Many nitrogen containing substances, ions, CO2, water, etc. that accumulate in the body have to be eliminated. Nature of nitrogenous wastes formed and their excretion vary among animals, mainly depending on the habitat. Ammonia, urea and uric acid are the major nitrogenous wastes excreted.
HUMAN EXCRETORY SYSTEM
In humans, the excretory system consists of one pair of kidneys, a pair of ureters, a urinary bladder and a urethra.
Each kidney has over a million tubular structures called nephrons. Nephron is the functional unit of kidney and has two portions- glomerulus and renal tubule. Glomerulus is a tuft of capillaries formed from arterioles, fine branches of 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 open into the renal pelvis through the medullary pyramids. The Bownam’s capsule encloses the glomerulus to form Malpighian or renal corpuscle.
Urine formation involves three processes i.e. filtration, reabsorption and secretion. Filtration is a non-selective process performed by the glomerulus using the glomerular capillary blood pressure. About 1200 mL of blood is filtered by the glomerulus per minute to form 125 mL of filtrate in the Bowman’s Capsule per minute. This is known as the Glomerular Filtration Rate (GFR).
A counter current mechanism operates between the two limbs of the loop of Henle and those of vasa retca (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 which is 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 urethra.
In addition to this, the skin, lungs and liver also assist in excretion.