The nephron is the functional unit of the kidney and has two portions – the glomerulus and renal tubule. The glomerulus is a tuft of capillaries formed from afferent arterioles, and fine branches of the renal artery.
The renal tubule starts with a double-walled Bowman’s capsule and is further differentiated into a proximal convoluted tubule (PCT), Henle’s loop (HL), and distal convoluted tubule (DCT). The DCTs of many nephrons join to a common collecting duct many of which ultimately open into the renal pelvis through the medullary pyramids.
The Bowman’s capsule encloses the glomerulus to form Malpighian or renal corpuscle. Urine formation involves three main processes, i.e., filtration, reabsorption, and secretion.
Filtration is a non-selective process performed by the glomerulus using the glomerular capillary blood pressure. A counter current mechanism operates between the two limbs of the loop of Henle and those of the vasa recta (capillary parallel to Henle’s loop). The filtrate gets concentrated as it moves down the descending limb but is diluted by the ascending limb. Electrolytes and urea are retained in the interstitium by this arrangement.
DCT and collecting duct concentrate the filtrate about four times, i.e., from 300 mOsmolL–1 to 1200 mOsmolL–1, an excellent mechanism of conservation of water.
Urine is stored in the urinary bladder till a voluntary signal from CNS carries out its release through the urethra, i.e., micturition. Skin, lungs, and liver also assist in excretion.