Iolene awakens suddenly to the sound of her alarm clock. Realizing that she is late for class, she jumps to her feet, feels light headed, and falls back on her bed. What probably caused this reaction? Why doesn't this happen all the time?
When Jolene stood up rapidly, gravity caused her blood volume to move to the lower parts of her body away from the heart, decreasing venous return. The decreased venous return resulted in a decreased end-diastolic volume (EDV), leading to a decreased stroke volume and cardiac output. In turn, blood flow to the brain decreased so the diminished oxygen supply caused her to be light-headed and feel faint. This reaction doesn't happen all the time because as soon as the pressure drops due to inferior movement of the blood, baroreceptors in the aortic arch and carotid sinus trigger the baroreceptor reflex. Action potentials are carried to the medulla oblonga where appropriate responses are integrated. In this case, we would expect an increase in peripheral resistance to compensate for the blood pressure. If this doesn't compensate enough for the drop, then an increase in heart rate and force of contraction would occur. Normally, these responses occur so quickly that changes in pressure following changes in body position go unnoticed.