Chapter 29: Structure and Function of the Respiratory System

The primary purpose of the respiratory system is the exchange of gases, facilitating the uptake of oxygen into the blood and the elimination of carbon dioxide, while also acting as a host defense mechanism and a metabolic organ. This complex function relies on the structural organization that separates the conducting airways, responsible for filtering, warming, and moistening air, from the respiratory tissues—the alveoli—where gas diffusion takes place. Ventilation, the physical movement of air, is achieved by changes in the size of the chest cavity, primarily through the contraction of the diaphragm and accessory muscles, which establish the necessary pressure gradients. The efficiency of ventilation is critically dependent on lung compliance (the ease of inflation), which is enhanced by pulmonary surfactant produced by Type II alveolar cells, reducing surface tension at the air-epithelium interface. Airflow is also influenced by airway resistance, which is greatest in the medium-sized bronchi. Pulmonary function is quantified using static lung volumes—such as the tidal volume (VT) and residual volume (RV)—that combine into capacities, as well as dynamic measures like the Forced Expiratory Volume in 1 second (FEV1.0). Effective gas exchange requires the matching of ventilation and perfusion (V/Q matching) in the pulmonary circulation; failure to match results in dead air space (ventilation without perfusion) or shunt (perfusion without ventilation). Oxygen is transported primarily bound to hemoglobin (98%–99%), and its release to tissues is regulated by the oxygen-hemoglobin dissociation curve, which shifts to the right (decreasing affinity) in response to increased metabolic demands, such as higher temperature, lower pH (acidosis), or elevated PCO2. Carbon dioxide, conversely, is mainly transported as bicarbonate ions. The automatic regulation of breathing is coordinated by the respiratory center in the brainstem, which receives crucial input from central chemoreceptors sensitive to PCO2/pH changes in the cerebrospinal fluid, and from peripheral chemoreceptors that monitor arterial oxygen levels. Protective reflexes, including the cough reflex, and subjective symptoms, like dyspnea (difficulty breathing), further integrate the system's function.