Functions of the skin:

Skin is stretched over the entire body to form a continuous protective layer.

Functions of skin fall under 3 categories,

1. Protection.

2. Temperature regulation.

3. As a sense organ.
 Protection:
It protects the various organs of the body that lie beneath it; prevents mechanical injury to the
organs.It prevents the excessive loss of moisture from the body through evaporation.It serves
as a barrier to the harmful UV rays of the sun, to pathogens and other harmful microorganisms
and prevents them from entering the body.

Temperature regulation:
The skin's immense blood supply helps regulate temperature.Dilated vessels allow for heat
loss, while constricted vessels retain heat.The skin regulates body temperature with its blood
supply. Sweating is one mechanism by which the body keeps cool in hot summer months.The
skin assists in homoeostasis.

As a sense organ:
The skin is the sense organ for touch and pressure. It contains numerous sensory neurones
which are sensitive to temperature, touch and pressure (pain). These neurones relay the
changes in the surroundings to the brain which instructs the effector organs to act accordingly.

Apart from this, the skin plays a role in the synthesis of vitamin D in the presence of sunlight
and helps the process of excretion eliminating excess water and salts in the form of sweat.
The skin is the largest organ of the body, with a total area of about 2 square metres. The skin
protects us from microbes and the elements, helps regulate body temperature and permits the
sensations of touch, heat and cold.

The derivatives of the skin include sweat glands, sebaceous glands, and mammary glands, hair,
hair follicles, and nails. All are epidermal invaginations into the dermis.

The role of Skin in Heat Regulation:

The skin’s immense blood supply helps regulate temperature: dilated vessels allow
for heat loss, while constricted vessels retain heat. The skin regulates body temperature with its
blood supply. Humidity affects thermoregulation by limiting sweat evaporation and
thus heat loss.

Heat Production: Heat is produced during metabolism. The breakdown of food releases a
large amount of energy inside the body due to oxidation.Warm air in the environment causes
our bodies to heat up due to conduction and convection. A small amount of heating also occurs
due to radiation from the surroundings.
Heat loss: Heat is lost from our body in the following ways,
 By convection, conduction and radiation through the skin.
 By evaporation of sweat from the surface of the skin.
 Along with urine, faeces or exhaled air.
Cold-blooded animals (fish, amphibians, reptiles) cannot balance the heat loss and the heat
gain. Their body temperatures vary with the surroundings, suffer heat loss in cold winters. This
difficulty overcome by hibernation. They are generally sluggish and less active compared to
warm-blooded animals.

Warm-blooded animals (mammals, birds) balance the heat loss and gain; maintain constant
body temperature.

Example: Humans show a constant temperature of 37˚F irrespective of outside conditions.

Enzymes work optimally at this temperature.

Vasodilation:
The part of the brain that controls body temperature is called the hypothalamus. When the
hypothalamus senses that the body temperature is too high, it sends impulses which cause
blood vessels supplying the capillaries in the skin to dilate. This is called vasodilation.
Effects of Vasodilation:
 Increased sweat production -> sweat evaporates to produce the cooling effect.
 Increased breathing rate -> exhaled air carries away some body heat.
 More blood diverted to the skin -> metabolic activities are slowed down because of
lesser blood flow to digestive system so the amount of heat generated is lowered.
Vasoconstriction:
When the hypothalamus senses that the body temperature is too low, it sends impulses which
cause blood vessels supplying the capillaries in the skin to contract. This is
called vasoconstriction and it reduces the blood flow to the surface tissues under the skin. As a
result, less heat is lost.
Effects of vasoconstriction:
 Decreased blood flow to the skin.
 Decreased activity of sweat glands results in less amount of water and salts removed
from blood and amount of sweat reduced results in loss of heat due to evaporation
of sweat.
 An increase in metabolic rate will increase internal body temperature.

Maintaining body temperature

The body can survive environmental changes in temperature ranging from -
52°C to +49°C (Hinchliff, 1996). Maintaining a constant core body temperature
(37°C) is an important function of the skin and this regulation is essential for
the normal functioning of cellular enzymes throughout the body. The
hypothalamus, which contains the temperature regulating centre, can
orchestrate a number of changes within the skin in response to temperature
changes.

Response to increased temperature

Arteries and veins in the subcutaneous layer immediately beneath the dermis
(Fig 2) are linked to small arterioles and capillaries, which supply blood to the
dermis. Blood flow can be varied by dilatation and constriction of these dermal
blood vessels. Blood flow to the skin can vary from about 250ml/min to 3l/min
in response to the need to lose or conserve heat. When the body’s core
temperature rises, blood vessels in the dermis dilatate and the skin becomes
warm to the touch, and pink or red in colour. The blood carries heat with it to
the skin surface. Heat is lost from the body in four ways:

- Convection - heat is lost to air currents, for example, when a fan is used to
cool a patient;

- Conduction - heat is lost to cooler solid objects which are in direct contact
with the skin;

- Radiation - heat from a warm body is lost to the cooler surrounding air;

- Evaporation - heat is lost as a liquid becomes a gas, for example, through

evaporation of sweat.

Eccrine sweat glands assist in reducing body heat. There are about 2-5 million
of these glands (Martini, 2003) and their distribution varies in different body
areas. There are 400/cm2 on the palms and soles of the feet and only 70/cm2
on the back (Hinchliff, 1996). These glands have their own nerve and blood
supply and produce sweat when the skin’s temperature rises above 35°C.

Sweat is composed of 99 per cent water, with sodium, urea, lactic acid and
potassium making up the remaining one per cent. We sweat an average of
500ml a day in the UK climate (Marieb, 2003), whether we are hot or not. But
this can rise to 10 or more litres a day in very hot climates. Spicy foods and
exercise also increase water loss through sweat. The watery sweat drips off
the skin, is absorbed into clothing or evaporates from exposed skin surfaces.
Problems arise if the air is very humid, because sweat cannot evaporate from
the skin’s surface, reducing heat loss.

Response to a fall in temperature

In cold weather, blood supply to the skin is reduced (Marieb, 2003), as is
sweat production. This conserves heat in the deeper organs, which are also
insulated by the layers of subcutaneous tissue and fat.
Another method used in the skin to conserve heat is the mechanism that
erects the body’s hair. Tiny arrector pili muscles, around the shaft of the hair,
contract and pull the hair vertically (Martini, 2003). In animals with lots of hair,
this results in the trapping of a layer of warm air around the body, and is an
important means of insulation. However, in humans it only results in goose
pimples.

Shivering occurs when the body becomes cold - this involuntary muscle action
throughout the body is a metabolic process, which produces heat.

Receiving stimuli from the outside world

The skin is the body’s largest sensory organ and its sensory (or afferent)
nerve receptors detect a number of different stimuli: mechanical, such as
pressure or stretching; and thermal, in terms of heat and cold (Marieb, 2003).
This ability to sense and provide information about contact of the skin with the
outside world allows the brain to interpret and act upon the stimuli and helps
to protect the body and avoid damage to it.

The skin is the main organ involved in the regulation of body temperature. Now let see how the
skin changes to keep the body temperature stable under cold and hot conditions

Under cold conditions

Different structure in the skin makes responses to reduce heat loss from the body

Firstly, Erector muscles in the skin contract to pull hairs upright. This traps a thicker layer of still
air close to the skin surface. Since air is a good insulator the heat loss to the surroundings is
reduced. Secondly, sweating is reduced. This helps reduce heat loss from the body due to
evaporation. Thirdly, the arterioles in the skin constrict so that less blood flows to the capillaries
near the skin surface. As a result less heat is lost from the skin to the surroundings by
conduction, convection and radiation. Fourthly the layer of subcutaneous fat becomes thicker.
This better insulates the body. This a long -term response to a cold environment, Because the
subcutaneous fat takes time to develop

Under hot conditions

The skin makes responses to promote heat loss from the body. Firstly, erector muscles in the
skin relax that hairs lie flat. This traps a thinner layer of still air close to the skin surface. As a
result, heat is lost more quickly from the body to the surrounding. Secondly, sweating is
increased the evaporation of more sweat from the skin absorbs more heat from the body
surface so that more heat is lost to the surroundings. thirdly, the Arterioles in the skin dilate so
that more blood flows to the capillaries near the skin surface. As a result, more heat is lost from
the skin to the surroundings. Fourthly, the layer of subcutaneous fat becomes thinner.
Therefore, the insulating effect is reduced. this is a long-term response to a hot environment