Grade by Grade
E. Animals: Structure and Function · Animals: Structure and Function

Breathing Mechanics: Pressure, Volume and Lung Capacity

E3.1 E2.1 E2.3 A1.5 A1.6 A1.12 A1.13About 70 minutes
Learning goals
  • I can explain inhalation and exhalation using Boyle's law and the actions of the diaphragm and intercostal muscles.
  • I can explain the roles of the pleural membranes and surfactant.
  • I can read a spirogram and define tidal volume, inspiratory and expiratory reserve volumes, vital capacity and residual volume.
  • I can calculate minute ventilation and alveolar ventilation.

Words to know

ventilation
The movement of air into and out of the lungs (breathing).
inhalation (inspiration) / exhalation (expiration)
Breathing in / breathing out.
Boyle's law
At constant temperature, the pressure of a gas is inversely proportional to its volume: P₁V₁ = P₂V₂.
intercostal muscles
Muscles between the ribs. External intercostals lift the ribs during inhalation; internal intercostals pull them down in forced exhalation.
pleural membranes
Two thin membranes, one on the lung and one lining the chest wall, with a film of fluid between them that makes the lungs follow the chest wall.
surfactant
A substance made in the alveoli that lowers surface tension so the alveoli don't collapse.
tidal volume
The volume of air breathed in or out in one normal, relaxed breath (about 500 mL in an adult).
vital capacity
The largest volume of air that can be breathed out after the deepest possible breath in: tidal volume + inspiratory reserve volume + expiratory reserve volume.

You don't suck air in

It feels as if you pull air into your lungs. You don't. Your lungs have no muscles. What you actually do is make your chest cavity bigger, which lowers the pressure inside your lungs, and the atmosphere pushes air in. Breathing is a pressure problem, and the physics behind it is a gas law you may meet again in chemistry.

Remember from Grade 10

Air travels nose → pharynx → larynx → trachea → bronchi → bronchioles → alveoli. The diaphragm contracts and flattens and the rib muscles lift the ribs to inhale; they relax to exhale. Now we add the numbers: pressures, Boyle's law, and the volumes of air your lungs move.

Boyle's law

For a fixed amount of gas at constant temperature, pressure × volume stays constant: P₁V₁ = P₂V₂. If the volume of a container gets bigger, the gas particles spread out and hit the walls less often, so the pressure falls. If the volume gets smaller, the pressure rises.

Air always moves from higher pressure to lower pressure. So:

  • Inhalation: the diaphragm contracts and moves down; the external intercostal muscles contract and lift the ribs up and out. The chest cavity's volume increases, so pressure in the lungs drops slightly below atmospheric pressure (by only about 1 to 3 mm Hg in quiet breathing). Air flows in until the pressures are equal.
  • Exhalation (at rest): the muscles relax. The diaphragm domes up, the ribs fall, and the stretchy lung tissue recoils. Volume decreases, pressure rises slightly above atmospheric, and air flows out. Quiet exhalation is passive: it uses no muscle contraction.
  • Forced exhalation (blowing out candles, coughing, hard exercise) is active: the internal intercostals pull the ribs down and abdominal muscles push the diaphragm up.
air inair outdiaphragm contracts, flattensINHALATIONchest volume ↑, lung pressure ↓EXHALATIONchest volume ↓, lung pressure ↑diaphragm relaxes, domes upribs move up and outribs move down and in
Inhalation (left): the diaphragm flattens and the ribs move up and out, enlarging the chest. Exhalation (right): the diaphragm relaxes into a dome and the ribs fall, shrinking the chest. (Simplified front view; not to scale.)
outside air760 mm Hginside lungsinhaling: about 758exhaling: about 762inhalationexhalationAir always flows from higher pressure to lower pressure.
Normal air pressure at sea level is about 760 mm Hg (101.3 kPa). Very small pressure differences are enough to move air in and out. (Typical values for quiet breathing.)

Why the lungs follow the chest

Each lung is wrapped in a thin pleural membrane, and another lines the inside of the chest wall. A thin film of pleural fluid between them lets them slide smoothly, but also makes them stick together, like two wet panes of glass. So when the chest wall moves out, the lungs are pulled out with it. If air gets into the space between the membranes (for example, from a chest wound or a burst air blister), the seal is lost and the lung can collapse. This is called a pneumothorax.

Inside each alveolus, the moist lining has surface tension, which tends to pull the tiny sacs closed. Special alveolar cells make surfactant, which lowers the surface tension and keeps alveoli open. Babies born very early may not have made enough surfactant yet and can have serious trouble breathing; they can be given surfactant through a breathing tube.

Worked example

Example 1: Boyle's law and the chest

Before a breath, the lungs hold 2.40 L of air at 760 mm Hg. Imagine the chest expands the lungs to 2.45 L before any air flows in (a thought experiment). (a) What would the new pressure be? (b) Why is the real pressure drop much smaller?

  1. Use P₁V₁ = P₂V₂, so P₂ = P₁V₁ ÷ V₂.
  2. P₂ = (760 mm Hg × 2.40 L) ÷ 2.45 L = 1824 ÷ 2.45 = 744.49… ≈ 744 mm Hg (three significant figures).
  3. That would be a drop of about 16 mm Hg.
  4. (b) In reality, air starts flowing in as soon as the pressure begins to fall, so the pressure never drops that far. In quiet breathing the drop is only about 1 to 3 mm Hg.

(a) About 744 mm Hg. (b) Air flows in continuously as the chest expands, so the pressure difference stays small.

Measuring the lungs: the spirogram

A spirometer measures the volumes of air you breathe. Its graph is a spirogram. Typical values for a healthy young adult (they vary with body size, age, sex and fitness):

  • Tidal volume (TV): a normal relaxed breath, about 500 mL.
  • Inspiratory reserve volume (IRV): the extra you can breathe in after a normal breath, about 3000 mL.
  • Expiratory reserve volume (ERV): the extra you can force out after a normal breath, about 1200 mL.
  • Vital capacity (VC) = TV + IRV + ERV, about 4700 mL here.
  • Residual volume (RV): air that always stays in the lungs, even after the hardest breath out, about 1200 mL. It keeps the alveoli from collapsing and keeps gas exchange going between breaths.
  • Total lung capacity = VC + RV, about 5900 mL here.

Doctors also measure how fast you can blow out. FEV₁ is the volume you can force out in the first second; in healthy lungs it is most of the vital capacity (the FEV₁/FVC ratio is usually above 0.70 in adults). In diseases that narrow the airways, such as asthma and COPD, the ratio drops (see the respiratory disorders lesson).

0100020003000400050006000lung volume (mL)timeIRV3000 mLTV 500ERV1200 mLRV1200 mLVC4700
A spirogram: three normal breaths, one deepest breath in and one hardest breath out, then normal breathing again. Brackets show the lung volumes in mL. (Typical adult values; vital capacity = 500 + 3000 + 1200 = 4700 mL.)

How much air do you move?

  • Minute ventilation = tidal volume × breathing rate. At rest: 500 mL × 12 breaths/min = 6000 mL/min (6 L/min).
  • Not all of that air reaches the alveoli. About 150 mL of each breath stays in the trachea, bronchi and bronchioles, where no gas exchange happens. This is the anatomical dead space.
  • Alveolar ventilation = (tidal volume − dead space) × breathing rate = (500 − 150) × 12 = 4200 mL/min.

That is why deeper breaths are more efficient than faster, shallow ones: each breath "pays" the same 150 mL of dead space.

Worked example

Example 2: Shallow vs deep breathing

Two people each have a minute ventilation of 6.0 L/min. Person A breathes 30 times a minute with a tidal volume of 200 mL. Person B breathes 10 times a minute with a tidal volume of 600 mL. Dead space is 150 mL for both. Find each person's alveolar ventilation.

  1. Check minute ventilation: A: 200 × 30 = 6000 mL/min ✓. B: 600 × 10 = 6000 mL/min ✓.
  2. A: (200 − 150) × 30 = 50 × 30 = 1500 mL/min.
  3. B: (600 − 150) × 10 = 450 × 10 = 4500 mL/min.
  4. B gets 3 times as much fresh air to the alveoli, even though both move the same total air.

A: 1.5 L/min; B: 4.5 L/min. Slow, deep breathing ventilates the alveoli far better than fast, shallow breathing.

Hands-on activity

Measure your vital capacity (bottle method)

Grown-up note: do this sitting down; stop if you feel dizzy. Use your own clean tube; don't share.

You need: a large clear plastic jug or bottle (4 L is ideal; a 2 L bottle works if you stop when it's empty), a marker, a measuring cup, a sink or large basin, and about 60 cm of clean flexible tubing (from a hardware or aquarium store) or several bendable straws taped together.

  1. Calibrate: pour water into the jug 250 mL at a time, marking the level each time.
  2. Fill the jug to the top, cover the opening with your hand, turn it upside down in a basin half full of water, and remove your hand under water. It should stay full.
  3. Slide one end of the tube up inside the jug.
  4. Breathe in as deeply as you can, then breathe out as hard and as long as you can through the tube. The air pushes water out.
  5. Read the volume of air in the jug from your marks. Refill and repeat 3 times. Record all trials and the average.
  6. Then measure a tidal volume: breathe out normally through the tube after a normal breath in.

What to look for: many teens find a vital capacity of roughly 3 to 5 L, depending on size and the equipment; a tidal volume of roughly 0.4 to 0.6 L. Discuss sources of error: leaks at your lips, air you breathed out before reaching the tube, reading the marks at an angle, and the jug being too small. A medical spirometer is more precise; this method is a model.

Just for fun

Riddle: I'm always in your lungs, even when you've blown out every bit of air you can. What am I?

Your residual volume, about 1.2 L of air that just refuses to leave the party.

Your turn

1

State Boyle's law.

2

Describe what the diaphragm and external intercostal muscles do during inhalation.

3

Why does air flow into the lungs when the chest expands?

4

Is quiet exhalation active or passive? Explain.

5

What is the job of surfactant?

6

A person has TV = 450 mL, IRV = 2800 mL and ERV = 1100 mL. Find the vital capacity.

7

Calculate minute ventilation for a tidal volume of 0.50 L and a rate of 15 breaths/min.

More practice: warm-up

1

What does a spirometer measure?

2

What is the residual volume?

3

What is normal atmospheric pressure at sea level in mm Hg?

4

Which muscles help with forced exhalation?

5

What do the pleural membranes do?

More practice: on your own

1

A balloon holds 1.20 L of air at 760 mm Hg. It is squeezed to 1.00 L at the same temperature. What is the new pressure?

2

Explain why a stab wound to the chest can cause a lung to collapse.

3

A person has a tidal volume of 450 mL, a dead space of 150 mL and a rate of 14 breaths/min. Find the alveolar ventilation.

4

Explain why a very premature baby may need help to breathe.

5

Why can't vital capacity be measured by breathing in from a spirometer after a normal exhale, then out? (Hint: think about ERV.)

Challenge

1

A trained swimmer and an untrained student both breathe at rest with a minute ventilation of 6 L/min. The swimmer breathes 8 times a minute; the student breathes 15 times a minute. Dead space is 150 mL for both. Compare their alveolar ventilation, and explain why the comparison favours slow, deep breathing.

2

The lungs and a closed jar both follow Boyle's law. Explain why the jar's pressure changes a lot when you change its volume, but the lungs' pressure changes only slightly.

3

In the bottle activity, a student measures 3.2 L, 3.6 L and 3.7 L. Report the result with the right precision, and suggest why the first trial may be low.

Check your understanding

Quiz

1.

According to Boyle's law, if the volume of the chest cavity increases, the pressure in the lungs:

2.

During quiet inhalation, the diaphragm:

3.

Vital capacity equals:

4.

What does surfactant do?

5.

A person with a tidal volume of 500 mL breathes 16 times a minute. What is the minute ventilation?

6.

Why is quiet exhalation called passive?

For parents and teachers

Big idea: E3.1 asks students to explain ventilation and the roles of pressure differences. This lesson connects breathing to Boyle's law and teaches lung volumes, which students measure (E2.3) and use in calculations with correct units and precision (A1.12, A1.13). Terms from E2.1: inhalation, exhalation.

How to help: Do the bottle activity together; it is memorable and works best with a helper holding the jug. Ask your teen to explain why breathing is a "pressure problem."

Common difficulties:

  • Saying the lungs suck air in. The chest enlarges and the atmosphere pushes air in.
  • Mixing up vital capacity and total lung capacity. Residual volume is not part of vital capacity.
  • Forgetting dead space. Not all inhaled air reaches the alveoli.

Safety: sit down for breathing tests and stop if dizzy. Don't share tubes. Students with asthma or other lung conditions should take part only as comfortable.