EXCRETION: Definition, Importance, Need for Excretory Systems, Organisms and Their Excretory Organs, Excretory Organs and Their Waste Product

Excretion can be defined as the process by which metabolic waste products are removed from the body of living things. Waste product from metabolic activities are always produced by the bod and their removal are carried out by special excretory systems or organs. 


[Photo credit:  Hämbörger. License: GNU Free Documentation License]


The waste products include:

• by-products formed during metabolic processes in the body;
• excess food substances which cannot be stored in the body; and
• unwanted materials present in food which re absorbed into the body.

The main waste substances come from metabolic reactions which digested food substances undergo in the cells of the body.

However, egestion and secretion should not be confused with excretion. 

Egestion is the removal of solid undigested food substances which are not by-products of metabolism, e.g. the removal of feces(faeces) from the anus. 

Secretion is the production of useful substances in the body, such as enzymes and hormones by metabolic processes.

NEED FOR EXCRETORY SYSTEMS

All parts of a unicellular organism are in close contact with the external environment. This is not the case with most of the cells in a multicellular animal. These cells are bathed in tissue fluids which provide their immediate environment. Known as the internal emvironment, tissue fluids play an important role in proper functioning of the cells of multicellular animals.

The conditions in the internal environment, such as temperature, pH and concentration of solutes, must be maintained at levels that are optimal for the cells. Poisons and unwanted substances can pollute the internal environment and harm the cells, and hence the animal as a whole.

To get rid of the unwanted materials from the internal environment, multicellular animals have developed excretory systems and mechanisms.

IMPORTANCE OF EXCRETION

Waste products of metabolism must not be allowed to remain in the body for the following reasons:

1. The excretory products are harmful to the body and so must be removed.
2. Some are poisonous and must never be allowed to accumulate within the body.
3. Excretion helps to maintain water balance in the body.
4. Excretion also helps to maintain salt balance. i.e homeostasis, in the body.
5. Waste products when not removed can interfere with normal metabolic activities of the body.

ORGANISMS AND THEIR EXCRETORY ORGANS

Organisms	Excretory organs
Protozoa, e.g. Amoeba	Contractile vacuole, by diffusion
Flatworms, e.g. tapeworm	Flame cells
Annelids, e.g. earthworms	Nephridia
Insects	Malphigian tubules
Crustaceans	Green glands
Fishes	Kidneys
Amphibians, e.g. toad	Kidneys
Reptiles	Kidneys
Birds	Kidneys and lungs
Mammals	Kidneys, skin, liver and lungs
Flowering plants	Stomata and lenticles

EXCRETORY ORGANS AND WASTE PRODUCTS EXCRETED

Excretory organs	Waste products excreted
Contractile vacuole	Carbon dioxide, ammonia and water
Flame cells	Carbon dioxide, ammonia and water
Nephridia	Water, urea, carbon dioxide and nitrogenous wastes
Malphigian tubules	Water, carbon dioxide and uric acid
Green glands	Water, urea, carbon dioxide and ammonia salts
Gills	Carbon dioxide, water and urea
Skin	Sweat containing urea, salts and water
Liver	Bile salts, water and urea
Lungs	Carbon dioxide and water vapor
Kidneys	Urine containing urea, salts, water, hormones, and uric acid
Stomata and lenticles	Water, carbon dioxide and oxygen
Bark of trees	Tannins, mucilage, gum, crystals, anthocyanin, alkaloid, resin, oil and latex

Categories: Biology, Excretion, Excretory System