Baroreceptors are stretch receptors that detect changes in blood pressure. They are found in the wall of the aortic arch and in the carotid sinuses (small swellings at the base of the internal carotid arteries, which supply the brain). When blood pressure rises, the artery wall is stretched more and the baroreceptors send impulses more frequently along sensory neurones to the cardiovascular control centre in the medulla oblongata.
When blood pressure is too high:
1. The artery wall is stretched more, so baroreceptors send more impulses.
2. The control centre sends more impulses along the vagus nerve and fewer along the accelerator nerve.
3. Acetylcholine released at the SAN reduces its rate of firing.
4. Heart rate falls, cardiac output falls and blood pressure returns towards normal.When blood pressure is too low:
1. The wall is stretched less, so baroreceptors send fewer impulses.
2. The control centre sends more impulses along the accelerator nerve and fewer along the vagus nerve.
3. Noradrenaline released at the SAN increases its rate of firing.
4. Heart rate rises, cardiac output rises and blood pressure returns towards normal.This is an example of negative feedback: the change in blood pressure causes a response that reverses the change. The baroreceptor reflex acts within seconds. It explains why you may feel dizzy for a moment when you stand up quickly: blood pools in the legs, the pressure in the carotid sinuses falls, and the reflex speeds up the heart to restore blood flow to the brain.The control centre can also change the diameter of arterioles, which alters blood pressure as well.
When blood pressure is too high:1. The artery wall is stretched more, so baroreceptors send more impulses.
2. The control centre sends more impulses along the vagus nerve and fewer along the accelerator nerve.
3. Acetylcholine released at the SAN reduces its rate of firing.
4. Heart rate falls, cardiac output falls and blood pressure returns towards normal.When blood pressure is too low:
1. The wall is stretched less, so baroreceptors send fewer impulses.
2. The control centre sends more impulses along the accelerator nerve and fewer along the vagus nerve.
3. Noradrenaline released at the SAN increases its rate of firing.
4. Heart rate rises, cardiac output rises and blood pressure returns towards normal.This is an example of negative feedback: the change in blood pressure causes a response that reverses the change. The baroreceptor reflex acts within seconds. It explains why you may feel dizzy for a moment when you stand up quickly: blood pools in the legs, the pressure in the carotid sinuses falls, and the reflex speeds up the heart to restore blood flow to the brain.The control centre can also change the diameter of arterioles, which alters blood pressure as well.