CHAPTER 19 : EXCRETION AND OSMOREGULATION

Excretion is the removal of toxic (nitrogenous) waste products of metabolism from the body. The waste end products of catabolism contain carbon, hydrogen, oxygen and nitrogen. Carbon atoms are eliminated in carbon dioxide, hydrogen in water (H₂O), and oxygen in carbon dioxide and water. Nitrogen, which is a highly toxic end product of protein metabolism, is found in large quantities and needs to be excreted as soon as it is formed. The kidneys (also liver) carry out detoxification, altering toxic substances into forms that are not poisonous to the body.

The process of maintenance of osmotic and ionic concentration of body fluids is called osmoregulation. Animals regulate this process in accordance with their external environment. Osmoregulation and excretion are intimately related; these processes together maintain homeostasis (i.e. staying the same), and are performed by the same set of organs. The kidney is the major organ of osmoregulation and excretion in vertebrates.

An osmoregulatory animal is generally in an osmotic steady state, and maintains relatively constant concentration of internal salts and water. The intake and outflow of water and salts are equal and it is maintained at the cost of energy.

The term 'excretion' is correctly applied only to substances which must cross the cell membrane to leave the body. This does not include expulsion of undigested food material (egestion) since the food passes down the digestive tract without ever passing through a cell membrane.

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Table of Contents

19.0 - Introduction
19.1 - Ammonotelism, Ureotelism and Uricotelism
19.2 - Excretory System of Man
19.3 - Skin and Lungs as Accessory Excretory Organs

Chapter 20