Glossary
Half-life.
Direct answer
Half-life is the time in which the concentration of a substance in blood falls to half its value. Half-life summarises how quickly the body distributes, transforms and excretes that substance, and it allows an estimate of how long measurable amounts persist in the organism.
The arithmetic matters: the decline is not linear but removes the same proportion in each interval. After five half-lives, less than a twentieth of the starting amount remains. A half-life also applies to a substance under stated conditions rather than in the abstract: age, liver and kidney function all shift it.
Half-lives can differ substantially between species, and allometry gives only a rough estimate. In the horse, passage through the hindgut and a high proportion of muscle tissue add further variation. The body's own substances have half-lives too: serum amyloid A disappears fast once the stimulus ends, fibrinogen far more slowly.
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