Fainting

When standing, because of gravity, blood pools at the feet due to the generation of hydrostatic pressure. Thus, blood does not return to the heart as quickly. End diastolic volume decreases (EDV). If this decreases, then cardiac output decreases because Q = HR * SV (EDV-ESV). Blood pressure also drops because (BP = Q * TPR). This leads to less flow because flow is proportional to a change in pressure / resistance - decreased flow to brain. As a result the pressure is higher in the heart. All these events decrease O2 (oxygen) delivery to brain. This is why we pass out because of lack of oxygen in the brain. We c prevent fainting by using our skeletal muscle pump (by wiggling toes, bending knees slightly, etc.). This skeletal muscle pump will generate more pressure and restore our end diastolic volume and reverse the effects.