Control of capillaries
Local control of blood flow – Capillaries regulate the local blood supply to the tissues according to the need of the tissues.
Micro circulation
Metabolic control of blood flow to an organ is the most important local control mechanism. Increased blood flow in tissues in response to increased metabolic rate is called as active hyperemia.
When the metabolic rate of a tissue increases, its O2 consumption also increases leading to increased rate of production of metabolic products like CO2, adenosine and lactic acid. Simultaneously, O2 concentration decreases in the interstitial fluid. This causes the dilatation of arterioles, relaxation of precapillary sphincters and opening of more capillaries to facilitate increased blood flow to the tissues to deliver more O2 and removal of accumulated metabolic products.
Autoregulation of blood flow is also a metabolic control phenomenon. If metabolic rate of an organ does not change but arterial-venous pressure is increased above normal, additional blood flow to that organ is achieved due to increased pressure. The additional blood flow accelerates removal of metabolic products and increases oxygen delivery to the tissues. Hence, the concentration of metabolic vasodilator products decreases. These changes cause the arterioles to constrict and blood flow returns to normal.