Chapter 18: Gas Exchange and Transport

Gas Exchange and Transport on gas exchange and transport examines the physiological mechanisms responsible for moving oxygen and carbon dioxide between the atmosphere, the alveoli, the blood, and the tissues. The discussion begins with the physics of gas diffusion, explaining how partial pressure gradients drive gas movement and how factors such as surface area, barrier permeability, and diffusion distance influence the rate of exchange as described by Fick's law of diffusion. It categorizes different types of hypoxia—hypoxic, anemic, ischemic, and histotoxic—and analyzes how respiratory pathologies like emphysema, fibrotic lung disease, pulmonary edema, and asthma impair alveolar gas exchange by altering compliance, resistance, or diffusion barriers. The text then shifts to gas transport in the blood, utilizing the Fick equation to relate mass flow and metabolic oxygen consumption. A major focus is placed on the role of hemoglobin, which transports more than 98 percent of arterial oxygen as oxyhemoglobin, compared to the small fraction dissolved in plasma. The summary details the sigmoid shape of the oxyhemoglobin saturation curve and explains how physiological variables such as pH (the Bohr effect), temperature, partial pressure of carbon dioxide, and 2,3-BPG cause right or left shifts in the curve, thereby regulating oxygen binding affinity to meet metabolic demands. Carbon dioxide transport is explored through its three primary methods: dissolved in plasma, bound to hemoglobin as carbaminohemoglobin, and converted into bicarbonate ions. This section elucidates the crucial role of the enzyme carbonic anhydrase in catalyzing the reversible reaction of carbon dioxide and water, as well as the chloride shift mechanism that maintains electrical neutrality in red blood cells. Finally, the chapter outlines the neural regulation of ventilation, identifying control centers in the medulla oblongata and pons, including the dorsal and ventral respiratory groups and the pre-Botzinger complex. It explains how the body maintains homeostasis through peripheral chemoreceptors in the carotid and aortic bodies that sense low oxygen and pH, and central chemoreceptors that monitor carbon dioxide levels via hydrogen ion concentration in the cerebrospinal fluid, alongside protective reflexes like the Hering-Breuer inflation reflex.