Grade 12 Zoology Note

Excretion and Osmoregulation

The process of elimination or removal of harmful substances from the body is known as excretion. The organs associated with the removal of harmful substances known as excretory system. Major excretory system is urinary system because most of the nitrogenous waste products, excess water and other toxic substances are thrown outside as urine. Skin, lungs liver and intestine are known as accessory excretory organs. Nitrogenous waste products are eliminated by urinary system (kidney).Volatile substances (alcohol, water vapour) are eliminated by lungs. Skin removes salts, water and fat derivatives.

Excretory system of man:

Excretory system of human are:

1.Apair of kidney

2.Apair of ureter

3.Urinary bladder



FIG: Urinary system of human being




Each kidney is dark red bean shaped measured about 11cm long and 150gm weight. They either side of vertebral column of lumber region. Right kidney is slightly lower than left. Each kidney is enclosed by renal fascia, adipose capsule and renal capsule. Blood vessels, Lymph vessels, nerves and ureter enter or leave from hilus renalis of kidney.

Microscopic structure of kidney:

Histologically, each kidney is composed of about one million nephrons.

L.S. of kidney shows two distinct regions-

1. Cortex:

It is outer dark part of kidney. It consists of Bowmann’scapsule and nephrons.


 Medulla is made up of conical modularly pyramids (15-16).Between the pyramids cortex extends as renal column of bertini. These modularly pyramid are connected with minor calyces then major calyces. The calyces open into renal pelvis.,+LS.jpg



These are one pair. Ureters are thin and muscular tube of about 30 cm long. These arise from hilum of kidney and open in to urinary bladder. Upper portion is renal pelvis and lower is ureter proper.

Urinary bladder:

It is thin muscular elastic bag located in abdominal cavity. Longitudinal fibers and circular fibers make the bag expanding. The collection of urine in bladder and discharging time from urethra is called micturition. Internal sphincter and external sphincter (voluntary) control the micturition.

Internally, the bladder has a triangular area called trigone in which three openings are opened.   Two are openings of ureters one is opening of ureters and one is opening of urethra. A urinary bladder can collect about 300ml urine.

FIG: LS of urinary bladder.




It is a short canal. Its length is 20 cm long in male and 4cm in female. It is called urinogenital duct in male as it serves for passage of urine and semen.


Structure of a Nephron (Uriniferous tubule):

Nephrons are the structural and functional units of kidney. These are about one million (ten lakh) uriniferous tubules or nephrons in kidney. Nephron differentiated into:

                    FIG: different parts of a nephrone

1.Malpighian body:

It has a cup like Bowman’s capsule and a network of blood capillaries called glomerulus. Bowman’s capsule is a small double walled cup. Outer layer is parietal and inner layer is visceral. Visceral cells form passage of fluid to filtrate in to Bowman’s capsule. Glomerulus is a blood capillary network in Bowman’s capsule. Entering is afferent renal arteriole and exists as efferent arteriole. Afferent lumen is wider than arteriole. The capillaries have small pores of about 100A0diameter.

2.Renal tubule: It is coiled tubular located behind the Bowman’s capsule. It has following parts:

   1.Proximal convoluted tubule (PCT):

   Internally it is lined with brush bordered cuboidal epithelium. It increases the surface area for absorption. Mitochondria provide the energy for active absorption. It is located in cortex and responsible for reabsorption.

   2.Henle’s loop:

   It is middle U-shaped. It lies in the medulla. It has thin descending limb and a thick ascending limb. Both limbs are supplied with parallel capillary system called vasa recta. These supply nutrient and carry reabsorbed water away.

  3.Distal convoluted tubule (DCT):

  It is posterior part and located in cortex. It maintains the concentration of urine.

  4.Collecting duct (CD):

   The DCT opens in to collecting tubule. It is present in the medulla region. The collecting tubule joins to form large Ducts of Bellini and to Renal pelvis. These ducts drain all the urine towards the pelvis.

About 85% nephrons lie in cortex (cortical nephrons) they don’t have vasa rectae. About 15% of total nephrons lie in medulla (juxtamedullary nephrons).These are highly supplied with vasa rectae.





Mechanism of Excretion or Urine Formation

Urine formation is complete in three steps:

1.Ultrafiltration (Glomerular filtration)

It is the first process of urine formation. It takes place in the glomerulus. Richards (1942) explained, the dissolved substances are filtered out in to the Bowmen’s capsule due to the pressure of blood.

The afferent arteriole enters the glomerulus and exist the form of efferent arteriole. The useful and harmful substances are filtered in Bowmen’s capsule. These are glucose, amino acids, vitamins and harmful substances---nitrogenous wastes (like uric acid, ammonia, creatine, large amount of salts) etc. These are the low molecular weight substances. Large molecules like protein, fats, and carbohydrates are not filtered.

The diameter of afferent arteriole is wider than diameter of efferent arteriole. So that more blood enters into the glomerulus and less blood volume exists. Which create hydrostatic pressure of blood in capillaries and force tend to move fluid out of the glomerulus. It is called ultra filtration. The hydrostatic pressure of blood (Pb) in the afferent arteriole is about 70mmHg.

About 180 liters of fluid are filtered from plasma but only about 11/2litre of urine is produced every day.

FIG: structure of nephrone showing vasa rectae

2.Selective Reabsorption

This is the second step in urine formation. It takes place in PCT and Henle Loop. PCT posses many brush bordered microvilli for absorption of filtrate. Mitochondria present it provide the energy for active absorption.

When the filtrate flows through PCT all useful substances are selectively absorbed from the filtrate to the blood capillaries by both passive and active transport mechanisms.

About 50% of urea in the filtrate diffuses back in blood capillaries by diffusion and about 80% of water by osmosis. Then much reduced volume of filtrate passes into Henle’s loop.

        FIG:showing selective absorption and tubular secretion

3.Tubular Secretion

It is the final step in urine formation. It takes place in distal convoluted and collecting. When tubular flows through the distal convoluted tubule(DCT) unwanted substances present in the blood such as uric acid, hippuric acid creatine, ammonia, K+ and H+ are secreted by the blood in to the tubular fluid by the process of active transport. At the same time, Na, Cl, and Ca are moved from the urine into blood to regulate the concentration of ions in plasma. Water also reabsorbed or secreted in the DCT according to the need of water by the body. The regulation of water is controlled by antidiuretic hormone (ADH) released from posterior pituitary gland.

Counter Current Mechanism:

The two limbs of Henle loops are opposite in property and the flow of filtrate is opposite in direction. This process is Counter Current Mechanism. The mechanism concentrates the urine due to the water diffusion. Vasa rectae are the veins present in between the limbs, act as counter current exchangers. The endothelial lining of vasa rectae is freely permeable to ions, urea and water. Role of Henle’s loop and Role of vasa rectae are important for Counter Current Mechanism.

Descending limb of the loop of Henle is fully permeable to water but slightly permeable to Na+ and Cl-. The wall of Ascending limb is largely impermeable to water but fully permeable to Na+ and Cl-.

As the filtrate passes through the descending limb of Henle loop, water moves out in the interstitial fluid (the fluid between vasa rectae and loop) and to the vasa rectae. The phenomenon maintains the high concentration of solutes in the in interstitial fluid. In the ascending limb Na+ from the filtrate and Cl- pass out passively. Concentrations of NaCl become high in interstitial fluid, deep in medulla and lowest in cortex. The ions draw out water from descending limb and from collecting duct. The ions move out from the ascending limb, the filtrate becomes progressively hypotonic(less concentrated).

Composition of Urine:

      1.About 96% water

      2.Urea 2%

      3.Urochrome, urinod, uric acid, creatine, K+, H+ phosphates, Oxalates etc are 2-3%. Urochrome makes the urine yellow colour and urinod gives bad smell to urine.

Isotonic urine is the concentration of water in urine= concentration of water in blood plasma.

Hypotonic urine is the concentration of water in urine > concentration of water in blood plasma.

Hypertonic urine is the concentration of water in urine< concentration of water in blood plasma.

Q. Write differences between descending limb and ascending limb of Henle loop, PCT and DCT, Tubular reabsoption and Tubular secretion.


Osmoregulation by kidney:

Osmoregulation is the maintenance of constant osmotic condition to regulate the water content and solute concentration in body.

      1.Osmoregulation in bony fishes (freshwater):

The fresh water bony fishes have gills and kidney for the osmoregulation. The fishes have gills and kidney for the osmoregulation. The fishes have hypertonic body fluids. Therefore there is a constant loss of ions from body. It balances the volume of water and salts by three means;

     i.Produce large amount of glomerular filtrate.

     ii.The large amount of selectively reabsorbed solute passes through the renal tube so fish excrete hypotonic urine.

     iii.Freshwater fish can take up Sodium ion and Chlorine ion from water directly due to the patience of Ionocytes cells in their gills.

     2.Osmoregulation in marine bony fishes:

     The fishes have hypotonic body fluid. There is constant loss of water from body. Marine fish conserve the water and loss ion to overcome dehydration of tissue by following ways:

      i.It constantly drinks sea water. Kidney lack of glomeruli so nitrogenous wastages are secreted directly into renal tubules and water is absorbed by osmosis.

      ii.Ionocytes help to expel monovalent anions from body fluids to seawater and divalent cations from faeces.

      3.Osmoregulation in human:

      In case of human hypertonic urine is excreted. This minimizes the water loss from    their body. The filtrate fluid in Bowman's capsule (isotonic) passes through the tubules of nephrones. Then a large amount of water and solutes are reabsorbed during this course.

During cold month, hypotonic urine is excreted but in warmer month hypertonic urine is excreted due to sweating.


ADH and regulation of water reasbsorption:

Antidiuretic hormone (ADH) or vasopressin is the hormone released by posterior part of pituitary gland. The main function of ADH is to increase permeability of distal convoluted tubule (DCT) and collecting duct (CD) due to which reabsorption of water increases. There are two conditions to balance water:

      1.When a person takes small amount of water:

      In this condition large amount of salt is ingested in diet or excessive sweating then solute potential of the body fluids become more negative (Osmotic pressure rises in the blood). The change in the osmotic potential is detected by osmoreceptors in the hypothalamus and carried to the brain. The brain detects such changes in the body and pituitatory gland releases ADH in the blood.

A large amount of water is reabsorbed rapidly from the filtrate into the cortex and medulla and passes back into the blood capillaries to maintain osmotic pressure normal. So that urine becomes highly concentrated and reduced volume of urine is released from kidney. It is generally called anti-diuresis.

     2.When a person takes large amount of water:

     When a person takes large amount of water or little sweating or extremely low salt intake in diet then the solid potential of the blood becomes less negative. (Osmotic pressure becomes low in blood) This condition is detected by osmoreceptors and carried to the brain. It sends the message to the pituitary gland to inhabit the

In absence of ADH walls of DCT and CD are impermeable to water and less water is reabsorbed as the osmotic pressure of filtrate is normal and large volume of diluted urine is excreted. It is generally called diuresis.

The regulation of water by ADH is an example of homeostatic feedback mechanism.

Other functions of kidney:

     1.Regulation of fluid balance: The kidney controls osmotic pressure of extra cellular body fluids by regulating the amount of water lost from body.

     2.Regulation of electrolyte concentrations: The concentration of electrolytes like Sodium, Potassium, Chloride Bicarbonates etc in blood also regulated. It is performed by selective tubular reasbsorption process in proximal tubule.

     3.Maintenance of acid-base balance

     4.Removal of other substances like mineral salts, iodides, drugs, arsenic and bacteria are recovered of the blood by kidney only.

     5.Kidney secretes rennin which is an enzyme but acts as hormone which changes the plasma protein.

      6.Kidney secretes erythropoietin which stimulates the formation of RBC.

      Functions of skin:

     1.Barrier against the invasion of microorganism and chemicals

     2.Regulate the dehydration

     3.Barrier against UV light

     4.Regulate the body temperature

     5.Acts as to regulate excretion of some substances

     6.Absorption of some substances

     7.Sensitivity in response to stimuli

Functions of liver:

1.Carbohydrate, fat, protein metabolism.

2.Deoxification of drugs.

3.Production of heat.

4.Secretion of bile product.

5.Storage of vitamins.

6.Breakdown of erthrocyst and defence.

7.Inactivation of hormones.

8.Synthesis of plasma protein.


The regulation or maintenance of a constant body fluid or internal environment is called homeostasis. The temperature, amount of water and glucose concentration are at almost constant in homeostasis. At the temperature of 37 oC enzymes work perfectly, division of cell and metabolism is also perfect.

The term homeostasis was first put forward by French biologist Claude Bernard in 1859. In 1929, American physiologist Walter Cannon first used the term homeostasis and studied about it. Homeostatic organs are skin, liver, kidney lungs, endocrine glands etc.

1.Skin as homeostatic organ: Skin helps in temperature regulation. It possesses pigment cells (chromatophores and melanophores), sweat glands and sebaceous glands which help in controlling the heat and fluid balance. The melanin pigment helps to absorb solar heat and increase body temperature. 

2.Kidney as homeostatic organ: Kidneys are the chief excretory and osmoregulatory organs. These also play key role in homeostasis. For example, (a) Regulation of water content by ADH (b) Regulation of salt or ion concentration in blood (c) Maintain acid base balance in body (Lactic acid, ketones, sulphuric acid etc) (d) Blood volume is regulated by the kidney

3.Liver as homeostatic organ: Liver is a key homeostatic organ due to the following reasons: (a) Regulation of Carbohydrate, lipid and amino acid metabolism. (b) Regulation of amount of glucose in blood by gluconeogenesis process. (c) It maintains the optimum temperature (d) Liver produces the bile. Bilirubin and Biliverdin, by products of dead RVC are excreted from the liver.

4.Lungs as homeostatic organ: It balances the concentration of O2 and Co2 in blood at the best level for the cells’ chemical reaction.

Kidney transplantation:

 Kidney transplantation is the organ transplant of a kidney into a patient with end-stage renal disease. Kidney transplantation is typically classified as deceased-donor (formerly known as cadaveric) or living-donor transplantation depending on the source of the donor organ. Living-donor renal transplants are further characterized as genetically related (living-related) or non-related (living-unrelated) transplants, depending on whether a biological relationship exists between the donor and recipient. Exchanges and chains are a novel approach to expand the living donor pool.

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