Between you and the rest of the world lies an interface that makes up 16% of your physical weight. This is your skin, the largest organ in your body: laid out flat, it would cover close to 1.7 square meters of ground. Its purpose may seem obvious— to keep our insides in. But a look beyond the surface reveals that it plays a surprising number of roles in our lives. First, the basics.
Skin is the foundation of the integumentary system, which also incorporates your hair, nails, and specialized glands and nerves. Made up of three layers, the epidermis, dermis, and hypodermis, skin’s thickness varies from 0.5 millimeters at its thinnest and up to four millimeters at its thickest. It also carries out three key functions: protecting, regulating, and sensing the world beyond its limits.
On a daily basis, its huge surface processes hundreds, if not thousands, of physical sensations, relying mostly on large, pressure-sensitive skin components called Merkel cells. In your fingertips alone, there are 750 Merkel cells per each square-centimeter of skin, coupled with over 2,500 receptors that give you your sense of touch. This surface is also the body’s first major line of defense. Without it, you’d be a soggy mass of tissue and fluids, fatally exposed to the elements.
Skin effectively seals off your insides and also absorbs pressure and shock with flexible collagen that makes up most of its dermal layer. The epidermis is made up mainly of skin cells called keratinocytes that are completely replaced every four weeks. As new cells form at the base of the epidermis, older ones are pushed up. When these cells move upwards, they’re filled with a hardened protein called keratin. Once they reach the surface, they form a tightly-overlapping, waterproof layer that’s difficult for invading microbes to breach.
Any harmful microbes that make it into the epidermis will encounter Langerhans cells. This group of protective skin cells detects invaders and communicates their presence to resident immune system T-cells, which react by launching an immune response. A crucial feature of this immune defense is the several thousand species of microorganisms that inhabit the planes, folds, and crevices of your skin.
These microbes, which include bacteria and fungi, thrive in the sebum, an oily substance that’s secreted onto the skin’s surface by sebaceous glands nestled inside the dermis. These skin microbes keep the immune system in a state of constant surveillance, ensuring that it’s ready to react if the body really is at risk. Beyond this protective role, your skin is also a sensory organ that helps regulate your body’s temperature, two roles that are closely interlinked.
Nerves detect whether your skin is warm or cold and communicate that information to your brain. In return, the brain instructs localized blood vessels to either expand if the body is too warm, releasing heat from the blood through the skin, or to constrict if the body is cold, which retains heat. At any given time, up to 25% of the body’s blood is circulating through the dermis, making this process extremely efficient. Under warm conditions, the skin’s sweat glands will secrete sweat via ducts onto the surface, transferring heat out of the body. Hair can also be stimulated to conserve or release body warmth.
The average human has 5 million hair follicles embedded everywhere on the body except the palms of your hands and soles of your feet. Ninety to 150,000 of those are on your scalp, where they help shield the large surface area of your head from physical damage and sunburn.
When you’re cold, tiny muscles called arrector pilli cause hair to stand upright across the body. That’s the phenomenon known as goosebumps and it traps body heat close to your skin. Skin’s vast surface isn’t just a shield; it also enables us to interact and connect with the world. Its multifunctional layer cools us down and keeps us warm. The integumentary system may be many things, but it’s certainly more than skin deep.