In response to cold temperatures, the body adapts by restricting blood flow to the skin. This is done as a thermoregulating mechanism to prevent further loss of body heat and to sustain the core body temperature. In Raynaud phenomenon, blood-flow restriction occurs during cold temperatures and emotional stress. Specifically, in Raynaud phenomenon, there is vasoconstriction of the digital arteries and cutaneous arterioles. This phenomenon was first described by Maurice Raynaud in 1862 and later studied by Sir Thomas Lewis in 1930. Overall, Raynaud phenomenon is a transient and peripheral vasoconstrictive response to cold temperatures or emotional stress. Raynaud phenomenon can be categorized as either primary or secondary.