BLOOD FLOW AND SKIN COLOR CHANGES
Because blood vessels to the skin are extensive and lo-cated close to the surface, alterations in blood flow can be visually observed as changes in skin color. Color changes are better observed in those persons with light-colored skin and may not be as distinct in those persons with dark-colored skin. You can experiment on yourself or your colleagues and observe these changes. The characteristic pink color or reddish tint of the skin is a result of the oxygenated hemoglobin in the red blood cells. When blood flow is reduced temporarily, the skin becomes pale. If pressure is applied to the skin, the blood in the vessels of the skin, stagnates. Oxygen in the hemoglobin is quickly used by the tissue in the area, and the hemoglobin becomes darker as a result of deoxyhemoglobin formation. When observed through the skin, this reaction gives a bluish hue, termed cyanosis. The bluish discoloration is more prominentin areas where the epithelium is thin, such as the lips, tongue, beneath the nails, and conjunctiva. Watch the color change when you obstruct blood flow by tying a string or rubber band around a finger.
When a person is exposed to a cold environment, the blood vessels to the skin constrict to conserve heat and the person appears pale. If the temperature is low, however, cell injury occurs in exposed areas such as the tip of the nose or ear. The metabolites lib-erated by the injured cells cause the smooth muscle of the of blood vessel walls to dilate, producing the typical redness seen after frostbite.
During exercise, blood vessels in the skin dilate to dissipate heat, while blood vessels in most other parts of the body constrict. This is in response to the hypo-thalamus, which monitors temperature changes. This reflex response overrides all other reflex responses that may be triggered in the blood vessels in the skin.
Draw a pointed object lightly over your skin and ob-serve what happens. The stroke line becomes pale— the white line. The mechanical stimulus causes the smooth muscle guarding blood flow through the cap-illaries, called the precapillary sphincter, to contract. As a result, blood drains out of the capillaries and small veins and the skin turns pale.
Now, draw a pointed object more firmly across the skin and observe. The stroke line turns red in about 10 seconds. This is called the red reaction. In a few minutes, swelling (wheal) and diffuse redness (flare) occur around the stroke line. The red reaction is caused by dilation of capillaries as a result of the stroke pressure. The wheal is a result of the increase in permeability of the capillaries and movement of fluid into the interstitial tissue caused by the release of histamine from mast cells located in the region. The flare is a result of arteriolar dilation. Together, the three responses are known as the triple response and are part of the normal response to injury.
Tie a piece of string (or you may use a rubber band) firmly around your finger. Leave it in place for one minute and then remove it and observe what hap-pens. The skin turns fiery red soon after the occlusion is removed. This is known as reactive hyperemia. When blood flow to an area is restricted, the arteri-oles in that area dilate as a result of the release of chemicals (products of metabolism) by the oxygen-deprived cells. When blood flow is no longer re-stricted, blood rushes into the dilated blood vessels.
At times, the skin appears red after injury, inflamma-tion, or exposure to heat. This redness is a result of dilatation of capillaries in the dermis and is termed erythema.