Summary

The body constantly produces substances it must get rid of, including many nitrogen-containing compounds, ions, carbon dioxide and water. The kind of nitrogenous waste an animal forms, and the way it is excreted, differs from one animal to another and depends largely on its habitat, especially on how much water is available. The three major nitrogenous wastes are ammonia, urea and uric acid. Across the animal kingdom the common excretory structures include protonephridia, nephridia, malpighian tubules, green glands and kidneys, and these organs not only remove nitrogenous wastes but also help keep the ionic balance and the acid-base balance of the body fluids steady.

In humans the excretory system is made up of one pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Each kidney holds over a million tiny tubular units called nephrons, the nephron being the functional unit of the kidney. A nephron has two parts, the glomerulus and the renal tubule. The glomerulus is a tuft of capillaries fed by an afferent arteriole, itself a fine branch of the renal artery. The renal tubule begins as a double-walled Bowman's capsule and continues as the proximal convoluted tubule, then the loop of Henle, and then the distal convoluted tubule. The distal tubules of many nephrons drain into a shared collecting duct, and these ducts eventually open into the renal pelvis through the medullary pyramids. Together the Bowman's capsule and the glomerulus it encloses form the Malpighian or renal corpuscle.

Urine is formed through three main processes: filtration, reabsorption and secretion. Filtration is a non-selective step carried out by the glomerulus using the blood pressure in its capillaries; about twelve hundred millilitres of blood is filtered every minute to yield roughly one hundred and twenty-five millilitres of filtrate in the Bowman's capsule per minute, a value called the glomerular filtration rate. A specialised region of the nephron, the juxtaglomerular apparatus, helps regulate this rate. Nearly ninety-nine per cent of the filtrate is reabsorbed as it passes along the nephron. The proximal convoluted tubule is the chief site of reabsorption and of selective secretion, while the loop of Henle mainly sets up an osmotic gradient within the kidney interstitium. The distal convoluted tubule and the collecting duct carry out extensive reabsorption of water and certain electrolytes for osmoregulation, and hydrogen ions, potassium ions and ammonia can be secreted into the filtrate to keep the ionic balance and pH of the body fluids in order. …