Skin is the outer layer of soft tissue covering the body of vertebrates and performing three key functions of protection, heat regulation, and sensation. It can perform these vital functions because of its structure, which consists of specialized cells each with its own set of uses.
It is the interface between the human body and its surroundings and acts as a barrier protecting the body from the harsh conditions of the environment. It also prevents the loss of important constituents from the body such as water.
It is classified as a human organ like the heart, liver, and kidneys and the largest one at that. It weighs 4 kg on average and the total surface area of the average adult skin is about 22 square feet or 2 square meters.
The structure of the skin consists of three layers, each armed with its own type of cells and a definite function.
Any extensive damage to the skin as in burns can result in death. This only emphasizes that besides offering protection to the body, the skin serves important and diverse functions, which are vital.
Structure (Anatomy) of skin
The skin consists of three main layers, each with a different anatomy and function.
1) The Epidermis
The epidermis is the outermost layer of the skin. The epidermis and the dermis together form what is called the cutis. It acts as a protective and waterproofing layer of the body.
The epidermis also helps to regulate body temperature. It is avascular, which means it contains no blood vessels and derives its nourishment from the blood vessels that are present in the uppermost layer of the dermis.
The epidermis consists of four specialized types of skin cells.
- Keratinocytes constitute 95% of the epidermal cells. They produce keratin and form a water barrier by secreting lipids. Keratinocytes also help in regulating calcium absorption upon activation of the cholesterol precursors by UVB light of the sunlight to form vitamin D
- Melanocytes produce melanin. This is the pigment, which gives color to the skin.
- Langerhans cells are defensive in nature and alert the immune system of any invasive attack by infectious agents like the virus or the bacteria.
- Merkel cells. Their function is still not known but it is speculated that they may have a neuroendocrine function.
2) The Dermis
The dermis lies beneath the epidermis and consists of connective tissue (fibrous tissue). Between the epidermis and the dermis is a membrane called the basement membrane, which tightly binds the epidermis and dermis.
The thickness of the dermis varies between 0.3 mm thickness on the eyelids to 3.0 mm on the back. The dermis contains many specialized cells and structures, each performing its own function.
- The hair follicles are placed in the dermis each attached with erector pili muscle.
- Sebaceous glands secrete an oily secretion called sebum, which acts as a lubricant.
- Apocrine glands are the sweat glands and they drain into the hair follicle.
- Eccrine glands are also sweat glands but are not connected to the hair follicles. They are the major sweat glands situated virtually all over the skin and open directly onto its surface and not into the hair follicles like the apocrine glands.
- Blood vessels and nerves. Blood vessels give nourishment to the dermis and the epidermis while nerves make the skin feel temperature, pain, touch, pressure, and itching. The skin is literally supplied with one million nerve fibers.
3) The Hypodermis
The hypodermis is technically not part of the skin and serves the function of attaching the skin to the underlying muscle and bone.
It consists of abundant subcutaneous fat, which accounts for 50% of the body fat. This helps it to function as a regulator of body temperature and absorb shock due to trauma.
Functions of skin
By virtue of its anatomical position and structure, the role of skin in being the largest organ of the body is diverse and goes beyond just protecting and giving color to our body.
By virtue of its anatomical position and structure, the skin protects the internal parts of the body against harm from environmental forces, such as mechanical injury, thermal injury, physical harm, hazardous substances and prevents the infective organism from entering the body
Prevent loss of moisture
The skin is a semi impermeable barrier, which prevents loss of body moisture. Damage to it as in burns results in a big loss of body fluids.
Sensation of feel
This function of the skin is possible due to the rich network of nerve endings present in it.
- The skin gives us the feeling of touch as when you touch something or someone or something touches you. This is very useful when you are in complete darkness or when a person is blind.
- The sensation of feeling hot or cold
- The feeling of pressure on the skin
- Feeling vibrations
- Feeling pain as during an injury
Regulates body temperature
By its rich supply of blood vessels, the skin is able to lower or preserve body temperature. During hot weather, the blood vessels in the skin dilate thus facilitating sweating and loss of heat through the sweat. During cold weather, the blood vessels constrict thereby reducing or stopping sweating, thereby conserving body heat.
The skin gives the complexion to the appearance and attractiveness to the person’s personality. Similarly, it can also help gauge a person’s mental state as in blushing, which shows shyness, or redness, which shows anger.
Produces vitamin D
With the help of the sunrays, the skin helps to synthesize vitamin D, which is so necessary for body nutrition.
Protects against UV rays
The skin protects the body against the harmful effects of ultraviolet radiation, which comes from exposure to sunlight.
Helps in excretion
The secondary function of sweating is the excretion of urea, which is present in sweat. However, the concentration of urea in sweat is very small.
In its very small way, the outer layer of skin absorbs oxygen thus aiding in procuring oxygen for the body. However, this function of absorption becomes very useful during topical applications of medicines being used for cure, such as ointments or nicotine patches.
The skin is water-resistant so it does not allow essential nutrients to be washed out from the body. This is helped by the sebum secretion, which is oily in nature and disallows absorption of water, and a protein called keratin, which is waterproof in nature and present in the outer layer of the skin.