In this activity we’ve seen that animals need to balance the amounts of water and chemicals in their bodies, and must excrete unneeded salts and organic materials. We’ve learned about osmotic balance, and that animals are either osmoconformers, which are isotonic with the surrounding environment, or osmoregulators. We’ve also learned that excretion depends on active transport across epithelia, and that animals excrete nitrogenous wastes as ammonia, uric acid, or urea.

The simplest excretory systems are the flame cells of flatworms and some other invertebrates. Insects excrete through Malpighian tubules connected directly to the hindgut. Earthworms have tubular nephridia that have a basic structure similar to nephrons— the filtering units of the vertebrate kidney. In mammals, the kidneys contain of an array of nephrons that drain into collecting ducts, which lead to the pelvis of the kidney.

Dissolved wastes in the blood flowing through the nephrons is filtered into the kidney’s duct system, and water, salts, and other nutrients are reclaimed from the ducts by reabsorption. Several hormonal mechanisms control the amount of water excreted from the body by adjusting the reabsorption process in the ducts. Diabetes and high blood pressure are the main causes of kidney failure. External filtrations using a dialysis machine is an effective method of replacing kidney function for short term use.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education