Your skin even plays a part in nutrition because, when it is exposed to sunlight, it makes vitamin D.
Over most of your body your skin is about 2mm thick, though over your eyelids it is only about 0.5mm thick and on the soles of your feet, where it gets a lot of wear, it is about 6mm thick. The skin is arranged in two main layers: the outer epidermis and the inner dermis.
The cells at the bottom of the epidermis are constantly dividing and pushing the ones above them up towards the surface of your skin. As the cells move further away from the blood vessels in the dermis, they die through lack of food and oxygen. All that remains is a hard protein called keratin. When they reach the surface (after about three weeks), the dead, hardened cells form a strong protective covering for your body. They then get worn away as you come into contact with things, and more new cells come up to take their place. Dandruff is an extra accumulation of cells from your head.
Blood vessels in the dermis supply your skin with food and oxygen. When you are hot, the blood vessels widen so that more blood can flow near the surface of your skin and be cooled by the air outside your body. This is why you look red when you are cold. When you are cold, the blood vessels narrow to prevent heat loss and you look paler.
Your epidermis contains special cells which produce a dye or “pigment” called melanin. The more melanin you have, the darker your skin color. In strong sunlight extra melanin is produced to protect your skin. This is why people whose races originated in hot and sunny climates have darker skin and why people tan in the sun. Freckles are small patches of skin which make more melanin than the surrounding area.
Sweat consists mainly of water and salt which are absorbed into your sweat glands from nearby capillaries. Sweat is constantly passing up to the surface of your skin and coming out through your pores. As it evaporates (dries up) on your skin, you become cooler. When you are hot, more sweat is produced to cool you down more.
Your hair grows out of pits, known as follicles, in the epidermis. Cells at the root of the hair divide and push it upwards. As it grows further away from the blood supply, it dies and becomes hardened by keratin. Having your hair cut is painless because the hair is dead. About every two years, the cells in the follicle stop dividing and the hair falls out. You are losing hair from your head at the rate of about 50 a day. After a few months rest, the cells start to divide again and a new hair grows. When this does not happen, people eventually go bald. The color of your hair depends on the amount of melanin in the cells. The shape of the follicles determines whether it is straight, wavy, or curly.
Opening into your hair follicles are small groups of cells called sebaceous glands. These produce an oily substance called sebum, which coats your hair and surrounding skin, helping to keep them waterproof and supple. There are no sebaceous glands in your fingertips and toes, which is why they sometimes wrinkle up in the bath.
Each of your hair follicles has a tiny muscle attached to it. You get goose pimples because these muscles contract and make your hair stand on end when you are cold. Goose pimples are not much use to humans, but in animals with fur, they trap air and this helps to keep them warm.
Nails are the remnants of claws and are formed in a similar way to hair. Each one develops from a row of dividing cells called a root. Your nails grow about 0.1mm a day and the part you can see is made of dead cells hardened by keratin.
You have hair over almost the whole of your body. The palms of your hands and the soles of your feet are without hair but are covered with tiny ridges instead. This makes them extra sensitive. The patterns made by the ridges on your fingers are your fingerprints. They are formed months before you are born and no two people’s are identical.