Red blood cells provide oxygen to body tissues. Most of the time, nearly all red blood cells in the arteries carry a full supply of oxygen. These blood cells are bright red and the skin is pinkish or red.
Blood that has lost its oxygen is dark bluish-red. People whose blood is low in oxygen tend to have a bluish color to their skin. This condition is called cyanosis.
Depending on the cause, cyanosis may develop suddenly, along with shortness of breath and other symptoms.
Cyanosis that is caused by long-term heart or lung problems may develop slowly. Symptoms may be present, but are often not severe.
When the oxygen level has dropped only a small amount, cyanosis may be hard to detect.
In dark-skinned people, cyanosis may be easier to see in the mucous membranes (lips, gums, around the eyes) and nails.