Eli5, what is the biological reason lips chap and crack and bleed

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Eli5, what is the biological reason lips chap and crack and bleed

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Same reason any other skin does. They’re exposed to the air and the air can get dry and make them dry. When tissue gets dry it cracks easy.
In the case of lips, friction is also a factor.

Another potential cause of dry tissue that is less common for lips is poor blood circulation.
Blood is the fluid of the body. It carries the water where it goes and causes hydration. If blood vessels become blocked tissue can dry out and crack. And in severe cases causes tissue death (ischemia).

Many people experience this with the skin of the legs later in life, particularly heavy smokers.

They get dehydrated. That’s why chapstick works, it provides a waxy barrier that prevents the wind from dehydrating your lips.

Relative to other primates, our lips are turned inside out. This exposes some of the more tactile-ly and environmentally sensitive tissue to the air. Apparently this is beneficial but it also means that this delicate surface has the potential to chap more readily than other surfaces that are covered with a more heavily cornified (like a thin callous) epidermis.

They are at the very edge of the mucous membrane in the mouth, which is thin and supposed to be moist. But since they’re exposed to the outside they dry out easily and split easily because of how thin they are. This is made worse when you lick your lips to moisten them again, because your saliva contains enzymes that actually partially digest your skin.

As some others have more or less mentioned, ephitelium (“””skin”””) of your lips is not the same as your fingers. Your “general” skin has a very thick protective layer called stratum corneum, it’s basically made of dead skin cells full of queratine, and they work as a barrier. It’s called queratinized ephitelium.
On the contrary, your lips are “unqueratinized”, like your buccal mucosa, meaning that they lack that last protective layer, that’s why they are less protected and they suffer more, in terms of humidity, friction…
Hope that helped!