Structure and Function of the Skin | Skin temperature Skin: Layers, Structure and Function | physiology of pain | SECRETIONS OF SKIN
Skin is the outer covering of body which is in contact with external environment.
Functions of skin
1. Protection of underlying structures from injury.
2. Excretion of salts like sodium chloride and metabolites like urea
3. Provides sensation which gives the awareness of environment.
4. Secretion of sweat and sebum.
5. Regulation of body temperature.
6. Synthesis of Vitamin D from ergosterol of skin by the action of ultraviolet rays of sun.
Structure of skin
Skin consists of:
1. An outer layer called epidermis.
2. An inner layer called dermis.
Epidermis: It is made of stratified epithelium and con- tains the following layers.
1. Stratum cornium- containing scale like cells which are castantly replaced. These cells have a protein called keratin.
2. Stratum lucidum- a glistening layer.
3. Stratum granulosum- has spindle shaped cells with granules in the cytoplasm.
4. Stratum germinativum- contains cuboidal cells. The skin cells multiply in this layer.
Dermis: It is the inner layer which forms true skin. It contains the following structures.
1. Melanophore cells containing melanin pigment and some elastic fibres which maintain the texture of skin.
2. Arterial and venous capillaries and sensory nerve endings.
3. Sweat and sebaceous glands.
4. Hair roots and erector pili muscles (contraction of these muscles produce straightening of the hair).
SECRETIONS OF SKIN
The two secretions of skin are sweat and sebum.
Secretion of sweat
Sweat is secreted by sweat glands which arise from dermis. They are twisted tubular glands and their ducts open in epidermis. Sweat glands are more numerous in the palms of hands and sole of feet. About 500 ml of sweat is formed in 24 hours.
Sweat contains mainly water, some salts and trace of other waste products.
Perspiration or sweating can be classified into:
1. Insensible perspiration which evaporates quickly and so it is not observed.
2. Sensible perspiration which occurs during excessive sweating. So production of sweat is more than evaporation.
The secretion of sweat is controlled by sympathetic nerves.
Secretion of sebum :
Sebum is a greasy secretion produced by sebaceous glands. They are small, flask shaped glands present in dermis. They have a duct which opens into a hair follicle. The sebaceous glands are present in the skin of many parts except the palm of hands and sole of feet. Sebum keeps the skin oily and prevents it from drying.
REGULATION OF BODY TEMPERATURE:
Regulation of body temperature is an important function performed by the skin. The normal body temperature is 98.4°F (37°C) It may be slightly lower in the morning and it may increase in severe muscular exercise. The normal body temperature is maintained by a balance between heat production and heat loss.
Heat production
Heat can be produced in the body by Allowing factors:
1. Increased activity of muscles as in severe exercise.
2. Increased activity of liver and other glandular structures.
3. Increased metabolism like oxidation of food stuffs and combustion of fat.
Heat loss
Loss of heat occurs through the skin by:
1. Conduction to objects in contact like clothing.
2 Convection by which the hot air around the body moves up and it is replaced by cool air.
3. Radiation by which heat is given to the surrounding air.
4. Evaporation which is the skin cool and leads to heat loss.
Body temperature is controlled by 'heat regulating centre' present in the hypothalamus.
PHYSIOLOGY OF PAIN
Pain in a protective mechanism which warns the body against disorders and defects in its parts. Pain can be clastified into: i) cutaneous pain i) visceral pain iii) deep pain iv) head-ache.
1)Cutaneous pain:
It may be produced by: 1)pathological states of akin 2) injury to skin 3)release of chemical substances such as those which produce itching. In some cases, the painful area may extend beyond the damaged or diseased area.
2)Visceral pain:
It may occur due to an altered physiology of an internal organ like kidney, liver etc. sometimes sensation of pain is displaced from the affected ace and felt it adjacent Areas and it is called as referred pain e.g. pain arising to liver is felt in the skin of right shoulder.
3)Deep pain:
It arises from the receptors of muscles, tendons and joints e.g. powerful contraction of a muscle may occlude blood supply in the muscle and this may cause pain. Like visceral pain, deep muscle pain can also by referred on the surface of body.
iv) Head-ache:
Headache may occur due to a variety of conditions like anxiety, tension, pathological changes in intracranial blood vessels or visual defects.
Pathway of pain:
The sensation of pain is carried by non myelinated afferent nerves (which originate from the area of pain) to dorsal horn of grey matter of spinal tract. From here, the sensation is carried to thalamus through spinothalamic tract. From the thalamus, the impulses are carried to subcortical area for crude sensation and then to cerebral cortex.