Excretion and Excretory Products

Every living cell constantly runs a huge set of chemical reactions collectively called metabolism — the breaking-down (catabolic) reactions and the building-up (anabolic) reactions that keep the organism alive. These reactions inevitably leave behind by-products that the cell has no further use for, and many of these by-products are actively harmful if allowed to build up. Anything the body must get rid of because it has been generated inside the body's own cells is called a metabolic waste product, and the process of eliminating it is excretion.

Excretion must be kept conceptually separate from simple digestive elimination. Undigested or unabsorbed food residues (faecal matter, essentially) never actually entered a living cell — they only ever travelled down the gut. Metabolic wastes, by contrast, are generated inside the cells themselves as reaction products, so removing them is a genuinely different physiological job even though both eventually leave the body.

Human metabolic waste comes in several very different chemical forms, and each is handled by whichever organ is best suited to it: water and dissolved mineral salts (sodium, potassium, calcium and others, when present in excess) leave through urine, sweat and faeces; carbon dioxide, a gaseous waste of cellular respiration, leaves through the lungs; nitrogenous wastes such as ammonia, urea, uric acid and creatinine leave mainly through urine; breakdown pigments of haemoglobin — bilirubin (via faeces) and urochrome (via urine, and responsible for urine's yellow colour) — are excreted along with dietary pigments (such as those from beetroot), excess vitamins, hormones and drugs; and volatile substances absorbed from spicy food are breathed out through the lungs.

Of all these, nitrogenous waste deserves special attention because the body cannot simply stockpile it the way it stockpiles fat or glycogen. Carbohydrates and fats in excess can be stored, but excess amino acids cannot be stored as such — they are broken down by deamination, a reaction in which the amino (-NH2) group is stripped off the amino acid, releasing highly toxic ammonia. That ammonia must then either be excreted directly or first converted into a less toxic compound (urea or uric acid) before being excreted. Interestingly, an animal's chief nitrogenous waste product tracks its habitat and water availability far more closely than it tracks its evolutionary lineage — the aquatic tadpole of a frog excretes ammonia, while the same species as an adult frog is mostly a urea-excreter; some terrestrial turtles excrete uric acid while other terrestrial turtles excrete urea or even ammonia. Because water availability is the deciding factor, biologists recognise three broad modes of nitrogenous excretion, covered in the next section.