The rate of breathing, also known as respiratory rate, is controlled mainly by the CO2 level in the blood. This is due to a mechanism known as the respiratory drive or the chemoreceptor response.

The respiratory system plays a crucial role in maintaining the balance of gases in our body, particularly oxygen (O2) and carbon dioxide (CO2). The respiratory drive is responsible for regulating the rate and depth of breathing, ensuring that the body receives an adequate supply of oxygen and removes excess carbon dioxide.

Here is a detailed explanation of how the CO2 level in the blood controls the rate of breathing:

1. Central Chemoreceptors:
- Located in the medulla oblongata, the central chemoreceptors monitor the chemical composition of the cerebrospinal fluid (CSF) that surrounds the brain.
- They are primarily sensitive to changes in the pH of the CSF, which is influenced by the concentration of carbon dioxide dissolved in the blood.
- When the CO2 level in the blood increases, it diffuses across the blood-brain barrier and combines with water to form carbonic acid (H2CO3).
- Carbonic acid then dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). The increase in H+ ions leads to a decrease in pH, which stimulates the central chemoreceptors.
- The chemoreceptors send signals to the respiratory centers in the medulla oblongata, which in turn increase the rate and depth of breathing to remove excess CO2 and restore the pH balance.

2. Peripheral Chemoreceptors:
- Located in the carotid bodies and aortic bodies, the peripheral chemoreceptors monitor the chemical composition of the arterial blood.
- They are mainly sensitive to changes in the partial pressure of oxygen (PO2) and carbon dioxide (PCO2) in the blood.
- While the peripheral chemoreceptors primarily respond to changes in the PO2 level, they also indirectly influence the respiratory rate by detecting changes in the PCO2 level.
- When the PCO2 level increases, it leads to a decrease in the blood pH. This decrease in pH stimulates the peripheral chemoreceptors, which send signals to the respiratory centers to increase the rate and depth of breathing.

In summary, the rate of breathing is mainly controlled by the CO2 level in the blood. The chemoreceptors in the medulla oblongata and peripheral chemoreceptors respond to changes in the pH and PCO2 levels, respectively, and regulate the respiratory centers to maintain a proper balance of gases in the body.