When humans first rose and walked on their hind legs, they markedly increased their vulnerability to the effects of gravity on the circulatory system. The brain, the organ most susceptible to hypoxia, was placed at quite a disadvantageous location, making humans more susceptible than other animals to syncope, the clinical manifestation of reduced blood flow to the brain's reticular activating system.

The challenge posed by upright posture is the vertical displacement of blood below the heart. A series of cardiovascular regulating mechanisms are activated to prevent an excessive decline in venous return and thus maintain arterial blood pressure and cerebral perfusion. In normal individuals, a carefully controlled balance between sympathetic and parasympathetic components of the autonomic nervous system allows rapid adjustments to take place and maintain near-constant cerebral perfusion. Other more subtle mechanisms, such as volume and osmotic regulation as well as release of endogenous hormones, contribute to maintenance of arterial blood pressure during prolonged standing.<sup>41 Wieling W., Lieshout J. Maintenance of postural normotension in humans Clinical Autonomic Disorders Boston: Little, Brown (1993).  69-78

Haga clic aquí para ir a la sección de Referencias</sup> Given the complexity of the adjustments involved, it is not surprising that dysfunction of the autonomic nervous system accounts for a large number of clinical syncopal syndromes.