Because blood is a fluid, changing body position in a gravitational field challenges the cardiovascular system. For example when standing up from a supine position, central blood volume (pool) decreases as the blood (400 ml) flows into the capacitance vessels of the abdomen and legs. This decreases heart preload and results in decreases in central venous pressure, stroke volume, and arterial blood pressure (Figure 1.04). If the latter change is large enough (orthostatic hypotension), it can produce a "greying out", or a complete "black-out" with syncope.
Normally the carotid sinus baroreflex acts within less than 1/2-1 sec to return blood pressure to, or nearly to normal, by increasing heart rate, abdominal and limb vascular resistance, and total peripheral resistance. With respect to the central blood pool, the effect and resultant response is not unlike that of hemorrhage (Table 1.06), (Figure 1.11).
An antigravity suit, an inflated suit worn by a pilot flying a high performance jet aircraft, eliminates the changes normally seen during orthostasis. This demonstrates that the changes are due to gravity, not just the result of muscular effort or the flexing of joints.
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