exterior by excretory pore.

Excretory materials diffuse from the surrounding tissues into the flame cells. Vibrations of the cilia cause these materials to remove in the excretory ducts. The walls of the ducts reabsorb useful substances and remaining excretory materials (e.g., ammonia) are expelled out through the excretory pores.

  • Aschelminthes : The round worms such as Ascaris have H-shaped excretory system. It is made up of a single Renette cel It consists of two longitudinal excretory canals connected anteriorly by a network of transverse canals. A short terminal duct opens outside via excretory pore. Ascaris is excretes both ammonia and urea.







































Fig. – Flame cell of Platyhelminthes

Fig. – Renette cell of Ascaris




  • Annelids : In annelids like Nereis, earthworm, leech, etc., the tubular coiled structures, the nephridia are excretory A typical nephridium starts from a rounded ciliated funnel, the nephrostome which opens into coelom (body cavity). The nephrostome leads into a nephridial tubule with ciliated cells. A typical nephridium opens outside




CANAL                                DISTAL





the body through a small aperture called nephridiopore. However, in earthworm three types of nephridia are found. The septal nephridia situated on the septa (behind 15th segment) and pharyngeal nephridia (











in three pairs of bundles in the 4th, 5th, and 6th segments) open into the alimentary canal and pour their excretory materials there. It is an

Fig. – Septal nephredium of Earthworm


adaptation for conservation of water. The integumentary nephridia (found scattered in the body wall in each segement except the first two segments) open directly on the body surface. Excretory materials help the earthworm in


keeping the skin moist for cutaneous respiration.

  • Arthropods : (a) The excretory system of the adult Prawn (crustacean) consists of a pair of antennary or green glands, a pair of lateral ducts and a single renal Each green gland consists of an end sac, labyrinth (glandular plexus) and bladder. The end sac extracts nitrogenous waste products and excess water from the blood. The excretory fluid is transferred from the sacs to the labyrinth in which useful materials are absorbed and carried to the blood. The remaining excretory fluid called urine, flows from the labyrinth to the bladder. The excretory fluid also comes here from the renal sac. Urine is temporarily stored in the bladders. Later on urine is expelled out through ureters and

renal pores.















  • Most insects, centipedes and millipedes, possess Malpighian

Fig. – Antennary gland of Prawn


tubules as their principal excretory organs. They are fine, spiral or convoluted, thread-like tubules which are attached to the alimentary canal. The distal closed end of each Malpighian tubule float freely in the haemolymph (blood). These tubules extract metabolic wastes like potassium and sodium urate, water and carbon dioxide from the blood. In the Malpighian tubules bicarbonates of potassium and sodium, water and uric acid are formed. A large amount of water and bicarbonates of potassium and sodium are reabsorbed by the cells of Malpighian tubules and then





transferred to the blood (haemolymph). Uric acid is carried to the alimentary canal of the insect and is finally passed out through anus.

  • Spiders and scorpions possess Malpighian tubules or coxal glands or both for excretion.
  • Molluscs : They have one or two pairs of kidneys which discharge excretory matter into the mantle cavity which is finally passed out of the body along with the out flowing
  • Echinoderms : Specialized excretory organs are absent in echinoderms (g., Starfish). The excretory products, chiefly


ammonia, are eliminated by diffusion through dermal branchae (primitive gills) and tube feet.


Fig. – Malpighian tubule of insecta





























Fig. – Coxal gland or Scorpion






Fig. – Organ of Bojanus (Pila – mollusca)


Excretory organs of different organisms


S.No.PhylumExcretory/osmoregulatory Organ/Organelle and

principal N2-waste

I. Invertebrates
(1)ProtozoaContractile vacuole






(2)PoriferaGeneral surface of bodyAmmonotelicSycon, Leucon
(3)CoelenterataAmmonia, General surface of bodyAmmonotelicHydra
(4)Platyhelminthisflame cells (=Solenocytes)

form the protonephridial system

AmmonotelicTaenia, fasciola
(5)NematodaH-shaped excretory organ, Renette cellsAmmonotelicAscaris
(6)AnnelidaNephridial system,

(Metameric), various types

a.Class-InsectaMalpighian tubuleUricotelicPeriplaneta






  (Uric acid)  
b.Class crustaceaAntennary (=green) gland

Uric acid

c.Class ArachnidaCoxal glands Malpighian tubule Hepato pancreas


(8)Mollusca(a)    Kidney (=organ of Bojanus) or Renal organ

(b)    Keber’s organ Aquatic forms excrete Ammonia

Terrestrial forms

Excrete uric acid



Ammonotelic Uricotelic




Pulmonate Mollusc


(9)EchinodermataDermal branchiae (primitive gills) tube feet,

body surface (Ammonia)




 Excretory system of man.



Mammalian (human) urinary system consists of a pair of kidneys, a pair of ureter, a urinary bladder and a urethra.

  • Kidneys : The kidneys are dark-red, bean-shaped organs about 11 cm long, 5 cm wide and 3 cm thick, each weight about 150 gm in an adult male and about 135 gm in adult They are placed against the back wall of the abdominal

cavity just below the diaphragm, one on either side opposite the

















last thoracic and first three lumber vertebrae. The lower two pairs of ribs protect them.

The kidneys are covered by peritoneum on the front (ventral) side only. thus, they are retroperitoneal. The right kidney is attached more anterior than the left in rabbit. This asymmetry is just the reverse of that found in man.

In man left kidney occurs at a slightly higher level than the




















right one, because right side has prominent right liver lobe. In

Fig. – Human urinary system


rabbit the condition is little differ due to quadropedilism i.e. left kidney is in normal position while the right kidney shift ached to provide place for stomach below it.

In mammals, the kidney is bean-shaped i.e. concavo convex. The center of concave inner surface is called as hilum or hilus which gives out a ureter. From this hilus surface the renal artery enters into the kidney, the renal vein comes out and the renal nerves enter into the kidney.






  • Structure of kidney : The kidneys are metanephric in The kidney is divisible into two parts outer-cortex and inner-medulla.

Renal pyramids or medullary pyramids : The medulla is subdivided into 10 to 12 conical masses – the renal pyramid, each having broad base towards the cortex and a narrow end called renal papilla towards the pelvis.

Renal columns of bertini : Between the pyramids, the cortex extends into the medulla or renal columns of bertini.

Calyx : Each renal papilla projects into the cavity of a minor calyx, minor calyx join to form major calyx. The major calyx open into a wide funnel like structure, the pelvis. The latter






















leads into the ureter. In rabbit, the pelvis is unbranched hence, it is without calyx.

In frog ventral surface of each kidney has many ciliated

MEDULLARY       RENAL COLUMNS PYRAMIDS                                OF BERTINI


funnels called nephrostomes. They drain wastes from body cavity

Fig. – H.L.S. of human kidney


(coelom) and connect to renal veins in frog or to uriniferous tubules in tadpoles.

Histology of kidney : Histologically a kidney is made of innumerable thin, long, much convoluted tubular


units called uriniferous tubule or nephron.

Nephron is the structural and functional unit of kidney. One human kidney may contain about one million (10 lac nephron) nephron (In rabbit each kidney bear about 2 lac nephron). In frog each kidney bears about 2 thousand nephron.

  • Structure and types of nephron : A nephron or uriniferous tubules is made of two parts –
    • Malpighian body : The proximal end of each nephron forms a blind or closed, enlarged and double walled cup, the Bowman’s capsules in the (name Bowman’s capsule is based on english physiologist and histologist William Bowman).

Each capsule contains a network of blood capillaries the glomerulus which receives blood through afferent arteriole and the blood comes out through the efferent arteriole .The diameter of the efferent arteriole














































is comparatively lesser. (Bowman’s capsule and glomerulus receives about 20 – 25% of






the cardiac out put (blood) at rest.

The composite structure of Bowman’s

Fig. – Position, structure and blood supply of cortical and juxtamedullary nephrons is a mammalian kidney





capsule and glomerulus is known as Malpighian body or Malpighian corpuscles after the Italian microscopist Marcello Malpighi.

  • Tubule : The tubule is differentiated in to 3 parts C.T., Henle’s loop and D.C.T.

The Bowman’s capsule opens into a proximal convoluted tubule (P.C.T.) the anterior part of the P.C.T. is more coiled where as its posterior part is almost straight. The P.C.T. opens into a Henle’s loop. The Henle’s loop is a U- shaped structure which has a distinct descending limb and an ascending limb. The ascending limb opens in to the distal convoluted tube. The D.C.T. is a coiled structure. Many D.C.T. unit to form a collecting duct. The collecting ducts of one pyramid unit to form a duct of Bellini. The duct of Bellini lead into the pelvis part.

Arrangement of nephron : The malpighian body and a part of P.C.T. and D.C.T. are situated in the cortex.

Most of the part of P.C.T. and D.C.T., Henle’s loop and collecting ducts are found in the medulla.

Vasa recta : The efferent arteriole of juxta-glomerular nephron forms a peritubular capillary system around the Henle’s loop which is called vasa recta. Each of the vasa recta makes U turn at the inner most part of the medulla and return to the venous circulation near the junction of medulla and cortex. The efferent arteriole and peritubular capillaries technically constitute a renal portal system. In all amniotes as reptiles, birds and mammals have a renal portal system.

Types of nephron : Nephrons are of two types cortical and juxtamedullary, with regard to their location in the kidney. The cortical nephrons form about 80% to 90% of total nephron. They lie in the renal cortex and have very short loops of Henle that extend only little into the medulla.

The juxta medullary nephron have their Bowman’s capsule close to (Juxta) the junction of the cortex and the medulla and have very long loops of Henle, extending deep into the medulla. This type of nephron is present in only birds and mammals. The cortical nephrons control the plasma volume when water supply is normal. The juxtamedullary nephrons regulate the plasma volume when water is in short supply (In advarse condition).

Differences between cortical and Juxtamedullary nephrons

Cortical NephronsJuxtamedullary Nephron
1.      Form 80% of total nephrons.1.      Form only 20% of total nephrons.
2.      Are small in size.2.      Are large in size.
3.      Lie mainly in the renal cortex.3.      Have Bowman’s capsules in the cortex near its junction

with the medulla.

4.      Henle’s loops are very short and extend only a little into the medulla4.      Henle’s loop are very long and extend deep into the medulla.
5.      Control plasma volume when water supply is normal.5.      Control plasma volume when water supply is short.


(c)  Histology of nephron

Glomerulus : Glomerulus is a network of up to 50 parallel branching and anastomosing capillaries covered by endothelium, basement membrane and epithelium made of podocytes which has slit pores that restrict passage of colloids. However, small molecules and water can easily pass through them in to the P.C.T.

Bowman’s capsule : The podocytes forming the inner wall of the Bowman’s capsule have gaps (about 25 nm wide) the slit pores.

The outer wall of the Bowman’s capsule consists of unspecialized squamous epithelium (flattened).


























Proximal convoluted tube : P.C.T. is made up of simple

Fig. A glomerular capillary entwined

by processes of three podocytes





columnar epithelium. It has microvilli so it is also known as brush border epithelium.

Loop of Henle : The epithelium of descending limb of loop of Henle is very thin and composed of squamous epithelium and ascending limb is lined by cuboidal epithelium. The ascending limb is impermeable to water and permeable to NaCl.

Distal convoluted tube : It is made up of cuboidal epithelium which is glandular in nature.

Collecting ducts : The collecting ducts are lined


by cuboidal and columnar epithelium in different regions. At intervals, the cuboidal cells are ciliated.

Juxta-glomerular apparatus : This specialized cellular apparatus is located where the distal convoluted tube passes close to the Bowman’s capsule and afferent arteriole. Cells of the D.C.T. epithelium in contact with afferent arteriole are denser than other epithelial cells known as maculla densa. Maculla densa has special Lacis cell or Polkisson’s cell. These cells secrete renin hormone that modulate blood pressure and thus renal blood flow and G.F.R. are regulated.

(d)    Origin and  types  of  kidneys  in  different






















































vertebrate : Kidney tubules (nephrons) arise in the embryo in a linear series from a special part of mesoderm called mesomeare or nephrotome.

Number, complexity and arrangement of Nephrons are differ in different groups of vertebrates. A nephron is

Fig. Juxta glomerular apparatus








differentiated into three parts – peritoneal funnel, tubule and

malpighian body. Peritonial funnel (nephrostome) are normally present in embryos and larvae and considered as vestigeal organ of hypothetical primitive kidneys.

Archeonephros kidney : Archeonephros is the name given to the hypothetical primitive kidney of ancestral vertebrate. It is also called as holonephros or complete kidney. (It extended entire length of coelom) It tubules are segmentally arragned and nephrostome is present. Glomerulus is external (without capsule). It duct is called as archeonephric duct. Ex. Larva of myxine.

Modern vertebrates exhibits three different kinds of adult kidney Pronephros, Mesonephros and Metanephros.

  • Pronephros : It originates from the anterior part of the nephrotome. It is also termed head kidney due to its anterior There are only 3 pronephrine tubule (nephron) in frog embryo, 7 in human embryo, and about 12
































in chick embryo which are segmentary arranged. Nephrostome present, glomerulus is external and unite to form glomus in




some cases. Duct is pronephric duct or mullerian duct.

Fig. – Juxtamedullary nephron and epithelial cells in the wall of its various parts





Pronephros is functional in all embryos and larval stages. It is mostly transitory and soon replaced by the next stage or mesonephros.

Example – Adult myxine and petromyzones (cyclostomes) and some fishes but non urinary and lymphoid in function.

Note :
  • Mesonephros : It originates from the middle part of the nephrotome. Duct is mesonephric or Wolffian duct. Nephrostome is absent except some embryos of anamniotes. Example – In amniotes (reptiles, birds and mammals) mesonephros is functional only in the embryos, replaced by metanephros in the In anamniotes (fishes and amphibian) mesonephros is functional in both embryo as well as adults.
    • In shark and caeccilians, tubules extend posteriorly throughout the length of coelom. So it is also called posterior or opisthonephric kidney.
    • In frog mesonephric duct is also known as Bidder’s canal which carry sperm and urine
  • Metanephros : It originates from the posterior part of the nephrotome. When metanephric tubules develop, all the mesonephric tubules disappear except those associated with the testes in male and forming vasa efferentia. Nephrostome absent. A thin, U-shaped loop of Henle forms between P.C.T. and D.C.T. which is incomplete in Reptiles and Birds and well developed in mammals. Duct is metanephric or ureter. Reproductive duct is separate. The kidney is highly compact which possesses innumerable nephrons. Example – All amniotes – Reptile, Birds and
  • Ureters : From the hilum of each kidney emerges a whitish tube the The ureters are about 28 cm long. Their wall consists of transitional epithelium surrounded by a layer of muscle fibres. Openings of the two ureters in the bladder are separate, but closely placed. These are oblique, so that the urine cannot regurgitate into the ureters when the bladder contracts. Peristalsis of ureters also cheeks regurgitation of urine.
  • Urinary bladder and Urethra : The urinary bladder is pear-shaped which is made up of smooth and involuntary muscles. The muscles is also known as detrusor muscles (muscles that has the action of expelling a substance). The lower part or neck of the bladder leads into the urethra. There is a smooth triangular area, called trigonium vesicae. The lumen of the urinary bladder is


lined by transition epithelium which has great power of streaching. The neck of bladder is guarded by two sphincters, inner is involuntary controlled by spinal reflex and outer is voluntary controlled by cerebral cortex. A person feels the sensation of micturation when the quantity of urine in the bladder is about 300 c.c.

Urethra : The urinary bladder leads into the urethra. In a female, it is quite short, only about 3 to 5 cm long, and carries only urine. It opens by urethral orifice, or urinary aperture in the vulva infront of the veginal or genital aperture. In a male urethra is much longer, about 20 cm and carries urine as well as spermatic fluid. It








































passes through the prostate gland and the penis. It opens out at the tip of the penis by urinogenital aperture.

Fig. – Parts of ureters, trigone of the bladder, sphincters and urethra


Differences between male and female urethra

Male urethraFemale urethra
1.      It is about 20 cm long.1.      It is just 3 – 5 cm long.






2.      It has 3 regions : prostatic urethra (3–4 cm), membranous

(1 cm) and penial (15 cm)

2.      It is not differentiated into regions.
3.      It opens out at the tip of the penis by urinogenital aperture.3.      It opens into the vulva by urinary aperture.
4.      It carries urine as well as semen to the exterior.4.      It carries only urine to the exterior.
5.      It has 2 sphincters.5.      It has a single sphincter.


 Physiology of excretion.

Major nitrogenous excretory substance in frog, rabbit and human is urea, i.e. these are ureotelic animals. The excretory physiology in these animals may be considered under two phases, viz urea synthesis and formation and excretion of urine.

Protein ¾¾Di¾ges¾tio¾n ¾in ¾ali¾me¾nt¾ar¾y ® Aminoacid ¾¾Ab¾so¾rp¾tio¾n ®Blood circulation

canal                                  intestine






  • Synthesis of urea in liver : Urea is formed in liver by two

(a) Deamination                             (b) Ornithine cycle

  • Deamination : The amino acid is oxidised using This result in removal of the amino group

(NH 2 ) and leaves pyruvic acid. the pyruvic acid can enter the Krebs cycle and be used as a source of energy in cell respiration. The amino group is converted to ammonia (NH 3 ) during deamination. Deamination is also known as oxidative deamination.






  • 1O



¾¾® CO + NH3


2      2   2









(Amino acid )                                        (Pyruvic acid )


With the help of a number of enzymes and energy of A.T.P. two molecules of ammonia are combined with to form urea according to the following cycle.




  • Ornithine cycle (Kreb-Henseleit cycle) : In liver one molecule of


is activated by biotin and


combines with two molecule of NH2 in the presence of carbamyle phosphate synthatase enzyme (C.P.S.) and 2


ATP to form carbamyle phosphate and one molecule of


release. Carbamyle phosphate react with ornithine





and form citrulline. Citrulin combines with another molecule of ammonia and form arginine. Arginine is broken into urea and arnithine in the presence of an enzyme arginase and water.

2NH 2  + CO2  ¾¾Ar¾gin¾as¾e ® NH 2 – CONH 2 + H 2O
























UREA           Fig. – Ornithine cycle


Liver cells, thus, continuously remove ammonia and some


from blood and release urea into the blood.


Kidneys continuously remove urea from the blood to excrete it in urine.

  • Urine formation : Urine formation occurs in the It involves three processes glomerular filtration, reabsorption and tubular secretion.

(a)  Ultra filtration or (Starlin hypothesis)

  • It is passive process which takes place from the glomerulus into the Bowman’s The glomerular epithelium has various micropores (diameter = 0.1


  1. m) which increase the rate of
  • The non colloidal part of the plasma as urea, water, glucose, and salts are forced out from the glomerular capillaries into the Bowman’s capsule by the high pressure of the blood in the glomerular The pressure is high because the glomerular capillaries are narrower than the afferent renal arteries.
  • The effective filtration pressure that causes ultrafiltration is determined by three

Glomerular  hydrostalic   pressure   :  The

G.H.P. is the blood pressure in glomerular capillaries due to the efferent arteriole is narrower than afferent arteriole. It is the chief determinent of effective filtration pressure, i.e. the main driving force to cause filtration.

G.H.P. = +70 mm Hg.




GHP®65-75 MM HG











10 MM HG


10 MM HG




Fig. – Ultra filtration









15-25 MM HG EFP





Blood colloidal osmotic pressure : The B.C.O.P. is the osmotic pressure created in the blood of glomerular capillaries due to plasma proteins (mainly albumin). It resists the filtration of fluid from the capillaries.

B.C.O.P. = – 30 mm Hg.

Capsular hydrostatic pressure : C.H.P. is the pressure caused by fluid (filtrate) that reaches into Bowman’s capsule and resists filtration.

C.H.P. = –20 mm Hg.

Effective filtration pressure : E.F.P. is glomerular hydrostatic pressure minus the colloidal osmotic pressure of blood and capsular hydrostatic pressure.

E.F.P. = G.H.P. – (B.C.O.P. + C.H.P.)

= 70 mmg – (30 mmg Hg + 20 mm Hg)

= 70 – 50

Note :

E.F.P. = 20 mm Hg

  • Net opposing filtration pressure (N.O.F.P.) = C.O.P.+C.H.P.

= 50 mm Hg.

Glomerular filtrate : The plasma fluid that filters out from glomerular capillaries into Bowman’s capsule of nephrons is called glomerular filtrate. It is a non colloidal part and possess urea, water, glucose, amino acid, vitamins, fatty acid, uric acid, creatin, creatinine, toxins, salts etc.

R.B.Cs, W.B.Cs platelets and plasma proteins are the colloidal part of the blood and do not filtered out from glomerulus. Glomerular filtrate is isotonic to blood plasma.

Glomerular filtrate or Nephric filtrate = Blood – (Blood cells + Plasma protein)


= Blood – (R.B.Cs + W.B.Cs+platelets + plasma protein)


= Plasma – Protein

Gomerular filtration rate (G.F.R.) : G.F.R. is the amount of filtrate formed per minute in all nephrons of the paired kidney. There is a sexual difference. In male the rate is 125 ml/min, in female it is 110 ml/min. G.F.R. is affected by volume of circulating blood, neural activity, stretch response to pressure of the wall of the arteriole.

180 litre of filtrate is formed per day, out of it, only 1.5 litre of urine is produced per day which is 0.8% of the total filtrate.

Renal plasma flow : About 1250 ml (25% of cardiac output or total blood) blood circulates through kidneys each minute and of this blood, about 650 ml is the plasma. The latter is called the renal plasma flow (R.P.F.)

R.P.F. = 650 ml.

Filtration fraction : This is the ratio of G.F.R. to R.P.F., and it is called filtration fraction.


Filtration fraction =





  • Selective reabsorption : Discovered by Richard and supporters.

P.C.T. : P.C.T. is the pivotal site for reabsorption.


Glucose, amino acid and

Na + , K +

ions are reabsorbed by active transport.


Cl – are reabsorbed by passive transport following the positively charged ions.

Active uptake of ions reduces the concentration of the filtrate and an equivalent amount of water passes into the peritubular capillaries by osmosis. (Here 80% water is reabsorbed by passive transport. It is also known as


obligatory water reabsorption). Most of the important buffer bicarbonate (HCO– ) is also reabsorbed from the filtrate.





P.C.T. absorb nearly 80–90% of filtered bicarbonate. Some urea is reabsorbed by diffusion. The rest reman in the filtrate for removed in the urine.

Henle’s loop : See counter current mechanism.

D.C.T. : When the level of plasma water falls, the posterior pituitary lobe release the antidiuretic hormone (ADH) which increases the permeablity of the distal convoluted tubule and the collecting duct to water. Water is reabsorbed from the filtrate by osmosis and a reduced amount of concentrated urine is produced (Here 13% water is reabsorbed by facultative reabsorption)

The distal convoluted tubule and the collecting duct actively reabsorbed sodium from the filtrate under influence


of the adrenal hormone aldosteron which makes their walls permeuble to ions. The reabsorption of


brings about


the uptake of an osmotically equivalent amount of water. But duct of Bellini is relatively impermeable to water. Bicarbonate ions are also reabsorbed in D.C.T.

  • Tubular secretion : It occurs as under –
  • Creatinine, hippuric acid and foreign substances (pigments, drugs including penicillin) are actively secreted into the filtrate in the PCT from the interstitial Hydrogen ions and ammonia (NH 3) are also secreted into the PCT.


  • Potassium, hydrogen, NH +and


ions are secreted by active transport, into the filtrate in the DCT.




  • Urea enters the filtrate by diffusion in the thin region of the ascending limb of Henle’s loop.


Removal of H +


NH +

from the blood in the PCT and DCT helps to maintain the pH of the blood



between 6 to 8. Any variation from this range is dangerous.

Tubular secretion probably plays only a minor role in the function of human kidneys, but in animals, such as marine fish and desert amphibians which lack glomeruli and Bowman’s capsules, tubular secretion is the only mode of excretion. When the blood pressure, and consequently the filtration pressure, drop below a certain level, filtration stops and urine is formed by tubular secretion only.

High threshold substances : Such substances are absorbed almost all. Example – Sugar, amino acids, vitamins etc.

Low threshold substances : They are absorbed in low concentration. Example – Urea, creatinine, phosphate.


Non threshold substances : They are not reabsorbed. Example – Uric acid.








Diuretic substances : Normally, the

amount of urine formed depends on the




intake of water, dietary constituents, environmental temperature, mental and physiological states of the person. However,

ISOTONIC                                                 HYPERTONIC OR ISOTONIC


there are some substances which increase






the volume of urine to be excreted, these substances are called diuretic substances. Exmaple – Tea, Coffee, alcohal etc.

(iii)               Mechanism       of        urine

ISOTONIC                                                               ISOTONIC




concentration (Counter current mechanism of urine concentration) : Mammals form hypertonic urine. The urine is made hypertonic with the help of counter current multiplier system. This process takes














place in the Henle’s loop and vasa recta

Fig. – Counter current multiplier in Henle’s loop






and it involves mainly

Na +

and Cl – . In P.C.T. urine is isotonic. The descending limb of loop of Henle is permeable


to water. Its surrounding tissue fluid is hypertonic. Hence, the water moves out and the

Na +



moves in the


descending limb by passive transport. Therefore, the filtrate in the descending limb finally becomes hypertonic.


The ascending limb of the Henle’s loop is impermeable to the water. The

Na +



moves out by active


transport. Hence the filtrate finally becomes hypotonic. The

Na +



re-enter into the descending limb of the


Henle’s loop. The collecting duct always passes through the hypertonic tissue fluid. Hence, water comes out osmotically making the filtrate hypertonic. Now in collecting duct glomerular filtrate is known as urine. Term urine first time use in collecting duct.

Summary of events occurring in a nephron


Materials transferredNephron regionProcess involvedMechanism
1. Glucose, Amino acids, Vitamins, Hormones, Na+, K+, Mg2+, Ca+2, H2O, Urea, Uric Acid, Creatinine, Ketone Bodies.Bowman’s capsuleGlomerular filtrationUltrafiltration
2. Glucose, Amino Acids, Hormones, Vitamins, Na+, K+, Mg2+, Ca+2Proximal convoluted tubuleReabsorptionActive transport
3. ClProximal convoluted tubuleReabsorptionPassive transport
4. WaterProximal convoluted tubuleReabsorptionOsmosis
5. UreaProximal convoluted tubuleReabsorptionDiffusion
6. H2ONarrow region of descending limb of Henle’s loopReabsorptionOmosis
7. Na+,K+,Mg+2,Ca+2,ClNarrow region of ascending limb of Henle’s loopReabsorptionDiffusion
8.Inorganic ions as aboveWide part of ascending limb of Henle’s loopReabsorptionActive transport
9.H2ODistal convoluted tubule, collecting tubule, collecting ductReabsorption with ADH HelpOsmosis
10. Na+Distal convoluted tubule, collecting tubule, collecting ductReabsorption with aldosterone help reabsorption secretionActive transport
11. UreaLast part of collecting ductReabsorption with aldosterone help reabsorption secretionDiffusion
12. Creatinine, Hippuric Acid, Foreign substancesProximal convoluted tubuleReabsorption with aldosterone help reabsorption secretionActive transport
13. K+, H+Distal convoluted tubuleReabsorption with aldosterone help reabsorption secretionActive transport
14. NH3Distal convoluted tubuleReabsorption with aldosterone help reabsorption secretionDiffusion
15. UreaAscending limb of Henle’s loop (Thin part)Reabsorption with aldosterone help reabsorption secretionDiffusion






The fluid and dissolved waste substances excreted by the kidneys constitute urine.

Quantity : An adult man normally passes about 1 to 1.8 litres of urine in 24 hours. The volume of urine depends upon (i) the fluid intake, (ii) level of physical activity, (iii) type of food taken and (iv) environmental temperature increase urine output. Less fluid intake and profuse sweating due to heavy physical work and high temperature reduce urine output. Certain substances, such as tea, coffee and alcohol, increase urine output. These are said to be diuretic.

  • Physical properties : Urine is transparent yellowish fluid, its shade depending on its concentration. Its colour is due to a pigment urochrome derived from the breakdown of haemoglobin from the worn-out RBCs. Colour of the urine is altered by certain materials taken such as beet, vitamin B complex and some drugs. It is hypertonic to blood plasma. Its specific gravity ranges between 003 and 1.04, being slightly higher than that of water. Its pH is 6. It depends on the diet. High protein food and fruits increase acidity whereas vegetables increase alkalinity. Urine has a characteristic unpleasant odour. If allowed to stand, urea is degraded by bacteria to ammonia which imparts a strong smell to urine.
  • Chemical composition : Urine consists of water and organic and inorganic substances. Water alone forms about 95% of it, other substances form only 5%. The organic substances are mainly nitrogenous organic compounds include urea, uric acid, creatinine and hippuric acid. Of these, urea is the principal component of human The non nitrogenous organic compounds include vitamin C, oxalic acid, phenolic substances include ammonia, and mineral salts such as chlorides, sulphates and phosphates of sodium, potassium, calcium and magnesium. Sodium chloride is the principal mineral salt of the urine. Urine also contains some other substances, such as pigments and drugs, and some epithelial cells and leucocytes.
  • Abnormal materials : Presence of proteins (albumins), bile salts, bile pigments, ketone bodies, blood, pus, microbes and more than a trace of glucose in the urine is pathological condition. Presence of glucose, protein, blood, ketone bodies and pus in the urine is called glucosurea, proteinuria, haematuria, ketonuria and pyuria respectively.
  • Renal threshold : A negligible amount of glucose is present in the The highest concentration of a substances in the blood upto which it is fully reabsorbed from the glomerular filtrate is called its threshold. If its concentration in the blood exceeds its renal threshold, some of the filtered out substance is not reasborbed and is excreted in the urine. For example, the renal threshold of glucose is 180 mg. per 100 ml. of blood. If its blood level exceeds 180 mg., some of the filtered out glucose is not reabsorbed and is passed in urine.
  • Conduction of urine and Micturition : Urine is produced and drained continuously by the nephrons into the renal From here, it is carried down the ureters by peristaltic waves into trigonum vesicae and then into the body of the urinary bladder. The bladder serves to store the urine temporarily and also to pass it out at suitable intervals. The process of passing out urine from the urinary bladder is called urination or micturition, As urine collects, the muscular walls of the bladder distend to accommodate it. Distension of its walls stimulates the sensory nerve endings in the bladder wall and this sets up reflexes, which cause an urge to pass out urine. During the discharge of the urine, the bladder and urethral sphincters relax and the smooth muscles of the bladder wall gradually contract. This slowly drives the urine from the bladder through the urethra to the exterior. Reflux of the




urine into the ureters is prevented because the terminal parts of the ureters pass obliquely through the bladder wall and are consequently closed when the bladder wall contracts around them. Relaxation and contraction of the urinary bladder are caused by impulses from the sympathetic and parasympathetic nerve fibres.

Micturition may be voluntarily postponed for some time until the pressure in the bladder rises too high to control. Micturition may also be voluntarily achieved even before sufficient urine has accumulated in the bladder. Normally an urge for micturition starts when the bladder is a little more than halffull of urine.

Urine constituents in man (in gram)


1.Total volume1,200 ml – per 24 h
2.Water1,140 ml
3.Total solids50 gm
7.Urea30 gm
8.Creatinine1.6 gm
9.Creatine0.1 gm
10.Hippuric acid0.7 gm
11.Urobilinogen0.4 mg
12.Porphyrins50 – 300 mg
13.Uric acid0.7 gm
14.NaCl15.0 gm
15.K3.3 gm
16.Ca0.3 gm
17.Mg0.1 gm
18.Fe0.1      gm

0.2    0.005 gm

19.SO42.5 gm
20.PO42.5 gm
  • pH of urine = 6
  • Yellow colour of urine is due to Urochrome
  • volume of urine is one day = 1 litre – 5 litre per day
  • Specific gravity = 1 – 04

Urine constituants in man (in %)


3.Uric acid0.2%
6.Hippuric acid0.025%






7.Salt1 %


 Hormonal control of renal function.

Hormonal controls of the kidney function by negative feedback circuits can be identified :

  • Control by antidiuretic hormone (ADH) : ADH, produced in the hypothalamus of the brain and released into the blood stream from the pituitary gland, enhances fluid retention by making the kidneys reabsorb more The release of ADH is triggered when osmoreceptors in the hypothalamus detect an increase in the osmolarity of the blood above a set point of 300 mosm L–1. In this situation, the osmoreceptor cells also promote thirst. Drinking reduces the osmolarity of the blood, which inhibits the secretion of ADH, thereby completing the feedback circuit.


  • Control by Juxtaglomerular apparatus (JGA) : JGA operates a multihormonal Renin-Angiotensin- Aldosterone System (RAAS). The JGA responds to a decrease in blood pressure or blood volume in the afferent arteriole of the glomerulus and releases an enzyme, renin into the blood In the blood, renin initiates chemical


















reactions   that    convert   a    plasma   protein,   called

  • Na+ AND H2O



angiotensinogen, to a peptide, called angiotensin II, which works as a hormone. Angiotensin II increases blood pressure









by causing arterioles to constrict. It also increases blood volume into ways : firstly, by signaling the proximal convoluted tubules to reabsorb more NaCl and water, and secondly, by stimulating the adrenal gland to release


















aldosterone, a hormone that induces the distal convoluted

tubule to reabsorb more Na+ and water. This leads to an





increase in blood volume and pressure, completing the feedback circuit by supporting the release of renin.

Fig. – Regulation of renal function by feedback circuits : (a) control by ADH: (b) Control by RAAS


  • Parathormone : The hormone increases blood Ca++ (Hypercalcium) and decreases PO4 accordingly, it increases absorption of Ca+, increases excretion of PO4.
  • Thyrocalcitonin : It increases excretion of Ca++ in the
  • Prostaglandin : The renal pyramids produce fatty acids of prostaglandins (P.G.) which participates in blood pressure
  • Erythropoiotin : It is secreted by juxtaglomerular apparatus and plays an important role in erythropoiosis (blood production).

Differences between Rennin and Renin


1.It is secreted by peptic (zymogen) cells of gastric glands into the stomach.It is secreted by specialised cells in the afferent arterioles of the kidney cortex.
2.Its secretion is stimulated by food.Its secretion is stimulated by a reduction of Na+ level in tissue fluid





3.It is secreted as an inactive form prorennin which is activated to rennin by HCl.It is secreted as renin.
4.It is a proteolytic enzyme.It is a hormone that acts as an enzyme
5.It helps in the digestion of milk protein casein.It   converts angiotensin.theproteinangiotensinogeninto


 Homeostatic regulatory functions of kidneys

By continuously eliminating metabolic wastes and other impurities, and even the surplus quantity of useful materials from blood plasma in the form of urine, kidneys play a vital role in homeostasis. Kidneys also operate certain other homeostatic regulatory mechanisms. Proper maintenance of the internal environment is knows as homeostasis. All regulatory functions of kidneys can be enumerated as follows –

  • Osmoregulation : Being the universal solvent, water is the actual vehicle in ECF to transport materials between various parts of body. Water volume in ECF tends to vary considerably due to several reason, such as drinking, perspiration, diarrhoea, vomiting, etc. As described in previous pages, the kidneys maintain the water balance in ECF by diluting or concentrating
  • Regulation of osmotic pressure : Osmolality of cytoplasm is mainly due to proteins and potassium and phosphate ions, whereas that of the ECF is mainly due to sodium, chloride and bicarbonate ions. Inspite of marked difference in chemical composition, the two fluids – intracellular (cytoplasm) and extracellular (interstitium) – must be isotonic, because if ECF becomes hypotonic, cells will absorb water, swell retaining apropriate number, mainly of sodium and chloride ions, kidneys maintain the normal osmolality of
  • Regulation of pH : Concentration of hydrogen ions (NaH2 PO4) in ECF is to be regulated at a constant value usually expressed as pH (minus log of H+). The normal pH of ECF is about 7.4. A low pH, e. a high H+ concentration causes acidosis, while a high pH, i.e. a low H+ concentration causes alkalosis. Both of these conditions severely affect cellular metabolism. Several special control systems, therefore, operate in the body to prevent acidosis and alkalosis. These system are called acid-base buffer system. Kidneys play a key role in maintenance and operation of these systems. Further, the kidneys regulate hydrogen ion concentration in ECF by excreting acidic or basic urine.
  • Regulation of electrolyte concentrations in ECF : The kidneys regulate, not only the total concentrations of water and electrolytes in ECF, but also the concentrations of individual electrolytes This regulation is complex and is accomplished by tubular reabsorption and secretion under the control of hypothalamic and adrenal hormones.
  • Regulation of RBC-count in blood : In oxygen deficiency (hypoxia), kidneys secrete an enzyme into the This enzyme reacts with plasma globulin to form erythropoietin. The latter substance stimulates bone marrow to produce more RBCs for enhancing O2-intake in lungs.
  • Regulation of renal body flow : See (R.A.A.S.).

 Excretory products in different organisms.

(i)  Waste products of protein metabolism





  • Amino acids : These are end products of protein digestion absorbed into the blood from small Certain invertebrates, like some molluscs (eg Unio, Limnae, etc.) and some echinoderms (eg Asterias) excrete excess amino acids as such. This is called aminotelic excretion or aminotelism.
  • Ammonia (NH +or NH ) : In most animals, excess amino acids are deaminated, e. degraded into their

keto and ammonia groups. The keto groups are used in catabolism for producing ATP, whereas ammonia is excreted as such or in other forms. Ammonia is highly toxic and highly soluble in water. Its excretion as such, therefore, requires a large amount of water. That is why, most of the aquatic arthropods, bony and freshwater fishes, amphibian tadpoles, turtles, etc excrete ammonia. This type of excretion is called ammonotelic excretion or ammonotelism.


(c)   Urea

CO (NH 2 )2 :  This is less toxic and less soluble in water than ammonia. Hence, it can stay for some


time in the body. Many land vertebrates (adult amphibians, mammals) and such aquatic animals which cannot afford to lose much water (e.g. elasmobranch fishes), turn their ammonia into urea for excretion. This type of excretion is called ureotic excretion or ureotelism.

  • Uric acid : Animals living in dry (arid) conditions, such as land gastropods, most insects, land reptiles (snakes and lizards), birds etc have to conserve water in their These, therefore, systhesize crystals of uric acid from their ammonia. Uric acid crystals are nontoxic and almost insoluble in water. Hence, these can be retained in the body for a considerable time before being discharged from the body. Uric acid is the main nitrogenous excretory product discharged in solid form. This excretion is called uricotelic excretion or uricotelism.
  • Trimethylamine oxide : Certain marine molluscs, crustaceans and teleost fishes first form trimethylamine from their ammonia by a process known as methylation. Then, the trimethylamine is oxidised to trimethylamine oxide for This oxide is soluble in water, but nontoxic.
  • Guanine : Spiders typically excrete their ammonia in the form of guanine. Some guanine is also formed in amphibians, reptiles, birds and earthworms. It is insoluble in water. Hence, no water is required for its
  • Wasteproducts of nucleic acid metabolism : As a result of nucleic acid digestion, nitrogenous organic bases – purines (adenine and guanine) and pyrimidines (cytosine, thymine and uracil) – are absorbed from intestine into the blood. Most of these are excreted out. About 5% of the total excretion of body accounts for these In man, purines are changed to uric acid for excretion. In most other mammals, nitrogenous organic bases are excreted in the form of allantoin. Insects, amphibians, reptiles and birds also excrete these bases in the form of uric acid. Some freshwater molluscs and crustacean arthropods excrete these in the form of ammonia.

(iii) Some sundry excretory substances (Others excretory products)

  • Hippuric and ornithuric acids : Sometimes food of rabbit and other mammals may contain traces of benzoic acid, or this acid may be formed in small amounts during fat metabolism. It is highly toxic. As it is absorbed in blood, it is combined with glycine and changed into less toxic hippuric acid for excretion. In birds, benzoic acid is combined with ornithine and changed into ornithuric acid for
  • Creatine and creatinine : Muscle cells contain molecules of creatine phosphate, which are high energy molecules and serve for storage of bioenergy like ATP. It is synthesised by 3 amino acids (A.M.) (Glycine, Argenine and Methionine). Excess amount of this phosphate is, however, excreted out as such, or after being changed into creatinine.





Differences between ammonotelism, ureotelism and uricotelism


1.Means excretion of nitrogenous waste mainly as ammonia.Means excretion of nitrogenous waste mainly as urea.Means excretion of nitrogenous waste mainly as uric acid.
2.Uses very little ammonia.energyinformingUses more energy in producing urea.Uses far more energy in producing uric acid.
3.Its product is very toxic.Its product is less toxic.Its product is least toxic.
4.Causes considerable loss of body’s water.Causes less loss of body’s water.Causes least loss of body’s water
5.Occurs in aquatic animals.Occurs in aquatic animals.aswellas landOccurs in land animals.
6.Examples : Amoeba, Scypha, Hydra, Earthworm, Unio, Prawn, Salamander, Tadpole or frog, bonyfish.Examples : Earthworm, Cartilaginous fishes, frog, turtles, alligators, mammals (man).Examples : Insects, land crustaceans, land snails, land reptiles birds.
7.Animals   excreting ammoniotelic.NH3arecalledAnimals excreting uroetelic.ureaaretermedAnimals excreting uric acid are called uricotelic.


 Disorders of kidneys.

  • Artificial kidney : Artificial kidney, called haemodialyser, is a machine that is used to filter the


blood of a person whose kidneys are damaged. The process is called haemodialysis. It may be defined as the separation of small molecules (crytalloids) from large molecules (colloids) in a solution  by interposing a

semipermeable membrane between the solution and










VEIN                                    CELLOPHANE


water (dialyzing solution). It works on the principle of dialysis, i.e. diffusion of small solute molecules through a semipermeable membrane (G. dia = = through, lyo = separate). Haemodialyser is a cellophane tube suspended in a salt-water solution of the same

composition as the normal blood plasma, except that no








CO2 AND AIR           FRESH




















urea is present. Blood of the patient is pumped from one





of the arteries into the cellophane tube after cooling it to 0oC and mixing with an anticoagulant (heparin). Pores

Fig. – Flow of blood through an artificial kidney for haemodialysis


of the cellophane tube allow urea, uric acid, creatinine, excess salts and excess H+ ions to diffuse from the blood into the surrounding solution. the blood, thus purified, is warmed to body temperature, checked to ensure that it is isotonic to the patient’s blood, and mixed with an antiheparin to restore its normal clotting power. It is then pumped into a vein of the patient. Plasma proteins remain in the blood and the pores of cellophane are too small to permit the passage of their large molecules. The use of artificial kidney involves a good deal of discomfort and a risk of the formation of blood clots. It may cause fever, anaphylaxis, cardiovascular problems and haemorrhage. Kidney transplant is an alternative treatment.

C.A.P.D. : Continuous ambulatory peritoneal dialysis.




(ii)  Kidney (Renal) Transplantation

Meaning : Grafting a kidney from a compatible donor to restore kidney functions in a recipient suffering from kidney failure is called renal transplantation.

History : First kidney transplant was performed between identical twins in 1954 by Dr. Charles Hufnagel, a Washington surgeon, India’s first kidney transplant was done on December 1, 1971 at Christian Medical College, Vellore, TamilNadu. The recipient was a 35 years old person Shaninughan.

Eligibility : All patients with terminal renal failure are considered eligible for kidney transplantation, except those at risk from another life-threating disease.

Donors : A living donor can be used in a kidney transplant. It may be in identical twin, a sibling, or a close relative. If the living donors are not available, a cadaveric donor may be used (cadaver is a dead body). Over half of the kidney transplants are from cadavers.

Success rate : A kidney transplant from an identical twin, called isogeneic graft or isograft, is always successful. A renal transplant from a sibling or a close relative or a cadaver, termed allogeneic graft or homograft, is usually successful with the use of an immunosupressant that prevents graft rejection by body’s immune response. Many renal transplant recipients are known to have retained functional grafts for over 20 years. Earlier, renal transplantation was limited to patients under 55 years. Now, however, with better techniques, kidney grafting has been done in selected patients in the 7th decade of life.

Pretransplant preparation : It includes haemodialysis to ensure a relatively normal metabolic state, and provision of functional, infection-free lower urinary tract.

Donor selection and kidney preservation : A kidney donor should be free of hypertension, diabetes, and malignancy. A living donor is also carefully evaluated for emotional stability, normal bilateral renal function, freedom from other systematic disease, and histocompatibility. Cadaveric kidney is obtained from previously healthy person who sustained brain death but maintained stable cardiovascular and renal function. Following brain death, kidneys are removed as early as possible, flushed with special cooling solutions, such as mannitol and stored in iced solution. Preserved kidneys usually function well if transplanted within 48 hours.

Recipient-Donor Matching : Recipient and donor are tested for 3 factors :

  • Blood groups : Recipient’s blood group should be compatible with donor’s blood
  • Human leucocyte antigen (HLA) : It is a genetic marker located on the surface of leucocytes. A person inherits a set of 3 antigens from the mother and three from the father. A higher number of matching antigens increases the chances that the kidney graft will last for a long
  • Antibodies : Small samples of recipient’s and donor’s blood are mixed in a If no reaction occurs, the patient will be able to accept the kidney.

Transplant procedure : Transplantation is done under general anaesthesia. Operation takes 3 or 4 hours. Cut is given in the lower abdomen. Donor’s kidney is transplanted retroperitonealy in the iliac fossa. Artery and vein of new kidney are connected to the iliac artery and vein of the recipient. Ureter of the new kidney is connected to the urinary bladder of the recipient. Often the new kidney starts producing urine as soon as blood flows through it, but sometimes it may take a few weeks before it starts working. A week’s stay in the hospital is necessary to recover from surgery, and longer if there are complications.

The new kidney takes over the work of two failed kidneys. Unless they are causing infection or high blood pressure, the old kidneys are left in place.




Immunosupression : Immunosupression means to depress the immune response of the recipient to graft rejection. Prophylactic immunosuppressive therapy is started just before or at the time of renal transplantation. An ideal immunosuppressant suppress immunity against foreign tissue but maintains immunity against infection and cancer. The drug, named cyclosporin, in such an immunosupressant. Use of antiserum to human lymphocytes is equally useful. It destroys T-cell mediated immune responses, but spares humoral antibody responses.

(iii) Kidney diseases

Pyelonephritis : It is an inflammation of renal pelvis, calyces and interstitial tissue (G.pyelos = trough, tub; nephros = kidney; itis = inflammation). It is due to local bacterial infection. Bacteria reach here via urethra and ureter. Inflammation affects the countercurrent mechanism, and the victim fails to concentrate urine. Symptoms of the disease include pain the back, and frequent and painful urination.

Glomerulonephritis : It is the inflammation of glomeruli. It is caused by injury to the kidney, bacterial toxins, drug reaction, etc. Proteins and R.B.Cs pass into the filtrate.

Cystitis : It is the inflammation of urinary bladder (G.kystis = bladder, –itis = inflammation). It is caused by bacterial infection. Patient has frequent, painful urination, often with burning sensation.

Uremia : Uremia is the presence of an excessive amount of urea in the blood. It results from the decreased excretion of urea in the kidney tubules due to bacterial infection (nephritis) or some mechanical obstruction. urea poisons the cells at high concentration.

Kidney stone (Renal calculus) : It is formed by precipitation of uric acid or oxalate. It blocks the kidney tubule. It causes severe pain (renal colic) in the back, spreading down to thighs. The stone may pass into the ureter or urinary bladder and may grow, and cause severe pain of blackade. When in bladder, the patient experiences frequent and painful urination and may pass blood in the urine. Surgery may be needed to remove stone and relieve pain.

Kidney (Renal) failure (RF) : Partial or total inability of kidneys to carry on excretory and salt-water regulatory functions is called renal or kidney failure. Result kidney failure leads to (i) uremia, i.e., an excess of urea and other nitrogenous wastes in the blood (G.ouron = urine, haima-blood); (ii) Salt-water imbalance; and (iii) stoppage of erythropoietin secretion.

Causes : Many factors can cause kidney failure. Among these are tubular injury, infection, bacterial toxins, glomerulonephritis (inflammation of glomeruli) arterial or venous obstruction, fluid and electrolyte depletion, intrarenal precipitation of calcium and urates, drug reaction, heammorrhage, etc.

  Accessory excretory organs

  • Skin : Many aquatic animals, such as Hydra and starfish, excrete ammonia into the surrounding water by diffusion through the body In land animals, the skin is often not permeable to water. This is an adaptation to prevent loss of body’s water. Mammalian skin retains a minor excretory role by way of its sudoriferous, or sweat, glands and sebaceous, or oil glands.
  • Sweat gland : Sweat glands pass out The latter consists of water containing some inorganic salts (chiefly sodium chloride) and traces of urea and lactic acid. It also contains very small amounts of amino acids and glucose. Sweat resulting from heavy muscular exercise contains a lot of lactic acid. The latter is produced in the muscles by glycolysis. Loss of salt by sweating produces no immediate problem because water is also lost, and the salt concentration of body fluids is not much changed. However, taking a lot of water after heavy sweating dilutes





the tissue fluid, causing ‘electrolyte imbalance’. This may cause muscle cramps. A dilute salt solution should be taken in case of heavy sweating.

  • Sebaceous glands : Oil glands pass out sebum that contains some lipids such as waxes, sterols, other hydrocarbons and fatty acids.


  • Lungs : Carbon dioxide and water are the waste products formed in Lungs remove the



and some water as vapour in the expired air. Lungs have access to abundant oxygen and oxidise foreign substances, thus causing detoxification and also regulate temperature.

  • Liver : Liver changes the decomposed haemoglobin of the worn-out red blood corpuscles into bile pigments, namely, bilirubin and biliverdin. These pigments pass into the alimentary canal with the bile for elimination in the faeces. The liver also excretes cholesterol, steroid hormones, certain vitamins and drugs via Infected or damaged liver does not remove bile pigments which accumulate in the blood and cause jaundice. The bile pigments impart yellowish tinge to the skin and mucosa (known as jaundice). Liver deaminates the excess and unwanted amino acids, producing ammonia, which is quickly combined with CO2 to form urea in urea or ornithine cycle. Urea is less toxic than ammonia. It is removed by the kidneys.
  • Large intestine : Epithelial cells of the colon transfer some inorganic ions, such as calcium, magnesium and iron, from the blood into the cavity of the colon for removal with the
  • Saliva : Heavy metals and drugs are excreted in the saliva.
  • Gills : Gills remove CO2 in aquatic They also excrete salt in many bony fish.


The regulation of solute movement, and hence, water movement, which follows solutes by osmosis, is known as osmoregulation. Osmosis may be defined as a type of diffusion where the movement of water occurs selectively across a semipermeable membrane. It occurs whenever two solutions, separated by semipermeable membrane (the membrane that allows water molecules to pass but not the solutes) differ in total solute concentrations, or osmolarity. The total solute concentration is expressed as molarity or moles of solute per litre of solution. The unit of measurement for osmolarity is milliosmole per litre (mosm L–1). If two solutions have the same osmolarity, they are said to be isotonic. When two solutions differ in osmolarity, the solution with higher concentration of solute is called hypertonic, while the more dilute solution is called hypotonic. If a semipermeable membrane separates such solutions, the flow of water (osmosis) takes place from a hypotonic solution to a hypertonic one.

Osmoconformers are the animals that do not actively control the osmotic condition of their body fluids. They rather change the osmolarity of body fluids according to the osmolarity of the ambient medium. All marine invertebrates and some freshwater invertebrates are strictly osmoconformer. Osmoconformers show an excellent ability to tolerate a wide range of cellular osmotic environments.

Osmoregulators, on the other hand, are the animlas that maintain internal osmolarity, different from the surrounding medium in which they inhabit. Many aquatic invertebrates are strict or limited osmoregulators. Most vertebrates are strict osmoregulators, i.e. they maintain the composition of the body fluids within a narrow osmotic range. The notable exception, however, are the hagfish (Myxine sp., a marine cyclostome fish) and elasmobranch fish (sharks and rays).




Osmoregulators must either eliminate excess water if they are in hypotonic medium or continuously take in water to compensate for water loss if they are in a hypertonic situation. Therefore, osmoregulators have to spent energy to move water in or out and maintain osmotic gradients by manipulating solute concentrations in their body fluids.

  • Water and solute regulation in freshwater environment : Osmolarity of freshwater is generally much less than 50 mosm L–1 while the freshwater vertebrates have blood osmolarities in the range 200 to 300 mosm L–1. The body fluids of freshwater animals are generally hypertonic to their surrounding environment. Therefore, freshwater animals constantly face two kinds of osmoregulatory problems : they gain water passively due to osmotic gradient, and continuously lose body salts to the surrounding medium of much lower salt content.

However, the freshwater animals prevent the net gain of water and net loss of body salts by several means, Protozoa (Amoeba, Paramoecium) have contractile vacuoles that pump out excess water. Many others eliminate water from the body by excreting large volume of very dilute urine. As a general rule, animals do not drink water, including freshwater fish do not drink water to reduce the need to expel and salt loss are minimised by a specialised body covering (subcutaneous fat layer of scaleless fish and scales over the body of fish or crocodile). Freshwater animals have remarkable ability to take up salts from the environment. The active transport of ions takes place against the concentration gradient. Specialised cells, called ionocytes or chloride cells in the gill membrane of fresh water fish can import Na+ and Cl from the surrounding water containing less than 1mM NaCl, when their plasma concentration of NaCl exceeds 100 mM.

  • Water and solute regulation in marine environment : Sea water usually has an osmolarity of about 1000 mosm L–1. Osmolarity of human blood is about 300 mosm L–1. The osmoregulatory problems in marine situation are opposite to those in freshwater environment. Marine bony fish have the body fluids hypotonic to seawater, and thereby, they tend to lose water from the body through permeable surfaces (gill membranes, oral and anal membranes). To compensate for the water loss, marine bony fish drink seawater. However, drinking seawater results in a gain of excess The ionocytes or chloride cells of the gill membrane of marine bony fish help to eliminate excess monovalent ions from the body fluid to the seawater. Divalent cations are generally eliminated with faeces. Hilsa, salmon and other fish that migrate between seawater and freshwater, when in ocean, drink and excrete excess salt through the gill membrane. A number of hormones play a key role in this switching over process.

In general, the body fluids of marine invertebrates, ascidians and the hagfish are isotonic to seawater. In elasmobranch fish (sharks and rays) and coelocanths (lobefin fish), osmolarity of the body fluids is raised by accumulating certain organic substances (osmolytes). Retention of osmolytes in body fluids reduces the osmoregulatory challenges. The best known examples of such organic osmolytes are urea and trimethylamine oxide (TMAO). Body fluids of sharks and coelocanths are slightly hyperosmotic to seawater due to retention of urea and TMAO while hypoionic to seawater as they maintain far lower concentration of inorganic ions in the body fluids.

  • Water and solute regulation in terrestrial environment : Land animals are always subject to osmotic desiccation, like the marine animals. Air-breathing animals constantly lose water through their respiratory surfaces. However, animals utilise various means to minimise this water Good examples are the waxy coatings of the exoskeletons of insects, the shell of the land snails and the multiple layers of dead, keratinised skin cells covering most terrestrial vertebrates. Despite such protective measures, a considerable amount of water is lost through oral, nasal and respiratory surfaces. This may even be fatal for the animal concerned. Humans, for examples, die if they lose around 12 per cent of the body water. Therefore, water loss must be compensated by drinking and eating moist food. Desert mammals are well adapted to minimise water loss. Kangaroos rats, for example, lose so little water that




they can recover 90 percent of the loss by using metabolic water (water derived from different cellular metabolic processes.) The nasal countercurrent mechanism for conserving respiratory moisture is also important. Behavioural adaptations, such as nervous and hormonal mechanisms that control thirst, are important osmoregulatory mechanisms in terrestrial animals. Many desert animals are nocturnal to avoid the heat of day-time, another important behavioural adaptation that minimises dehydration. The camels, however, reduce the chance of overheating by orienting to give minimal surface exposure to direct sunlight. They produce dry faeces and concentrated urine. When water is not available, the camels do not produce urine but store urea in tissues and solely depend on metabolic water. When water is available, they rehydrate themselves by drinking up to 80 litres of water in 10 minutes.

Important Tips

  • Anuria Failure of kidney to form
  • Oligourea – is less urine
  • Cystitis – Inflammation of urinary
  • Filtration fraction Ratio between GFR (glomerular filtration rate) and RPF (renal plasma flow).
  • Gout Painful great toe (arthiritis) due to deposition of uric
  • Haematuria – Presence of blood cells in
  • Oedema Increased volume of interstitial
  • Polynephritis Inflammation of large number of
  • Renal stone Stone formation in the nephrons of kidney due to accumulation of mainly calcium oxalates some phosphates and uric
  • Trimethylamine Excretory product of marine teleosts (bony fishes).
  • Uraemia High concentration of urea (about 10 times) in
  • Chloragogen cells – Found in coelomic fluid of earthworm and are analogous (functionally similar) to human liver as are excretory in
  • Contractile vacuole – Osmoregulatory apparatus of fresh-water protozoans like Amoeba, Parmaecium So contractile vacuole is functionally analogous to vertebrate kidney.
  • Glomerulonephritis Chronic inflammation of glomeruli due to streptococcal
  • Aminoaciduria – Urine with amino acids like cystine, glycine,
  • Polyuria Increased urine
  • Allantoin and allantoic acid are nitrogenous excretory products formed during embryonic development of amniotes with shelled Allantoin is also called embryonic waste by allantoic acid is stored in allantois foetal membrane.
  • Chances of infection of urinary tract are more in women due to shroter
  • Urate cells These are excretory cells of fat body of These store excretory waste permanently called storage excretion.
  • Bright disease – Characterised by nephritis caused by streptococal
  • Ptosis Displacement of
  • Dysuria Painful

Certain animals are both ammonotelic and ureotelic e.g. Ascaris, earthworm, lung fish (African toad), etc.

  • Aminotelism Expelling of amino acids as nitrogenous waste g. molluscs like Unio, Echinoder like Asterias, etc.
  • Chordate with flame cells is Branchiostoma (also called Amphioxus).
  • Nocturia Increased volume of urine at



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