Osmo Regulation And Excreation In Animal

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.


  • The matabolic wastes which are excreted in the animals are of several kinds and vary from animal to animal and in the same animal from time to time. The difference in the nature of waste products in animal is correlated with the matbolic processes taking place in the body of the animals.

  • In different animals the types of nitrogenous waste in which form these are eliminated, vary as per their mode of life. These are of following types :

Ammonotelism :

  • Ammonia formed during deamination of protein (amino acid) is most toxic hence, needs to be eliminated immediately.

  • Being highly soluble in water, it diffuses and dissolves in surrounding water, hence in aquatic animals e.g. protozoan, sponges, coelenterates, molluscs, earthworm, most teleosts and fresh water bony fishes, aquatic amphibians (salamanders), etc, this is the common process.

  • In fishes most NH3 is lost as NH+4 ions through gill membrane, kidney plays a minor role.

Ureotelism :

  • In terrestrial condition NH3 can not come out readily and its staying is very toxic. Urea being 1 lakh times less toxic than NH3, NH3 is converted into urea in liver by urea cycle. To be eliminated in solution form, urea requires sufficient amount of water.

  • Ex. Mammals, amphibians (frogs or toads) cartilaginous fishes (sharks) and aquatic reptiles. Turtles and terrapins excrete both urea and ammonia. Some tortoises can modify nitrogenous waste as per change in the environment.

  • Sharks retain urea as major osmolyte to raise its body fluid concentration to maintain its osmolarity with sea water. This minimises the water loss.

  • Lung fishes,Xenopus (toad) are ammonotelic in water but ureotelic in mud during summer.

  • Tadpole of frog / toads are ammonotelic but adult is ureotelic.

Uricotelism :

  • Uric acid which is least toxic of all and hence can be retained (storage excretion) in the body for even longer time without any harmful effect.

  • It is thousand times less soluble in water than NH3 and urea, hence implies minimum loss of water.

  • It’s also a means for conservation of water since it is eliminated in semi solid precipitate form hence is useful for the animals with less traffic of water in their body.

  • Ex. insects, reptiles, birds, land snails, land crustaceans.

  • White material in the droppings of birds, lizards consist of insoluble crystals of urates. In this form it is commercially obtained from birds droppings (guano) used as fertilizer.

  • It is specially important for the land vertebrates (birds, reptiles) which lay shelled egg. Shell is impermeable to nitrogenous waste hence during embryonic development it is stored within the egg and is left behind when animal hatches out.

  • Crocodiles, excrete both uric acid and ammonia.

                                                                 Excretory organs of different organisms



Excretory/osmoregulatory Organ/Organelle and

principal N2-waste



I. Invertebrates



Contractile vacuole








General surface of body


Sycon, Leucon



Ammonia, General surface of body





flame cells (=Solenocytes)

form the protonephridial system


Taenia, fasciola



H-shaped excretory organ, Renette cells





Nephridial system,

(Metameric), various types







Malpighian tubule

(Uric acid)




Class crustacea

Antennary (=green) gland

Uric acid




Class Arachnida

Coxal glands

Malpighian tubule

Hepato pancreas






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

(b) Keber's organ

Aquatic forms excrete


Terrestrial forms

Excrete uric acid















Dermal branchiae (primitive gills) tube feet,

body surface (Ammonia)




Excretory system of man.

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 female. 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.

Structure of kidney :

Structure and types of nephron : A nephron or uriniferous tubules is made of two parts –

(1) Malpighian body : The proximal end of each nephron forms a blind or closed, enlarged and double walled cup, the Bowman's capsules in the cortex. (name Bowman's capsule is based on english physiologist and histologist William Bowman).

(2) Tubule : The tubule is differentiated in to 3 parts P.C.T., Henle's loop and D.C.T.

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.

                                                                 Differences between cortical and Juxtamedullary nephrons

Cortical Nephrons

Juxtamedullary 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 medulla

4.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.

                                                              Differences between male and female urethra

Male urethra

Female 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.

                                                                        Summary of events occurring in a nephron

Materials transferred

Nephron region

Process involved


1. Glucose, Amino acids, Vitamins, Hormones, Na+, K+, Mg2+, Ca+2, H2O, Urea, Uric Acid, Creatinine, Ketone Bodies.

Bowman's capsule

Glomerular filtration


2. Glucose, Amino Acids, Hormones, Vitamins, Na+, K+, Mg2+, Ca+2

Proximal convoluted tubule


Active transport

3.  Cl

Proximal convoluted tubule


Passive transport

4. Water

Proximal convoluted tubule



5. Urea

Proximal convoluted tubule



6. H2O

Narrow region of descending limb of Henle's loop



7. Na+,K+,Mg+2,Ca+2,Cl

Narrow region of ascending limb of Henle's loop



8.Inorganic ions as above

Wide part of ascending limb of Henle's loop


Active transport


Distal convoluted tubule, collecting tubule, collecting duct

Reabsorption with ADH Help


10. Na+

Distal convoluted tubule, collecting tubule, collecting duct

Reabsorption with aldosterone help reabsorption secretion

Active transport

11. Urea

Last part of collecting duct

Reabsorption with aldosterone help reabsorption secretion


12. Creatinine, Hippuric Acid, Foreign substances

Proximal convoluted tubule

Reabsorption with aldosterone help reabsorption secretion

Active transport

13. K+, H+

Distal convoluted tubule

Reabsorption with aldosterone help reabsorption secretion

Active transport

14. NH3

Distal convoluted tubule

Reabsorption with aldosterone help reabsorption secretion


15. Urea

Ascending limb of Henle's loop (Thin part)


Reabsorption with aldosterone help reabsorption secretion




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.

(i) 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 1.003 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.

(ii) 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 urine. 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.

(iii) 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.

(iv) Renal threshold : A negligible amount of glucose is present in the urine. 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.

(v) Conduction of urine and Micturition : Urine is produced and drained continuously by the nephrons into the renal pelvis. 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.

                            Urine constituants in man (in gram)


Total volume

1,200ml – per 24h





Total solids





















Hippuric acid







50 – 300 g


Uric acid
















    1. gm

    2. 0.005gm








                                                     Differences between Rennin and Renin





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.


Its secretion is stimulated by food.

Its secretion is stimulated by a reduction of Na+ level in tissue fluid


It is secreted as an inactive form prorennin which is activated to rennin by HCl.

It is secreted as renin.


It is a proteolytic enzyme.

It is a hormone that acts as an enzyme


It helps in the digestion of milk protein casein.


It converts the protein angiotensinogen into angiotensin.

                                           Differences between ammonotelism, ureotelism and uricotelism






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.


Uses very little energy in forming ammonia.

Uses more energy in producing urea.

Uses far more energy in producing uric acid.


Its product is very toxic.

Its product is less toxic.

Its product is least toxic.


Causes considerable loss of body's water.

Causes less loss of body's water.

Causes least loss of body's water


Occurs in aquatic animals.

Occurs in aquatic as well as land animals.

Occurs in land animals.


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.


Animals excretingNH3 are called ammoniotelic.


Animals excreting urea are termed uroetelic.


Animals excreting uric acid are called uricotelic.

  • Elasmobranchs can retain 2.5% of urea in their body fluids to make them hyperosmatic to counter the highly concentrated sea water. For this, these have urea absorbing segments in their nephrons.

  • Bidder’s canal : Inside the kidney of male frog and carries the sperms only.

  • Contractile vacuole is absent in marine and parasitic protozoans as these are found in isotonic media.

  • All the flat worms e.g. planarians, flukes and tape worms have protonephridia (Flame cells or solenocytes) for excretion and osmoregulation. Each flame cell has a bundle of cilia whose beating gives the appearance of flickering of a candle flame.

  • Fresh-water fishes have large glomeruli to expel excess water so urine is highly hypotonic, while marine bony fishes are either aglomerular or have small glomeruli and expel small amount of isotonic urine.

  • Marine bony fishes have salt secreting cells in their nasal chambers and gills.

  • Gout : Excess of uric acid is deposited in the joints (especially those of great toes) and causes painful movements and arthritis.

  • Ornithinic acid : Nitrogenous excretory waste of certain birds.

  • In human urine, uric acid is mainly formed from the purines of nucleic acids.

  • Rennet cell : Excretory cell ofAscaris.

  • Septal nephridia and Pharyngeal nephridia of earthworm are enteronephric as they release wastes in gut.

  • Neural gland : Excretory structure of Urochordates e.g.Herdmania.

  • Guanin is the nitrogenous waste of many spiders, so called guanotelic.

  • Study of structure, working and disorders of kidneys is called nephrology while study of female urinary tract and male urinogenital tract is called urology.

  • Urography : Radio x-ray photograph of urinary tract.

  • Urinometer : Instrument used to measure specific gravity of urine.

  • Hepatic vein is with maximum amount of urea, while renal vein is with minimum amount of urea.

  • Incomplete loop of Henle is found in birds while complete loop of Henle is found in mammals. There is no loop of Henle in nephrons of fishes, amphibians and reptiles.

  • Urinary bladder is absent in cyclostomes, cartilage fishes, snakes, alligators, crocodiles and birds.

  • Sweating is mainly meant for cooling of body and not for excretion.

  • Urate cells : Excretory cells of fat body of insects. These store the nitrogenous wastes permanently called excretion by storage.

  • In lower animals, ammonia is formed in all the cells.

  • Desert mammals have long loop of Henle for more conservation of water.

  • Podocytes : Specialized cells of visceral (inner wall) layer of Bowman’s capsule.

  • Ornithine cycle occurs in mitochondria of liver cells.

  • Renal blood flow : It is amount of blood that passes through the human kidneys. It is about 1100-2000 liters of blood each day.

  • Renal Plasma Flow (RPF) : Amount of blood plasma which passes through glomerular capillaries of all the nephrons of both the kidneys in one minute and is about 650 ml.

  • Filtration coefficient (Kf) : Glomerular filtration per mm Hg of GFP and is about 12.5 ml per mm Hg.

  • Alcohol acts as a diuretic as it lowers the secretion of ADH so lowers the water reabsorption, so alcoholic generally suffers from dehydration.

  • Normally urine is about four times concentrated than blood plasma of man.

  • About 99% of water from nephric filtrate is reabsorbed and only 1% of water of nephric filtrate is lost in urine.

  • No water is reabsorbed in ascending limb (as is impermeable to water), while very little water is reabsorbed from descending limb.

  • Normal level of urea in human blood is 17-38 mg/100 ml of blood (i.e. about 4.0 gm/litre of blood).

  • In adult man, urge for urination can be suppressed upto 1-1.5 liters of urine in urinary bladder.

  • pH of urine depends upon the nature of diet. High protein food increases acidity while fruits and vegetables increase alkalinity.

  • Some marine birds like sea gulls expel excess of salts through their nasal glands by active transport.

  • Compensatory hypertropy : Enlargement of a kidney to compensate damaged other kidney.

  • Lithotripsy : Surgeryless treatment of renal stones. It is bloodless, painless and low-costing. In this, renal stones are broken with radiations.

  • Anhydrobiosis : Ability of certain animals to live without water. They can lose almost all their body water and survive in a dormant state when their habitat dries up. They can survive in an inactive form for a decade or more. It is also called cryptobiosis.

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