The siphon controversy: an integration of concepts and the brain as baffle.

Whether gravity challenges blood supply to the brain in standing man is a much-disputed topic in physiology. Burton (3) stated that “it is no harder in the circulation for the blood to flow uphill than downhill” and “differences in the level of different parts of the vascular bed do not in any way affect the driving forces for flow and so do not directly affect the circulation” (3). The prerequisite for the existence of a vascular siphon is a continuous column of blood in both the arterial and venous limbs of the loop; for the brain, a siphon could exist from the thoracic aorta via the filled cerebral veins where they leave the skull to the right atrium. The siphon concept implies that no work is done on blood to increase its gravitational potential energy because the pressure gradients are equal and opposite in direction in the ascending and the descending limbs of the loop. Studies addressing the possibility of a siphon include hydrostatic models using rigid and flexible tubing in a laboratory setup; animal studies, especially measurements in giraffes (as a model of considerable heart-to-head difference in height) and snakes; and human studies. We will discuss 1) the siphon concept and the supporting evidence; 2) the “vascular waterfall” and evidence that there is no siphon functioning in blood flow to and from the brain; and 3) based on recent advances, an integration of these seemingly controversial concepts and address the role of the brain itself as interruption of the siphon. The latter part of the discussion is limited to studies in humans.