Excretory Products and their Elimination — Class 11 Biology

This chapter explains how animals remove nitrogenous and other wastes, focusing on the human excretory system. Ammonia, urea and uric acid are the major nitrogenous wastes; ammonotelic animals excrete ammonia, ureotelic animals such as mammals excrete urea, and uricotelic animals such as reptiles and birds excrete uric acid with minimum water loss. Excretory structures include protonephridia, nephridia, Malpighian tubules and green glands in invertebrates, and kidneys in vertebrates. The human excretory system comprises a pair of kidneys, ureters, a urinary bladder and a urethra; each kidney has about a million nephrons, the functional units, each consisting of a glomerulus enclosed in Bowman's capsule and a renal tubule (proximal convoluted tubule, loop of Henle and distal convoluted tubule) leading to a collecting duct. Urine formation involves glomerular filtration (ultrafiltration of blood, about 125 mL per minute), selective reabsorption of nearly 99 per cent of the filtrate, and tubular secretion of ions to maintain ionic and acid-base balance. A counter current mechanism between the loop of Henle and the vasa recta concentrates the urine. Kidney function is regulated by ADH, the renin-angiotensin mechanism and atrial natriuretic factor. Micturition releases urine, other organs such as lungs, liver and skin assist in excretion, and disorders include uraemia, treated by haemodialysis, and kidney stones.