Describe the mechanism of breathing in humans, including the role of the ribs and diaphragm

What this dot point is asking

SEAB wants you to describe the mechanism of breathing (ventilation): how the diaphragm and the intercostal muscles change the volume and pressure of the chest to move air in and out. You should clearly distinguish breathing from respiration and explain why ventilation is needed to keep gas exchange going.

The answer

Breathing is not respiration

Breathing (ventilation) is the movement of air into and out of the lungs. Respiration is the chemical release of energy from glucose in cells. Breathing supplies the oxygen that respiration needs and removes the carbon dioxide it makes; they are linked but different.

The parts involved

The chest (thorax) is a sealed box. Its floor is the diaphragm, a sheet of muscle. Around the sides are the ribs, moved by the intercostal muscles between them. Changing the size of this box changes the air pressure inside, which moves air in or out.

Breathing in (inhalation)

1. The diaphragm contracts and flattens (moves down).
2. The external intercostal muscles contract, pulling the ribs up and out.
3. These movements increase the volume of the chest.
4. A larger volume means a lower pressure inside the chest, below the air pressure outside.
5. Air moves into the lungs from the higher pressure outside, down the pressure gradient, and the lungs inflate.

Breathing out (exhalation)

1. The diaphragm relaxes and curves up; the intercostal muscles relax, so the ribs move down and in.
2. These movements decrease the volume of the chest.
3. A smaller volume means a higher pressure inside the chest, above the pressure outside.
4. Air moves out of the lungs, down the pressure gradient.

In short: chest gets bigger, pressure drops, air rushes in; chest gets smaller, pressure rises, air pushes out.

Why ventilation matters

Breathing constantly brings in fresh oxygen-rich air and removes carbon-dioxide-rich air. This keeps the oxygen high and carbon dioxide low in the alveoli relative to the blood, so the steep gradients for gas exchange are maintained and the blood can keep supplying oxygen for respiration.

Examples in context

Example 1. The bell-jar model. A bell jar with balloons inside and a rubber sheet across the bottom models the chest: pulling the sheet down (like the diaphragm flattening) increases the volume, lowers the pressure, and inflates the balloons, just as inhalation inflates the lungs.

Example 2. Breathing harder during exercise. During exercise the diaphragm and intercostal muscles work harder and faster, moving more air per breath and more breaths per minute. This supplies extra oxygen for the increased respiration in the muscles.

Try this

Q1. State the difference between breathing and respiration. [2 marks]

• Cue. Breathing is the movement of air into and out of the lungs; respiration is the chemical release of energy from glucose in cells.

Q2. Describe what the diaphragm does during inhalation. [1 mark]

• Cue. It contracts and flattens (moves down), increasing the volume of the chest.

Q3. Explain why air enters the lungs when the chest volume increases. [2 marks]

• Cue. A larger volume lowers the pressure inside the chest below the pressure outside, so air moves in from the higher outside pressure down the gradient.