Cyanosis is a medical problem. When a person has cyanosis, their skin and mucous membranes turn blue or purple. Cyanosis is a sign that the cyanotic (blue) parts of the body are not getting enough blood and oxygen. This can happen because there is not enough oxygen in the blood. It can also happen when small blood vessels in the body get smaller because of a medical problem (this is called vasoconstriction).
Usually cyanosis is first seen in the extremities (the parts of the body that are farthest away from the heart). Usually the first parts of the body to become cyanotic are the fingertips (especially under the fingernails), the toes, the lips, the tip of the nose, and the earlobes. As the body goes longer without oxygen, more of the body becomes cyanotic.