When a person moves from a sitting position to laying down blood pressure decreases within a few minutes. What causes this decrease in blood pressure?

1. When a person is sitting, gravity causes blood to pool in the lower parts of the body. This requires a higher sympathetic tone to maintain blood pressure. 2. When the person lies down, the effect of gravity is reduced. As a result, there is a more even distribution of blood, and the central blood volume increases. 3. The increase in central blood volume is sensed by the baroreceptors. These stretch receptors, located mainly in the carotid sinus and aortic arch, respond to the increased pressure. 4. The baroreceptor reflex then reduces sympathetic nervous system activity and increases parasympathetic activity. Mathematically, if we consider the net autonomic influence on heart rate \( HR \) as \[ HR \propto \text{Sympathetic Activity} - \text{Parasympathetic Activity}, \] a reduction in sympathetic drive leads to a decrease in heart rate and vasodilation. 5. The reduction in sympathetic tone results in vasodilation and a lower systemic vascular resistance. This, in turn, causes the observed decrease in blood pressure. Thus, the blood pressure \( BP \) becomes \[ BP = CO \times SVR, \] where \( CO \) is cardiac output and \( SVR \) is systemic vascular resistance. With vasodilation (a decrease in \( SVR \)), \( BP \) decreases. In summary, the decrease in blood pressure when moving from sitting to lying down is primarily due to the baroreceptor-mediated reduction in sympathetic outflow and the resulting vasodilation.