Clinical meaning
The respiratory system facilitates gas exchange through the process of ventilation (movement of air into and out of the lungs) and diffusion (transfer of oxygen and carbon dioxide across the alveolar-capillary membrane). The alveoli, numbering approximately 300 million in healthy adults, provide a vast surface area of about 70 square meters for gas exchange. Oxygen diffuses from alveoli into pulmonary capillary blood driven by a partial pressure gradient (alveolar PO2 ~100 mmHg vs. venous PO2 ~40 mmHg), while carbon dioxide diffuses in the opposite direction. The respiratory centers in the medulla oblongata and pons regulate breathing rate and depth through chemoreceptor feedback responding to PaCO2, pH, and PaO2 levels.