Local blood flow regulation

There are two major means of local regulation of blood flow, which are described below. • Metabolic control, which consists of metabolites and paracrine agents released from surrounding tissue that act on the blood vessel(s). For example, as tissue metabolism increases, driving up oxygen demand, the amount of available oxygen decreases, driving down the pH and triggering a release in adenosine, which triggers the blood vessel to vasodilate. • Myogenic control, which originates from the wall of the blood vessel itself and consists of both muscle reflexes and products released from endothelial cells that line the vessel. These endothelial products include nitric oxide and endothelin-1 that are released in response to either chemical stimuli, like histamine, or increased shear stress on the blood vessel (meaning the amount of stress exerted by blood on the blood vessel walls). While nitric oxide causes vasodilation, endothelin-1 causes vasoconstriction. == Examples of local blood flow regulation ==

Below are several examples of differing types of local blood flow regulation by specific organ type or organ system. In each case, there is a specific type of intrinsic regulation occurring in order to maintain or alter blood flow to that given organ alone, instead of creating a systemic change that would affect the entire body. • Cerebral (brain) circulation is highly sensitive to changes in pCO2, meaning the amount of dissolved carbon dioxide (CO2) present in the blood vessel, as well as the hydrogen ion concentration. Both of these factors affect pH and, in turn, the balance between vasodilation versus vasoconstriction in the brain. So, the blood vessels found specifically in the brain respond changes in dissolved carbon dioxide levels. • Coronary (heart) circulation is controlled at the local level primarily by metabolic control mechanism. More specifically it is regulated by adenosine, a local vasodilator produced by neighboring cells. • Pulmonary (lung) circulation undergoes hypoxic vasoconstriction, which is a unique mechanism of local regulation in that the blood vessels in this organ react to hypoxemia, or low levels of dissolved oxygen in blood, in the opposite way as the rest of the body. While tissues and organs tend to increase blood flow by vasodilating in response to low oxygen supply, the blood vessels in the lungs actually vasoconstrict to decrease blood flow in response to low oxygen. • Splanchnic circulation, which supplies blood to several gastrointestinal organs (liver, gallbladder, pancreas, intestines) and the spleen, is influenced by gastrointestinal hormones and metabolites, such as vasodilatory kinins, released from the cells lining the intestines, bile acids from the gallbladder, and by products of digestion. == See also ==