Fig. 2

Theoretical vascular pressure profile from aortic values through the circulation to the great veins. Note that mean arterial pressure (MAP) is constant for most of the length of the large arteries, because those vessels serve mainly as vascular capacitors holding stored blood under pressure. Whereas vascular pressure drops rapidly as blood traverses the smallest arteries, arteriole and precapillary sphincters. The point at which arterioles spontaneously collapse limiting arterial pressure drop is referred to as the critical closing pressure (Pcc) and approximates a vascular waterfall, in that water flowing over a waterfall is unaffected by how far it falls once over the edge. Thus, shown as a dashed line, the pressure fall from arterioles to venules; changes in the downstream venous pressure do not influence either arterial pressure or blood flow. While the mean systemic filling pressure (Pmsf) represents the upstream pressure driving venous return against a downstream central venous pressure (CVP). These concepts were recently validated in post-operative humans where Pcc was estimated to be about 40 mmHg and Pmsf at 20 mmHg [14]