Hicks J W, Badeer H S
Department of Physiology, School of Medicine, Creighton University, Omaha, Nebraska 68178-0224.
Am J Physiol. 1989 Feb;256(2 Pt 2):R567-71. doi: 10.1152/ajpregu.1989.256.2.R567.
Controversy exists over the principles involved in determining blood flow to the head of a giraffe, specifically over the role of gravity pressure (pgh) in the collapsible jugular vein in facilitating uphill flow in arteries. This study investigated the pressures within vertically oriented models containing both rigid and collapsible tubes. An inverted U tube was constructed (height = 103 cm) of thick rubber tubing in the ascending limb and collapsible dialysis tubing in the descending limb. Water flow was induced by a variable speed pump maintained at the reservoir level such that the descending limb was partially collapsed. Pressure measurements were made at various levels within the U tube by two methods: 1) with the transducer at same level as the tip of the water-filled catheter and 2) with the transducer at the reservoir level. During flow, the pressure at any point was nearly atmospheric along the length of the descending limb. Such methods of obtaining pressure indicated that the pressure gradient within the partially collapsed descending limb was the sum of viscous flow pressure (P1-P2 of Poiseuille) and gravitational pressure (pgh). To study the facilitatory effect of a siphon, the descending limb was compared with a horizontally placed limb (length = 100 cm), and the flow was kept constant. Calculations of hydraulic "work" (pressure x flow) indicated that with a partially collapsed descending limb, work of the pump was reduced by 15% compared with uphill flow to the elevated horizontal position. It is concluded that the siphon mechanism operates in a partially collapsed descending limb of a siphon loop.
关于确定长颈鹿头部血流量所涉及的原理存在争议，特别是关于重力压力（pgh）在可塌陷颈静脉中促进动脉向上血流的作用。本研究调查了包含刚性和可塌陷管道的垂直定向模型内的压力。构建了一个倒U形管（高度 = 103厘米），其上升支由厚橡胶管制成，下降支由可塌陷的透析管制成。通过维持在储液器水平的变速泵诱导水流，使下降支部分塌陷。通过两种方法在U形管内的不同水平进行压力测量：1）将换能器置于与充满水的导管尖端相同的水平，2）将换能器置于储液器水平。在流动过程中，沿着下降支的长度，任何一点的压力几乎都是大气压。这种获取压力的方法表明，部分塌陷的下降支内的压力梯度是粘性流动压力（泊肃叶定律中的P1 - P2）和重力压力（pgh）之和。为了研究虹吸管的促进作用，将下降支与水平放置的支（长度 = 100厘米）进行比较，并保持流量恒定。水力“功”（压力×流量）的计算表明，与向上流动到升高的水平位置相比，下降支部分塌陷时泵的功减少了15%。得出的结论是，虹吸管机制在虹吸管回路的部分塌陷下降支中起作用。
