Control of Blood Flow Dr. Yasir M. Khaleel, M.Sc., PhD The College of Medicine, University of Mosul

Local (intrinsic) Control of Blood Flow 1. Autoregulation Examples: heart, brain, kidneys Decreased perfusion leads to vasodilation to restore blood flow. 2. Active hyperemia Blood flow to an organ is proportional to its metabolic activity. e.g. During exercise….. More blood flow to exercising muscles. 3. Reactive hyperemia Blood flow to an organ will be increased after a period of occlusion.

Mechanisms that explain local control of blood flow I- Myogenic hypothesis This can explain (Autoregulation), but not active or reactive hyperemia. - Vascular smooth muscles contract on stretching e.g. increase blood flow will stretch vascular smooth muscles which contract in response (vasoconstriction)

II- Metabolic Hypothesis Tissue supply of oxygen is matched to its demand for oxygen Vasodilator metabolites (CO2, H+, K+, lactate, adenosine) produced as a result of metabolic activity of the tissues e.g. during exercise, muscle demand for O2 is increased and accumulation of metabolites also increased, this lead to vasodilation and increased blood flow. e.g. Increased blood flow to certain organ due to increased O2 demand will washout vasodilator metabolites, which then lead to vasoconstriction and return blood flow back to normal.

Hormonal (Extrinsic) Control of Blood Flow I- Sympathetic innervation of vascular smooth muscles Inc symp tone = vasoconstriction Dec symp tone = vasodilation The density of symp innervation varies widely among tissues * Skin has greatest innervation * Least innervation in Coronary, Pulmonary, and Cerebral vessels

II- Other vasoactive hormones 1. Histamine; causes arteriolar dilation & venous constriction It increases Pc, so increase filtration leading to local edema 2. Bradykinin; causes arteriolar dilation & venous constriction ( similar to histamine ) 3. Serotonin (5HT); causes arteriolar constriction and it is released in response to blood vessel damage to prevent blood loss. 4. Prostaglandins (PG); Prostacyclin, vasodilator of several vascular beds E-series PG, vasodilators F-series PG, vasoconstrictors TXA2, vasoconstrictor

Regulation of blood flow in the Coronary Circulation Coronary Cir exhibit Autoregulation, active and reactive hyperemia . It is almost entirely controlled by local metabolic factors Hypoxia and adenosine play essential role e.g. increase contractility…. Inc O2 demand…. Inc coronary blood flow (active hyperemia) During systole…. Compress coronary vessels (hypoxia)…. followed by increased coronary blood flow during diastole

Regulation of blood flow in the Cerebral circulation It is almost entirely controlled by local metabolic factors - Most important local vasodilator in cerebral circulation is CO2 and H+ - It exhibit autoregulation, active and reactive hyperemia .