Theory question 108
Excretion(For candidates in Nigeria only)
(a)(i) What is excretion? (ii) List the excretory organs in humans and name one waste product excreted by each organ.
(b) How does the mammalian kidney function as an excretory organ?
(c) Name the excretory organs in: (i) insects; (ii) earthworms.
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(a)(i) Excretion is the removal from the body of waste products of metabolism (e.g. urea, CO2, excess water and salts) that would be harmful if they built up.
(ii) Kidney — urea/urine; skin — sweat (water and salts); lungs — carbon dioxide and water vapour; liver — bile pigments.
(b) Blood enters the kidney through the renal artery; in the glomerulus it is filtered under high pressure (ultrafiltration) into the Bowman's capsule, giving a filtrate of water, glucose, salts and urea; in the tubules, useful substances (all glucose, amino acids, much water and salts) are selectively reabsorbed into the blood; more water is reabsorbed in the loop of Henle, distal tubule and collecting duct (controlled by ADH); the remaining fluid (urine: water, urea, salts) passes down the ureter to the bladder; purified blood leaves through the renal vein.
(c)(i) Malpighian tubules; (ii) nephridia.
Describe the kidney's work in three steps: filtration, reabsorption, then urine leaving.