(a) List three functions of the kidney. (b) Make a labelled diagram of the mammalian kidney tubule (nephron) (c) Describe how the kidney carries out two of ...

(a) Three functions of the kidney

1. Excretion: removal of nitrogenous waste (mainly urea), together with excess salts, water and other toxic substances, from the blood as urine.
2. Osmoregulation: regulation of the water and salt (osmotic) balance of the blood and body fluids, keeping the osmotic pressure of the plasma constant.
3. Homeostasis / regulation of the acid-base balance (pH): maintaining the constant internal composition of the blood by removing excess acids, bases and ions.

(b) Labelled diagram of the mammalian kidney tubule (nephron)

The nephron is the functional unit of the kidney. Each nephron consists of a cup-shaped Bowman's (renal) capsule enclosing a knot of blood capillaries called the glomerulus, fed by the afferent arteriole and drained by the narrower efferent arteriole. The capsule leads into the proximal (first) convoluted tubule, then the loop of Henle (with a descending and an ascending limb), the distal (second) convoluted tubule, and finally the collecting duct. The tubule is surrounded by a network of blood capillaries that reabsorb useful substances back into the blood.

(c) How the kidney carries out two of its functions

(i) Excretion / formation of urine (ultrafiltration and selective reabsorption): Blood is carried to the glomerulus under high pressure through the wide afferent arteriole; because the efferent arteriole leaving the glomerulus is narrower, a high pressure builds up. This high pressure forces water, glucose, salts, urea and other small molecules out of the blood, through the capillary and capsule walls, into the Bowman's capsule. This process is called ultrafiltration. Blood cells and large plasma proteins are too big to pass through and remain in the blood. The filtrate then flows along the tubule, where useful substances are taken back into the surrounding capillaries by selective reabsorption: all the glucose, most of the water and the needed salts are reabsorbed in the proximal convoluted tubule and the loop of Henle. The liquid that remains, now containing urea, excess salts and excess water, is urine. It passes through the collecting duct to the ureter and is stored in the bladder by peristalsis until it is expelled.

(ii) Osmoregulation: The kidney keeps the composition of the blood plasma constant by controlling how much water is reabsorbed. When the blood becomes too concentrated (through excess salt intake or heavy water loss by sweating), this rise in osmotic pressure is detected by sensitive cells in the hypothalamus of the brain, which stimulate the pituitary gland to release more antidiuretic hormone (ADH). ADH travels in the blood to the kidney, where it makes the walls of the distal convoluted tubule and collecting duct more permeable, so that more water is reabsorbed from the filtrate back into the blood. A small volume of concentrated urine is produced and water is conserved. When the blood is too dilute, less ADH is released, less water is reabsorbed, and a large volume of dilute urine is produced. In this way the kidney keeps the water and salt content, and therefore the osmotic pressure, of the blood constant.