How Breathing Works: The Mechanics of Inhalation and Exhalation

Neural Control

Breathing is driven by the respiratory centers in the brainstem (medulla and pons). They generate the basic rhythm and integrate inputs from:

• Chemoreceptors: Central (medulla) and peripheral (carotid/aortic bodies) detect CO₂, H⁺, O₂.
• Lung receptors: Stretch receptors (Hering-Breuer reflex), irritant receptors, J-receptors.
• Higher brain centers: Voluntary control (cortex), emotional influence (limbic system).
• Reflexes: Diving reflex, gag reflex, sneezing, coughing.

Mechanics of a Breath

1. Inspiration (active): Diaphragm contracts → descends; external intercostals lift ribs → thoracic volume ↑ → intrapulmonary pressure ↓ → air flows in.
2. Expiration (passive at rest): Diaphragm relaxes; elastic recoil of lungs/chest wall → volume ↓ → pressure ↑ → air flows out. Forced expiration uses internal intercostals and abdominal muscles.

Key Influences on Rate and Depth

• CO₂ (primary driver): ↑PaCO₂ → ↑ventilation (hypercapnia). ↓PaCO₂ (hypocapnia) → ↓ventilation.
• O₂ (secondary): Significant hypoxemia (PaO₂ <60 mmHg) stimulates breathing via peripheral chemoreceptors.
• pH: Acidosis ↑ ventilation; alkalosis ↓ ventilation.
• Exercise: ↑CO₂ production, ↑temperature, neural feedforward → ↑ventilation.
• Sleep: Reduced chemosensitivity, loss of voluntary drive → mild hypoventilation.

This integrated control maintains blood gas homeostasis across diverse conditions.