1. Excretion
1.1 Definition of the term ‘excretion’
Excretion is the process by which metabolic waste products and toxic substances are removed from the body of an organism.
1.2 Reasons for the need of excretion
Tissue fluid bathing the cells and blood contain various substances such as glucose, salts, amino acids dissolved in the water solvent. The conditions of the tissue fluid and blood - such as pH, temperature and osmotic pressure have to be kept optimal for normal functioning of the cells.
Excretory products that affect blood and tissue fluid pH like carbon dioxide, urea and creatinine need to be removed. Furthermore, high concentrations of urea would have toxic effects on the cells.
Excess water also needs to be excreted in the form of urine to maintain a constant osmotic pressure. Large amount of water in tissue fluid and blood causes a net movement of water into living cells. This causes the living cells. to burst.
1.3 Types of excretory products and excretory organs
Table 1 Different type of excretory products and excretory organs found in mammals.
1.4 Differences between excretion and egestion
Excretion
Involves the removal of metabolic waste products.
Egestion
Involves the removal of undigested food (faeces) material. As the undigested food material is not formed from substances within cells and has never been absorbed into cells.
NB: Undigested food material is not an excretory product while the bile pigment in the faeces is an excretory product formed during breakdown of haemoglobin.
Table 2 Differences between excretion and egestion.
Important note: Egestion is not a form of excretion.