Change in blood flow

After the initial injury, there is a transient period of vasoconstriction lasting from a few seconds to a few minutes with severe injuries. Then vasodilatation occurs, predominantly in arterioles. Capillary flow is also increased through the relaxation of pre-capillary sphincters. With decreased resistance, increased blood flow occurs: the clinical swelling and redness result.

After a variable period, usually minutes, the walls of the microcirculation become more permeable to protein-rich fluid. The loss of this fluid results in concentration of the intravascular cellular elements and increased blood viscosity. The slowing of flow facilitates approximation of leukocytes with the walls of the vessels, ready for the next stages.