Covering an average of 20 square feet, the skin is the body’s largest and heaviest organ. Its most obvious job is to protect our insides from the outside, but there is much more to the skin than that.
Alongside its role as a protective barrier, the skin helps us maintain the right internal temperature and allows us to sense the world through nerve endings.
Skin is a complex organ; an average square inch of skin contains 650 sweat glands, 20 blood vessels, and more than 1,000 nerve endings. Despite being just a few millimeters thick, skin makes up around one-seventh of our body weight.
In this article, we will cover the basics of skin, how it is constructed, what it does, and how it does it.
The skin has three basic levels — the epidermis, the dermis, and the hypodermis:
Main roles: makes new skin cells, gives skin its color, protects the body.
The epidermis is the outermost layer; it is a waterproof barrier that gives skin its tone.
Dead cells are shed continuously from the epidermis as new ones take their place.
We shed around 500 million skin cells each day. In fact, the outermost parts of the epidermis consist of 25–30 layers of dead cells.
New cells are made in the lower layers of the epidermis. Over the course of around 4 weeks, they make their way to the surface, become hard, and replace the dead cells as they are shed.
Keratinocytes are the most common cell type within the epidermis; their job is to act as a barrier against bacteria, parasites, fungi, viruses, heat, ultraviolet (UV) rays from the sun, and water loss.
The epidermis contains no blood vessels.
The color of our skin is produced by a pigment called melanin, which is produced by melanocytes; these are found in the epidermis and protect the skin from UV rays.
The epidermis is subdivided into five layers:
- stratum corneum
- stratum lucidum
- stratum granulosum
- stratum spinosum
- stratum germinativum
Between the epidermis and the dermis is a thin sheet of fibers called the basement membrane.
Main roles: makes sweat and oil, provides sensations and blood to the skin, grows hair.
The dermis is mostly connective tissue, and it protects the body from stress and strain; it gives the skin strength and elasticity. If the dermis is stretched a lot, for instance, during pregnancy, the dermis can be torn, and this shows up as so-called stretch marks.
Receptors that detect pressure (mechanoreceptors), pain (nociceptors), and heat (thermoreceptors) are based in the dermis.
The dermis houses hair follicles, blood vessels, and lymphatic vessels. It is also home to a number of glands, including sweat glands and sebaceous glands, which produce sebum, an oil that lubricates and waterproofs hair.
The dermis is further split into two layers:
Papillary region: made of loose connective tissue, it has finger-like projections that push into the epidermis. These projections give the dermis a bumpy surface and are responsible for the patterns we have on our fingertips.
Reticular region: made of dense, irregularly organized connective tissue. Protein fibers in the reticular region give skin its strength and elasticity.
Main roles: attaches dermis to the body, controls body temperature, stores fat.
The deepest layer is called subcutaneous tissue, the hypodermis, or subcutis. It is not technically part of the skin but helps attach the skin to underlying bone and muscle. Subcutaneous tissue also provides skin with nerves and blood supply.
The hypodermis is mostly made of fat, connective tissue, and elastin (an elastic protein that helps tissues return to their normal shape after stretching). The high levels of fat help insulate the body and prevent us from losing too much heat. The fat layer also acts as protection, padding our bones and muscles.
Some hormones are made by fat cells in the hypodermis, vitamin D, for instance.
Some of the many roles of skin:
- Protection: against pathogens. Langerhans cells in the skin are part of the immune system.
- Storage: stores lipids (fats) and water.
- Sensation: nerve endings detect temperature, pressure, vibration, touch, and injury.
- Control water loss: the skin prevents water from escaping by evaporation.
- Water resistance: it prevents nutrients from being washed from the skin
- Thermoregulation: by producing sweat and dilating blood vessels, the skin helps keep the body cool. “Goosebumps” and blood vessel constriction, help us retain heat.
Human skin color can vary from almost black through to almost white. Most of this variation is due to a pigment called melanin. It is worth noting that the coloration of light skin is mostly determined by the whitish-blue color of connective tissue below the dermis and hemoglobin in the veins of the dermis.
Melanin’s primary role is to protect the skin from damaging UV light from the sun, which can cause skin cancer. When skin is exposed to UV light, melanocytes start producing melanin, creating a suntan.
Populations that live in parts of the world that receive higher levels of UV light, for instance, nearer the equator, tend to have higher levels of melanin and, therefore, darker skin. Conversely, populations that receive less sunlight (toward the poles) tend to have lighter skin with less melanin.
In general, females have lighter skin than males. This may be because women need more calcium during pregnancy and while breast-feeding; vitamin D, which is produced when skin is exposed to the sun, is important for absorbing calcium.
As with any other organ of the body, the skin is susceptible to certain diseases; these include:
Atopic dermatitis: also known as eczema, this is an inflammatory skin disease characterized by dry, red, itchy patches of skin.
Acne: this is perhaps the most common skin disorder. It occurs when hair follicles become clogged with dead skin cells and oil.
Melanoma: a type of skin cancer caused by exposure to excess sunlight.
Rosacea: a common rash found in middle-aged people. They have a tendency to flush and have small red bumps on the center of the face.
Psoriasis: this is another inflammatory skin disease. It causes red, flaky patches to appear on the skin.
Scabies: an itchy skin condition caused by the human scabies mite.
Shingles: also called herpes zoster, it is a painful blistering rash caused by a virus.
Lichen planus: an itchy non-infectious rash. The bumps have flat shiny tops.
As we get older, our skin changes; it becomes thinner and more easily damaged. Also, the process of healing slows. Overall, we have less skin, and it is less elastic.
There are a number of reasons why the skin goes through these changes. One important factor is exposure to UV rays, which also increases the risk of skin cancer.
In a nutshell
The skin is a large, complex organ with a wide range of vital roles. From protecting us from pathogens to helping us maintain the right temperature, we certainly couldn’t do without our skin!