Explainer: What is skin? | Science News for Students

Explainer: What is skin?

The body’s biggest organ serves as a soft, flexible type of armor
Jan 22, 2016 — 7:15 am EST

The largest organ on your body is the skin that serves as a flexible barrier to the outside world.


The human body’s largest organ — skin — is active, living tissue. It serves as tough but flexible armor to keep harmful microbes, chemicals or strong rays of light away from more sensitive inner tissues. At the same time, nerves within the skin relay important information about the world around us by sensing pain, textures and temperatures.

The skin you scrub every day in the bath or shower is only the outermost layer, called the epidermis (Ep-ih-DER-mis). The epidermis is constantly shedding dead cells from its surface as new ones grow to take their places. Beneath that outer layer, the dermis contains blood vessels. An even deeper layer is called the subcutis (Sub-KEW-tis). It stores reserves of fat that act as a cushion to help protect muscles and bones from bumps and falls. 

Look closely at your nose in a mirror and you’ll see what looks like tiny pits on the skin. These are pores. The epidermis hosts some 5 million of them. Hairs grow from the dermis up and out of each pore. (Most of these pores and hairs are too small to see.) Organs called glands sit near the bottom of each hair. Some of these glands produce sweat to help cool the skin. Others pump sebum (SEE-bum), an oily substance, up to the outer surface of the skin. Sebum is important for skin health. It forms a protective barrier that holds in moisture and locks out many disease-causing microbes.

A clogged pore that hasn’t closed up completely can form a tiny pimple called a blackhead. A whitehead happens when the pore seals up and swells with inflammation. When this happens, some people may even develop hard lumps underneath, called nodules, or oozing pus-filled sores.

Teens going through puberty get pimples, known as acne, more often — and more severely — than anyone else. Blame hormones, those chemicals that are orchestrating the body changes that will transform a child into an adult. These hormones tend to make glands in the skin boost their production of sebum. That bonus oil means there’s a higher chance that pores will clog. What’s more, bacteria known as P. acnes, live on the skin of people. These germs dine on sebum. And some types of this bacterium promote the development of pimples. So the more of this greasy substance that builds up on the skin and in the pores, the more of these germs that can grow. This could promote the development of unsightly zits.

skin anatomy



Power Words

(for more about Power Words, click here)

acne    A skin condition that results in red, inflamed skin, commonly called pimples or zits.

bacterium (plural bacteria)  A single-celled organism. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals and humans.

blackhead    A type of pimple that forms when oil, dead skin and bacteria build up near the top of an open skin pore, pushing it open. Oxygen in the air causes the oil clogging the pore to blacken.

dermis  The middle layer of skin, which contains blood vessels to feed skin cells and keep them warm.

epidermis   The outermost layer of skin, which provides a moisture-holding barrier to germs and physical dangers.

gland  A cell, a group of cells or an organ that produces and discharges a substance (or “secretion”) for use elsewhere in the body or in a body cavity, or for elimination from the body.

hormone   (in zoology and medicine)  A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body. (in botany) A chemical that serves as a signaling compound that tells cells of a plant when and how to develop, or when to grow old and die.

inflammation  The body’s response to cellular injury and obesity; it often involves swelling, redness, heat and pain. It is also an underlying feature responsible for the development and aggravation of many diseases, including acne.

microbe   Short formicroorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

nodules   A small roundish bump or growth from the surface of a rock or other surface. (in dermatology) Large and painful pimples that are deep in the skin and feel hard to the touch.

pore    A tiny hole in a surface. On the skin, substances such as oil, water and sweat pass through these openings.

sebum   A type of oil secreted by glands (sebaceous glands) in the skin. It helps keep the skin moist and healthy.

subcutis    The fatty, innermost layers of skin, sometimes called the subcutaneous or hypodermis.

whitehead   A type of pimple that forms when oil, dead skin and bacteria block a skin pore completely.

zits   A colloquial term for the pimples caused by acne.

Further Reading

Learn more about acne from the National Institutes of Health.