Excretory Products and their Elimination
Class 11 Biology · Biology · Chapter 16
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Summary
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.
Nitrogenous wastes and excretory structuresHuman excretory system and the nephronUrine formation: filtration, reabsorption, secretionConcentration of urine and counter current mechanismRegulation of kidney function and micturitionRole of other organs and excretory disorders
Key terms
Nephron
The functional unit of the kidney, consisting of a glomerulus and a renal tubule.
Glomerular filtration
The ultrafiltration of blood in the glomerulus, the first step of urine formation.
Glomerular filtration rate
The amount of filtrate formed by the kidneys per minute, about 125 mL in a healthy individual.
Reabsorption
The return of nearly 99 per cent of the filtrate to the blood by the tubular cells of the nephron.
Counter current mechanism
The arrangement of the loop of Henle and vasa recta that maintains an osmotic gradient to concentrate urine.
Micturition
The process of release of urine from the urinary bladder, controlled by the micturition reflex.
Extra questions & answers
Try one from this chapter
Mammals that excrete nitrogenous waste mainly as urea are called: