SKIN

The entire surface of the body is covered by skin. In man, the skin is about 1 to 2mm thick except the skin of the palm and soles with thickness of about 6mm. The skin is the most extensive organ in the body.

The mammalian skin consists of two major layers: the outer epidermis and inner dermis.

 

Epidermis

This is further divided into three layers known as cornified, granular and malpighian layers.

 

• Cornified layer: This is the upper-most layer of the skin and consists of flat dead cells. Cornified layer protects the body against loss of water and entering of foreign bodies.
• Granular layer: Contains living cells. It lies just beneath the cornified layer. The cells become gradually thickened and longer until they are given up to the cornified cells.
• Malpighian layer: This is a layer of living cells and where active cell division takes place. This layer contains dark melanin pigments which give the skin its colour. Black people have more melanin pigments than white people. Oriental people have an additional pigment, carotene making their skin yellowish. Melanin pigments are absent in albinos. As the cells of the malpighian layer divide, they are pushed up. As they move up, they become filled with a substance called keratin and become flat and dead, thus forming the cornified layer.

 

Dermis

The dermis is a thick layer of connective tissues. This layer consists of blood vessels, sweat glands, sebaceous glands, hair follicles, erector muscles, sensory cells and fatty cells.

• Blood vessels: Blood vessels supply food and oxygen to the tissues of the skin.
• Sweat gland:  is a coiled tube becoming straight as it passes through the dermis and slightly drawn into folds as it goes through the epidermis. It opens on the surface as a pore. The sweat gland is well supplied with blood capillaries. As blood flows through the blood capillaries, waste products such as water, sodium chloride, small amount of urea are absorbed by the sweat gland. From the sweat gland the wastes are secreted to the sweat duct and finally to the sweat pore. It secretes waste products in form of sweat from the body. It is also greatly concerned with temperature regulation.
• Sebaceous gland: Each hair follicle is supplied with at least two sebaceous glands. Sebaceous glands secrete oily substance (sebum) which lubricates the hair. The sebum contains lysozyme which kills bacteria and viruses.
• Hair follicle: Each hair lies in a deep pit called hair follicle. The hair projects out of the skin. A hair is made up of keratin. Hair takes part in temperature regulation. The root of each hair is situated deep in the dermis, but the hair is an epidermal structure.
• Erector muscle: This is a slender muscle which runs from each side of the hair to the base of the Malpighian layer. The contraction of muscle makes the hair to stand up-right and its relaxation flattens the hair. It is concerned with temperature regulation.
• Fatty cells (adipose tissue): These cells are found below the dermis. They are round in shape. Fatty cells act as insulating layers. They also act as energy reserve and protect the skin from damage.
• Sense receptors: There are five sense receptors namely: touch, pressure, pain, heat and cold. Each receptor is sensitive to a particular type of stimulus.

 

Functions of the skin

1) Protection: The skin protects the body in the following ways:

• The skin covers and holds the body together.
• It forms barrier between external environment and the tissues below.
• Prevents harmful bacteria from invading the tissues. The sweat and sebaceous glands have certain constituents e.g (lysozyme) in them that are antagonistic to invading bacteria.
• Protects the body to some extent from physical injuries.
• It prevents the tissues from drying up.
• Melanin pigments prevent the ultra-violet rays from damaging the under-lying tissues.
• In some mammals such as porcupine, the hairs are modified into thick sharp spines that protect the animal from attack by predators.
• The hairs of many mammals have different colours and patterns, that camouflage the animals in their surroundings, making them difficult to be seen by their predators.

 

2) Sensitivity

• The skin contains different sense receptors which are sensitive to different environmental stimuli. The body can therefore react to changes in the environment. For example, on touching a hot object you quickly withdraw your hand to avoid burning injury.

 

3) Excretion and osmoregulation

• The sweat glands excrete surplus water containing salts and some little amount of waste nitrogenous substance (mainly urea).

 

4) Maintenance of a suitable constant body temperature

Maintenance of a suitable constant body temperature is very important to mammals and birds, since their metabolic activities are tied to a constant temperature. These animals are known as warm blooded animals or homoiothermic animals. No matter the external changes in temperature, the body temperatures of mammals and birds remain the same. The constant body temperature in man is 98.4 degrees F (37 degrees C). The heat gained must be equal to heat lost. Other animals such as fish, amphibians and reptiles are cold blooded or poikilothermic since their body temperature changes with external temperatures.

 

5) The skin manufactures and stores vitamin D.

 

6) Storage of reserved food: Fats are stored under the dermis. The fats form an insulating layer.

 

7) Nails, claws, hoofs, horns and scales are outgrowths of skin.

• Nails give extra support to the fingers so that they can hold or grasp things well, Nails are also used in scratching the skin.
• Claws are mostly used by carnivores such as cats and lions as weapons of defence, offence and for catching their prey.
• Horns are used as weapons of offence and defence in mammals such as cows, deer, goats and rams.
• Scales protect the body of some mammals (e.g pangolin) from losing water. They also protect the inner organs.

 

8) The mammary glands in mammals are modification of the skin. They help to produce milk for feeding the young.

 

The care of the human’s skin

1. Bathe regularly with soap to remove dirts, sweat and natural oils from the skin. This prevents bacteria, fungi, mites and dust from sticking to the skin and blocking the sweat pores. Dirty skin causes diseases such as, yaws, tinea and scabies. Do not bathe with harsh soap.
2. Dry the washed body with clean towel.
3. The feet must be washed frequently and cleaned with towel. Wearing clean stockings helps to absorb sweat from the clefts of the toes. Sweat staying in between toes inhabits fungus which causes tinea.
4. Wearing of shoes prevents hookworm and jigger from infecting the skin and its underlying tissues.
5. The skin must be kept moist always by using good creams especially during the harmattan period to keep the skin soft and flexible and protect it from cracking. On no account should bleaching creams be used on the skin. Bleached skin is proned to skin infection and cancer
6. Wearing clean light and loose clothes allows free circulation of air round the body. Dirty and heavy clothes cause discomfort; rashes and fungal infections of the skin.
7. Report any skin disease to a medical doctor for treatment.
8. The skin should not be exposed to unnecessary direct radiation of the sun more especially the albinos. Direct hot sunlight may cause sun burns.

 

How mammals maintain their constant body temperature

1) In hot weather

In a hot weather, the body temperature goes up. The following things happen to maintain a constant body temperature.

• More blood flows to the surface of the skin: The blood vessels in the skin dilate. More blood flows to the surface of the skin. The flow of blood stimulates the sweat glands to produce more sweat which is poured to the surface of the skin. The sweat evaporates thus producing cooling effect since the latent heat of vaporisation required to evaporate the sweat comes from the body. Failure of the skin to keep the body at constant normal temperature during hot weather leads to hyperthermia, heat stroke and death occurs.
• The hairs are lowered: The erector muscles of the hairs relax thus lowering the hairs and therefore the layer of air between the hairs and the skin is thin. This thinness of air allows more heat to be lost from the body (Note, air is a poor conductor of heat and if a thick layer of air is trapped more heat will be retained in the body.

 

2) In cold weather

In a cold weather, the body temperature is lowered and the following things happen to maintain a constant body temperature of 98.4 degrees F (37 degrees C).

• Less blood flows to the surface: In a cold weather, the blood vessels constrict, therefore, less blood flows to the surface of the skin; less sweat is produced by sweat glands. Little latent heat is lost from the body and therefore less heat is lost from the body. That is why people are pale in cold weather.
• The hairs stand erect: The erector muscles of the hairs contract making the hairs to stand erect. A layer of thick air is trapped between them. Air being a poor conductor of heat insulates the body and less heat is lost by conduction, convection and radiation. Failure of the skin to keep the body at constant normal temperature during cold lead to hypothermia. Death occurs by frost bite.
• Shivering: When the body temperature is low, some involuntary muscles contract and relax continuously, thus producing heat. This is circulated to all parts of the body by the blood stream and helps to raise the body temperature.