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The Diaphragm and Breathing: How It Works

The diaphragm is the primary muscle of inspiration. Its contraction increases thoracic volume, decreasing intrathoracic pressure (Boyle's law), drawing air in.

Mechanics

  1. Contraction: Diaphragm flattens, descends 1–2 cm (quiet) to 10 cm (deep). Thoracic cavity expands vertically.
  2. Pressure change: Palv drops below atmospheric → inflow.
  3. Relaxation: Elastic recoil of lungs/chest wall + abdominal pressure pushes diaphragm up → passive expiration (quiet breathing).

Accessory Muscles

  • Inspiration: External intercostals, scalenes, SCM, pectoralis minor (forced).
  • Expiration: Internal intercostals, abdominals (rectus, obliques, transversus) — forced expiration, cough, exercise.

Neural Control

  • Medullary centers: Dorsal respiratory group (inspiratory), ventral respiratory group (expiratory).
  • Pontine centers: Pneumotaxic (rate), apneustic (depth).
  • Chemoreceptor drive: Central (CO₂/pH), peripheral (O₂/CO₂/pH).
  • Voluntary override: Motor cortex → phrenic nerve.

Factors Affecting Rate/Depth

  • Metabolic demand (exercise, fever).
  • PaCO₂ (primary), PaO₂ (secondary), pH.
  • Emotion, pain, voluntary control, lung mechanics.

Key Takeaway

The diaphragm is the engine of breathing; its rhythmic contraction, driven by brainstem centers and chemoreceptor feedback, sustains gas exchange.

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