Head lice need to live on a human scalp in order to be able to feed on the human blood that they need to survive for their 30 or so days of life. When the head louse needs food, it moves around very close to the skin and checks it out until it finds a suitable vein. The head louse then pierces the skin with its mouthparts and adds saliva to the wound, to prevent the blood from clotting while the head louse eats. The louse ingests the blood via two small pumps in its head. As the head louse is partly transparent, you can actually see a thin thread of blood running from its mouth as it is pumped through the head and on into the intestine.
Unlike body lice, head lice need food relatively often, thus they never leave their hosts. How often they need blood is not fully known. We do know, however, that head lice, which feed only four times in 24 hours, will die after a few days. Therefore, the head lice probably have to eat at least six times in 24 hours in order to survive.