Introduction to Excretion

Every cell in the body is continuously carrying out metabolic reactions, and alongside the useful products these reactions yield, they also generate a stream of waste by-products that the cell cannot use and cannot safely retain. Among these, the wastes that contain nitrogen are of special concern, because proteins and nucleic acids — the only two classes of major biomolecule built with nitrogen atoms — are broken down (catabolised) at a steady rate throughout life, whether to replace worn-out tissue proteins or to supply energy when carbohydrate and fat reserves run low. When an amino acid is broken down, its amino group (-NH2) is first removed in a reaction called deamination, and this immediately yields ammonia (NH3), a very small, highly water-soluble, but extremely toxic molecule. Left to accumulate even briefly in the blood, ammonia disrupts the pH balance of body fluids and interferes seriously with cellular metabolism, particularly in the nervous system; no animal can allow it simply to build up. The breakdown of purine and pyrimidine bases from worn-out nucleic acids adds further nitrogenous waste to this stream, some of it eventually appearing as uric acid. Excretion is the collective name for the physiological processes an organism uses to remove this nitrogenous waste, together with several other classes of metabolic by-product that also need to be got rid of on a regular basis — carbon dioxide from cellular respiration, excess water and excess dissolved salts taken in with food and drink beyond what the body actually needs, and the pigments left over when old red blood cells are broken down and their haemoglobin is degraded.

Because the amount of water available in an animal's surroundings differs enormously between a freshwater fish, a desert reptile and a terrestrial mammal, different animal groups have evolved quite different excretory organs and quite different chemical strategies for handling nitrogenous waste, each representing a different balance between the metabolic energy it costs to convert ammonia into a less toxic form and the amount of water it costs to keep whatever form is excreted safely dilute. Even at the level of a single-celled organism such as Amoeba, a specialised organelle — the contractile vacuole — collects and periodically expels excess water and some dissolved waste; simple invertebrates such as flatworms use flame cells arranged along branching tubules to drive fluid (and waste dissolved in it) out through pores in the body wall; the earthworm and other annelids possess paired, segmentally repeated nephridia that filter fluid from the body cavity and modify it as it passes along a coiled tubule before it is voided; and insects rely on Malpighian tubules opening into the gut, which actively transport waste out of the surrounding blood (haemolymph) and into the gut for elimination alongside undigested food residue, an arrangement that conserves water particularly well. Vertebrates, including humans, instead rely on a pair of compact, highly efficient organs — the kidneys — together with an associated system of ducts, to carry out this same essential task on a much larger scale, filtering the entire blood volume many times over each day. The sections that follow first take up the three characteristic chemical patterns by which different animal groups excrete their nitrogenous waste, before turning to the detailed structure and working of the human excretory system itself.