Regulation of breathing is controlled not by the amount of oxygen circulating in the blood but by blood pH. When the blood becomes too acidic, receptors in the brain respond to changes in blood pH and increase breathing rate. Order the following events to show how the body maintains stable blood pH when blood carbon dioxide levels change. Assume a person is exercising and her muscles are releasing carbon dioxide. 1. There is an increase in carbon dioxide in the blood.
2. Receptors in the brain detect an increase in acidity of blood.
3. Increase in breathing rate, exhaling more carbon dioxide.
4. Carbon dioxide combines with water to form carbonic.
5. Carbonic acid dissociates into water and carbon dioxide.
6. pH of the blood increases (becomes more base)
7. Carbonic acid dissociates, forming bicarbonate and hydrogen ions.
8. pH decreases in the blood (it becomes more acidic).

1. There is an increase in carbon dioxide in the blood.

4. Carbon dioxide combines with water to form carbonic.

7. Carbonic acid dissociates, forming bicarbonate and hydrogen ions.

8. pH decreases in the blood (it becomes more acidic).

2. Receptors in the brain detect an increase in acidity of blood.

5. Carbonic acid dissociates into water and carbon dioxide.

3. Increase in breathing rate, exhaling more carbon dioxide

6. pH of the blood increases (becomes more base)

Explanation:

When the concentration of carbon dioxide in the body increases due to active metabolism as in the case of a runner, the carbon dioxide can be transported through the blood by three methods; (1) dissolved directly in the blood, (2) bound to plasma proteins or hemoglobin, or (3) converted into bicarbonate  which is the major route.

In the bicarbonate route, carbon dioxide diffuses into red blood cells and is converted to carbonic acid by carbonic anhydrase, which subsequently hydrolyzes the carbonic acid to bicarbonate  and hydrogen ions. The hydrogen ion binds to hemoglobin in red blood cells, and bicarbonate is transported out of the red blood cells into the plasma in exchange for a chloride ion. The increase in the concentration of hydrogen ions results in increase in the acidity of blood. Receptors in the brain detect an increase in acidity of blood and then send signals to increase ventilation rate in order to remove the excess carbon dioxide.  

In the lungs, bicarbonate is transported back into the red blood cells in exchange for chloride. Carbonic anhydrase in plasma first converts the  bicarbonate and hydrogen ion which dissociates from hemoglobin to carbonic acid, and then catalyzes the formation of carbon dioxide and water from the carbonic acid. The carbon dioxide is then expelled from the lungs with increased breathing rate.

With the removal of excess carbon dioxide, the blood pH decreases and breathing slows down to normal rate.