Gaseous Exchange in Humans and Plants
Senior 2 · Biology
Gaseous Exchange in Humans and Plants
Lesson 2 of 5
22 min
Let's begin
Run once round the school field and your chest heaves: you breathe faster and deeper to take in more oxygen and get rid of more carbon dioxide. But what actually moves the air? The lungs have no muscles of their own. They are filled and emptied by changes in the size of the chest.
Scientists use a simple model to show how breathing works. Two balloons (the lungs) hang from a Y-shaped tube (the trachea and bronchi) inside a glass bell jar (the chest). A rubber sheet across the bottom stands for the diaphragm.
Worked example
What happens when the rubber sheet is pulled down, and why?
The rule behind this is Boyle's law: in a closed space, when the volume increases the pressure decreases, and when the volume decreases the pressure increases.
The model has limits: the jar cannot move, while real ribs move up and out; and real lungs are elastic tissue with fluid-filled pleural membranes, not balloons in air.
Quiet breathing out is passive: no muscle has to contract. When you cough, shout or exercise hard, the internal intercostal muscles contract to pull the ribs down more forcefully, and the abdominal muscles push the diaphragm up.
| Inhalation | Exhalation (at rest) | |
|---|---|---|
| Diaphragm | contracts, flattens | relaxes, domes upwards |
| External intercostals | contract | relax |
| Ribs | up and out | down and in |
| Volume of thorax | increases | decreases |
| Pressure in lungs | falls below atmospheric | rises above atmospheric |
| Air | flows in | flows out |
| Component | Inhaled air | Exhaled air |
|---|---|---|
| Oxygen | about 21% | about 16% |
| Carbon dioxide | about 0.04% | about 4% |
| Nitrogen | about 78% | about 78% (not used by the body) |
| Water vapour | varies | saturated (fog forms on a cold mirror) |
| Temperature | that of the surroundings | close to body temperature |
Bubbling exhaled air through limewater turns it milky much faster than inhaled air does, which shows exhaled air has more carbon dioxide.
Worked example
At rest a student takes 15 breaths a minute, each of 0.5 dm³. Volume breathed per minute = 15 × 0.5 = 7.5 dm³. The oxygen content falls from 21% to 16%, so the body removes (21 − 16) ÷ 21 × 100 ≈ 24% of the oxygen in each breath.
Tip
Breathing IN: muscles Contract, ribs Up and out, Volume up, Pressure down, air In.
Breathing OUT: the exact opposite. In every answer, link volume to pressure and pressure to air movement; examiners mark each link.
Describe what the diaphragm does during inhalation, and how this helps air enter the lungs.
Solution: It contracts and flattens (moves down). This increases the volume of the thorax, so the pressure inside the lungs falls below atmospheric pressure and air flows in.
In the bell jar model, which part represents (a) the diaphragm and (b) the lungs?
Solution: (a) The rubber sheet. (b) The balloons.
During exercise a student breathes 30 times a minute, with 2.0 dm³ per breath. Calculate the volume of air breathed per minute and compare it with the resting value of 7.5 dm³.
Solution: 30 × 2.0 = 60 dm³ per minute, which is 60 ÷ 7.5 = 8 times the resting value.
Why is exhaled air still able to keep someone alive in mouth-to-mouth rescue breathing?
Solution: Exhaled air still contains about 16% oxygen, which is enough to supply the casualty's blood.
Remember
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
