Respiratory Gas Exchange and Oxygenation

Gas exchange is passive diffusion down partial-pressure gradients. On room air at sea level, alveolar oxygen tension is approximately 100 mmHg, while mixed venous blood arriving at the pulmonary capillary is approximately 40 mmHg; oxygen therefore moves into blood. Carbon dioxide moves in the opposite direction, from pulmonary capillary blood into the alveolus. The efficiency of this exchange follows the principles described by Fick’s law. A larger alveolar surface area and a steeper partial-pressure gradient favour transfer. A thicker alveolar–capillary membrane slows it. Emphysema reduces available surface area, while pulmonary oedema adds fluid and pulmonary fibrosis thickens the diffusion barrier. These disorders all produce impaired oxygenation, but by different anatomical routes. Ventilation is air reaching the alveoli; perfusion is blood reaching them. Their relationship, expressed as the ventilation–perfusion ratio, often explains the patient’s oxygenation pattern: - High V/Q or dead space: the alveolus receives air but little or no blood flow. Pulmonary embolism is the classic example; ventilation is wasted. - Low V/Q: blood passes regions that receive inadequate ventilation, as in pneumonia, asthma, COPD, or atelectasis. - True right-to-left...