Chapter 19: Gas Transfer & Transport in the Lungs

Gas Transfer & Transport in the Lungs distinguishes between perfusion-limited gas transfer, such as that of nitrous oxide and oxygen under normal conditions, and diffusion-limited transfer, exemplified by carbon monoxide. The concept of lung diffusing capacity (DL) is introduced as a critical clinical measure of the lung's ability to transfer gas, often assessed using carbon monoxide due to its unique binding properties. A significant portion of the chapter is dedicated to oxygen transport, differentiating between dissolved oxygen and the majority carried by hemoglobin. The sigmoid shape of the oxyhemoglobin equilibrium curve is analyzed to explain how loading occurs in the lungs and unloading happens at the tissues. Key concepts such as P50 and the Bohr effect are explored, illustrating how changes in pH, carbon dioxide tension, temperature, and 2,3-DPG shift the curve to alter hemoglobin's affinity for oxygen. The chapter contrasts these physiological shifts with pathological states like carbon monoxide poisoning and anemia, emphasizing the vital distinction between oxygen saturation and total oxygen content. Carbon dioxide transport is also detailed, describing its carriage as bicarbonate ions via the chloride shift, as carbamino compounds, and in dissolved forms, while also explaining the Haldane effect regarding oxygen's influence on CO2 loading. Finally, the text categorizes the respiratory causes of hypoxemia—regional hypoventilation (ventilation-perfusion mismatch), anatomical or physiological shunts, generalized hypoventilation, and diffusion blocks—and explains how the alveolar-arterial (A-a) oxygen gradient and response to 100% oxygen are used to differentially diagnose these conditions.