The excretory system consists of all the organs in the body which are responsible for the removal of all metabolic waste products. The excretory metabolic waste products include water, urine, carbon dioxide, salts, heat, etc.

The major excretory organs responsible for the removal of metabolic waste products from the body are skin, kidney and the liver.


Structure of Skin: The skin of farm animals has been described as the largest organ of the body. It covers the outer surfaces of all animals. The skin is made up of two layers – epidermis and dermis.


The epidermis is the outermost layer and it is made up of three layers: (i) cornified layer (ii) granular layer (iii) malpighian layer.

(i) The Cornified Layer: It is the outermost layer of the epidermis. It is made up of flat, hard and scaly cells. orod The cells of cornified layer are dead.

(ii) The Granular Layer: This contains the living cells where active division of adiny cells takes place. These cells replace the ni ale cells of the granular layer which in turn are replacing the cells of the cornified doir layer which are constantly being worn out

(iii) The Malpighian Layer: This contains granules of pigment called melanin. This pigment is responsible for skin colour The melanin also prevents the penetration of a lot of violet rays of sunlight. The malpighian layer also owlto 22contains another pigment called keratin which is responsible for the toughness and flexibility of the skin.

Dermis: The dermis is a thick layer of fibrous connective tissues. The dermis contains nerves which enable the skin to be sensitive to changes in its environment such as pain, heat, temperature, touch, cold, etc. The blood and lymph vessels supply materials to the skin, remove wastes from the skin cells, and carry out temperature regulation. The sweat glands which consist of a coiled tubular gland opening into the epidermis excrete water and salts as sweat, sebaceous or oil gland which produces an oily substance called sebum. This helps to lubricate the hair and make it waterproof, while the erector muscle controls the erection of the hairs on the skin surface by its contraction and relaxation. Underneath the skin is a layer of fat called subcutaneous fatty tissue. This fat layer varies in thickness, depending on the part of the body.

Functions of the Skin The functions of the skin include the following:

Protection: The skin protects the inner skin or tissue from mechanical injuries, bacterial infection, ultra-violet rays of the sunlight and against decication.

Excretion: The skin excretes excess water, mineral salts and nitrogenous wastes through the sweat glands as sweat.

Sensitivity: The skin is sensitive to its environment through the specialised sensory nerve endings scattered in the dermis. The skin is sensitive to such stimuli as change in temperature, pressure, pain and cold.

Production of Vitamin D:The skin slid a manufactures Vitamin D by using infra dig red rays from sunlight. 002 ai doids

Production of Milk in Females: The atmammary gland which is a modification (non of the skin produces milk which is used for feeding the young ones.

Storage of Preserved Foods: Fats are stored under the dermis of mammals, especially those of pigs. Fats also form an insulating layer.

Regulation of Body Temperature: When an animal is hot, blood vessels under the epidermis of the skin dilate (vasodilation) and more blood is brought to the surface to be cooled through the process of convection and radiation. On the other hand, the blood vessels of the skin constrict (vasoconstriction) in cold weather, thereby conserving heat.

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