18: Transport and Exchange of Respiratory Gases
- Page ID
- 178721
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Breathing brings fresh air into the lungs, but ventilation is only the first part of the body’s challenge. To support life, oxygen must diffuse from alveoli into the blood, travel to the tissues, and unload where it is needed, while carbon dioxide produced by cells must be carried back to the lungs for removal. This chapter explores the movement of these gases and the physiological principles that allow the body to maintain adequate oxygenation and carbon dioxide clearance under resting conditions, during exercise, and in extreme environments such as high altitude.
Gas exchange depends on pressure gradients, alveolar ventilation, surface area, and the thinness of the respiratory membrane. Any condition that disrupts these variables, such as emphysema, pulmonary edema, or fibrosis, can reduce diffusion efficiency and lead to hypoxia. Gas solubility plays a central role in determining how much oxygen or carbon dioxide dissolves in blood, and the differing solubilities of these gases explain why carbon dioxide moves so easily across respiratory surfaces.
Transport of gases in the blood reflects the principles of mass flow and mass balance. Oxygen delivery depends on both the concentration of oxygen carried by hemoglobin and the rate at which blood flows to tissues. Hemoglobin acts as a reversible oxygen buffer, loading oxygen in the lungs and unloading it to active tissues based on factors such as pH, temperature, and the presence of 2,3 bisphosphoglycerate. Carbon dioxide transport relies on dissolved gas, carbaminohemoglobin, and the bicarbonate system, powered by the enzyme carbonic anhydrase within red blood cells.
Ventilation is regulated by networks of neurons in the medulla and pons, which coordinate rhythmic breathing through feedback from chemoreceptors that monitor oxygen, carbon dioxide, and pH. Carbon dioxide is the most powerful driver of ventilation, ensuring that even small increases stimulate breathing. Protective reflexes and higher brain centers add additional layers of control that allow breathing to respond to environmental demands and emotional states.