Expansion
Blood pools in the legs, venous return falls, and the baroreflex compensates
Expansion
On standing, gravity shifts about 500 to 800 ml of blood into the legs and splanchnic veins within seconds.
- Venous return falls, so stroke volume and cardiac output fall
- Arterial pressure falls transiently, unloading the baroreceptors
- Baroreflex responds: sympathetic outflow rises and vagal tone falls
- Result: tachycardia, increased contractility, arteriolar and venoconstriction
- Slower back-up: renin-angiotensin activation and antidiuretic hormone release
Net effect in health: heart rate rises by 10 to 15 beats per minute, diastolic pressure rises slightly, and systolic pressure is nearly unchanged.
Orthostatic hypotension results when compensation fails: hypovolaemia, autonomic neuropathy, drugs (antihypertensives, diuretics, alpha blockers, tricyclics), prolonged bed rest and age.
A fall in pressure without a compensatory rise in heart rate points specifically to autonomic failure rather than simple hypovolaemia.