When blood vessels constrict at the surface of the skin, little blood is able to flow there. A purple tinge to the skin indicates a lack of oxygen caused by restricted blood flow to the region. There are a number of reasons why blood vessels at the surface of the skin may constrict, and it is not uncommon for this to happen during swimming, especially in the hands and feet.
Water Temperature
When a person is swimming or completely immersed in cool water, he begins to lose body heat, and the body fights this by retaining blood flow in the core and restricting blood flow to the extremities. The hands are considered an extremity -- an end point to a limb or body part -- and when the body's surroundings are cool, blood flow to the extremities is restricted to allow blood flow and heat to be concentrated within the body's core. Blood vessels in women begin to constrict at the extremities at 70 degrees Fahrenheit. In men, these blood vessels constrict at 67 degrees.
Stress
Stress is another factor that may cause blood vessels to constrict at the extremities. Stress stimulates the sympathetic nervous system -- whose job it is to incite a fight-or-flight response and make the most of the body's energy -- and the production of adrenaline. Therefore, if swimming is a source of stress for someone, it may cause blood vessels to constrict at the surface of the skin and the hands to turn purple.
Diet
Certain foods may cause you to be more sensitive to the lower temperatures that may be encountered while swimming. Foods that may cause constriction of the blood vessels include cold foods and foods rich in tyramine such as chocolate, cheese, salami, caffeine and beverages with alcohol. Dehydration and an excess of calcium also encourage blood vessel constriction. Foods rich in omega-3 fatty acids, such as oily fish and walnuts, discourage constriction of the blood vessels, so they may help to prevent purple hands while swimming.
Medications and Recreational Drugs
Medications that may trigger constrictions of the blood vessels under certain conditions that may be present while swimming include antihistamines, amphetamines, decongestants and hypotensive drugs. Recreational drugs that may cause this same effect include cocaine, caffeine, LSD, amphetamines and tobacco.
Reynaud's Syndrome
Reynaud's syndrome is a rare disorder of the arteries. Blood flow to the extremities can be severely restricted as a result of cool temperatures, stress or no reason at all. In extreme cases, the fingers turn white after they turn purple, as blood flow continues to decrease.
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