The structure and functions of the skin, kidney and liver are discussed below in detail.
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:
- cornified layer.
- granular layer.
- malpighian layer.
- The cornified layer: It is the outermost layer of the epidermis. It is made up of flat, hard and scaly cells. The cells of cornified layer are dead.
- The granular layer: This contains the living cells where active. division of cells takes place. These cells replace the cells of the granular layer which in turn are replacing the cells of the cornified layer which are constantly being worn-out.
- 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 contains 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 desiccation.
- 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 manufactures Vitamin D by using infra-red rays from sunlight.
- Production of milk in females: The mammary gland which is a modification 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.
The kidneys are two reddish brown, bean-shaped structures that lie asymmetrically on the posterior dorsal body wall of the lumber-upper region of the abdomen below the adrenal gland. At the concave edge is a depression called the helium. Arising from the helium is a small tube called the ureter.
The helium also serves as the point of entry of the renal artery and leaving of the renal vein. The ureter connects the kidney to the urinary bladder and from the bladder to the urethra. The urethra opens through the penis in male and separately in female as the urinary tract.
The kidney is made up of two distinct regions: the outer cortex and inner medulla. This contains thousands of urinary tubules. Each malpighian tubule consists of malpighian body situated in the cortex and convoluted tube.
Each malpighian body, consists of a cup-like chamber, the Bowman’s capsule, into which a knot of blood capillaries, the glomerus, fit. Beyond the malpighian body, each tubule makes a U-shaped loop into the medulla, re-enters into the cortex and bends again into the medulla where its course is completed.
The U-shaped loop is the Henle’s loop. The tubule transverses both regions and opens at the tips (papillae) of triangular-shaped masses called pyramids, which open into the pelvis.
Functions of the kidney
The functions of the kidney include:
- Excretion: The kidney- helps to remove unwanted nitrogenous wastes like urea, ammonium compounds, water and salts from the body.
- Detoxication: The kidney also helps to eliminate poisonous substances from the body, e.g toxins, drugs and alcohol.
- Removal of excess glucose: The kidney also aids the removal of excess glucose from the body.
- Maintenance of acid/base balance: The kidney helps to maintain the blood pH and osmotic pressure. Normal urine is slightly acidic.
- Osmo-regulation of the body: The kidney also functions in the osmoregulation of the body, i.e keeps the concentration of the blood plasma and conditions of the body cells fairly constant.
- Conservation: The kidney aids the conservation of some useful materials by re-absorbing them from the glomerula filtrate which is the first step in urine formation. Materials conserved include water, glucose, sodium ions, chlorine ions and Vitamin C.
The liver is usually regarded as the most powerful organ in the body because it is constantly at work, controlling major activities going on in the body. It is located on the right side of the upper abdomen and partly overlaps the stomach. It is basically divided into lobes.
Functions of the Liver
The liver performs a number of functions which include:
1) Digestion: The liver produces bile which is secreted into the duodenum through the bile duct. The functions in digestion include:
- It adds water to chyme (less watery form of food undergoing digestion), because of its high percentage of water.
- Its alkaline (sodium) salts neutralise hydrochloric acid of the chyme, thereby providing a right medium for the action of pancreatic juice enzymes.
- It reduces the surface tension of fats and emulsifies them, i.e, splits them into minute droplets.
2) Deamination: Proteins are not stored in the body and so excess amino acids must be eliminated. Amino acids which are not built up into proteins and used for growth and replacement of cells are broken down (deaminated) by the liver into carhohydrate and urea by the removal of the amino group. The urea is secreted through the kidney while the carbohydrate can be converted into glycogen to be stored or oxidised to release energy.
3) Storage of iron: Iron derived from the broken down red blood cells (erythrocytes) is completed and stored in the liver.
4) Regulation of blood sugar: The liver has the role in carbohydrate metabolism and so is able to convert glucose, amino acid and other substances to an insoluble carbohydrate called glycogen. Some of the glucose may be taken from the hepatic portal vein carrying blood which is rich in digested food from the small intestine to the liver. Their reserve of glycogen is converted to glucose so as to maintain the level of glucose circulating in the blood.
5) Regulation of body temperature: Many chemical activities taking place in the liver release energy in form of heat which is distributed round the body by the circulatory system.
6) Fat metabolism: The liver contains about 6% stored lipid and when required for use in providing energy and in starvations, it travels in the blood stream from the fat deposits, leading to a fall in fat content of the liver. This happens after exhaustion of all other body fats. Some are used directly or changed to other substances that can be oxidised for energy.
7) Detoxication: Poisonous compounds and other chemical substances transported in the blood to the liver are converted to harmless substances and later excreted in the urine.
8) Manufacture of plasma proteins: The liver produces most of the protein found in blood plasma, including fibrinogen which forms an important part in the clotting action of blood.
9) Storage of vitamins: Vitamins A and D are stored in the liver. (Livers of fish are richer in vitamins especially vitamin D than livers of mammals). The liver also stores vitamin B 12, an antianaemic factor which is necessary for the production of red blood cells in the bone marrow.