Local Blood Flow Regulation - Intrinsic Mechanisms
- Intrinsic mechanisms of local blood flow regulation contribute to the precise matching a tissue's metabolic needs to the quantity of blood flow delivered by the microcirculation. These mechanisms operate completely within the tissue itself and are thus independent of outside physiological inputs.
- The Vasodilator Mechanism posits that insufficient blood flow to a tissue results in the build-up of vasodilatory substances that locally diffuse and induce vasodilation of adjacent arterioles and pre-capillary sphincters. Some vasodilatory substances are produced as a result of normal cellular metabolism while others are produced in contexts of low oxygen tension. Lactic Acid, adenosine, CO2, hydrogen ions, and potassium ions all appear to display properties of vasodilatory substances. These substances are leaked in small amounts by cells and their concentrations locally increase when they are not washed away by normal levels of blood flow.
- As mentioned, local increases in these vasodilatory substances result in vasodilation of adjacent arterioles and pre-capillary sphincters, resulting in increased blood flow to the tissues. The converse also appears to be true, when blood flow is excessive, the baseline concentrations of these substances decreases, resulting in vasoconstriction of arterioles and pre-capillary sphincters, thus allowing fine-tuning of blood flow levels to a tissue's metabolic demands
- The oxygen-deficiency mechanism posits that vascular smooth muscle cells require sufficient oxygen and nutrients to maintain tension. Consequently, in the absence of sufficient oxygen and nutrients vascular smooth muscle cells will naturally relax, causing vasodilation of arterioles and pre-capillary sphincters. This quite simple mechanism may account for some of the vasodilation observed when blood flow to a tissue is reduced
- An important feature of physiology is that local blood flow through tissues remains relatively constant over a wide range of systemic arterial pressures. Some of this is undoubtedly due to the vasodilatory mechanism discussed above as increased blood pressure would cause a transient increase in local blood flow, washing out the vasodilatory substances, and thus resulting in vasoconstriction of arterioles and pre-capillary sphincters.
- Additionally, it appears that arterioles inherently react to sudden changes in blood pressure by undergoing vasoconstriction. While the precise mechanism of this reactive vasoconstriction is not currently understood, it is thought to play a role in the ability of arterioles to autoregulate local blood flow inspite of changes in systemic arterial pressure.
- While a combination of all of the mechanisms above likely contribute to local blood flow regulation in most tissues, certain tissues possess specialized mechanisms of blood flow regulation which are critical for the proper functioning of the organs.
- Kidneys: In addition to the above, Tubuloglomerular Feedback is an important regulator of blood flow to the kidneys and is important for guaranteeing a relatively constant level of glomerular filtration
- Lungs: As discussed in Pulmonary Blood Flow Regulation, reductions in oxygen concentration tend to cause vasoconstriction of pulmonary arterioles rather than the vasodilation seen in other tissues
- Brain: Cerebral blood flow is much more sensitive to changes in CO2 and hydrogen ions than other metabolites likely because neuronal function is highly sensitive to changes in pH which are determined by the balance between local CO2 and hydrogen ion concentrations
- Heart: As discussed in Cardiac Blood Flow, the Vasodilator Mechanism appears to be the dominant process regulating blood flow in the coronary vasculature