Explain inhalation and exhalation pressure changes

WHY does air move at all?

WHAT: Air moves because of a pressure gradient between the atmosphere and the alveoli (air sacs inside lungs).

WHY: Gases spread to fill space. If one region is at lower pressure, gas from a higher-pressure region rushes in to equalise. The atmosphere is the "outside" at a fixed pressure ($P_{atm} \approx 760$ mmHg at sea level). Your lungs are the "inside" at pressure $P_{alv}$.

• If $P_{alv} < P_{atm}$ → air flows IN (inhalation)
• If $P_{alv} > P_{atm}$ → air flows OUT (exhalation)
• If $P_{alv} = P_{atm}$ → no net flow

So the whole game is: how do we make $P_{alv}$ go below or above $P_{atm}$? Answer: change the lung volume.

HOW volume controls pressure — deriving from Boyle's Law

Derive it from first principles. Pressure is caused by gas molecules hitting the container walls. If you shrink the container (smaller $V$), the same number of molecules hit the walls more often per second → higher pressure. If you enlarge the container, hits become rarer → lower pressure. The number of molecules $n$ and temperature $T$ (their speed) stay fixed during a quick breath, so:

$PV = nRT = \text{constant}$

$\Rightarrow \boxed{P_1 V_1 = P_2 V_2}$

This is the ENTIRE mechanism. Everything below is just which muscles change the volume.

HOW the muscles change chest volume

The lungs stick to the inside of the chest wall via a thin fluid layer (pleural fluid), so when the chest expands, the lungs are dragged open with it.

Two muscle groups:

Inhalation (active — needs energy)

1. Diaphragm contracts → flattens, moves down.
2. External intercostals contract → ribcage moves up and out.
3. Thoracic volume increases.
4. By Boyle: $P_{alv}$ drops below $P_{atm}$ (about $-1$ mmHg, i.e. 759 mmHg).
5. Air flows IN down the gradient.

Exhalation (usually passive — no energy at rest)

1. Diaphragm and external intercostals relax.
2. Elastic lungs and ribcage recoil back to smaller shape.
3. Thoracic volume decreases.
4. By Boyle: $P_{alv}$ rises above $P_{atm}$ (about $+1$ mmHg, i.e. 761 mmHg).
5. Air flows OUT down the gradient.

(Forced exhalation adds: internal intercostals + abdominal muscles squeeze harder — this one is active.)

Intrapulmonary vs Intrapleural pressure

WHY intrapleural is always negative: the elastic lungs constantly try to collapse inward while the chest wall pulls outward — this tug-of-war creates a partial vacuum in the pleural space. If it were punctured (pneumothorax), the lung would collapse.

Worked examples

Common mistakes

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Flashcards

Connections

• Boyle's Law — the physics engine behind breathing
• Diaphragm and Intercostal Muscles — the volume changers
• Gas Exchange in Alveoli — what happens after air arrives
• Pressure Gradient and Diffusion — same "high→low" principle
• Spirometry and Lung Volumes — measuring these changes
• Pneumothorax — what happens when intrapleural pressure fails

Concept Map

Hinglish (regional understanding)