Haldane effect

The Haldane effect is a fundamental principle of respiratory physiology explaining how oxygen and carbon dioxide transport are physiologically linked. When hemoglobin is in its deoxygenated state (deoxyhemoglobin), it has a higher affinity for CO2 and can carry more CO2 than when it is oxygenated (oxyhemoglobin). This occurs through two main mechanisms: (1) Deoxyhemoglobin is a weaker acid than oxyhemoglobin, making it a better buffer for H+ ions produced when CO2 forms bicarbonate, and (2) Deoxyhemoglobin can directly bind CO2 to form carbaminohemoglobin more readily. In peripheral tissues where O2 is released, the resulting deoxyhemoglobin facilitates CO2 uptake. Conversely, in the lungs where O2 binds to hemoglobin, the newly formed oxyhemoglobin releases CO2, promoting CO2 elimination. This effect is crucial for efficient gas exchange and is the physiological basis for why venous blood carries significantly more CO2 than arterial blood (despite similar CO2 partial pressures).