Physiology

Mechanics of Breathing

How Air Moves In and Out

Amos Asamoah
October 2025
4 min read
Respiratory System

Every single breath you take — about 12–16 times per minute — is a perfect example of physics meeting biology. Breathing (or pulmonary ventilation) is all about pressure gradients: air moves from high pressure → low pressure areas. So when the pressure inside your lungs drops below atmospheric pressure, air rushes in (inspiration). When it rises above it, air flows out (expiration). Simple, yet profoundly elegant.

⚙️ The Two Phases of Breathing

  1. Inspiration (Inhalation) — active process
  2. Expiration (Exhalation) — usually passive, but can be active when forced

🫁 1️⃣ Inspiration — The Act of Drawing Air In

Inspiration is like opening a bellows — the chest expands, lung volume increases, and air gets sucked in.

Muscles Involved

Type Muscles Role
Primary (quiet breathing) Diaphragm, External intercostals Main drivers
Accessory (deep/forced inspiration) Sternocleidomastoid, Scalenes, Pectoralis minor Lift ribs & sternum to further expand chest

🫀 Step-by-Step Sequence (Quiet Inspiration)

  1. Diaphragm contracts → moves downward ~1.5 cm, increasing thoracic volume.
  2. External intercostals contract → ribs move upward & outward, expanding chest laterally.
  3. Thoracic cavity volume increases → intrapulmonary (alveolar) pressure drops (~–1 to –2 mmHg).
  4. Air flows into lungs until pressures equalize.
Key equation: Airflow = ΔP/R
Where ΔP = pressure difference between atmosphere & alveoli, R = airway resistance.
Clinical insight: During inspiration, intrapleural pressure becomes more negative (~–6 mmHg), keeping lungs expanded.

💨 Forced Inspiration

When exercising or in distress:

  • Accessory muscles (sternocleidomastoid, scalenes, serratus anterior) lift the chest even higher.
  • Diaphragm descends up to 10 cm, expanding lungs massively.

🌬️ 2️⃣ Expiration — The Act of Letting Air Out

Expiration is usually passive — driven by the elastic recoil of lungs and relaxation of inspiratory muscles.

🔹 Quiet Expiration

  1. Diaphragm relaxes → moves upward.
  2. External intercostals relax → ribs move down and in.
  3. Thoracic volume decreases → alveolar pressure rises (+1 to +2 mmHg).
  4. Air flows out until pressure equalizes with atmosphere.
No muscle actively contracts during quiet expiration — it’s purely elastic recoil.

💨 Forced Expiration

During coughing, exercise, or labored breathing, expiration becomes active.

Muscles Involved Action
Internal intercostals Pull ribs downward & inward
Abdominal muscles (rectus abdominis, obliques) Push diaphragm upward by compressing abdominal contents
Clinical tie: In conditions like asthma or COPD, airway resistance ↑ → forced expiration becomes prolonged and difficult.

🧩 Pressures Involved in Breathing

Pressure Type Definition Normal Values
Atmospheric pressure (Patm) Pressure of air around us 760 mmHg (sea level)
Intrapulmonary (alveolar) pressure (Palv) Pressure within alveoli –1 mmHg (inspiration) → +1 mmHg (expiration)
Intrapleural pressure (Pip) Pressure in pleural cavity ~ –4 mmHg at rest; becomes more negative during inspiration
Transpulmonary pressure (Ptp) Difference between alveolar and intrapleural pressure Keeps lungs inflated (Palv – Pip)
High-yield: When Pip = Patm, the lung collapses → pneumothorax.

🎯 Role of Elastic and Surface Forces

🧠 1. Lung Compliance

  • The ease with which lungs expand.
  • Defined as: Compliance (C) = ΔV / ΔP
  • Normal: ~0.2 L/cmH₂O.
↓ Compliance: Fibrosis, pulmonary edema. ↑ Compliance: Emphysema (loss of elastic tissue).

🫧 2. Elastic Recoil

  • Tendency of lungs to collapse after being stretched.
  • Provided by elastin fibers and surface tension inside alveoli.

🫁 3. Surfactant

  • Produced by Type II pneumocytes.
  • Reduces alveolar surface tension → prevents collapse, especially during expiration.
Clinical note: Surfactant deficiency in premature infants → Neonatal Respiratory Distress Syndrome (RDS).

⚙️ Lung and Chest Wall Relationship

The lungs and chest wall are connected by pleural fluid tension. They move together — when the chest expands, the lungs follow. This interaction creates the negative intrapleural pressure essential for breathing.

Clinical link: If air enters the pleural space → intrapleural pressure = atmospheric → lung collapses (pneumothorax).

🧠 Work of Breathing

Breathing isn’t free — it consumes energy, especially when resistance or stiffness increases.

Component Description
Elastic work Expanding lungs against recoil forces
Airway resistance work Overcoming friction in airways
Tissue resistance work Overcoming viscosity of lung and chest tissues
Normally breathing uses 2–3% of total body O₂, but in respiratory distress → may rise to 30%!

🩺 Factors Affecting Breathing Mechanics

Factor Effect
Airway resistance ↑ in asthma, COPD → difficult expiration
Lung compliance ↓ in fibrosis, ↑ in emphysema
Surfactant ↓ surface tension → prevents collapse
Elastic recoil Helps expiration
Chest wall elasticity Stiffness ↑ with aging or deformity

💨 High-Yield Summary Table

Phase Muscles Active Pressure Change Air Movement
Quiet inspiration Diaphragm, external intercostals ↓ Alveolar pressure Air in
Quiet expiration None (passive recoil) ↑ Alveolar pressure Air out
Forced inspiration SCM, scalenes, pectoralis ↓↓ Pressure Deep air in
Forced expiration Abdominals, internal intercostals ↑↑ Pressure Air out fast
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