Trong khi các mạch máu chính xác cơ chế (s) liên quan đến vẫn là một vấn đề của cuộc tranh luận giữa các myogenic và lý thuyết trao đổi chất, tự điều đặc biệt quan trọng để bảo vệ tưới máu các cơ quan như não. | 34 D. M. Payen creases the vessel caliber increases to maintain the flow. While the precise vascular mechanism s involved remain a matter of debate between the myogenic and the metabolic theory autoregulation is of particular importance to protect perfusion of organs such as the brain. The Concept of Waterfall The flow within an organ can be seen as a function of the difference between the inflow pressure and the outflow pressure. For a given perfusion pressure the flow depends on the regional vascular tone or resistance. While this remains true for many organs especially the musculo-cutaneous territory it may not be so for others. The above concept is no longer correct when organ vessels are surrounded by a pressure different to atmospheric pressure. If this pressure is positive it can at least induce vessel collapse. The perfusion pressure flow relation is then more complex and surrounding pressure has to be integrated. If such external pressure becomes higher than intravascular pressure the vessel is narrowed with a reduced flow. The outflow pressure is not the venous pressure but the intra-vascular pressure elevated by the positive pressure surrounding the tissue. The waterfall phenomenon occurs in the lung the heart the brain and to a lesser extent the portal vein in the liver. Phasic Blood Flow 1 Arterial flow is a phasic phenomenon with systolic and diastolic components. At the aortic level flow is only present during systole with no flow in diastole. At the microvessel level flow is more continuous which is a witness to the buffer role of arterial vessels that transform phasic flow into continuous flow. It is important to note that arterial organ blood flow is phasic with systolic and diastolic components that differ from organ to organ 2 . For example forearm blood flow is essentially during systole with no flow during diastole whereas cerebral blood flow is systolo-diastolic and left coronary blood flow is purely diastolic Fig. 2 3 . This implies .
