Breathing Mechanics

7 min read Updated Mar 26, 2026

Here is a question that trips up many students: do you actively suck air into your lungs? No. You create a pressure difference, and the atmosphere pushes air in for you. Breathing is entirely about pressure gradients, and the physics behind it is Boyle’s Law - one of the simplest gas laws you will encounter on the MCAT.

Diagram showing the mechanics of inspiration and expiration, with diaphragm and intercostal muscle positions during each phase, and corresponding pressure changes — Mechanics of breathing: during inspiration, the diaphragm contracts and flattens, expanding the thoracic cavity and drawing air in. During expiration, the diaphragm relaxes and the elastic recoil of the lungs pushes air out. *Credit: OpenStax Anatomy & Physiology 2e, CC BY 4.0*

Detailed illustration of the diaphragm and rib cage positions during inspiration and expiration, showing the bucket-handle motion of the ribs — The diaphragm and rib cage during breathing. Note the bucket-handle motion of the ribs during inspiration (external intercostals contract, lifting ribs up and out). *Credit: Lumen Learning / OpenStax Anatomy & Physiology, CC BY 4.0*

Inspiration (Inhalation)

Inspiration is an active process. It requires muscle contraction and energy expenditure. Here is what happens in sequence:

Step 1: Muscle contraction. The diaphragm (the dome-shaped muscle separating the thoracic and abdominal cavities) contracts and flattens downward, increasing the vertical dimension of the thoracic cavity. Simultaneously, the external intercostal muscles contract, pulling the ribs upward and outward (think of a bucket handle lifting), increasing the anteroposterior and lateral dimensions.

Step 2: Volume increases. The thoracic cavity expands in all three dimensions.

Step 3: Pressure drops. Per Boyle’s Law, as volume increases, intrapulmonary pressure (pressure inside the lungs) drops below atmospheric pressure. This creates a negative pressure gradient.

Step 4: Air rushes in. Air always flows from high pressure to low pressure. Since atmospheric pressure (760 mmHg at sea level) is now higher than intrapulmonary pressure, air flows into the lungs until pressures equalize.

Expiration (Exhalation)

Quiet expiration is a passive process. It requires no muscle contraction:

Step 1: Muscles relax. The diaphragm relaxes and domes upward. The external intercostal muscles relax, and the ribs move down and inward due to gravity and elastic recoil of the chest wall.

Step 2: Volume decreases. The thoracic cavity shrinks.

Step 3: Pressure rises. Per Boyle’s Law, decreased volume means increased intrapulmonary pressure, which now exceeds atmospheric pressure.

Step 4: Air is pushed out. Air flows from the higher-pressure lungs to the lower-pressure atmosphere.

The key insight: quiet expiration is driven entirely by elastic recoil of the lungs and chest wall. The lungs are stretched during inspiration like a rubber band, and they snap back passively. No energy required.

Forced Expiration

During exercise, coughing, or any situation requiring rapid, forceful exhalation, forced expiration becomes an active process:

Internal intercostal muscles contract, pulling the ribs downward and inward
Abdominal muscles (rectus abdominis, obliques) contract, pushing the abdominal organs upward against the diaphragm

These muscles actively compress the thoracic cavity, rapidly decreasing volume and increasing pressure far above atmospheric pressure, forcing air out quickly.

Pressure Changes During Breathing

Understanding the three pressures involved in breathing is essential:

Pressure	Definition	During Inspiration	During Expiration
Atmospheric ( $P_{\text{atm}}$ )	Pressure of outside air	760 mmHg (constant)	760 mmHg (constant)
Intrapulmonary ( $P_{\text{alv}}$ )	Pressure inside the alveoli	Drops below 760 mmHg	Rises above 760 mmHg
Intrapleural ( $P_{\text{ip}}$ )	Pressure in the pleural space	Drops further negative (~-6 mmHg)	Less negative (~-4 mmHg)

The critical relationship: intrapleural pressure is always more negative than intrapulmonary pressure during normal breathing. This keeps the lungs inflated. If intrapleural pressure ever equals atmospheric pressure (as in a pneumothorax), the lung collapses.

The Role of the Diaphragm

The diaphragm is the primary muscle of respiration. It is innervated by the phrenic nerve, which arises from cervical spinal cord levels C3, C4, and C5.

Compliance and Elastance

Two opposing properties determine how easily the lungs move:

Compliance = how easily the lungs stretch. High compliance = inflate easily. Emphysema destroys alveolar walls and elastic tissue, so compliance goes up.
Elastance = how readily the lungs snap back. High elastance = strong recoil. Pulmonary fibrosis deposits stiff scar tissue, so elastance goes up.

The two trade off: emphysema is easy to inflate but hard to deflate (air trapping). Fibrosis is hard to inflate but easy to deflate (restrictive pattern). Surfactant raises compliance by lowering surface tension, so its absence (preterm RDS) makes the alveoli hard to inflate.

During heavy exercise or respiratory distress, accessory muscles (neck muscles for inspiration, abdominals for forced expiration) join in to help the primary respiratory muscles.

Is quiet expiration active or passive, and what drives it?

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Quiet expiration is passive. It is driven by the elastic recoil of the lungs and chest wall, not by muscle contraction. The stretched lung tissue snaps back like a rubber band, decreasing thoracic volume, increasing intrapulmonary pressure above atmospheric, and pushing air out. Muscle contraction is only needed for forced expiration.

A patient with a C4 spinal cord injury can still breathe independently, but a patient with a C2 injury cannot. Why?

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The phrenic nerve originates from C3-C5. A C4 injury preserves some phrenic nerve function, allowing partial diaphragm contraction. A C2 injury is above all phrenic nerve roots, completely paralyzing the diaphragm. Without diaphragm function, the patient cannot generate the negative pressure needed for inspiration and requires mechanical ventilation.