Section 1 of 9The gas exchange system
Humans have a large, active body, so every cell needs a rapid supply of oxygen for aerobic respiration and a way of removing carbon dioxide.
- It is lined with ciliated cells and mucus-secreting goblet cells that trap dust and microbes and sweep them away.
Section 2 of 9Adaptations of the alveoli
Each lung has millions of alveoli, giving a huge surface area (about 70 m2 in total). Oxygen diffuses from the air in the alveolus, across the thin wall, into the blood in the surrounding capillaries; carbon dioxide diffuses from the blood into the alveolus and is breathed out. The alveoli are adapted in several ways:
| Gas | Moves from | Moves to |
|---|---|---|
| Oxygen | Alveolar air | Blood |
| Carbon dioxide | Blood | Alveolar air |
Section 3 of 9Ventilation: how breathing works
Breathing is the movement of air in and out of the lungs. It depends on changes in the volume and pressure of the thorax.
| Inhalation (breathing in) | Exhalation (breathing out) | |
|---|---|---|
| Intercostal muscles | Contract, ribs move up and out | Relax, ribs move down and in |
| Diaphragm | Contracts and flattens | Relaxes and curves upward |
| Volume of thorax | Increases | Decreases |
| Pressure in lungs | Falls below atmospheric | Rises above atmospheric |
| Air movement | Air flows in | Air flows out |
Air always moves from a region of higher pressure to lower pressure, so when the pressure inside the thorax is lower than outside, air flows in. Breathing out at rest is mostly passive, but during exercise the internal intercostal and abdominal muscles can contract to force air out more quickly.
Section 4 of 9Lung volumes and measurements
The tidal volume is the volume of air in one normal breath (about 500 cm3 at rest).
Section 5 of 9Health and disease
Lung diseases reduce the efficiency of gas exchange.
The gas exchange system links to transport in cells (diffusion across the alveolar wall), the circulatory system (blood carries the gases) and respiration (cells use the oxygen to release energy).
Section 6 of 9Interpreting data and the effect of exercise
Questions often present a spirometer trace, which is a graph of lung volume against time.
Why does this happen? Muscles respire faster during exercise, producing more carbon dioxide. The rise in carbon dioxide in the blood is detected by the brain, which increases the rate and depth of breathing to remove it and to supply more oxygen. This keeps the concentration gradients at the alveoli steep and so speeds gas exchange.
A model of the lungs using a bell jar, a balloon and a rubber sheet is commonly used to explain pressure changes.
- which you should mention if asked to evaluate it.
Section 7 of 9Exam-style questions
Describe how air moves into the lungs during inhalation. [3]3 marks
The intercostal muscles and diaphragm contract [1]; the volume of the thorax increases [1] so the pressure falls below atmospheric pressure and air flows in [1].
Explain three ways in which alveoli are adapted for efficient gas exchange. [3]3 marks
Large surface area from millions of alveoli [1]; thin walls (one cell thick) giving a short diffusion distance [1]; many capillaries and ventilation keep the concentration gradient steep [1].
A person takes 14 breaths per minute with a tidal volume of 450 cm3. Calculate the volume of air breathed in one minute. [2]2 marks
450 x 14 [1] = 6300 cm3 per minute [1].
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Alveolus | Tiny air sac in the lung where gas exchange occurs. |
| Trachea | Windpipe, a tube that carries air to the bronchi. |
| Bronchus | One of the two tubes that carry air into the lungs. |
| Diaphragm | Sheet of muscle below the lungs that helps breathing. |
| Intercostal muscles | Muscles between the ribs. |
| Ventilation | Movement of air in and out of the lungs. |
| Tidal volume | Volume of air in a normal breath. |
| Cilia | Tiny hairs that sweep mucus. |