CH-19 (EXCRETORY PRODUCTS AND THEIR ELIMINATION) BIOLOGY CLASS-XI, NCERT Topper Notes

Chapter-19
Excretory Products and Their Elimination

Points to Remember

Excretory Products :

Ammonia (most harmful) and needs large amount of water for excretion. Urea (less harmful) and Uric acid (least harmful) needs least amount of water for excretion.

Nephrons :

The structural and functional unit of kidneys. Each kidney contains about one million of nephrons.

Structure of Nephron :

A nephorn consists of Glomerulus, Bowman's capsule, PCT (Proximal Convoluted Tubule). JG A (Juxtaglomerular Apparatus), DCT (Distal Convoluted Tubule) and the collecting duet.(Refer fig., 19.3, page 292 (NCERT Text Book of Biology for Class XI)

Structure of kidney :

Size 10-12cm in length, 5-7cm in width, 2-3cm thick, average weight about 120-170 g.

• The blood vessels, ureter and nerves enter in the kidney through hilum (a notch).


• The outer layer of kidney is a tough capsule.


• The outer zone of kidney is cortex and the inner is medulla.


• The medulla is divided into few conical masses (medullary pyramids) projecting into calyces.


• The cortex extends between medullary pyramids called columns of Bertini, (Refer figure 19.2, page 292 (NCERT - Class XI Biology)

Glomerular Filration :

The filration of blood in glomerulus, about 1100-1200ml of blood is filtered by the kidney per minute.

Glomerular Filration Rate (GFR) :

The amount of filtrate formed by the kidney per minute is called GFR. In a healthy individual it is about, 125 ml/minute, i.e., 180 litres per day.

Types of Nephrons :

(i) Juxtamedullary Nephron :- About 15% of total nephorns, Gloeruli are found in inner region of cortex, large in size, long loop of Henle and found deep in medulla, associated with vesa recta control plasma volume when water supply in short.

(ii) Cortical Nephron :- About 15% of total nephorn mainly lie in renal cortex, glomeruli found in outer cortex, short loop of Henle, extends very little in medulla. They do not have vasa recta or vasa recta in highly reduced.

Functions of Tubules :

(i) PCT - Absorption of all essential nutrients and 70-80% of electrolytes and water, helps to maintain the pH and ionic balance of body fluids by selective secretion of H⁺, ammonia and K⁺ into filrate.

(ii) Henele's Loop - Reabsorption in this segment is minimum, it plays a significant role in maintenance of higher molarity of medullary interstitial fluid.

(iii) DCT - Conditional reabsorption of Na⁺ and water takes place here, reabsorption of HCO₃^- and selective secretion of H⁺ and K⁺ and ammonia to maintain the pH and sodium-potassium balance in blood.

(iv) Collecting duct - Large amount of water is absorbed from this region to produce concentrated urine, it plays a role in maintenance of pH and ionic balance of blood by selective secretion of H⁺ and K⁺ ions.

Steps of Urine Formation :

1. Glomerular Filration - Blood is filtered by glomeruli through three membranes i.e., endothelium of blood vessel, filration slits of Bowman's capsule and basement memebrane between these two layers. This filtration is called ultrafiltration as all constituents of plasma comes into filterate except proteins.

2. Reabsorption - 90% of filtrate is reabsorbed by the renal tububles by active or passive mechanism.

3. Secretion - Tubular cells secrete H⁺, K⁺, ammonia into the urine. It maintains acid-base balance of body fluids

Mechanism of Concentration of the Filtrate (Countercurrent Mechanism) :

Refer fig 19.6 page 296 (NCERT-Class XI Biology)

• This mechanism is said to be countercurrent mechanism because the out flow (in the ascending limb) runs parallel to and in the opposite direction of the inflow (in the desecending limb).


• NaCl is transported by the ascending limb of Henle's loop which is exchanged with the desecending limb of vasa-recat.


• NaCl is returned to the interstitium by the ascending portion of vasa recta.


• Henle's loop and vasarecta as well as the counter current in them help to maintain an increasing osmolality towards the inner medullary interstitium i.e., from 300 mosmol/L in cortex to about 1200 mosmol/L in inner medulla.


• Small amount of urea enter, the thin segment of ascending limb of Henle's loop which is transported back to the interstitium by the collecting tubule.


• This mechanism helps to maintain a concentration gradient in the medullary tubule interstitium.


• It helps in an easy passage of water from the collecting tubule to concentrate the filrate i.e. urine.

Renin Angiotensin System :

Micturition :

The expulsion of urine from the urinary bladder is called micturition. It is a reflex process but can be controlled volunraily up to some extent in grown up children and adults.

• The CNS (Centrall Nervous System) sends the signal which cause the stretching of the urinary bladder when it gets filled with urine.


• In response, the stretch receptors on the walls of the bladder sends signals to the CNS.


• The CNS passes on motor message to initiate the contraction of smooth muscules of the bladder and simultaneous relaxtion of the urethral sphincter causing the release of urine.


• An adult human excretes on an average 1 to 1.5 Litres of urine pre day.


• On an average 25-30 gram of urea is excreted out per day.

Role of other organs in excretion :

• Lungs - removes CO₂ (18L/day) and water.


• Liver - secretes bilirubin, biliverdin etc. helps to eliminate these substances along. with cholesterol, vitamins, drugs and dergraded steroid hormones through digesive wastes.


• Sweat and sebaceous glands - These glands of skin help to eliminate small amount of urea, NaCl and lactic acid etc. through sweat while sebaceous glands help to eliminate some substances like steroids, hydrocarbons and waxes through sebum.


• Saliva - It can help to eliminate small amount of nitrogenous wastes.

Disorders of Excretory System :

• Uremia - The accumulation of urea in blood due to malfunctioning of kidney.


• Hemodialysis - The process of removal of urea from the blood artificially.
In this process the blood from an artery is passed into dialysing unit after adding an anticoagulant like heparin. The blood passes through coiled cellophane tube, surrounded by dialysing fluid. The nitrogenous wastes form the concentration gradient and the blood becomes clear. This blood is pumped back to the body through vein after adding anti-heparin to it.


• Renal calculi - The formation of insoluble mass of crystallised salts (oxalates or phosphates of calcium).


• Glomerulonephrities - Inflammation of glomeruli of kidney.

Kidney Transplantation :

• Kidney transplantation is done in a patient in which both the kidneys fail to work i.e., at total failure of kidney. Kidney transplantation is the ultimate method for treatment of renal failure. In case of kidney transplantation both the damaged kidneys of patient are removed by surgery. And a functional kidney from a healthy donar preferably from close relative is taken and transplantation in the body of patient. After successful transplantation thee patient and donar can survive on one kidney.

Precautions taken for successful transplantation of Kidney :

1. Kidney should be taken from a healthy donor preferably from close relative.

2. Matching of blood group and other factor and compatibility should be done carefully before transplantation.

3. The patient (recipient) has to take some prescibed medicines immunosuppressants through out the life to suppress the immune system.