Figure 1.

Illustration of the circulation, with the venous system depicted as having a much larger capacity and compliance than arterial. Arterial blood pressure is intimately related to the arterial blood volume. Veins blue: capacity 5 × that of arteries. Arteries red: A small change in venous volume causes a large change in arterial volume—and hence pressure. Arterial volume and hence pressure are sustained by the level of venous tone. Venous tone depends mainly on the intensity of the sympathetic outflow from the brain to venous wall musculature. The portal blood flow is shown stippled red and blue, an intermediate drainage from gut to liver. Apart from the emphasis on the different venous and arterial volumes, the tissues, heart, and lung (L) dimensions are arbitrary.