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

Abstract
Using a model of both rigid and collapsible tubes, Hicks and Badeer (10) reported that the siphon mechanism is still operating within vertically oriented models, even when the descending limb is flexible and partly collapsed (10). This implies that partially collapsed descending veins do not interrupt the siphon as long as there is a continuous column of fluid. They emphasize the importance of the interaction of the viscous and the hydrostatic components in the interpretation of pressure measurements in a vessel. They attribute the pressure gradient of 13 to 4 mmHg down the jugular veins of a standing giraffe (9), where approximately −93 to −27 mmHg would be expected based solely on the prevailing hydrostatic gradient, as related to the sum of gravitational and viscous pressures. In a more recent study, the authors further support the concept that the heart does not have to overcome the weight of the blood pumped to the head, only the viscous resistance of the blood vessels (11). They state that the mechanical advantage of a closed system in relation to gravitational effects is similar to the operation of the loop of a siphon, but to avoid confusion of the physics of open vs. closed systems, the term “siphon” should be avoided: “in ‘open' systems, gravity hinders uphill flow and causes downhill flow, in which the liquid acts as a falling body. In contrast, in ‘closed' systems, like the circulation, gravity does not hinder uphill flow, nor does it cause downhill flow, because gravity acts equally on the ascending and descending limbs of the circuit” (11). Bearing in mind the difference between open vs. closed systems, for historical reasons, we will continue to use the term “siphon” here.