Baroreceptors are nerve endings embedded in blood vessels that are sensitive to the stretching of vessel walls. They are most abundant in the walls of the internal carotid arteries and carotid sinus. When the vessel walls stretch in response to increasing blood volume, they signal the central nervous system to inhibit the vasoconstrictor center in the medulla and to stimulate the vagal center. This in turn causes peripheral vasodilation, decreased heart rate and less vigorous contractions, thereby decreasing arterial blood pressure. When baroreceptors detect decreased blood pressure they induce peripheral vasoconstriction, increased heart rate and more vigorous contraction.
Baroreceptors quickly respond to small changes in blood pressure.
They fail to respond to higher pressure levels after one or more days, thus they lose their ability to control arterial pressure in chronic hypertension. Chemoreceptors are nerve endings located in the walls of the carotid arteries, the aorta and the medullary area of the brainstem. Responding to abnormally low levels of dissolved oxygen and carbon dioxide in the blood, the nerve endings stimulate sympathetic activity and inhibit parasympathetic activity which in turn causes a reflex increase in arterial pressure.