Respiratory physiology is the branch of physiology dedicated to the comprehensive study of the mechanics and chemical exchanges that occur during breathing. It encompasses the processes of pulmonary ventilation, gas diffusion across the alveolar-capillary membrane, gas transport via the bloodstream, and the neural-chemical regulation of breathing patterns. This system is fundamental to homeostasis, maintaining arterial pO₂, pCO₂, and blood pH within narrow physiological limits.
Respiration in physiology refers specifically to the cellular and systemic processes of oxygen uptake and carbon dioxide elimination, distinct from cellular respiration which describes intracellular ATP production via oxidative phosphorylation.
Anatomical Substrates of Gas Exchange
The respiratory tract is functionally divided into conducting and respiratory zones. The conducting zone, extending from the nasal cavity through the terminal bronchioles, conditions inhaled air by warming, humidifying, and filtering it. It contributes approximately 150 mL to the anatomical dead space.
The respiratory zone begins at the respiratory bronchioles and includes alveolar ducts, alveolar sacs, and roughly 480 million alveoli. The alveolar-capillary membrane is exceptionally thin (~0.5 μm) and highly vascularized, providing a total gas exchange surface area of approximately 70 m² in a healthy adult. Type I pneumocytes facilitate diffusion, while Type II pneumocytes secrete pulmonary surfactant, a phospholipid-protein complex that reduces surface tension and prevents alveolar collapse during exhalation.
Mechanics of Pulmonary Ventilation
Ventilation is driven by pressure gradients established through changes in intrathoracic volume. According to Boyle's Law (P₁V₁ = P₂V₂), an increase in thoracic volume during inspiration decreases intrapulmonary pressure below atmospheric pressure, drawing air inward. This volume change is primarily achieved by diaphragmatic contraction and external intercostal muscle expansion of the rib cage.
V̇A = (VT - VD) × f
Where V̇A is alveolar ventilation, VT is tidal volume, VD is dead space volume, and f is respiratory frequency. Efficient gas exchange depends on maximizing V̇A rather than total minute ventilation (V̇E), as dead space ventilation does not participate in gas exchange.
Lung Volumes & Capacities
Pulmonary function testing quantifies several standard volumes and capacities:
- Tidal Volume (VT): ~500 mL per breath at rest
- Residual Volume (RV): Air remaining after maximal exhalation (~1,200 mL)
- Forced Vital Capacity (FVC): Maximal exhalation after maximal inhalation
- Functional Residual Capacity (FRC): Equilibrium volume where elastic recoil equals chest wall expansion
Gas Exchange & Transport
Diffusion across the alveolar membrane follows Fick's Law and is driven by partial pressure gradients. At sea level, alveolar pO₂ averages ~100 mmHg, while venous blood entering pulmonary capillaries has pO₂ ~40 mmHg. Oxygen binds reversibly to hemoglobin in erythrocytes, forming oxyhemoglobin (HbO₂), which accounts for ~98.5% of oxygen transport. The remaining 1.5% dissolves directly in plasma.
Carbon dioxide transport occurs via three pathways: dissolved in plasma (7%), bound to hemoglobin as carbaminohemoglobin (23%), and as bicarbonate ions (HCO₃⁻) (70%). The latter is catalyzed by carbonic anhydrase within red blood cells, enabling rapid buffering and transport.
Cross-disciplinary analysis reveals that the oxygen-hemoglobin dissociation curve shifts rightward during fever, acidosis, or increased 2,3-BPG levels, enhancing oxygen unloading in metabolically active tissues. This Bohr effect is dynamically modulated by local tissue demands.
Neural & Chemical Regulation
Respiratory rhythm originates in the brainstem respiratory centers: the dorsal respiratory group (DRG) drives inspiration, while the ventral respiratory group (VRG) modulates forced expiration. The pneumotaxic and apneustic centers in the pons fine-tune respiratory rate and depth.
Chemoreceptors provide continuous feedback. Central chemoreceptors in the medulla respond primarily to changes in cerebrospinal fluid pH (driven by pCO₂), while peripheral chemoreceptors in the carotid and aortic bodies detect hypoxemia (pO₂ < 60 mmHg), acidosis, and hypercapnia. This integrated feedback loop maintains arterial blood gas homeostasis within seconds of metabolic perturbation.
Clinical Relevance & Pathophysiology
Disruptions in respiratory physiology manifest across obstructive, restrictive, and vascular disease categories:
- COPD & Asthma: Characterized by airflow limitation, increased airway resistance, and dynamic hyperinflation. Spirometry shows reduced FEV₁/FVC ratios.
- Pulmonary Fibrosis: Restrictive pattern with reduced lung compliance, decreased total lung capacity, and impaired diffusion capacity (DLCO).
- ARDS: Diffuse alveolar damage causing non-cardiogenic pulmonary edema, severe V/Q mismatch, and refractory hypoxemia.
- High-Altitude Physiology: Decreased inspired
pO₂triggers hypoxic ventilatory response, polycythemia, and pulmonary vasoconstriction (HVPM).
Arterial blood gas (ABG) interpretation requires simultaneous assessment of pH, pCO₂, and HCO₃⁻ to differentiate respiratory vs. metabolic acid-base disorders and determine compensation status.
References
- West, J. B. (2012). Respiratory Physiology: The Essentials (9th ed.). Lippincott Williams & Wilkins.
- Merck Manual Professional. (2024). Overview of the Respiratory System. Retrieved from merckmanuals.com
- Coleman, S. A. (2021). Gas Exchange and Transport. In Pulmonary Physiology (3rd ed.). Oxford University Press.
- American Thoracic Society. (2023). Guidelines for the Interpretation of Pulmonary Function Tests. American Journal of Respiratory and Critical Care Medicine, 207(4), 450–462.
- Levine, B. R., & Cherniack, N. S. (2020). Control of Breathing. In Respiratory Care Principles & Practice. Jones & Bartlett Learning.