How does breathing work, and how is oxygen exchanged at the alveoli during exercise?
The pathway of air and the mechanics of breathing, gas exchange at the alveoli, lung volumes, and how breathing changes during exercise.
A focused answer to the WJEC GCSE PE topic on the respiratory system, covering the pathway of air, the mechanics of breathing (the role of the diaphragm and intercostal muscles), gas exchange at the alveoli, lung volumes including tidal volume, and the changes during exercise.
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What this dot point is asking
WJEC wants you to describe the pathway of air and the mechanics of breathing, explain gas exchange at the alveoli, name the main lung volumes, and explain the changes during exercise.
The pathway of air
Air follows this route into the lungs:
Mouth and nose -> trachea (windpipe) -> bronchi -> bronchioles -> alveoli.
The trachea splits into two bronchi (one to each lung), which branch into smaller bronchioles, ending in clusters of tiny air sacs called alveoli where gas exchange happens.
The mechanics of breathing
- Breathing in (inspiration): the diaphragm and intercostal muscles contract, the ribcage moves up and out, the chest volume increases and air is drawn in.
- Breathing out (expiration): the diaphragm and intercostal muscles relax, the chest volume decreases and air is pushed out.
Gas exchange at the alveoli
The alveoli are tiny air sacs perfectly adapted for exchanging gases:
- a large surface area for lots of exchange,
- walls only one cell thick for a short diffusion distance,
- a rich blood supply of capillaries to maintain the concentration gradient.
Oxygen diffuses from the alveoli (high concentration) into the blood (low concentration). Carbon dioxide diffuses from the blood (high concentration) into the alveoli (low concentration) to be breathed out. Both move by diffusion down a concentration gradient.
Lung volumes
- Tidal volume: the volume of air breathed in or out per breath (at rest, about 0.5 litres).
- Breathing (respiratory) rate: the number of breaths per minute.
- Minute ventilation: the total air moved per minute, equal to tidal volume multiplied by breathing rate.
Changes during exercise
During exercise, the muscles need more oxygen and make more carbon dioxide, so:
- breathing rate increases (more breaths per minute), and
- tidal volume increases (each breath is deeper).
Together these greatly increase minute ventilation, taking in more oxygen and removing more carbon dioxide.
How oxygen reaches the muscles
Gas exchange at the alveoli is the first step in a chain that links the respiratory and cardiovascular systems:
- air is breathed into the alveoli,
- oxygen diffuses into the blood in the surrounding capillaries,
- the oxygen binds to haemoglobin in the red blood cells,
- the heart pumps the oxygenated blood to the working muscles,
- oxygen diffuses out of the blood into the muscles, where it is used to release energy aerobically.
This is why the respiratory and cardiovascular systems are often described as working together as the cardio-respiratory system.
Why this matters
The respiratory system works with the cardiovascular system to supply oxygen for aerobic exercise: the lungs load oxygen into the blood, and the heart delivers it to the muscles. Regular training improves the respiratory system, a long-term effect covered under the effects of exercise.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe how gas exchange takes place at the alveoli.Show worked answer →
A 4-mark question: reward the structure of the alveoli, the gases moved and the process of diffusion.
Gas exchange happens in the alveoli, which are tiny air sacs in the lungs. They have a very large surface area, walls that are only one cell thick, and a rich supply of capillaries, which makes them ideal for exchange. Oxygen diffuses from the high concentration in the alveoli into the blood in the capillaries, where the concentration is lower. At the same time, carbon dioxide diffuses from the blood (high concentration) into the alveoli (low concentration) to be breathed out.
Markers reward: the adaptations of the alveoli (large surface area, thin walls, good blood supply), oxygen diffusing into the blood, and carbon dioxide diffusing out, by diffusion down a concentration gradient.
WJEC style4 marksExplain how breathing changes during exercise and why this change is needed.Show worked answer →
A 4-mark question: two marks for the change and two for the reason.
During exercise both breathing rate and tidal volume (the depth of each breath) increase, so breathing becomes faster and deeper. This increases the volume of air moved in and out of the lungs each minute (minute ventilation).
This change is needed because the working muscles use more oxygen and produce more carbon dioxide during exercise. Breathing faster and deeper takes in more oxygen for the muscles and removes the extra carbon dioxide more quickly, allowing exercise to continue. Markers reward the rate and depth increase and the link to more oxygen in and more carbon dioxide out.
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Sources & how we know this
- WJEC GCSE Physical Education specification (from 2016) — WJEC (2016)