Blood vessels in the skin play a critical role in regulating body temperature.  Blood from the heart enters the aorta.  From there blood is delivered to smaller arteries and then arterioles in the skin.  These arterioles contain smooth muscle that allows them to constrict or dilate in response to nerve signals from the body.  Blood exiting the arterioles enters the smallest type of blood vessel called a "capillary."  Capillaries are so small that you need a microscope to see them.  In fact, the smallest capillaries only allow red blood cells to travel one-by-one in single file!  Capillaries drain into the smallest veins called venules and this blood is eventually delivered back to the heart.

How do blood vessels in the skin keep us cool?

When the environment is hot and the body is generating too much heat, this heat needs to be removed from the body.  Arterioles in the skin open wide, allowing blood to flow freely close to the surface of the skin.  This is called vasodilation.  Remember, blood is the same temperature as the rest of the body (typically 98.6 degrees Fahrenheit).  As long as the outside temperature is lower than this, the heat from the blood can be released into the environment.  For outside temperatures greater than 98.6F, sweating is required to remove heat. You can see this process in action.  The next time you find yourself in the hot sun or when you are exercising, look at your skin.  Light skinned people will notice that the skin has a more pink appearance.  People of all skin colors can feel the warmth radiating from the skin.

How do blood vessels in the skin keep us warm?

When the environment is cold, the body needs to conserve heat.  Arterioles near the skin contract, limiting the blood supply close to the surface of the skin.  this is called vasoconstriction.  This prevents unnecessary body heat from being delivered to surface of the skin.  You may have noticed that your fingers and toes are the first body parts to get cold when it is cold outside.  This is because the body knows that it is more important to keep the vital organs warm (heart, lungs, kidneys and brain).

Why do we turn pale when we are scared or sick?

The body has a procedure to deal with stressful situations called the "fight or flight" response.  When we are scared (imagine running into angry bear in the woods), the body sends nerve signals to the skin arterioles telling them to contract (vasoconstriction).  This allows the blood in the body to stay close to more important body organs, such as the muscles needed to fight the bear or run away!  I suggest running away!  

When we are sick, the body has a similar response.  Blood and heat are required to help the body heal.  The skin is less important than the vital organs in this situation also.  This is why you look pale when you are sick.

What happens when blood vessels near the skin are damaged?

When these blood vessels are damaged, they can leak clear fluid (causing swelling) or they can leak blood (causing bruising).  Bruises appear when blood leaks into the surrounding tissues.

When pressure is applied to the skin, you can temporarily make the skin look pale.  If this pressure stays too long, the skin will die and form a pressure ulcer.

Photo by Dan Ferber, scanning electron microscope picture of capillaries, 12 June 2007

Illustration credit http://training.seer.cancer.gov/module_anatomy/images/illu_capillary.jpg

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