The baroreceptor reflex is only a short-term regulator of blood pressure because the receptors adapt by raising the threshold and lowering discharge rate.
8. Describe the reflex compensations when someone suddenly stands up from a supine position. What would happen in a patient who just had a sympathectomy?
Sudden standing causes pooling of blood in the leg veins. This results in decreased venous return to the heart, which leads to decreased cardiac output (Frank-Starling mechanism), which leads to decreased MAP. This decrease in MAP is detected by the carotid sinus baroreceptors, which relay a message to the medullary cardiovascular control center, which increases sympathetic outflow and decreases parasympathetic outflow, this causes:
* An increase in HR and myocardial contractility, tending to restore cardiac output.
* Vasoconstriction in skeletal musculature, skin, kidneys and gut, reducing blood flow to these organs and increasing TPR.
* Venoconstriction decreasing capacitance and increasing venous return
A patient with a sympathectomy would experience what's referred to as orthostatic hypotension (which might lead to syncope). Orthostatic hypotension is a decrease in arterial pressure when going from supine to a standing position. A person with a normal baroreceptor mechanism will try to restore MAP. In a person who had a sympathectomy, the sympathetic component of the baroreceptor mechanism is absent.
Heart Physiology II
M.A.S.T.E.R. Learning Program, UC Davis School of Medicine
Date Revised: Jan 16, 2002
Revised by: Gordon Li and Carolyn Nguyen