Skin, Skin Temperature, Heat Loss And Gain, And Temperature Control

 


THE skin is a continuous layer of tissue over the surface of the body. It has three principal functions:

(a) it protects the tissues beneath from mechanical injury, ultravíolet rays in sunlight, bacterial infection and desiccation; 

(b) it contains numerous sense organs which are sensitive to temperature, touch and pain and so make the organism aware of changes in its surroundings; 

(c) it helps to keep the body temperature constant.

SKIN STRUCTURE

The skin consists of two main layers, (1) an outer epidermis and (2) an inner dermis. The relative thickness of the layers and abundance of structures within the dermis varies with the position on the body. For example, the skin on the soles of the feet has a very thick epidermis and no hair follicles, The account given below is a generalized one.

1. Epidermis 

(a) The Malpighian layer is a continuous layer of cells which can divide actively and so produce new epidermis. Also in this layer are the pigment granules, melanin, that determine the skin colour and act as a screen against ultraviolet light.

(b) The granular layer contains living cells but towards the outside it gives way gradually to the cornified layer.

(c) Cornified layer. In this region the cells are dead and form a tough outer coat which offers resistance to damage and bacterial invasion and reduces the loss of water by evaporation. The cells of the cornified layer are continually being worn away and replaced from beneath. On the palms of the hand and soles of the feet it may become very thick, particularly when the hands are used for heavy manual work.

2. Dermis

The dermis is a thicker layer of connective tissue with many elastic fibres in it. There are also blood capillaries, nerve endings or sensory organs, lymphatics, sweat glands and hair follicles.

Capillaries. The capillaries supply the skin with the necessary food and Oxygen and remove its excretory products. The sweat glands and hair follicles have a network of capillaries supplying them. The capillaries beneath the epidermis play an important part in temperature control.  

Sweat glands. The sweat gland is a coiled tube consisting of secretory cells which absorb fluid from the surrounding cells and capillaries and pass it into the duct through which it reaches the skin surface. The fluid is water with some salts, notably sodium chloride, dissolved in it and also small quantities of urea and lactic acid.

Although the body loses water vapour through the skintairly constantly at most normal temperatures, the 2-3 million Sweat glands do not operate until the body temperature rises about 0-2-05 Cabove normal. In a hot climate a man at work may lose about 1 kg per hour in sweat. Since salts, particularly sodium chloride and other solids, constitute up to 0-5 per cent of the sweat, these must be replaced by the food or in the drink of workers who lose much sweat; otherwise, if water alone is taken to replace that lost by sweating, the salt and water balance of the blood and tissues is upset leading to "heat Cramp".

Hair follicle. The hair follicle is a deep pit of granular and Malpighian layers the cells of which multiply and build up a hair inside the follicle. The cells of the hair become impregnated with a horny substance, keratin, and die. The constant adding of new cells to the base of the hair causes it to grow. Growth continues for about four years; the hair then falls out and a new period of growth begins. The hairs of the body form a protective and heat-insulating layer in the regions where they grow thickly. The layer of stationary air held between the hairs reduces evaporation and heat loss. The follicle is supplied with sensory nerve-endings which respond to movements of the hair. This sensory function of the hair is well developed in the whiskers or vibrissae that grow on the sides of the face in mammals such as the cat or mouse.

Sebaceous glands. The sebaceous glands open into the hair follicles and produce oily secretion which gives the hairs water-pelling properties, keeps the epidermis supple, and reduces the tendency for it to become too dry as a result of evaporation. It also has antiseptic properties against certain bacteria 

Sub-cutaneous fat. The layers beneath the dermis contain numerous fat cells where fat is stored. The tat may also act as a heat-insulating layer.

Fish, amphibia, reptiles and all the invertebrates are poikilothermic, that is, their body temperature is the same as or only a few degrees above their surroundings, and varies accordingly. This makes them very dependent on temperature changes; for example, in cold conditions their low body temperature slows down all chemical changes and reduces the organism to a state of inactivity. Insects can be immobilized by a sudden fall in temperature.

Homoiothermic or constant-temperature animals are more independent of their surroundings because their body temperature is higher and does not alter with fiuctuations in external temperature.


Heat loss and gain

Many of the chemical activities in living protoplasm release heat energy. Chemical changes in the liver and in contracting muscle produce a good deal of the body's heat which the circulatory system distributes round the body. At the same time the body loses heat from its surface to the atmosphere, mainly by convection and radiation. Evaporation of water from the surface of the skin also removes heat from the body. An outer layer of fur, feathers or clothing reduces the heat losses.

Normally a balance is maintained so that the rates of heat loss and gain are the same; hence man's body temperature, although varying in different parts of the body, remains at about 36:8°C, as shown by readings taken from under the tongue. There are regulating mechanisms in the body, under the control of the brain, that compensate for over-heating or cooling.

Over-heating

Vigorous activity, disease, absorption of radiation from the sun, and many other external causes may bring about ove- heating. If the blood reaching the brain is a fraction of a degree higher than normal, nerve impulses are sent to the skin and produce two marked effects.

(1) Vasodilation. The dilating, or widening of the arterioles which supply the capillary network beneath the epidermis, causes more blood to flow near the surface. In consequence,more heat escapes into the air by convection and radiation Vasodilation causes flushing of the skin because of the incréased volume of blood beneath the epidermis.

(2) Sweating. Nerve impulses, starting mostly in the brain, increase the rate of sweat production so that a continuous layer of moisture may be produced on the skin surface. The latent heat absorbed by the sweat as it evaporates is taken from the body, so reducing the body's temperature. Any air movement over the body helps to speed up the evaporation of Sweat which is why fans, while not necessarily reducing the temperature of a room, have a cooling effect on the body.

In humid conditions, the air contains so much water vapour that the sweat may not evaporate rapidly enough to produce an adequate cooling effect, and may lead to heat stagnation in which the body temperature rises to over 41° C, causing co- lapse and sometimes death. Heat stroke is a similar result of extreme over-heating when, after prolonged sweating due to vigorous activity at high temperatures, sweat production ceases and the body temperature rises to a lethal level. Both conditions may be called sun-stroke" but it is not the effect of direct sunlight on the body so much as the high temperatures produced.

Over-cooling

If the body tends to lose more heat than it is generating the following compensatory changes may take place: 

(1) Decrease in sweat production, thus minimizing heat lost by evaporation.

(2) Vasoconstriction. Constriction of the arterioles which supply the surface capillaries reduces the volume of blood flowing near the surface and hence diminishes heat losses. Vasoconstriction makes a person look pale or blue.

(3) Increased metabolism. An increase in the rate of chemical changes, particularly in the liver, releases more heat.

(4) Shivering. This reflex action operates when the body temperature begins to drop. It is a spasmodic contraction of the muscles. These contractions produce heat which helps to raise the body temperature.

Furry mammals and birds can fluff out their fur or feathers by contraction of the erector muscles which are attached to them in the skin. This increases the layer of trapped air round the skin and so improves insulation by reducing convection and conduction. In man, a similar contraction of the muscles of the hairs only produces "goose-pimples


Hibernation

In cold climates or at high altitudes some small mammals pass the coldest months in a state of hibernation. When a mammal hibernates it falls asleep in some specially prepared burrow or nest and its body temperature falls well below normal, so that it may be only a few degrees above that of its surroundings. Breathing is often imperceptible, and all the chemical activities in the body go on very slowly, using food stored as fat and gycogen.

The animal is quite insensible and cannot be awakened by touching it, in fact it is likely to die if such attempts are made. At the end of its hibernation period its temperature rises spontaneously to normal, starting from the innermost regions of the body. Hedgehogs and dormice are examples of hibernating mammals.

Hibernation allows the small mammal to survive the period when, because of the heat losses from its body, its energy requirements are high, but at the same time, food is scarce.

Some reptiles and amphibia in temperate climates also hibernate during the cold months, but being poikilothermic they do not undergo the drastic change in temperature that occurs in mammals.


Skin, Skin Temperature, Heat Loss And Gain, And Temperature Control Skin, Skin Temperature, Heat Loss And Gain, And Temperature Control Reviewed by Admin on May 30, 2022 Rating: 5

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