Human Skin: Definition, Layers, Types, & Diseases

Skin is the soft external surface covering of the human body. It is the largest organ of the body.

The skin is a connective tissue that consists of cells, fibers, and an extracellular matrix.

It is the foundation of the integumentary system which made up of different tissues that are joined to perform a specific function.

It is waterproof, stretchable, tough and washable. It covers the surface area of 1.5 – 2 square meters and it’s thickness varies from 3-3.5 mm.

The proportion of skin covering different parts of the human body can be explained in percentages as :

  • Head and Neck = 9%
  • Each upper limb = 9%
  • Front of the trunk = 18%
  • Back of the trunk (including buttocks) = 18%
  • Each lower limb = 18%
  • Perineum =1%

There are various accessory structures of the skin include hair, nails, sweat glands, and sebaceous glands.

These structures embryologically originate from the epidermis and can extend down through the dermis into the hypodermis.

The skin is continuous with the mucous membrane at the orifices of the body.

The skin and its accessory structures perform a variety of important functions, such as protecting the body from surrounding microorganisms, chemicals, and other environmental factors; preventing dehydration; acting as a sensory organ; modulating body temperature and electrolyte balance, and synthesizing vitamin D.

The underlying hypodermis has important roles in storing fats, forming a “cushion” over underlying structures, and providing insulation from cold temperatures.

The skin has mesodermal cells, pigmentation, such as melanin provided by melanocytes, which absorb some of the potentially dangerous ultraviolet radiation  (UV) in sunlight.

It also contains DNA repair enzymes that help reverse UV damage, such that people lacking the genes for these enzymes suffer high rates of skin cancer.

One form predominantly produced by UV light, malignant melanoma, is particularly invasive, causing it to spread quickly, and can often be deadly.

Human skin pigmentation varies among populations in a striking manner due to which we can classify the person on the basis of skin color.

This assessment is very important in cases involving burns, where the patient requires treatment according to the magnitude of the damaged area of skin.

Functions of skin


  • The skin protects the rest of the body from the basic elements of nature such as wind, water, and UV sunlight.
  • It acts as a protective barrier against water loss, due to the presence of layers of keratin and glycolipids in the stratum carenum.
  • It also is the first line of defense against abrasive activity due to contact with grit, microbes, or harmful chemicals.
  • Sweat excreted from sweat glands deters microbes from over-colonizing the skin surface by generating dermcidin, which has antibiotic properties.


  • The skin contains a variety of nerve endings that react to heat and cold, touch, pressure, vibration, and tissue injury.

Heat regulation

  • The skin contains a blood supply far greater than its requirements which allows precise control of energy loss by radiation, convection, and conduction.
  • Dilated blood vessels increase perfusion and heat loss, while constricted vessels greatly reduce cutaneous blood flow and conserve heat.

Water resistance

  • The skin acts as a water-resistant barrier so essential nutrients are not washed out of the body.


  • Sweat contains urea, however, its concentration is 1/130th that of urine, hence excretion by sweating is at most a secondary function to temperature regulation.

Prevent dehydration

  • The skin forms a waterproof layer all around the body thus preventing excessive loss of water, which is vital for people living in deserts and dry.

Vitamin D synthesis

  • The epidermal layer of human skin synthesizes vitamin D when exposed to UV radiation.

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