Haemoglobinic Acid

When carbonic acid dissociates inside a red blood cell, it releases hydrogen ions (H⁺). If these built up, the pH inside the cell would fall, which could denature enzymes and other proteins. Haemoglobin prevents this.The hydrogen ions bind to haemoglobin, forming haemoglobinic acid (HHb). As they bind, oxyhaemoglobin releases its oxygen:HbO₈ + H⁺ → HHb + 4O₂This has two important effects.1. Buffering. By taking up free hydrogen ions, haemoglobin acts as a buffer, keeping the pH of the red blood cell and the blood fairly constant.2. Oxygen release. Hydrogen ions binding to haemoglobin change its shape, which lowers its affinity for oxygen. Oxyhaemoglobin therefore unloads its oxygen. This happens in respiring tissues, exactly where carbon dioxide levels are high and oxygen is needed. It is the molecular basis of the Bohr effect.In the lungs the process reverses. The partial pressure of oxygen is high, so haemoglobin binds oxygen and releases its hydrogen ions. These combine with hydrogencarbonate ions returning from the plasma to re-form carbonic acid, which carbonic anhydrase breaks down into carbon dioxide and water. The carbon dioxide diffuses into the alveoli.So a tissue that respires faster produces more carbon dioxide, more hydrogen ions and more haemoglobinic acid, and more oxygen is unloaded to it.