SKIN
The skin is not just a simple covering to hold the body substance inside. It is one of the most active organs of the body, functioning in numerous ways. Many of our recognising features come from the complexion, hair patterns, etc.
The skin is the outermost covering of the body. It is stretched all over in the form of a layer. There are many structures and glands derived from the skin.
FUNCTIONS OF THE SKIN
The skin (or the integument) serves a number of important functions as follows:
1. Protection.
Protection is the primary function of the skin. It affords protection in four
different ways :
(i) It protects the underlying tissues from mechanical shocks.
(ii) It holds the body
fluids inside and prevents excessive loss of water by evaporation.
(iii) It prevents the
entry of harmful substances or disease - causing germs.
(iv) It protects the
body against excess ultraviolet light which is potentially very harmful.
2. Sensation. Our skin
serves as a sense organ for touch, pain, pressure, heat, etc.
3. Temperature regulation. The skin prevents loss of heat in cold weather and facilitates loss of heat in hot weather. (You will read about the actual mechanism of temperature regulation later in this chapter).
4. Storage of food. The
skin stores reserve food in the form of a layer of fat contained in special
cells.
5. Excretion. The skin
assists in the process of excretion (through sweating) eliminating water, salts
and, to a very limited extent, urea. (Since sweating from the skin occurs
mainly for temperature regulation, we cannot really consider the skin as an
excretory organ).
6. Synthesis of Vitamin
D. The skin can synthesise vitamin D when exposed to sunshine. This is a minor
function and its mechanism is not yet fully understood. In fact, too much
sunshine may cause tanning and other ill-effects including even skin cancers.
7. The skin on our
fingers and palms forms ridges and grooves which provide a more efficient grip.
STRUCTURE OF THE SKIN
The principal components of the human (mammalian) skin are as follows.
THE SKIN PROPER
Microscopically, the
skin proper is composed of two layers —the outer epidermis and the inner dermis.
EPIDERMIS
The epidermis is the outer thinner part of the skin. It is formed of stratified epithelium piled up layer after layer. At places, the epidermis becomes thick and hard as on the palms, soles and specially on the heels. It is devoid of blood vessels at all places. The epidermis shows three regions (or sublayers)
Depth wise :
(a)
outermost cornified layer,
(b) middle granular layer and
(c) inner malpighian
layer.
(a) The cornified layer (stratum corneum) is the outermost layer consisting of several piled up layers of flattened dead cells. These cells are made of a horny protein called keratin (also found in nails, hairs, horns, hoofs and silk). Its cells are continually worn away or shed and are replaced from beneath by those arising from the deeper malpighian layer. The cornified layer is tough and offers resistance to three things :
(i) mechanical damage,
(ii) bacterial infection and (iii) loss of water by evaporation.
(b) The granular layer is a very thin middle layer consisting of two or three sub-layers of flattened cells. Gradually, it gives way to the outermost cornified layer.
(c) The malpighian layer (stratum malpighi, also called germinative layer) is the innermost region of the epidermis. Its cells can actively divide to produce new cells which press and shift outward to replace the worn-out cells of the outermost cornified layer. Colouration of the skin (e.g. the complexion of the face) is due to a pigment melanin contained in the cells of the malpighian layer. The different quantities of this pigment in different human races lead to form very light-brown to dark colouration of the skin.
African negroes have genetically dark coloured skin, the Europeans light coloured (whitish) and most Indians an intermediate coloured (wheatish) skin. In fair-coloured people also, on continued exposure to sunlight, the face or other parts of the body may get tanned, but revert to original condition when the exposure is cut down.
Two abnormal conditions of skin pigmentation.
* Leucoderma also called “ igo” : Skin pigmentation (melanin) is lost from smaller or larger patches at different regions of the body, exact cause of this disease is not yet known.
* Albinism : Complete loss of pigmentation of the skin all over the body including hair, eyebrows, eyelashes and even the iris. The skin of such persons appears pinkish because of the underlying blood capillaries. Albinism is a recessive trait caused due to inheritance; an albino couple would get all albino children.
DERMIS
‘The dermis is the inner thick layer of connective tissue made of elastic fibres. It is tough and flexible.
At certain places the
dermis in our skin is very thick as on the palms and soles, and very thin at
other places as in the eyelids. The dermis contains several other
structures — blood vessels, nerve fibres, sensory organs, hair follicles, sweat glands, etc. [Leather obtained from the hides of animals is actually the dermis part of the skin].
The outer region of the dermis which lies next to the epidermis is raised into numerous small processes called papillae which contain blood capillaries and nerve endings. The nerve endings and sense organs here are concerned with sensations of touch and pain. The sharp sense of touch in the skin of finger tips enables the blind to read the Braille characters. There are some more sense receptors in the deeper parts, which are concerned with the sensations of pressure, pain, heat, cold, etc.
DERIVATIVES OF THE SKIN
The special derivatives of the human skim include the following :
1. Hair 2. Nails 3. Sebaceous glands 4. Sweat glands 5. Mammary glands
HAIR
A hair consists of three parts.
(i) Hair shaft is the part which projects from the skin and may extend slightly below the surface of the epidermis. In normal situations, they lie obliquely on the skin.
(ii) Hair root is the part embedded within the dermis. The lowest part of the hair root is expanded to form a. (iii) hair bulb which contains a projection of the dermis called hair papilla, with capillary blood supply.
The hair follicle is a structure enclosing the hair root. It is composed of an epithelial and a connective tissue sheath. The hair bulb and the hair follicle together are responsible for the growth and elongation of the hair. The growth of hair occurs by addition of cells at the base, which soon die.
The colour of the hair is due to varying quantities of melanin. The gray or silvery colour of the hair is due to minute air spaces formed in the hair when the pigment is lost.
Everyone has sometimes experienced “goose flesh’” during winter or during some emotion. In this, the hair is lifted called piloerection, (pilo : hair) and the surface of the skin presents a somewhat contracted and wrinkled appearance. This is caused by the erector (also called arrector) muscle of the hair which runs obliquely between the hair follicle and the outer part of the dermis. The contraction of this muscle at one end pulls the hair to a somewhat vertical position, and at the other end, depresses the epidermis.
Hair in human beings
are continuously lost and regrown. The duration of scalp hair is 2-5 years and that
of the eyebrows and eyelashes is 3-5 months. Hairs from the different parts of
the body (head, chest, arm pit, beard, nose, etc.) show subtle differences.
These are helpful in forensic (crime detection) investigations.
Hairs also provide a sensation of touch because nerve fibres extend up to their bases.
There are hairs eyelashes along the edges of the eyelids helping to prevent entry of particles, raindrops. Similarly, there are hairs in the nose, again to prevent dust particles from entering nasal passages. Facial hairs in human males, i.e. moustaches and beard, help in distinguishing the male sex (sexual dimorphism).
NAILS
Nails are hardened keratinous plate-like structures which grow as dead cells from the nail root, which lies below the skin at the base.
(i) Plate — It is hard and outer part of the nail. It is made up of dead, keratinized cells.
(ii) Bed (root) — It lies below the nail plate.
(iii) Matrix — It lies just below the skin surface at the base of the nail. It is usually visible as whitish half-moon at the base of nail, it produces new cells which on maturation push out the older one towards the tip of the nail and causes growth of the nail.
SEBACEOUS GLANDS
These branched glands usually open into a hair follicle though, sometimes they even open directly to the outside. They give out an oily secretion (called sebum) which makes the hair and the outer surface of the skin oily and waterproof to keep the epidermis supple and to prevent loss of water by evaporation. In cold and dry weather, the skin may become rough and leave a powdery surface when scratched; this is due to reduced secretion of oil from the sebaceous glands. In hot and humid weather, the skin becomes extra oily due to increased secretion of sebum.
Three common problems
related to sebaceous glands :
1.Pimples — Sebum accumulation, causes growth of bacteria because it is nutritive, gets infected and results into the formation of boils and pimples.
2. Acne — Sebaceous glands get inflamed due to hormonal influence. It is one of the commonest adolescence problems.
3. Black head — Sebaceous glands of the face get enlarged due to accumulated sebum. On oxidation melanin and sebum give it a black colour named black head.
SWEAT GLANDS
Each sweat gland is a simple coiled tube consisting of a deeper secretory part and an excretory part which runs upwards to open on the surface. The outer openings are called the sweat pores. Their total number in the body is estimated at about two million. Human races belonging to hotter countries usually have more sweat pores than those belonging to colder ones.
The secretory part of a sweat gland absorbs fluid from the surrounding cells and blood capillaries of the dermis and passes it into the excretory sweat duct which pours it out on the surface. Sweating (or perspiration) goes on at all times in minute quantities (incipient or invisible perspiration), or sometimes in large quantities as during strenuous exercise or during hot and humid weather, when a person may lose as much as 1 kg per hour. The major function of sweating is to lose body heat by evaporation.
Sweat consists of about 99 percent water, 0.2 to 0.5 percent salts (mainly sodium chloride) and traces of urea (0.08 per cent). The urea lost through the sweat is about 1 percent of the total urea excreted by the body.
“Cold sweat” may be due to psychic influences such as fright and nervousness. Sweating may also accompany nausea and severe pain (due to loss of body salts).
MAMMARY GLANDS
The mammary (or milk) glands are modified sweat glands. These glands are present both in males and females. But in males, they persist only in a rudimentary state, whereas in females at puberty, they enlarge in the form of a pair of breasts. Each breast carries a central conical projection called nipple. 15-20 milk ducts open on the nipple. Each milk duct is continued inward in a branching manner to join a cluster of 15 to 20 lobes of the mammary glands. The activity of the mammary glands is related to the reproductive hormones (prolactin) and pregnancy. The milk secreted by the mammary glands is highly nutritious for the new-born baby.
Meibomian glands : These are modified sebaceous glands which open on the margins of the eyelids. Their secretion is oily and serves to lubricate the margins of the lids and to prevent the overflow of tears.
Ceruminous glands : These are modified sebaceous glands found in the auditory canal and secrete wax like substance called cerumen or earwax which lubricates and protects the delicate eardrum from dust particles and germs.
Extra (supernumerary or accessory) nipples or breast tissue
Extra nipples or breast tissue is fairly common (1-6% of women) and is a result of incomplete regression of the mammary ridge (milk line) during the development of the embryo before birth.
Extra breast tissue is a common developmental variation in human anatomy. Breast development begins around week four of gestation, with two parallel lines of glandular tissue called the milk line (or mammary ridge) extending from slightly beyond the underarm area, down the chest and abdomen, to the groin area and ending near the groin at the inner sides of the thighs (see figure). Breasts eventually develop from these lines of tissue, and the remaining tissue regresses. Occasionally, there is an incomplete regression of the mammary ridge during embryo development, and extra (supernumerary) nipples and/or breast tissue forms, usually along this line but occasionally in other parts of the body. This can occur in both males and females. An extra nipple, areola, and/or glandular tissue may be present anywhere along this line (a nipple without glandular tissue is most common), and it is not at all unusual to have more than one.
SKIN AND HEAT REGULATION OF BODY
All mammals including
humans are warm-blooded (endothermal: body heat generated from inside); so also
are the birds. They all maintain a more or less constant body temperature even
if it is very cold or very hot outside.
[Animals other than mammals and birds are cold-blooded or (ectothermal: body heat gained from outside). Their body temperature fluctuates with the environmental temperature. This means that they must somehow escape both conditions of too hot or too cold environment and thus they hibernate (winter sleep) or aestivate (summer sleep) as in the case of frogs.]
Human body temperature is usually about 37°C (in the mouth), it is about 1°C higher in the rectum and about 1°C lower in the arm pits.
* Our body temperature may also show variation of 0.3°C to 0.5°C in the course of 24 hours. It is lowest in the early morning and highest in the late afternoon.
* Rise in body temperature above normal means fever and sickness and similarly, a fall in temperature may be dangerous. A suitable (optimum) temperature of 35°C — 40°C is essential for normal body activities, especially for the action of enzymes. The working of enzymes is slowed down with the lowering of temperature, while higher temperatures destroy them.
HEAT PRODUCTION
There are many sources of heat production in our body. Chemical reactions occurring in all body cells, especially in the liver by the oxidation of glucose, generally produce heat. Most of the heat produced in our body comes from the activity of our muscles. Vigorous activity makes you warm on a cold day and overheats you on a hot day. A small amount of heat comes from the ingestion of hot foods and beverages.
HEAT LOSS
Heat is lost from our body through the following four channels.
1. Skin- about 85 per cent of the body heat is lost through the skin by convection, conduction, radiation and through evaporation of sweat.
2, Lungs- Heat is lost in the warm air which is breathed out. This loss in warm air can easily be experienced by gently blowing on the back of your hand keeping the mouth wide open. Some heat is also lost during vaporisation of water from the lungs.
3. Urine and Faeces- These substances are eliminated at body temperature.
4. Foods- Heat is also lost when cold food, water, or cold beverages are taken into the body.
TEMPERATURE REGULATION
The principal heat-regulating centre is located in the hypothalamus, a portion of the fore-brain. This part acts like a thermostat. When the body tends to cool below the normal temperature, it switches on or speeds up the heat-producing process, and when the body tends to get overheated, it accelerates the cooling process, and switches off the heat-producing processes.
IN COLD WEATHER — When outside temperature is low, the blood vessels get narrowed (vasoconstriction). This reduces the blood supply to the skin. As a result, there is less loss of heat by convection, conduction and radiation, and also less loss of it through vaporisation of sweat because, with reduced blood supply, the sweat glands secrete less sweat. This is a natural method to conserve body heat. When vasoconstriction occurs, it makes a person look pale or bluish, because of reduced blood supply to the skin, Simultaneously, the heat production is increased by the increased metabolic rate and through increased muscular activity which is sometimes in the form of shivering.
IN HOT WEATHER — When the outside temperature is high or when a person is engaged in strenuous physical work, which means overproduction of heat within the body, the blood supply to the skin is increased by the dilation of blood vessels in the skin (vasodilation)
Also by vaporisation of sweat (latent heat) which is now produced in larger quantities due to the rich supply of blood to the skin. Air movements over the body help speed up evaporation of sweat; that is why the fans which do not cool the air of the room, have a cooling effect on our body.
Heatstroke or “sunstroke” is a condition in which sweat production is unable to keep pace with its evaporation in very hot winds. This results in the rise in body temperature (fever) which may sometimes be fatal. Drinking a lot of water and taking a little more of salt in summer is a good precaution against heatstroke.