Breathing changes the air: the air we breathe out has less oxygen, more carbon dioxide and more water vapour than the air we breathe in. This lesson tests the difference with limewater, measures how exercise changes the rate and depth of breathing and (Extended) explains how the ribs, intercostal muscles and diaphragm move air, and how the brain responds to carbon dioxide.
Expired air contains less oxygen (about 16% instead of 21%), more carbon dioxide (about 4% instead of 0.04%) and more water vapour than inspired air, and turns limewater milky much faster. During physical activity breathing becomes faster and deeper. Extended: muscles change the volume of the thorax, which changes the pressure, so air moves in and out; extra carbon dioxide in the blood during exercise is detected by the brain, which increases the rate and depth of breathing.
Extended. The lungs cannot move themselves; they follow the walls of the thorax. Press Breathe in, Breathe out and Forced breath out, and watch the external intercostal muscles (red), the internal intercostal muscles (blue) and the diaphragm.
| Breathing in (inspiration) | Breathing out (expiration) | |
|---|---|---|
| external intercostal muscles | contract | relax |
| internal intercostal muscles | relax | contract in forced breathing out |
| ribs | move up and out | move down and in |
| diaphragm | contracts and flattens | relaxes and domes upwards |
| volume of thorax | increases | decreases |
| pressure in lungs | falls below atmospheric | rises above atmospheric |
| air | flows in | flows out |
Air does not rush in and then make the chest expand. The order is: muscles contract, volume increases, pressure falls, then air flows in. And the diaphragm flattens when it contracts; it domes upwards when it relaxes.
In the limewater apparatus, air breathed in is drawn through tube A and air breathed out bubbles through tube B. Press Breathe in and out 10 times and compare the two tubes, then read the composition chart.
| Gas | Inspired air | Expired air | Extended: why |
|---|---|---|---|
| oxygen | 21% | 16% | cells use oxygen in aerobic respiration, so some is absorbed into the blood |
| carbon dioxide | 0.04% | 4% | respiration produces carbon dioxide, which diffuses from the blood into the alveoli |
| water vapour | varies | more (saturated) | water evaporates from the moist lining of the alveoli |
Expired air still contains plenty of oxygen (about 16%), which is why rescue breaths work. Do not write that expired air has "no oxygen" or is "all carbon dioxide".
Choose resting, walking, jogging or sprinting and watch the spirometer trace. The breathing rate is the number of breaths per minute; the depth is the volume of each breath.
| Activity | Rate / breaths per min | Depth / dm3 |
|---|---|---|
| resting | 12 | 0.5 |
| walking | 18 | 0.9 |
| jogging | 28 | 1.8 |
| sprinting | 40 | 2.6 |
In the Extended explanation the trigger is the rise in carbon dioxide in the blood, detected by the brain. Do not say the lungs detect a lack of oxygen, or that the muscles decide how fast to breathe.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the ventilation tab, what happens to the pressure in the lungs as you press Breathe in?
Which muscles light up (blue) when you press Forced breath out?
After 10 breaths in the limewater apparatus, which tube has turned milky?
In the exercise tab, what are the breathing rate and depth when sprinting?
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Which row correctly compares expired air with inspired air?
Extended: during breathing in, which changes happen?
Extended: why does expired air contain more water vapour than inspired air?
Extended: during exercise the rate and depth of breathing increase because:
This skill is now recorded on your profile. Keep the chain going.