Badeer H S
Comp Biochem Physiol A Comp Physiol. 1986;83(2):207-11. doi: 10.1016/0300-9629(86)90562-1.
Vascular pressure consists of the sum of two pressures: (a) pressure developed by the pumping of the ventricles against the resistance of vessels, designated as viscous flow pressure, and (b) pressure caused by gravity, traditionally called hydrostatic, better described as gravitational pressure. In a conduit, both of these pressures must be overcome when a liquid is discharged to a higher level of gravitational potential energy. If a liquid is returned to its original level, gravity neither helps nor hinders flow because of the siphon effect. This circumstance prevails in the circulatory system. Hence, P1-P2 in the Poiseuille equation excludes gravitational pressure between those points. The long neck of the giraffe, therefore, poses no impediment to blood flow in the erect posture. The giraffe has a high aortic pressure. This is not for driving the blood to its head but is for minimizing the gravitational drop of intravascular pressure and collapse of the vessels. The cerebral circulation is protected by the cerebrospinal fluid which undergoes parallel changes in pressure with posture. Other vessels in the head are less protected by connective tissue, surrounding muscles and other structures. The high aortic pressure in the giraffe is probably caused by the high total peripheral resistance of the systemic circuit due to vascular adaptations related to the overall height of the animal.
(a) 心室泵血对抗血管阻力所产生的压力，称为粘性流压力；(b) 由重力引起的压力，传统上称为静水压力，更确切地说是重力压力。在管道中，当液体被排放到重力势能更高的水平时，这两种压力都必须被克服。如果液体回到其原始水平，由于虹吸效应，重力既不促进也不阻碍流动。这种情况在循环系统中普遍存在。因此，泊肃叶方程中的P1 - P2排除了这些点之间的重力压力。因此，长颈鹿的长脖子对其直立姿势下的血液流动没有阻碍。长颈鹿的主动脉压力很高。这不是为了将血液输送到头部，而是为了最小化血管内压力的重力下降和血管的塌陷。脑循环受到脑脊液的保护，脑脊液的压力会随着姿势发生相应变化。头部的其他血管受到结缔组织、周围肌肉和其他结构的保护较少。长颈鹿的高主动脉压力可能是由于与动物整体高度相关的血管适应性导致全身循环系统总外周阻力较高所致。
