Bell Jar Model & Breathing Mechanics

What is the Boyle’s Law

When volume increases, pressure decreases

When volume decreases, pressure increases

What is lung pressure

Lung pressure is the force that air exerts inside the lungs.

What is lung volume

Lung volume is the amount of space the lungs take up or the amount of air inside them.

What happens when the diaphragm contracts (rubber sheet pulled down)?

• Thoracic volume ↑

• Lung volume ↑ (balloons expand)

• Lung pressure ↓ (Boyle’s Law)

• Air flows into lungs → inhalation

What happens when the diaphragm relaxes (rubber sheet pushed up)?

• Thoracic volume ↓

• Lung volume ↓

• Lung pressure ↑

• Air flows out of lungs → exhalation

Why does lung pressure decrease as you inhale?

Air molecules are always colliding with the lung walls, creating pressure. When you inhale, the lungs expand, so the molecules collide less often, which lowers the pressure.

What is a spirometer?

A device used to measure lung volumes, especially air exhaled.

What is tidal volume (TV)?

The amount of air inhaled or exhaled during normal breathing (~500 mL).

What is inspiratory reserve volume (IRV)?

Extra air you can inhale after a normal inhale.

What is expiratory reserve volume (ERV)?

Extra air you can exhale after a normal exhale.

What is residual volume (RV)?

Air left in lungs after maximum exhalation (you can’t get rid of it).

What is vital capacity (VC)?

The maximum air you can move in or out of lungs.

Formula for the Vital capacity

VC = TV + IRV + ERV

What is total lung capacity (TLC)?

The total air lungs can hold.

Formula for TLC

TLC = VC + RV

What does tidal volume look like on a graph?

Small, regular up-and-down waves (~500 mL).

How are all lung volumes related?

• TV + IRV + ERV = Vital Capacity

• Vital Capacity + RV = Total Lung Capacity

How does height affect lung volumes?

Taller people → larger lungs → higher VC and RV

How does age affect vital capacity?

Older age → decreased VC (lungs lose elasticity)

How does age affect residual volume?

Older age → increased RV (harder to fully exhale)

How does the respiratory system regulate pH?

By controlling CO₂ levels in the blood.

What happens when CO₂ increases in the blood?

CO₂ + H₂O → H₂CO₃ → HCO₃⁻ + H⁺

• ↑ H⁺ → ↓ pH → more acidic

What is the pH formula?

pH = −log[H⁺]

How does pH work?

Higher pH= Basic

Lower pH= Acidic

What is normal blood pH?

7.35 – 7.45 (slightly basic)

What is phenolphthalein?

A pH indicator:

• Pink = basic

• Clear = acidic

What happens when you blow CO₂ into phenolphthalein solution?

• CO₂ dissolves → forms H⁺

• pH decreases

• Color changes pink → clear

(Becomes acidic)

What does this experiment demonstrate?

CO₂ lowers pH, showing how breathing affects blood acidity.

How do breathing patterns affect pH?

Hyperventilation →

Lowered CO₂ levels (Because the body gets more oxygen)→ Higher pH (more basic)

Hypoventilation →

Higher CO₂ levels→ Lower pH (more acidic)

What is the MOST important sequence to remember for breathing?

Volume change → Pressure change → Air movement

Why can’t residual volume be removed?

It prevents lung collapse and keeps alveoli open.

What is the fastest way to remember lung capacity equations?

• VC = “all movable air” (TV + IRV + ERV)

• TLC = “everything” (VC + RV)