We can’t afford to lose any blood. Even though a healthy person can lose as much as one third of all his blood and stilt live, a steady loss of blood, or loss of blood while we are sick, would be very dangerous.

Nature has protected us against this danger by giving blood the capacity to clot. If this clotting took place inside our circulatory system, it would be equally dangerous. So blood does not coagulate, or clot, when it is in contact with the-smooth walls of the blood vessels.