CBSE Class 10 Science Excretion Notes

EXCRETION

6.1  EXCRETION :

There are various metabolic activities which take place inside the living organisms. All these activities are chemical reactions. As a result in animal body several end products are formed which are of no use to the cells. These are called as wastes. These must be removed from the body for proper functioning of the body. The elimination of these waste nitrogenous products form the body is called as excretion. Waste material is ammonia, urea, uric acid, carbon dioxide, pigments, salts digestive wastes, excess of water etc. Ammonia, urea uric acid are waste nitrogenous products, The excretory produces are both volatile and non-volatile. These are removed from the body by different methods.

6.1(a) Excretion in Amoeba :

Amoeba is an ammonotelic organism since the principal excretory product is ammonia. Special excretory organelle in Amoeba is lacking. CO₂ and ammonia are exerted by diffusing is solution through plasma membrane. The concentration of ammonia is always higher in Amoeba than in the surrounding water. The water enters through plasma membrane by “endosmosis”. Ammonia is formed in cytoplasm by metabolism. Surplus water enters contractile vacuole. This surplus water can rupture the animal’s body. Thus size of contractile vacuole increases, when the contractile vacuole is fully expanded with water, it moves towards the periphery. As it comes in close contact with the plasma membrane, the contractile vacuole bursts. Thus excess of water (surplus water) is discharged in the surrounding water, this phenomenon of controlling the amount of water in the body is called as “osmoregulation”.

6.1(b) Excretion in Earthworm :

In earthworm, the excretory organs are nephridia.  The internal funnel-like opening is called as “nephrostome”. The waste material from body cavity (coelom) enters the nephridium through nephrosome. In the inner lining of nephridium, the cells absorb useful substances like glucose.

6.2 STRUCTURE OF A TYPICAL NEPHIRIDIUM :

A typical nephridium consists of three pars : nephrostome, body and terminal duct. The nephridium communicates with the coelom (body cavity) through internal nephrostome. Nephrostome is a ciliated funnel which leads into body of nephridium through the neck. The body of nephridium consists of short straight lobe, a long spiral lobe with narrow apical part. Spiral lobe consists of proximal limb and distal limb. Neck of the nephridium leads into proximal part of spiral lobe and terminal duct leaves the proximal limb. The tubule of the neck enters the body of the nephridium and leaves the body as terminal duct. These tubules have ciliated tracts inside. The number of ciliated tracts depends upon the number of coils of the tubules. The terminal duct may open outside by nephridiopore or into the gut (alimentary canal).

6.2 (a) Functioning of Nephridium :

Nephridia are highly vascular and extract nitrogenous wastes from the blood. The nitrogenous wastes and useful substances (glucose) enter the body of nephridium through internal neprostome in the fluid form. The cilia present in the tubule beat to move the fluid. Useful substances like glucose are reabsorbed by cells, lining the tubule and is passed into the blood. The remaining waste is discharged into the alimentary canal or to exterior through nephridiopore. According to the position of nephridia in the body of earthworm, nephridia are of three types :

(i)  Septal nephridia are attached on septa. Nephridiopore is missing.

(ii) Integumentary nephridia are attached on inner side of the skin. Nephridiopore is present.

(iii) Pharyngeal nephridia are present as three p airs of groups of nephridia, on both sides of alimentary canal. Nephridiopore is absent. Septal and pharyngeal nephridia are endonephric as these open in the alimentary canal. Integumentary nephridia are ectonephric. Excretion is an adaptation to conserve water. Earthworm is ammonotelic (excrete ammonia) in excretion, in sufficient water while it is ureotellic (excrete urea) on land.

6.3 HUMAN EXCRETORY SYSTEM :

As a result of various metabolic process going on in our body a number of waste products are formed. These have to be eliminated as they are toxic to the body.

The waste products include :

(i)  Carbon dioxide which is liberated during respiration; and is eliminated by the lungs.

(ii) Nitrogenous metabolic wastes, such as urea and uric acid produced in the liver from excessive proteins.

(iii) Bile pigments : Bile pigments (e.g., billrubin) derived by the breaking down of hemoglobin of the erythrocyte.

(iv) Excess salts, water and vitamins : Concentration of these substance above the required level, is harmful to the body. Elimination of all metabolic nitrogenous wastes from the body is callers as excretion.

6.4 ORGANS OF EXCRETION :

(i) Lungs : Carbon dioxide produced by the oxidation of glucose or other food substances in the tissues is removed by the blood. This carbon dioxide is carried to the lungs through the blood vessels (veins) where it diffuses into the alveoli and out through the respiratory tract. Water vapour in small amount is also exhaled during expiration from the lungs.

(ii) Skin : Substances like soluble food mater, oxygen, water dissolved mineral salts, traces of urea and uric acid diffuse from the thin walls of capillaries into the walls of the sweat glands. Oxygen and food substances are used for metabolic activities of the cells of seat glands but the remaining metabolic wastes are excreted out of the gland through the sweat duct which opens on the surface of the skin through sweat pore. Sweat contains 99% water, traces of urea and uric acid. However, after heavy exercise, lactic acid forms a major constituent of seat. Profuse sweating may lead to sodium deficiency, leading to muscle cramps. An adaptation of prevention of water loss is the impermeability of our skin to water. However, in aquatic animals, skin is the major excretory organ. They excrete ammonia through their skin by diffusion as ammonia is highly soluble in water.

(i) Bowman’s capsule : It is a single-cells thick, double walled cup-shaped structure present in the cortex region of the kidney. The cup-shaped capsule contains a network of capillaries called Glomerulus’s. Glomerulus’s and Bowman’s capsule are together called as Renal corpuscle.

(ii) Proximal convoluted tubule (PCT) : It starts after the Bowman’s capsule and is greatly twisted. The whole P CT lies in the cortex region.

(iii) Henle’s loop : Henle’s loops is a U-shaped tubule located in the medulla region. it consists of

(A) a thin-walled descending limb in the medulla

(B)  a thick-walled ascending limb in the cortex. Henle’s loop is long in those animals which pass hypertonic urine.

(iv) Distal convoluted tubule : The ascending limb continues into the distal convoluted tubule which forms several coils in the cortex.

(v) Collecting duct : Collecting tubule receives distal tubules of several uriniferous tubules. Several such tubules  unite to form a large collecting duct. The collecting ducts are held together and converge to form a pyramid. The pyramid opens into the pelvis which leads into the ureter.

6.6 BLOOD SUPPLY TO NEPHTRONS :

Inside the kidney, the renal artery branches into a  number of renal arterioles. A branch from a renal arteriole enters each Bowman’s capsule, and is called the afferent arteriole. It breaks up into a network of capillaries which reunite to form a efferent arteriole. (Glomerulus is a mass of network of capillaries in the Bowman’s capsule). The efferent arteriole after emerging from the Bowman’s’ capsule runs a short distance and breaks up into a capillary network which surrounds the renal tubule and rejoins to form a vein. By reuniting again and again with other veins of the kidney it forms the renal vein which drains into the posterior venacava.

6.7 CHEMICAL COMPOSITON OF URINE :

Normal human urine consist of about 95% water and 5% of solid wastes. Besides the normal constituents , certain hormones and medicines like the antibiotic and excess vitamins are passed out with urine. Organic compounds (gm/l): Urea - 2.3; Creatinine - 1.5; Uric acid - 0.7; Ammonia - 0.6 Inorganic Compounds (gm/l) Nacl - 19.0; KI - 12.5; H2SO4 - 1.8; NH3 - 0.6. Normally a man excretes 1000 - 170 ml of urine daily, depending upon the water intake, diet, climate, mental state and physiological condition. Tea, coffee, alcohol and other beverages increases the formation of urine.

6.7 (a) Working of Nephron :

Main function of nephron is to form urine. There are three main  process involved in the urine formation :

(i) Glomerular ultrafiltration : If is the filtration of body fluids and solutes from the blood, out of he glomerular capillaries into the Bowman’s capsule due to the pressure in the glomerulus. All substances from the blood are filtered out except the large protein molecules. This fluid in the glomerular capsule is called as glomerular filtrate. It consists of water, urea, salts, glucose and other plasma solutes. Blood coming out of the efferent arteriole is therefore thick.

(ii) Tubular reabsorpiton : Glomerular filtrate contains a lot of useful materials like glucose, salts such as that of sodium and water. These substances are reabsorbed from the renal tubule at various levels and in varies proportions. Glucose is reabsorbed completely from the proximal convoluted tubule. More than 85% of water is  reabsorbed from the proximal, distal and even in collecting tubules. Sodium chloride is reabsorbed in the proximal and distal tubules. Potassium and phosphate is completely reabsorbed from the proximal tubule. Other substances reabsorbed are uric acid, sulpahtes, vitamin C, amino acids etc.

(iii) Tubular secretion : This occurs mainly in the distal convoluted tubule and the collecting duct of the nephron. It is an active, vital process performed by the cells of the cuboidsl epithelium lining the tubules which excrete additional wastes from the blood stream into the filtrate by active transport. In this process substances like potassium, hydrogen, creatinine and certain drugs like phenol, penciling etc. are directly exerted by the tubular cells from the blood. The fluid which now flows through the last parts of the tubule is urine which consist of water, urea, uric acid, mineral ions like sodium, potassium, chlorides, phosphates etc.

6.8 ARTIFICIAL KIDNEY :

In case of loss or damage of one kidney, the other kidney performs the function of both the kidneys and the person can lead a normal life. But the failure of both the kidneys leads to death. Artificial kidney is a dialysis machine which cleans blood of waste products, thus acting like a kidney. The patients’ blood is led from the radial artery of the arm through the machine where urea and other salts are removed and pure blood is returned to vein in the same arm. In case of permanent damage to the kidneys, dialysis has to be performed for about twelve hours, twice a week. Patients with chronic kidney failure have   been recorded to survive for more than 12 years on dialysis. Now a days, diseased kidney may be replaced with healthy one by kidney transplantation. To lead a normal life, one healthy kidney is more than enough. Therefore, a healthy person can donate his one kidney to patient who has both kidneys impaired