The skin is the largest organ of the human body. Expanding to around 20 square feet, it makes up approximately 16% of our bodyweight. The thickness of skin will vary depending on a person’s age, skin health and the particular site on their body. On average it is 1mm thick, with the thinnest skin on the eyelids (0.5 mm) and the thickest on the palms and soles (1.5 mm). Skin consists of multiple layers, the epidermis (outer layer), dermis (middle layer) and hypodermis (deepest layer).
Our skin is the major interface between our internal organs and the environment, so it serves many specialised functions which keep us alive. It keeps our organs contained and, to some extent, shields them from physical injury. Skin protects us from damage by a number of noxious substances, such as UV light, heat and micro-organisms (bacteria and viruses). It is also the most extensive sensory organ of the body, containing nerve endings which detect tactile (touch), thermal (temperature) and pain stimuli. The skin serves many important immune functions. It possesses several types of immune cells (i.e. Langerhans cells) which act to identify, destroy and eliminate foreign invaders and heals wounds to keep the physical barrier intact.
Skin regulates body temperature in several ways. When we get too hot it produces sweat which cools the body as it evaporates. In hot weather, the tiny capillaries in our skin also dilate (expand) to allow more blood to flow closer to the body surface where it can cool. Conversely, when we get cold, these same vessels constrict to keep the blood closer to the warm centre of the body. Muscles in our skin (erector pili muscles) also make fine hairs stand up on end. The erect hairs trap a layer of warm air close to the skin which helps to retain heat.
Water loss through the skin is partly controlled by its structure – the overlapping, tight-knit arrangement of cells and the lipids (fats) they contain. The amount of water held in each of the skin layers also changes depending on its availability, a mechanism which further conserves water to protect against dehydration.
The skin maintains the balance of fluids and nutrients in our bodies by secreting sweat containing water, various minerals and trace elements. Some substances are stored in the skin, such as fats and water, and small amounts of gases (i.e. oxygen, nitrogen and carbon dioxide) can be absorbed into the outer layer. A fraction of our waste products are also removed from the body across the skin surface (i.e. urea and lactate).
Vitamin D is a nutrient which is essential in small quantities for good health, including bone growth and the action of nerves and muscles. Its production begins in our skin when we are exposed to the UVB radiation in sunlight.
For more information on skin, watch our ‘What is skin?’ video on YouTube.
A cross section of skin highlighting keratinocytes, the epidermis and dermis (not to scale)