The skin is part of the integumentary system, which is the body’s largest organ system by surface area and weight. This system includes your skin, hair, nails, and several types of glands, all working together as the body’s outermost line of defense. In adults, skin alone covers roughly 1.6 to 1.8 square meters and makes up about 90% of the dermis layer’s thickness in structural tissue.
What the Integumentary System Includes
The integumentary system is more than just skin. It consists of skin, hair, nails, and glands, along with the nerves and blood vessels that support them. The glands fall into several categories:
- Sweat glands (sudoriferous glands) release sweat onto the skin surface or into hair follicles. Eccrine glands open directly onto the skin and cover most of the body. Apocrine glands release their product into hair follicle canals, concentrated in areas like the armpits.
- Sebaceous glands produce an oily substance called sebum, a mix of wax esters, cholesterol, and triglycerides. These glands release sebum through a process that destroys the entire secreting cell, known as holocrine secretion.
- Ceruminous glands in the ear canal produce earwax.
- Mammary glands on the chest are also technically part of this system.
The Three Layers of Skin
Skin itself has three distinct layers, each with a different job. The epidermis is the outermost layer, acting as the primary barrier between your body and the outside world. It keeps bacteria and germs from reaching the bloodstream, blocks UV radiation, and constantly produces new cells. The epidermis also contains melanin, the pigment responsible for skin color, and specialized immune cells called Langerhans cells that help fight off infections.
Beneath the epidermis sits the dermis, which makes up about 90% of your skin’s total thickness. This layer is packed with collagen (a protein that gives skin its strength) and elastin (which keeps it flexible). The dermis is where you’ll find blood vessels, nerve endings, hair follicles, and most of the glands listed above.
The deepest layer is the hypodermis, sometimes called the subcutaneous layer. It’s primarily composed of fat, which cushions your body against impact and helps insulate you from temperature extremes.
How Skin Protects You
The outermost part of the epidermis works like a wall of bricks and mortar. Tough, flattened skin cells (the “bricks”) are held together by proteins and surrounded by a mixture of specialized fats (the “mortar”) that prevents water from escaping. These fats are arranged in a precise lattice: roughly 40 to 50% ceramides, 25% cholesterol, and 10 to 15% free fatty acids. This lipid barrier accounts for about 20% of the outermost layer’s total volume and is essential for keeping skin hydrated and intact.
Inside these cells, a substance called natural moisturizing factor acts as a built-in humectant, pulling in and holding water to prevent the skin from drying out, cracking, or becoming rigid.
Your skin also maintains a slightly acidic surface, with a pH between 4 and 6. This “acid mantle” encourages the growth of beneficial bacteria while discouraging harmful organisms. When skin pH shifts toward alkaline, it becomes more hospitable to pathogens and less effective at defending itself.
Temperature Regulation
Your skin plays a central role in keeping your internal temperature stable. When you’re hot, blood vessels near the skin’s surface widen to bring more warm blood close to the surface, where heat can escape. Sweat glands kick in at the same time, and as sweat evaporates, it cools the blood flowing through those dilated vessels before it circulates back to the core. When you’re cold, those same blood vessels narrow to reduce heat loss. If cooling continues, the brain triggers shivering to generate heat through rapid muscle contractions. This entire process is coordinated by a temperature-sensing region in the brain that monitors both internal and surface temperatures.
Touch and Sensory Detection
The skin contains a dense network of specialized nerve endings that detect different types of stimuli. Some respond to light touch, others to sustained pressure, vibration, temperature changes, or pain. These receptors are distributed across both hairy and hairless skin, though the types and densities vary by location.
For example, some receptors respond quickly to a brief tap and then go quiet, letting you sense movement and vibration. Others fire continuously while pressure is applied, helping you feel when something is resting against your skin. Separate nerve endings detect warmth, cold, and pain. Together, these receptors give your brain a constantly updated map of what’s happening on your body’s surface.
Vitamin D Production
Your skin is the body’s primary factory for vitamin D. When UVB rays from sunlight hit the skin, they break apart a cholesterol-related molecule naturally present in skin cells. The resulting compound then converts to vitamin D3 through a heat-dependent process. From there, the liver and kidneys transform it into the active form that the body uses.
Vitamin D does far more than support bone health. Receptors for it have been found in nearly every tissue in the body, with thousands of binding sites across the genome regulating hundreds of genes. This makes the skin’s role in vitamin D synthesis relevant to a wide range of biological processes beyond calcium absorption.
Constant Self-Renewal
The epidermis replaces itself on a rolling cycle. New cells form at the base, gradually move upward, flatten, and eventually shed from the surface. On average, this full turnover takes about 27 to 28 days, but the timeline varies significantly with age. Teenagers cycle through new skin in as few as 14 to 21 days. Adults typically take 28 to 42 days. After age 50, the process can slow to 45 to 90 days or longer, which is one reason wounds heal more slowly and skin appears thinner with age.