How does the body take in oxygen and get rid of carbon dioxide?
Describe the parts of the respiratory system, explain breathing and gas exchange in the lungs, and state why we need oxygen
A clear answer to the N(T) Science point on breathing. The parts of the respiratory system, how we breathe in and out, gas exchange in the lungs, and why the body needs oxygen.
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What this dot point is asking
This dot point wants you to describe the parts of the respiratory system, explain how we breathe in and out, and explain how gases are exchanged in the lungs. You should also know why the body needs oxygen. The big idea is that the body needs oxygen to release energy from food, and it makes carbon dioxide as waste. The respiratory system takes oxygen in from the air and removes carbon dioxide, and this swap of gases happens in the lungs.
The answer
Why we need to breathe
Every cell in the body needs oxygen to release energy from food. This energy keeps the body moving, warm and working. As the cells release energy, they make a waste gas called carbon dioxide, which must be removed. Breathing brings oxygen into the body and removes carbon dioxide.
The parts of the respiratory system
When you breathe, air travels through these parts:
- The nose and mouth: where air enters. The nose warms, moistens and cleans the air.
- The windpipe (trachea): the main tube that carries air down toward the lungs.
- The two lungs: the large, spongy organs in the chest where gases are exchanged.
- The air sacs (alveoli): millions of tiny sacs inside the lungs where oxygen and carbon dioxide are swapped with the blood.
- The diaphragm: a sheet of muscle below the lungs that helps you breathe in and out.
How we breathe in and out
Breathing is caused by muscles changing the size of the chest.
When you breathe in, the chest muscles and the diaphragm move so the chest gets bigger. This makes more space inside, so air is drawn in through the nose or mouth, down the windpipe and into the lungs.
When you breathe out, the chest gets smaller again, which pushes the air back out of the lungs and out of the body. This in-and-out movement happens all the time, without you having to think about it.
Gas exchange in the lungs
The real purpose of breathing is gas exchange, which happens in the tiny air sacs (alveoli). The air sacs have very thin walls and are surrounded by tiny blood vessels (capillaries).
When fresh air reaches the air sacs, oxygen passes through the thin walls from the air into the blood. At the same time, carbon dioxide passes from the blood into the air sacs, ready to be breathed out. The thin walls and the huge number of air sacs give a very large surface area, so a lot of gas can be exchanged quickly. The blood then carries the oxygen to the rest of the body.
Breathed-in versus breathed-out air
Because of gas exchange, the air you breathe out is different from the air you breathe in. Breathed-in air has more oxygen and less carbon dioxide. Breathed-out air has less oxygen (some was taken into the blood) and more carbon dioxide (added as waste), and it is also warmer and more moist.
Examples in context
Example 1. Breathing harder during exercise. When you run, your muscles need much more oxygen to release energy and they make more carbon dioxide. So you breathe faster and deeper to take in more oxygen and get rid of the extra carbon dioxide. The panting after a sprint is the respiratory system working hard to keep up.
Example 2. Why smoking damages the lungs. Smoking coats and damages the tiny air sacs in the lungs, reducing the surface area for gas exchange. With fewer working air sacs, less oxygen can pass into the blood, which is why smokers often get short of breath. This shows how important the large surface area of healthy air sacs is.
Try this
Cue. Name the gas taken into the blood in the lungs and the waste gas removed. Oxygen is taken into the blood, and carbon dioxide is the waste gas removed.
Cue. Describe what happens to the chest when you breathe in. The chest gets bigger as the diaphragm and chest muscles move, which draws air into the lungs.
Cue. Explain why the air sacs in the lungs have thin walls and a large surface area. Thin walls let oxygen and carbon dioxide pass through quickly, and a large surface area means a lot of gas can be exchanged at once.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksAir is breathed into the lungs. (a) Name the gas the body takes from the air in the lungs. (b) Name the waste gas the body gets rid of in the lungs. (c) Name the tiny air sacs in the lungs where gases are exchanged. (d) State why the body needs oxygen.Show worked answer →
(a) The body takes oxygen from the air.
(b) The body gets rid of carbon dioxide.
(c) The tiny air sacs are the air sacs (alveoli) in the lungs.
(d) The body needs oxygen to release energy from food (this energy is used for movement, warmth and keeping the body working).
What markers reward: oxygen taken in, carbon dioxide given out, the air sacs (alveoli) as the place of exchange, and oxygen being needed to release energy from food.
Original3 marksWhen you breathe in, air travels down to the lungs. (a) Name the main tube that carries air from the throat toward the lungs. (b) Describe what happens to make air go into the lungs when you breathe in. (c) State one difference between the air you breathe in and the air you breathe out.Show worked answer →
(a) The main tube is the windpipe (trachea).
(b) When you breathe in, the chest muscles and the muscle below the lungs (the diaphragm) move so the chest gets bigger. This makes more space in the lungs, so air is drawn in through the nose or mouth and down to the lungs.
(c) Any correct difference, for example: breathed-in air has more oxygen and less carbon dioxide, while breathed-out air has less oxygen and more carbon dioxide (and is warmer and more moist).
What markers reward: naming the windpipe (trachea), describing the chest getting bigger to draw air in, and one correct difference (more carbon dioxide or less oxygen in breathed-out air).
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