The Excretory System (Elimination of Body Wastes) — ICSE Class 10 Biology Important Questions

Answer: (b) Nephron.

Each kidney contains about a million nephrons, and each one carries out filtration and the formation of urine.

Answer: (c) Urea.

Urea is formed in the liver from excess amino acids and is the chief nitrogenous waste removed by the human kidney.

Answer: (b) Completely reabsorbed in the renal tubule.

Glucose is filtered at the glomerulus but is entirely reabsorbed by selective reabsorption (mainly in the proximal convoluted tubule), so healthy urine contains no glucose.

Answer: (b) Efferent arteriole is narrower than the afferent arteriole.

Blood enters the glomerulus through a wide afferent arteriole and leaves through a narrower efferent arteriole. This creates high hydrostatic pressure in the glomerulus, driving ultrafiltration.

Answer: (a) On a hot day, heavy sweating removes water, so to conserve water the kidney reabsorbs more of it, producing a smaller volume of concentrated urine (osmoregulation). R correctly explains A.

The two main parts are the Malpighian body (glomerulus enclosed in the Bowman's capsule) and the renal tubule (proximal convoluted tubule, loop of Henle and distal convoluted tubule).

Filtration of blood (ultrafiltration) takes place in the Malpighian body, where the glomerulus filters blood into the Bowman's capsule.

Glomerular filtrate: the fluid formed in the Bowman's capsule after ultrafiltration; it contains water, glucose, salts, urea and other small molecules (no blood cells or proteins).

Urine: the final fluid after selective reabsorption and tubular secretion; it contains water, urea, uric acid and excess salts, but no glucose and much less water than the filtrate. Thus urine is more concentrated in wastes than the filtrate.

Ultrafiltration occurs in the Malpighian body:

1. Blood enters the glomerulus through a wide afferent arteriole and leaves by a narrower efferent arteriole, creating high hydrostatic pressure.

2. Under this pressure the blood is filtered through the walls of the glomerular capillaries and the Bowman's capsule.

3. Small molecules — water, glucose, amino acids, salts, urea and uric acid — pass into the Bowman's capsule as the glomerular filtrate.

4. Large molecules such as plasma proteins and blood cells are too big to pass and remain in the blood.

The excretory system consists of a pair of kidneys, two ureters, a urinary bladder and the urethra.

1. Kidney: filters the blood and forms urine, removing urea and other nitrogenous wastes.

2. Ureter: a tube that carries urine from each kidney to the urinary bladder.

3. Urinary bladder: a muscular sac that temporarily stores urine until it is passed out through the urethra.

Osmoregulation is the regulation of the water and salt content of the body fluids.

1. The kidney adjusts the amount of water reabsorbed from the filtrate in the tubules and collecting duct.

2. When the body has excess water, less is reabsorbed, so a large volume of dilute urine is passed.

3. When the body is short of water (e.g. after sweating), more water is reabsorbed under the influence of a hormone (ADH), so a small volume of concentrated urine is passed.

In this way the kidney keeps the concentration of the blood and tissue fluids constant.

Formation of urine — three steps:

1. Ultrafiltration (in the Malpighian body): Blood in the glomerulus is under high pressure, so water, glucose, salts, urea and other small molecules are filtered into the Bowman's capsule, forming the glomerular filtrate. Blood cells and proteins are retained.

2. Selective reabsorption (mainly in the PCT and loop of Henle): As the filtrate flows along the renal tubule, useful substances are reabsorbed into the surrounding blood capillaries — all the glucose and amino acids, most of the water, and the required salts. Wastes such as urea are left behind and become concentrated.

3. Tubular secretion (in the DCT): Additional wastes and excess ions (e.g. $H^+$H^+, $K^+$K^+, ammonia, certain drugs) are actively passed from the blood into the tubule to keep the blood pH and salt balance correct.

The fluid remaining is urine, which flows through the collecting duct to the ureter, is stored in the bladder and is finally passed out.

Kidney failure is the condition in which the kidneys stop filtering the blood properly, so nitrogenous wastes such as urea and excess salts and water build up in the blood, which can be fatal.

Haemodialysis (artificial kidney):

Principle: The wastes are removed from the blood by diffusion across a selectively permeable membrane into a dialysing fluid, exactly as the nephron filters and reabsorbs — but done by a machine.

Working:

1. Blood is drawn from an artery (usually in the arm) and mixed with an anticoagulant (heparin) to stop clotting.
2. It is pumped through long, coiled tubes made of a selectively permeable membrane, which are surrounded by the dialysing fluid.
3. Waste substances (urea, excess salts) diffuse from the blood, where they are in high concentration, into the dialysing fluid.
4. The purified blood is warmed and returned to a vein of the patient.

Composition of the dialysing fluid: It has the same concentration of glucose, salts and other useful substances as normal blood, but contains no urea or wastes. Because there is no concentration difference for glucose and essential salts, these do not diffuse out, so only the wastes are removed while useful substances are retained.

(i) Proteins are large molecules that cannot pass through the glomerular capillary walls during ultrafiltration, so they stay in the blood and the filtrate contains none.

(ii) Glucose is small enough to be filtered, so it appears in the filtrate at the same concentration as plasma (0.10). As the filtrate flows through the tubule, all the glucose is reabsorbed back into the blood by selective reabsorption, so none is left in the urine.

(iii) Urea is a waste and is not reabsorbed. As water is reabsorbed from the tubule back into the blood, the remaining urea becomes far more concentrated — rising from 0.03 in the filtrate to about 2.00 in the urine.

(iv) Glucose in the urine (glycosuria) suggests the blood glucose was so high that not all of it could be reabsorbed. This commonly indicates diabetes mellitus.