CBSE Class 11 Biology Excretory Products And Their Elimination Notes Set B

Points To Remember

Ammonotelism :
The animals which excrete ammonia are called ammonotelic and excretion of ammonia is known as ammonotelism eg. Amoeba, sycon, hydra, liver fluke, tapeworm, Leech, Prawn, bony fishes etc.

Ureotelism:
excretion of urea is known as ureotelism and the animals which excrete urea are ureotelic animals eg. mammals, many terrertrial amphibians and marine fishes and sting rays etc.

uricotelism :
Excretion of uric-acid is known as uricotelism and the animals are called uricotelic eg. most insects, land snails, lizards, snakes and birds.

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

Structure of Nephron :
A nephron consists of Glomerulus, Bowman’s capsule, PCT (Proximal Convoluted Tubule). JG A(Juxaglomerular Apparatus), DCT (Distal Convoluted Tubule) and the collecting duct. (Refer fig., 19.3, page 292 (NCERT Text Book of Biology for Class XI)

Structure of Kidney :

- Size 10-12 em in length, 5-7 em in width, 2-3 em 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.

Glomerular Filtration :
The filtration of blood in glomerulus, about 1100-1200 ml of blood is filtered by the kidney per minute.

Glomerular Filtration 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) Juxtamedullar·y Nephron-About 15% of total nephrons, Glomeruli are found in inner region of cortex, large in size, long loop of Henle and found deep in medulla, associated with vasa recta control plasma volume when water supply is short.
(ii) Cortical Nephr-on-About 85% of total nephron 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 is 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 filtrate.
(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 HC03- 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 Gromemlar Filtration-Blood is filtered by glomerulus through three membranes i.e., endothelium of blood vessel, filtration slits of Bowman’s capsule and basement membrance 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 tubules by active or passive mechanism.
It is evident by the fact that out of 180L of filtrate formed per day only 1. 5 L of urine released.
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) :

- 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 descending limb).
- NaCl is transported by the ascending limb ofHenle’s loop which is exchanged with the descending limb of vasa-recta.
- 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 filtrate i.e. urine.

Anti Diuretic Hormone (ADH) Controls the urine formation when there is less blood volume due to excessive loss of fluid from the body, osmoreceptors send the signal to hypothalamus to release ADH which in turn facilitates water reabsorption thus preventing diuresis (increase in frequency of urination)

Micturition :

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

- The CNS (Central 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 muscles of the bladder and simultaneous relaxation of the urethral sphincter causing the release of urine.
- An adult human excretes on an average 1 to 1.5 Litres res of urine per day.
- On an average 25-30 gram of urea is excreted out per day.

Renin Angiotensin System

Role of other organs in excretion :

Lungs – removes C02 (18L/day) and water.
Liver – secretes bilirubin, biliverdin etc. helps to eliminate these substances along. with cholesterol, vitamins, drugs and degraded 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 surrounding by dialysing fluid. The nitrogenous wastes from 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 fmmat ion of insoluble mass of crystallised salts (oxalates or phosphates of calcium.
Glomer·ulonephritis-Inftamation of glomeruli of kindney.

KidneyTransplantation

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 transplanted in the body of patient. After successful transplantation the patient and donar can survive on one kidney.

Precautions taken for successful tr·ansplantation 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 presctibed medicines immunosuppresses through out the life to suppress the immune system.