Baroreceptors are stretch receptors in the wall of some blood vessels. They are involved in the control of arterial pressure through the discharge of impulses to the cardiovascular centre when there is distension due to a change in the blood pressure. Baroreceptors are found in the carotid sinus (dilation in the left and right internal carotid arteries), the aortic arch, and the elastic arteries of the neck and chest and some veins.
Any decline in the blood pressure stretches the vascular wall which stimulates the baroreceptors. These receptors send impulses to the cardiovascular centre which in turn decreases parasympathetic stimulation of the heart via the vagus nerves and increases the sympathetic stimulation of the heart. In addition to that the cardiovascular centre stimulates the secretion of adrenaline and noradrenaline from the medulla of the adrenal gland. The effect on the heart and blood vessels is to accelerate heart rate and contractility and promote vasoconstriction, resulting in an increase in blood pressure.
If there is an increase in blood pressure, the baroreceptors send impulses to the cardiovascular centre. In response the cardiovascular centre increases the parasympathetic stimulation of the heart, and decreases its sympathetic stimulation. The heart rate and contractility will decrease leading to low cardiac output and the peripheral resistance will decline due to vasodilation. Low cardiac output and low peripheral resistance cause a decrease in blood pressure.
The baroreceptors in the carotid sinus are responsible for the regulation of the blood pressure in the brain, while those in the aortic arch are responsible for regulation of systemic blood pressure.
Chemoreceptors are found close to the carotid and aortic baroreceptors in small structures called carotid bodies and aortic bodies. They are sensitive to any change in the chemical composition of the blood, such as a decrease in oxygen level and pH of the blood or an increase in the carbon dioxide level. These receptors send impulses to the cardiovascular centre which in turn increases the sympathetic stimulation to the blood vessels causing an increase in blood pressure. Chemoreceptors also stimulate the respiratory centres in the brain to increase the rate of respiration.