Excretion

Excretion is the removal of metabolic wastes such as ammonia, carbon dioxide, ions and water as well as toxic xenobiotics and metals. The process involves the gills, kidney, liver and rectal gland (elasmobranchs and coelacanth). In the liver, amino acids, haemoglobin, steroids and molecules resulting from human activities are transformed to excretable products. The rectal gland excretes ions, notably Na+ and Cl−. The kidney in teleosts has a distinction between an anterior head-kidney containing haematopoietic tissue and endocrine tissue and the posterior region with nephrons (kidney tubules). Fish nephrons generally have a Malphigian corpuscle with a glomerulus but the structure varies between fish taxa and some marine teleosts lack a glomerulus. Control systems for fish excretion are unclear but it is expected that various hormones influence excretory homeostasis.