Layers of Skin
Skin is made of two main layers:
Epidermis (outer layer):
- Thin protective covering
- Top cells are dead and tough — they form a waterproof barrier
- Contains melanin (the pigment that gives skin its colour and protects against UV radiation)
- New cells are constantly made at the base and push upward to replace the old ones
Dermis (inner layer):
- Thicker layer beneath the epidermis
- Contains sweat glands, hair follicles, blood vessels, and nerve endings
- Gives skin its strength and flexibility
Below the dermis is a layer of fat that insulates the body and cushions organs.
You shed and replace the entire outer layer of your skin roughly every 2–4 weeks. The dust in your home is partly made up of shed skin cells!
What Skin Does
Skin carries out several vital functions:
| Function | How |
|---|---|
| Protection | Acts as a barrier to bacteria, chemicals, and UV light |
| Temperature regulation | Sweat glands release sweat that cools you; blood vessels widen or narrow to lose or keep heat |
| Sensation | Nerve endings detect touch, pressure, pain, temperature |
| Vitamin D production | Sunlight on skin triggers vitamin D synthesis |
Skin and Temperature Control
When your body gets too hot:
- Sweat glands produce sweat → evaporation cools the skin
- Blood vessels near the surface widen (vasodilation) → more blood near the skin → heat escapes
When your body gets too cold:
- Blood vessels narrow (vasoconstriction) → less blood near surface → less heat lost
- Tiny muscles at the base of each hair pull the hairs upright — goosebumps — trapping warm air (more effective in furry animals than in humans)
Skin is an organ — just like the heart or lungs. In fact, it is the largest organ in the human body, covering about 2 m² in an adult.
Real world: Sunscreen and skin protection
Sunscreen protects the skin from UV radiation by absorbing or reflecting UV rays before they damage melanocytes (pigment cells) and the DNA of deeper skin cells. Melanoma — the most dangerous form of skin cancer — results from UV damage to skin cells' DNA. Applying SPF 30+ sunscreen in summer is a direct real-world response to understanding the epidermis's role and its limits in UV protection.
Real world: Sweating and temperature regulation during exercise
During a 5 km run on a warm day, the body can produce several litres of sweat. Sweat glands (in the dermis) push water and salt to the skin surface, where evaporation removes heat energy. Simultaneously, blood vessels near the skin dilate (vasodilation), bringing hot blood close to the surface to lose heat to the air. This is the temperature regulation system of the skin operating at full capacity — visible as flushed, sweaty skin.
Real world: Burns and skin repair
Severe burns destroy both the epidermis and dermis, removing the protective barrier, sweat glands, nerve endings, and hair follicles. Patients lose the ability to regulate temperature, are vulnerable to infection, and feel no pain in the burned area (nerves destroyed). Skin grafts transplant undamaged skin from elsewhere on the body to cover the wound. This treatment is only necessary because skin's functions are so critical.
Where do you see this in real life? On a cold day, look at the skin on your arms — you'll likely see goosebumps and the hairs standing upright. This is your arrector pili muscles (in the dermis) pulling hairs up to trap warm air. Your skin is also paler than normal because blood vessels have narrowed (vasoconstriction) to reduce heat loss — temperature regulation in visible action.