Flow and pressure in the vasculature, especially in the veins, are affected by gravity (Fig. 23). First consider an individual who is lying down. In this position, the only force affecting pressure in the blood vessels is that generated by flow through them because all levels of the cardiovascular system are at about the same vertical height. Upon standing, however, the pressures generated by the hydrostatic columns of blood in the vessels become quite large. At the beginning of this chapter, we stated that 100 mm Hg are generated by every 136 cm (54 inches) of water (or in this case blood, which is only slightly more dense than water). Consider the arterial system of a 6-foot-tall individual. When standing, the hydrostatic column will cause the pressure in an artery at the base of his foot to be 133 mm Hg higher than that in an artery in the top of his head. That is remarkable considering that mean arterial pressure at the heart is only about 100 mm Hg. Because of the position of the heart high in the chest, arterial blood pressure is high enough to force blood to the head because there is only about a 30-mmHg hydrostatic column between the two structures. If the gravitational acceleration were increased, such as occurs in a high-performance aircraft during a tight turn, the arterial pressure loss to the head due to hydrostatic columns can be a problem. At three times the acceleration of gravity, the arterial pressure to the brain approaches zero and the pilot loses consciousness. Hydrostatic columns augment the pressure in the arteries below the heart. Figure 23 reveals that, upon standing, an additional 100 mm Hg are added to the arterial pressure at the feet.
Gravity markedly affects function of the veins as well. Pressure in the central veins near the heart is normally less than 5 mm Hg. Thus, upon standing, pressure in the veins in the head and neck tends to become negative, which causes these vessels to collapse. That is why the jugular pulse is not normally seen in a person in the erect posture but can be seen when supine. Inside the cranium, rigid venous sinuses actually allow venous pressure to become slightly negative at the top of the head, as shown in Fig. 23. If sinuses are ruptured during head trauma, air can actually be drawn into them. Figure 23 also shows that the hydrostatic column in the veins below the heart would cause venous pressure in the feet to increase to more than 100 mm Hg if compensatory mechanisms did not come into play. Such a high venous pressure would cause blood to pool in the dependent veins because of their high compliance. Also, the increased venous pressure would be reflected back into the
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