Seymour R S, Hargens A R, Pedley T J
Department of Zoology, University of Adelaide, South Australia.
Am J Physiol. 1993 Oct;265(4 Pt 2):R715-20. doi: 10.1152/ajpregu.1993.265.4.R715.
The circulatory systems of vertebrate animals are closed, and blood leaves and returns to the heart at the same level. It is often concluded, therefore, that the heart works only against the viscous resistance of the system, not against gravity, even in vascular loops above the heart in which the siphon principle operates. However, we argue that the siphon principle does not assist blood flow in superior vascular loops if any of the descending vasculature is collapsible. If central arterial blood pressure is insufficient to support a blood column between the heart and the head, blood flow ceases because of vascular collapse. Furthermore, the siphon principle does not assist the heart even when a continuous stream of blood is flowing in a superior loop. The potential energy gained by blood as it is pumped to the head is lost to friction in partially collapsed descending vessels and thus is not regained. Application of the Poiseuille equation to flow in collapsible vessels is limited; resistance depends on flow rate in partially collapsed vessels with no transmural pressure difference, but flow rate is independent of resistance. Thus the pressure developed by the heart to establish a given flow rate is independent of the resistance occurring in the partially collapsed vessels. The pressure depends only on the height of the blood column and the resistance in the noncollapsed parts of the system. Simple laboratory models, involving water flow in collapsible tubing, dispel the idea that the siphon principle facilitates blood flow and suggest that previously published results may have been affected by experimental artifact.
脊椎动物的循环系统是封闭的，血液在同一水平进出心脏。因此，人们常常得出结论，即使在虹吸原理起作用的心脏上方的血管环中，心脏的工作也只是克服系统的粘性阻力，而不是克服重力。然而，我们认为，如果任何下行血管是可塌陷的，虹吸原理并不能帮助血液在上方血管环中流动。如果中心动脉血压不足以支撑心脏和头部之间的血柱，由于血管塌陷，血液流动就会停止。此外，即使有一股连续的血液在上方血管环中流动，虹吸原理也不会帮助心脏。血液被泵送到头部时获得的势能在部分塌陷的下行血管中因摩擦而损失，因此无法恢复。泊肃叶方程在可塌陷血管中的应用是有限的；在没有跨壁压差的部分塌陷血管中，阻力取决于流速，但流速与阻力无关。因此，心脏为建立给定流速而产生的压力与部分塌陷血管中出现 的阻力无关。压力仅取决于血柱的高度和系统未塌陷部分的阻力。简单的实验室模型，涉及水在可塌陷管道中的流动，消除了虹吸原理促进血液流动的观点，并表明先前发表的结果可能受到实验假象的影响。
