Mechanism of Breathing — Inhalation and Exhalation Explained

Question

Explain the mechanism of breathing in humans. Describe the role of the diaphragm and intercostal muscles during inhalation (inspiration) and exhalation (expiration). How do pressure changes drive airflow?

Solution — Step by Step

Step 1: The Core Principle — Pressure Drives Airflow

The lungs themselves cannot move. They are elastic, passive organs that expand or compress based on the volume of the thoracic (chest) cavity. The key principle is Boyle's Law:

At constant temperature: Pressure × Volume = constant

So: Volume ↑ → Pressure ↓ (and vice versa)

When the thoracic cavity expands, the lungs expand too, intrapulmonary pressure drops below atmospheric pressure, and air flows in. When the thoracic cavity shrinks, lungs compress, pressure rises above atmospheric, and air flows out.

The muscles of breathing create these volume changes. Air does the rest.

Step 2: Muscles Involved in Breathing

Primary muscles:

• Diaphragm: Dome-shaped muscle forming the floor of the thoracic cavity. The most important respiratory muscle.
• External intercostal muscles: Between the ribs; tilt ribs upward and outward

Accessory muscles (for forced/deep breathing):

• Internal intercostal muscles: Between ribs; pull ribs downward (used in forced expiration)
• Sternocleidomastoid and scalene muscles: Neck muscles; used in deep inspiration
• Abdominal muscles: Compress the abdomen upward; used in forced expiration

Step 3: Inhalation (Inspiration)

Quiet inhalation — step by step:

1. Diaphragm contracts → dome shape flattens downward → vertical dimension of thoracic cavity increases (chest gets taller)

2. External intercostal muscles contract → ribs and sternum move upward and outward → anteroposterior and lateral dimensions of thoracic cavity increase (chest widens)

3. Net effect: Thoracic cavity volume increases → lungs expand (they are attached to thoracic wall via pleura and follow its movements)

4. Intrapulmonary pressure drops below atmospheric pressure (atmospheric ≈ 760 mmHg; intrapulmonary drops to ≈ 758 mmHg during inhalation)

5. Air flows in from atmosphere (high pressure) into lungs (lower pressure) until pressures equalise

Deep/forced inhalation: Additionally, accessory neck muscles (scalenes, sternocleidomastoid) and pectoralis minor contract → elevate ribs and sternum further → greater volume increase → more air inhaled.

Step 4: Exhalation (Expiration)

Quiet exhalation — a passive process:

1. Diaphragm relaxes → elastic recoil returns it to dome shape → vertical dimension decreases

2. External intercostal muscles relax → ribs and sternum return to resting position → thoracic cavity narrows

3. Lung elastic recoil: Lung tissue is elastic (contains elastin fibres). When lungs expand during inhalation, they store elastic potential energy. On exhalation, this energy is released → lungs recoil inward.

4. Net effect: Thoracic cavity volume decreases → lungs compress

5. Intrapulmonary pressure rises above atmospheric (≈ 762 mmHg)

6. Air flows out from lungs (high pressure) to atmosphere (lower pressure)

Normal quiet exhalation requires NO active muscle contraction — it is driven entirely by passive elastic recoil of the lungs and chest wall.

Forced exhalation (during exercise, coughing, sneezing, singing, playing wind instruments):

1. Internal intercostal muscles contract → pull ribs downward and inward
2. Abdominal muscles contract → compress abdomen upward → pushes diaphragm further into thoracic cavity
3. These actions forcefully decrease thoracic volume → greater rise in intrapulmonary pressure → faster, larger volume of air expelled

Why This Works — The Pleura's Role

The lungs don't attach directly to the chest wall — they're enclosed in the pleural cavity, a space between the visceral pleura (covering the lung) and parietal pleura (lining the chest wall).

The pleural cavity contains a thin film of pleural fluid that creates surface tension between the two membranes. This surface tension makes the lungs follow every movement of the chest wall — when the chest expands, the lungs expand with it; when the chest contracts, the lungs contract.

Pneumothorax (collapsed lung): If air enters the pleural cavity (e.g., from a stab wound or a ruptured air sac), the surface tension is broken, the lung separates from the chest wall, and it collapses. The lung can no longer follow the chest wall's expansion movements — no air enters → life-threatening if not treated.

Breathing Rates and Control

Normal breathing rate: 12–20 breaths/minute at rest During exercise: can increase to 40–60 breaths/minute

Breathing is controlled by the respiratory centre in the medulla oblongata (brainstem). The medulla detects rising CO₂ (falling pH) in the blood via chemoreceptors → sends nerve impulses to the diaphragm and intercostal muscles → increases breathing rate and depth.

It's the CO₂ concentration (not O₂) that primarily drives the urge to breathe. This is why you feel "out of breath" when CO₂ builds up during exercise, even before O₂ levels drop significantly.

Alternative Method — The Balloon in a Jar Model

A classic demonstration model:

• Large balloon (lung) inside a sealed jar (thoracic cavity)
• Elastic sheet at the bottom (diaphragm)
• Pull the elastic sheet down → jar volume increases → balloon inflates (inhalation)
• Release elastic sheet → jar volume decreases → balloon deflates (exhalation)

This model makes the pressure-driven mechanism intuitive: the balloon doesn't actively pump — it responds to the changing volume of the container around it.

Common Mistake