The skin is the biggest bit of flesh you own in both square inches and weight, protecting you from the outside-in and inside-out. Skin isn’t just the sausage casing that holds us together – it also keeps our temperature steady, and is also home to a lot of nerve endings so that we can sense the world, making up one-seventh of our body weight.
One inch of skin contains about:
- 650 sweat glands
- 20 blood vessels
- over 1,000 nerve endings
Skin layers and what they do
The three main layers of skin are the epidermis, dermis and hypodermis.
The epidermis, made up of five sublayers, makes new skin cells, provides colour (pigment, melanin), and protects the body from the outside, being the outer layer of skin. The epidermis is waterproof, sheds skin cells (500 million per day!), and produces keratinocytes to protect us from microbes, heat, UV rays, and water loss. The epidermis is actually 25-30 layers of dead skin cells, which are sloughed off after their four-week growth phase.
There is a membrane – the basement membrane – between the epidermis and the next layer down, the dermis.
The dermis’ role is to make sweat and oil, provide sensations and blood, and grow hair. This layer is mostly connective tissue, protecting us by providing strength and elasticity to the skin. When the dermis is stretched beyond its abilities, it creates stretch marks, like during pregnancy or puberty. We have a lot of nervous system work going on in the dermis – pressure detectors, pain receptors, and heat receptors.
Also in the dermis are our hair follicles, blood vessels, lymphatic vessels, and sweat and sebaceous glands. The dermis has two sublayers.
Hypodermis (subcutaneous tissue, subcutis)
This layer is where fat is stored, but also where the dermis attaches to our bodies and where our body temperature is controlled from. The hypodermis is not technically part of the skin, but it is an critical element in how skin works, so we add it in as a layer; the hypodermis is essential for the skin to function, providing nerves and blood supply. The hypodermis plays several roles in, for example, how fat cells near the skin produce vitamin D.