Elimination of CO₂ via Ventilation

In the lungs, ventilation affects the pH of the blood:

• Oxygen is inspired and diffuses from the alveoli into the blood. In the red blood cell, oxygen binds with hemoglobin (to form oxyhemoglobin) and releases a H⁺. The H⁺ combines with HCO₃^- to form carbonic acid (H₂CO₃), which is then converted to carbon dioxide (CO₂) and water.
• CO₂ diffuses into the alveoli from the blood and is eliminated via ventilation.

In the tissues, the reverse reaction occurs:

• CO₂, which is the end product of most metabolic processes, diffuses from the tissue into the red blood cells where it combines with H₂O to form H₂CO_3. It then dissociates into H⁺ and HCO₃^-.
• Bicarbonate ions diffuse into the plasma. To maintain electroneutrality, chloride diffuses into the red blood cells (known as the chloride shift).
• The H⁺ ions generated are buffered by binding with deoxygenated (HHb) hemoglobin.

The result is a minimal change in H⁺ concentration between the venous and arterial circulations.

• However, if the respiratory rate decreases, the reaction slows, causing an accumulation of CO₂ and a decrease in the blood pH.
• Conversely, if the respiratory rate increases, the CO₂ is removed at an increased rate (causing a decrease in H⁺ concentration) and causes an increase in the blood pH.

The lungs can therefore respond to nonrespiratory disorders by altering the ventilation rate to return the blood pH to normal.