In this article we will discuss about the roles of different organs in excretion.

Role of Lungs in Excretion:

Human lungs regularly remove about 18 L of CO2 per hour and about 400 ml of water per day in normal resting condition. Water loss through the lungs is small in hot humid climate and large in cold dry climate. Thus CO2 and water (both are metabolic wastes produced during oxidation of food in the cells) are removed via lungs.

Role of Skin in Excretion:

In many aquatic animals ammonia is mainly excreted out into the surrounding water by diffusion through the skin. Human skin has two types of glands: sudoriferous (sweat) glands and sebaceous (oil) glands.

(i) Sudoriferous glands (Sweat glands) secrete an aqueous fluid called sweat. Sweat contains water (99.5%), NaCl, urea, lactic acid, amino acids and glucose. Sweat does not contain uric acid.

The volume of sweat varies from negligible to 14L a day, depending upon activity and temperature. When sweat evaporates, it provides cooling to the body. Normal pH of sweat is 4.5. Sweat production is also influenced by atmospheric temperature.

(ii) Sebaceous glands (Oil glands) secrete an oily or wax-like secretion called sebum. It keeps the skin oily. Sebum removes some lipids like waxes, sterols, other hydrocarbons and fatty acids from the body.

Role of Liver in Excretion:

Urea is formed in the liver which is eliminated through kidneys. Liver cells also degrade the haemoglobin of worn out red blood corpuscles into bile pigments (bilirubin and bi-liver- din). Liver cells also excrete cholesterol, certain products of steroid hormones, some vita­mins and many drugs. Liver secretes these substances in the bile. The bile carries these substances to the intestine and are passed out with faeces.

Role of Intestine in Excretion:

The epithelial cells of the intestine (colon) excrete certain salts such as iron and calcium. These salts are eliminated with the faeces.

Role of Salivary glands in Excretion:

Salivary glands excrete substances like mercury, potassium iodide, lead and thiocyanate. In aquatic animals like fish, gills remove carbon dioxide. Gills of many bony fish also excrete salt.

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