A runner works out her eccrine glands.

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Pores, Sebum and Sweat Glands

We have pores all over our bodies, tiny holes that contain hair follicles and act as outlets for sweat and sebum to leave the body. Sebum is an oily substance that covers much of our body in a thin protective layer that both regulates body heat and makes it hard for bacteria to settle on the body's surface. Sebum is made by your body's sebaceous glands, which can be found everywhere on your body except the bottoms of your feet and the palms of your hands. These glands are most prevalent on our faces, backs, chests and groin areas.

Pore size varies, but your genes and your age are two big determinants for how large they'll be. People with large pores tend to have oily skin, and as skin ages, sun damage and the loss of collagen leads to bigger-looking pores. When your pores are clogged with dead skin cells and other debris, they can appear larger, too.

Like the size of our pores, whether our skin is dry or oily is often determined by genetics. Hormones also play an important role. Since our hormones activate sebum production, changes to our hormones can affect the level of oil our skin produces. In addition to puberty, menstruation and pregnancy also affect women's hormone levels and consequently the production of extra sebum [source: Bouchez].

Why does that level of oil matter? Because excess oil, combined with dead skin cells and bacteria, can lead to acne. When oil is unable to exit through the hair follicles because they're clogged with accumulated dead skin cells and sebum, you end up with a breakout. The extra skin cells and sebum form a small blockage beneath the surface of the skin that pushes outward, often in the form of small, upraised red bulges with white centers -- or what we (not so) fondly refer to as pimples.

Our body's other main secretion, sweat, exits the body through two different types of glands.

Eccrine glands are the most prevalent and are located all over the body. The sweat produced by them exits through the skin's pores, and is of the non-stinky variety. Apocrine glands are located in our armpits and groin-anal area. They begin producing sweat during puberty, which exits the body through hair follicles.

Next, let's find out why different people have different skin colors.