Excretion

Definition

The elimination of metabolic waste products and other non-useful substances from an organism or cell; in arthropods, this process involves specialized organs that maintain osmotic balance and remove nitrogenous wastes. Unlike secretion, which releases substances that serve active physiological roles, excretion disposes of harmful or spent metabolic byproducts.

Etymology

Latin excretus, past participle of excernere 'to sift out, separate', from ex- 'out' + cernere 'to sift, separate'

Example

In insects, Malpighian tubules filter nitrogenous waste from the hemolymph and discharge it into the hindgut for excretion; arachnids such as spiders produce guanine as their primary nitrogenous waste, excreted through coxal glands or Malpighian tubules depending on the group.

Synonyms

• elimination
• waste removal

Related Terms

• Malpighian tubules
• secretion
• Osmoregulation
• nitrogenous waste
• guanine
• Uric acid
• coxal glands
• Rectum
• metabolism

Usage Notes

Distinguished from secretion by function: secretion releases biologically active substances (enzymes, hormones, silk) for specific purposes, while excretion removes deleterious metabolic byproducts. In arthropods, the distinction can be subtle—some structures like the silk glands of spiders are secretory, whereas Malpighian tubules are strictly excretory. The term is sometimes used broadly in informal contexts to include defecation of undigested material, but physiologically, excretion proper refers only to metabolic waste elimination, not egestion of fecal matter.