excretion

excretion is the transfer of substances out of a living organism into its environment. At its simplest, for single-cell forms of life, this involves extrusion across the cell membrane of the unwanted or potentially toxic by-products of respiration and metabolism. This is also what is happening continually in the individual cells of the animal body, but from their immediate environment substances must move into the blood to be carried away to the site of their ultimate disposal. In the animal body there is also another type of excretion: expulsion of the residue of substances which have not been absorbed into the body proper from the gut (which can be considered a tunnel through the body of the external world).

In human terms ‘excreta’ normally refers only to urine and faeces, whereas the definition of excretion would also include both carbon dioxide and heat, and these will be considered first.

Carbon dioxide

carbon dioxide (CO₂), along with water, is the end-product in cells which use oxygen to release their energy supply from food sources, and those cells are in the vast majority. If this CO₂ were to accumulate the cells would become too acidic for their internal chemistry to proceed. Continual generation of CO₂ maintains a concentration gradient from inside to outside so that it moves by diffusion out of the cells into the surrounding fluid, and thence into the blood in the nearby capillaries. So the blood picks up CO₂ as it circulates, until it converges from the whole body into the right side of the heart, carrying an amount of CO₂ which varies with the total rate of energy release by body cells. This venous blood, low in oxygen and high in CO₂, is pumped through the lungs, where CO₂ is excreted by the reverse process to that of its uptake from cells — it diffuses out down a gradient, because breathing keeps the concentration lower in the gas in the lungs than it is in the incoming blood.

Heat

Heat is continually generated by resting metabolic activity, and to a much greater extent by working muscles. Unless conservation of body heat is required in cold conditions to maintain body temperature, it is ‘excreted’ from the surface of the body when there is a temperature gradient from the skin to the environment. This gradient, and therefore heat loss, is regulated by the mechanisms for temperature regulation: dilation of skin blood vessels brings heat to the surface and increases the gradient; when this mechanism is inadequate, sweating comes into play as well.

Excretion in the urine

The kidneys are responsible for filtering off a continual sample of the watery component of the blood plasma, with its solutes, at a rate equivalent to the whole of the plasma volume about every twenty minutes. The further processes within the kidneys could be likened to ‘quality control’ and correction. Not only the filtered water, but also many dissolved substances, are largely reabsorbed, but the reabsorption is fine-tuned according to any need for correction of the blood composition; nitrogenous waste (mainly urea) from protein breakdown is allowed to escape, and waste acid (H⁺) and other substances present in excess are actively secreted into the urine. The end result is production of urine at a variable rate depending on fluid intake, but on average less than one-hundredth the rate of filtration of fluid from the blood, and containing all that needs to be excreted minute by minute.

Excretion from the bowel

That which is voided consists of the residue that remains after digestion and absorption of food breakdown products in the stomach and small intestine, and after absorption of most of the remaining water in the large intestine. This is also the route for voiding of cholesterol, excreted by the liver into the bile. The colour of the faeces is derived from bile pigments: although these are recycled to a large extent, the remainder becomes stercobilin and leaves by this route.

Sheila Jennett

See also faeces; urine.