Central cyanosis occurs when the oxygenation of arterial blood in the lungs is reduced or when excessive amount of an abnormal haemoglobin derivative like methaemoglobin is present in the blood. Since the defect is in the central mechanism, cyanosis can be seen simultaneously in the mucous membranes as well as on the skin of the peripheral parts of the body.
Peripheral cyanosis is due to the abnormal excessive extraction of oxygen from the normally saturated arterial blood in the peripheral circulation. This usually results from diminished or sluggish peripheral blood flow due to vasoconstriction (e.g. exposure to cold, peripheral vascular disease, etc.). Hence cyanosis can be seen only on the skin of the affected part of the body where there is vasoconstriction.
Cyanosis occurring in conditions like cardiogenic shock is due to both the above mechanisms of production of cyanosis, and this type of cyanosis is referred to as mixed cyanosis.