Explain the mechanics of breathing and gas exchange, and describe how breathing rate and tidal volume change with exercise

What this dot point is asking

SEAB wants you to explain how breathing moves air in and out of the lungs, how gas exchange happens at the alveoli, and how breathing changes during exercise. The central idea is that breathing is driven by pressure differences created by muscle action, and that gas exchange relies on diffusion down concentration gradients, both of which ramp up when you exercise.

The answer

The mechanics of breathing

Breathing works by changing the volume of the chest, which changes the pressure inside the lungs.

Inspiration (breathing in):

• The diaphragm contracts and flattens, moving down.
• The external intercostal muscles contract, pulling the ribcage up and out.
• The chest volume increases, so the pressure inside the lungs falls below atmospheric pressure.
• Air flows in to equalise the pressure.

Expiration (breathing out):

• The diaphragm relaxes and curves back up.
• The intercostal muscles relax, so the ribcage drops down and in.
• The chest volume decreases, so the pressure inside the lungs rises above atmospheric pressure.
• Air flows out.

During hard exercise, expiration becomes active too, with extra muscles forcing air out faster.

Gas exchange by diffusion

Gas exchange at the alveoli happens by diffusion: gases move from where they are in high concentration to where they are in low concentration.

• Oxygen is high in the alveolar air and low in the blood arriving from the body, so oxygen diffuses from the alveoli into the blood.
• Carbon dioxide is high in that blood and low in the alveolar air, so carbon dioxide diffuses from the blood into the alveoli to be breathed out.

How breathing changes with exercise

Exercise raises the body's demand for oxygen and its production of carbon dioxide, so breathing increases in two ways.

• Breathing rate (breaths per minute) goes up.
• Tidal volume (the air per breath) goes up.

Together these raise minute ventilation, the total air breathed per minute:

Examples in context

Example 1. A games player after a hard sprint. Breathing is fast and deep: rate and tidal volume are both high, so minute ventilation is many times the resting value. The deeper breaths and faster rate clear the extra carbon dioxide produced and replace the oxygen the muscles used.

Example 2. A diver controlling their breathing before submerging. By breathing slowly and deeply, the diver maximises tidal volume to load oxygen, then holds the breath. Gas exchange continues in the alveoli during the hold, but with no fresh air the oxygen concentration in the blood gradually falls.

Try this

• Cue. Describe what the diaphragm and ribcage do during expiration. (The diaphragm relaxes and rises, the ribcage drops down and in, chest volume falls and air is pushed out.)

• Cue. State the direction oxygen and carbon dioxide each diffuse at the alveoli. (Oxygen diffuses from alveoli into blood; carbon dioxide diffuses from blood into alveoli.)

• Cue. Calculate minute ventilation for 25 breaths per minute at a tidal volume of 1.8 litres. ($25 \times 1.8 = 45$ litres per minute.)