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The human body also has four different way of excreting unwanted waste product. These include the skin, lungs, liver and the kidney. The skins is used to excrete sweat which is a solution of water, ions and urea, the lungs excrete carbon dioxide and water through the process of respiration.  The liver is used as an excretion source for bile, which contains mineral ions, bile pigments and cholesterol, the gut excretes mucus cells, water and bile waste. Last but not the least the kidney is used to excrete urine, which is a solution of urea, mineral ions, water, and other unwanted chemicals in the body;

1. Lung; Carbon dioxide is a catabolic waste product formed due to oxidation of food releasing energy. During expiration this carbon dioxide along with a small quantity of water vapour is expelled out of the lungs.

2. Skin; The skin of aquatic animals is permeable and excretory wastes pass out by diffusion. In terrestrial animals, the skin is less permeable and excretory products pass out only through ducts and pores. Human skin has two kinds of glands that perform the function of excretion.

The skin has a sweat glands which are long tubular glands, coiled at their ends, located in the dermis of the skin, embedded in the subcutaneous tissue.
They produce a watery fluid called sweat which consists of 98% water and 2% solid residue such as Urea, uric acid, creatinine, volatile fatty acids , Sodium chloride, potassium chloride and glucose.
Also skin has a Sebaceous glands which are attached to the hair follicle. Each gland consists of a small branched alveoli which produces an oily or waxy substance called sebum. Excretory substances like waxes, sterols, fatty acids and traces of other hydrocarbons are eliminated along with the sebum.

3. Liver; Bile pigments (biliverdin and bilirubin) are produced as a result of breakdown of haemoglobin in the worn out red blood cells in the liver and excreted with the bile into the duodenum and removed along with the faeces.

Liver also excretes cholesterol, various inactivated products of steroid hormones and harmful products like alcohol, nicotine and several drugs. The liver is also responsible for converting ammonia produced by deamination into less toxic urea by combining it with carbon dioxide.

4. Kidney; kidney is used to excrete urine, which is a solution of urea, mineral ions, water, and other unwanted chemicals in the body.

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