The idea is that cardiovascular collapse from both common syncope and life-threatening haemorrhage is caused by a loss of central blood volume, which in turn causes both central venous and arterial pressures to fall. As a result, mechanoreceptors located in the heart and lungs, aorta and carotid sinuses, are deactivated and trigger reflex reductions in vagal outflow to the heart and increases in sympathetic outflow to the peripheral circulation. Under normal conditions, these mechanisms are responsible for the remarkable ability of most humans to maintain their blood pressure in spite of marked reductions in central blood volume.
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