7.1 Respiratory Physiology and Mechanics

Respiratory physiology and mechanics are crucial for understanding how we breathe and exchange gases. This topic covers lung volumes, ventilation, and the physical forces involved in breathing. It's essential for grasping how the respiratory system works and how it's measured.

Understanding these concepts is key to interpreting spirometry results and diagnosing respiratory disorders. We'll explore how air moves in and out of the lungs, how gases are exchanged, and the factors that affect breathing efficiency.

Lung Volumes and Capacities

Tidal Volume and Vital Capacity

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• Tidal volume ($V_T$) represents the normal volume of air displaced between normal inhalation and exhalation when extra effort is not applied
  • Typical values are around 500 mL per breath at rest
• Vital capacity (VC) is the maximum amount of air a person can expel from the lungs after a maximum inhalation
  • VC is about 4600 mL in men and 3200 mL in women
  • Calculated as the sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume
  • VC can be used clinically as an index of pulmonary function (spirometry)

Residual Volume and Dead Space

• Residual volume is the volume of air remaining in the lungs after maximal exhalation (about 1200 mL)
  • Prevents alveolar collapse and maintains alveolar $\text{PO}_2$ between breaths
• Anatomical dead space is the volume of air that fills the conducting zone of airways (nose to bronchioles)
  • This volume is about 150 mL where no gas exchange occurs
  • Physiological dead space also includes alveolar dead space (alveoli that are ventilated but not perfused)

Ventilation and Gas Exchange

Ventilation and Diffusion

• Ventilation is the process of physically moving air in and out of the lungs
  • Includes both inspiratory and expiratory phases
• Diffusion is the primary mechanism for gas exchange between alveoli and pulmonary capillaries
  • Oxygen diffuses from alveoli into blood and carbon dioxide diffuses out of blood into alveoli
  • Net diffusion depends on partial pressure gradients of the gases

Perfusion and Alveolar Ventilation

• Perfusion refers to the flow of blood through pulmonary capillaries surrounding alveoli
  • Allows blood to pick up oxygen and offload carbon dioxide
• Alveolar ventilation ($\dot{V}_A$) is the rate at which new atmospheric air reaches the alveoli
  • \dot{V}_A = (\text{Tidal Volume} - \text{Dead Space}) \times \text{[Respiratory Rate](https://www.fiveableKeyTerm:Respiratory_Rate)}
  • Alveolar ventilation is essential for maintaining proper alveolar $\text{PO}_2$ and $\text{PCO}_2$

Respiratory Mechanics

Lung Compliance and Airway Resistance

• Lung compliance is the change in lung volume for a given change in pressure
  • Compliance = $\Delta$Volume / $\Delta$Pressure
  • Reduced compliance makes it harder to inflate the lungs (restrictive lung disease)
• Airway resistance refers to the resistance of the respiratory tract to airflow
  • Inversely related to airway radius (R) by Poiseuille's law: Resistance $\propto 1/R^4$
  • Increased resistance reduces airflow and increases work of breathing (obstructive lung disease)

Respiratory Rate and Minute Ventilation

• Respiratory rate is the number of breaths per minute
  • Normal respiratory rate at rest is around 12-20 breaths/min in adults
• Minute ventilation ($\dot{V}_E$) is the total volume of air entering the lungs per minute
  • $\dot{V}_E = \text{Tidal Volume} \times \text{Respiratory Rate}$
  • Minute ventilation can be increased by increasing tidal volume, respiratory rate, or both
  • Allows the respiratory system to adjust ventilation to meet metabolic demands