Explanation

(a)Functions of the kidney:
(1) Serves as excretory organ, e.g. urine
(2) As osmoregulatory organ
(3) Homeostasis
(4) Formation of urine
(5) Elimination of toxins, drugs and.harmful substances
(6) Heat production
(7) Water level regulator.

 (c)(i) Osmo regulation: Kidney keeps the composition of the body plasma constant by means of osmosis, e.g. if there is too much water in the plasma, the kidneys excrete more water and thus reduce the amount in the plasma. In this way, the osmotic pressure of the plasma is kept constant by the kidney and this is important because, in this way, the balance of osmotic pressure between the blood and cells through the intermediate lymph is maintained. Normally the composition of the plasma is controlled or varied as a result of the amount  of water and soluble substances absorbed from the alimentary canal or lost through sweating. The variation  is corrected by the kidney.The osmotic pressure of the blood may rise due to too much water loss or excess salt intake. This is detected by sensitive cells in the hypothalamus of the brain which stimulate the production and release of antidiuretic hormone (ADH) by the pituitary gland. As soon as the hormone reaches the kidney, it induces the kidney tubules to reabsorb more water from the glomerular filtrate into the blood, making the urine Cortex more concentrated and thus, reducing more loss of water  from the blood. This state helps to maintain the osmotic pressure of the blood.

(C)(ii) Excretion of urine: The unit of excretion is the nephron. Waste products are transported from the.renal artery to the kidney. Inside the Bowman's capsule is the glomerulus which serves as the ultrafilteration of the waste products. The filterate is transported down the kidney tubules. In the loop shaded position of the tubule, more salts are added to the fluid while water is removed from it, resulting in the formation of urine. The urine moves to the bladder by peristatsis and is stored until it is ready to be expelled. 
 

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