Chemoreceptors detect changes in the chemical composition of the blood, especially the carbon dioxide concentration and the pH. They are found in the carotid bodies (in the walls of the carotid arteries), the aortic bodies (in the aortic arch) and in the medulla oblongata itself. They also respond to large falls in oxygen concentration.During exercise, muscles respire faster and release more carbon dioxide into the blood. Carbon dioxide reacts with water to form carbonic acid, which dissociates to release hydrogen ions:CO2 + H2O → H2CO3 → H+ + HCO3− so the blood pH falls.
When carbon dioxide concentration rises:
1. Chemoreceptors detect the fall in pH and send more impulses to the cardiovascular control centre in the medulla.
2. The control centre sends more impulses along the accelerator (sympathetic) nerve to the SAN.
3. Heart rate increases, so blood flows faster through the lungs.
4. More carbon dioxide is removed and the pH returns to normal.At the same time, the chemoreceptors send impulses to the ventilation centre in the medulla, so the rate and depth of breathing increase. This removes carbon dioxide faster and brings in more oxygen.When the carbon dioxide concentration falls, for example after exercise, fewer impulses reach the control centre, more impulses pass along the vagus nerve and heart rate returns to its resting value.Chemoreceptors and baroreceptors work together: chemoreceptors mainly match heart rate to metabolic activity, while baroreceptors keep blood pressure stable.
When carbon dioxide concentration rises:1. Chemoreceptors detect the fall in pH and send more impulses to the cardiovascular control centre in the medulla.
2. The control centre sends more impulses along the accelerator (sympathetic) nerve to the SAN.
3. Heart rate increases, so blood flows faster through the lungs.
4. More carbon dioxide is removed and the pH returns to normal.At the same time, the chemoreceptors send impulses to the ventilation centre in the medulla, so the rate and depth of breathing increase. This removes carbon dioxide faster and brings in more oxygen.When the carbon dioxide concentration falls, for example after exercise, fewer impulses reach the control centre, more impulses pass along the vagus nerve and heart rate returns to its resting value.Chemoreceptors and baroreceptors work together: chemoreceptors mainly match heart rate to metabolic activity, while baroreceptors keep blood pressure stable.