Mechanisms of Breathing
High-Yield Summary
- Inhalation: diaphragm + external intercostals contract → intrathoracic volume increases → intrapleural pressure decreases → lungs expand, air flows in (negative pressure breathing).
- Passive exhalation: muscles relax, elastic recoil + alveolar surface tension collapse the chest. Active exhalation adds internal intercostal + abdominal muscle contraction.
- Surfactant reduces alveolar surface tension, preventing alveolar collapse during recoil.
- 6 lung volumes/capacities: TLC, RV, VC (= TLC − RV), TV, ERV, IRV (TV + IRV + ERV = VC). RV cannot be measured directly by spirometry.
- Medulla oblongata's ventilation center sets breathing rhythm, driven mainly by CO₂ (central chemoreceptors) rather than O₂ (mainly sensed by peripheral chemoreceptors).
- Hyperventilation drops CO₂ (can cause alkalosis/fainting); hypoventilation raises CO₂.
Inhalation vs. Exhalation
| Inhalation | Exhalation (Passive) |
|---|---|
| Diaphragm + external intercostals contract | Diaphragm + external intercostals relax |
| Intrathoracic volume increases | Intrathoracic volume decreases |
| Intrapleural pressure decreases | Intrapleural pressure increases |
| Air flows in (negative pressure breathing) | Lungs recoil (elastic tissue + alveolar surface tension), air flows out |
Lung Volumes and Capacities
- TLC (Total Lung Capacity)
- Max air the lungs hold after a full deep inhalation (~6–7 L).
- RV (Residual Volume)
- Air remaining after full exhalation; keeps alveoli from collapsing; cannot be measured directly by spirometer.
- VC (Vital Capacity)
- Total air moved in/out of the lungs; VC = TLC − RV.
- TV (Tidal Volume)
- Air moved during a normal, relaxed breath at rest.
- ERV (Expiratory Reserve Volume)
- Extra air forcibly exhaled after a normal exhale.
- IRV (Inspiratory Reserve Volume)
- Extra air forcibly inhaled after a normal inhale.
Lung Volume Relationships
VC = TV + IRV + ERV, and TLC = VC + RV
- VC = Vital capacity
- TV = Tidal volume
- IRV = Inspiratory reserve volume
- ERV = Expiratory reserve volume
- TLC = Total lung capacity
- RV = Residual volume (not spirometer-measurable)
Common MCAT Trap
- CO₂, not O₂, is the primary driver of everyday breathing rate — central chemoreceptors (medulla) mainly sense CO₂/pH; peripheral chemoreceptors are the main O₂ sensors but respond weakly.
- Hyperventilation lowers CO₂ → can cause alkalosis and even fainting/breathing pause. Hypoventilation raises CO₂. Don't flip these.
- RV can't be measured by spirometry directly — it must be inferred (e.g. via TLC − VC).
Quick Recall
What causes negative pressure breathing during inhalation?
What is the equation for vital capacity?
Which chemoreceptors are primarily responsible for day-to-day breathing regulation?
What does surfactant do?