Re: why does the skin go red?

Dear Becky-

Thank you for an interesing question! In order to answer your question, I need to tell you a bit about circulatory system physiology, and then get to your question. Your cardiovascular system is composed of arteries, capillaries and veins, connected to the heart, which moves the blood around the system. Blood travels from the left ventricle through the aorta (the largest artery), then on through smaller arteries, then arterioles, then capillaries (where gas and waste exchange occurs with the tissues of the body), then venules, then veins, and back to the heart. Blood will then take a loop through the lungs, and then back to the left atrium and down into the left ventricle of the heart. Then the process repeats, providing oxygen to the cells of the body. Now here's the interesting part: the average person has about 5 liters of blood in their body, and vessels that are estimated to stretch 100000 km. There is actually not enough blood in the body to fill all of the vessels to full capacity at any one time. This may sound surprising, but it is extremely evident in circumstances such as toxic shock syndrome, where bacterial products cause vasodilation in the whole circulatory system. When this systemic vasodilation occurs, the blood pressure drops (because now you're spreading the same amount of blood over more vessels - think of pouring 5 liters through a 5 inch wide hose rather than a 1 inch wide hose), and the patient goes into shock.

So why aren't we in shock all of the time? Well the answer is arterioles. Arterioles regulate the amount of blood that flows to the tissue they serve. If the tissue is not very metabolically active, such as muscle when you're sitting still or sleeping, the arteriole will constrict, reducing blood flow. If the tissue is very metabolically active, such as that same muscle when you're exercising, the arteriole will relax, increasing blood flow.

How does the arteriole know when to contract or relax? Well, when a tissue has run out of oxygen, the pH drops, interstitial potassium increases, carbon dioxide levels increase and adenosine increases. These changes are sensed by the vascular smooth muscle of the arteriole, which then relaxes, allowing more blood to enter the tissue.

Now, finally, for your question: Why does skin turn red when you've put pressure on it? Well, when you put pressure on an area of tissue, you cut off the blood flow to that area by physically collapsing the vessels (you don't completely, because of other factors, including collateral flow) depriving that tissue of oxygen. The tissue says "Hey!! I've run out of oxygen!!" and asks the arterioles to dilate. The arterioles dilate, but no more blood enters due to the physical blockage, so the arterioles dilate more. When you finally change position and allow blood flow into the area, the arterioles are wide open, allowing blood flow (and red color) into the area, almost as if the tissue were blushing.

I hope that answers your question!
Ingrid
MADScientist References: Lilly, Leonard S., Ed. Pathophysiology of Heart Disease, 2nd Ed. Baltimore: Williams & Wilkins, 1998. Campbell, Neil A., Biology, 3rd Ed. New York: The Benjamin/Cummings Publishing Co., Inc., 1993. For more information on cardiovascular biology, check out:
http://www.cardiovascular.cx/
McGill site
Oxford site