 Dear students, in this topic we shall discuss the regulatory controls of circulation. The regulation of circulation is necessary to assure adequate supply of blood to the vital organs that is brain and heart. It is also necessary to supply blood to other organs according to their activity levels. It is also necessary to control capillary pressure to maintain tissue volume and composition of interstitial fluid. The regulatory mechanisms focus on controlling the cardiac output and blood pressure. These regulatory mechanisms include auto-regulation, neural mechanisms and endocrine mechanisms. Auto-regulation causes an immediate localized homeostatic adjustment in the pattern of blood flow within the capillary bed. This auto-regulation occurs due to the effect of local vasodilators and vasoconstrictors, which are found in the interstitial fluid. These vasodilators and vasoconstrictors act on the pre-capillary sphincters causing them to open and close. The local vasodilators include various chemicals and various factors. For example, the decreased tissue oxygen level or increased carbon dioxide level acts as a local vasodilator. Similarly, lactic acid generated by tissue cells and nitric oxide released from endothelial cells act as local vasodilators. Moreover, the rising concentrations of potassium or hydrogen ions in the interstitial fluid and elevated local temperature also act as local vasodilators of capillaries in the tissues. The local vasoconstrictors include the prostaglandins and thrombocins, which are released by activated platelets and white blood cells. Apart from this, endothelins, which produce damaged endothelial cells, also act as local vasoconstrictors and control blood supply in the tissues. If blood supply is controlled in a tissue, then neural and endocrine mechanisms are activated. Neural mechanisms respond to changes in arterial pressure and blood gas levels. These changes in arterial pressure and gas levels are sensed at specific sites in the circulatory system. These changes are responded by the cardiovascular centers which are located in the medulla of long eta and pons of the brain. Cardiovascular centers adjust the cardiac output and peripheral resistance and result in the maintenance of blood pressure and adequate blood flow. The endocrine mechanisms include the release of various hormones which cause short-term adjustment in the circulatory patterns that is flow and pressure as well as direct long-term changes in the cardiovascular performance.