Summary — Excretory Products and their Elimination

This chapter has traced excretion in humans from the underlying chemistry of nitrogenous waste through to the structure of the excretory system and the physiological mechanisms that keep it working correctly, and it is worth drawing the whole sequence together as a single connected story before moving on.

Because proteins and nucleic acids are the only major biomolecules built with nitrogen, their breakdown generates nitrogenous waste that every animal must dispose of, and three broad strategies have evolved according to how much water is available: ammonotelism (ammonia, needing abundant water, used by aquatic animals), ureotelism (urea, needing only moderate water, used by mammals and other partly or wholly terrestrial vertebrates), and uricotelism (uric acid, needing almost no water, used by birds, reptiles, land snails and insects). Humans, as mammals, are ureotelic.

The human excretory system itself is built around a pair of kidneys, each internally organised into an outer cortex and an inner medulla of renal pyramids draining via calyces into the renal pelvis, and connected to the single urinary bladder by a ureter on each side, with a single urethra finally carrying urine to the exterior. Within each kidney, roughly a million nephrons — each a Malpighian corpuscle (glomerulus plus Bowman's capsule) followed by a long tubule of PCT, loop of Henle, DCT and collecting duct — carry out the actual work of urine formation in three successive steps: glomerular filtration (bulk, non-selective, driven by high glomerular blood pressure), tubular reabsorption (obligatory in the PCT, facultative and hormone-controlled further along), and tubular secretion (chiefly of H+, K+ and ammonia). The loop of Henle and the vasa recta, working together as a counter-current system, build and maintain the steep osmotic gradient of the medulla that lets the collecting duct concentrate urine far beyond the concentration of blood plasma — the physiological basis of osmoregulation. …