Breathing is the mechanical process of moving air into and out of the lungs by changing chest cavity volume. When volume increases, pressure falls below atmospheric and air rushes in; when volume decreases, pressure rises and air is pushed out. The diaphragm and intercostal muscles produce these changes.
What is the difference between breathing and gas exchange?
These two terms are frequently confused at GCSE:
- Breathing (ventilation) is the physical movement of air into and out of the lungs, driven by muscle contractions. It is a mechanical process.
- Gas exchange is the diffusion of oxygen from the alveoli into the blood, and carbon dioxide from the blood into the alveoli. This occurs across the alveolar surface and is driven by concentration gradients.
- Respiration is the chemical process inside cells that uses oxygen and glucose to release energy, producing carbon dioxide and water.
Breathing delivers fresh air to the alveoli so that gas exchange can maintain the concentration gradients needed for diffusion. Without continuous ventilation, oxygen concentration in the alveoli would fall and carbon dioxide would rise, ending gas exchange.
Which muscles are involved in breathing?
Two sets of muscles control the volume of the thorax (chest cavity):
1. Diaphragm A dome-shaped sheet of muscle forming the floor of the thorax. When it contracts, it flattens — increasing the volume of the chest cavity. When it relaxes, it returns to its dome shape — decreasing the volume.
2. Intercostal muscles Two layers of muscle between the ribs:
- External intercostal muscles: contract during inhalation, pulling ribs upward and outward.
- Internal intercostal muscles: contract during forced exhalation, pulling ribs downward and inward.
During normal quiet breathing, exhalation is passive — the intercostal muscles simply relax and the elastic recoil of the lungs pushes air out. During exercise or deep breathing, the internal intercostals contract actively to force air out faster.
What happens during inhalation?
Inhalation is an active process — it requires muscle contraction:
- The diaphragm contracts and flattens.
- The external intercostal muscles contract, pulling the rib cage upward and outward.
- The volume of the thorax increases.
- Because the lungs are attached to the thorax wall via the pleural membranes, they expand too.
- Lung volume increases → pressure inside the lungs falls below atmospheric pressure.
- Air flows from the higher pressure outside (atmosphere) into the lower pressure inside (lungs).
Pressure change: atmospheric pressure is approximately 101 kPa. During inhalation, lung pressure drops to about 100.7 kPa — a difference of just ~0.3 kPa is enough to drive airflow.
What happens during exhalation?
Quiet (passive) exhalation does not require muscle contraction:
- The diaphragm relaxes and returns to its dome shape.
- The external intercostal muscles relax; the rib cage falls inward and downward under gravity.
- The volume of the thorax decreases.
- Lung volume decreases → pressure inside the lungs rises above atmospheric pressure.
- Air flows from the higher pressure inside the lungs out to the lower pressure atmosphere.
During forced exhalation (e.g. blowing out a candle, coughing, playing a wind instrument), the internal intercostal muscles and abdominal muscles contract actively to push more air out more rapidly.
How is lung volume measured?
A spirometer is used to measure lung volumes. It records the volume of air breathed in and out over time, producing a trace called a spirogram.
| Lung volume measurement | Definition | Typical value (adult) |
|---|---|---|
| Tidal volume (TV) | Volume of air breathed in or out in one normal breath | ~500 cm³ |
| Vital capacity (VC) | Maximum volume that can be breathed in after a full exhalation | ~4,500 cm³ |
| Residual volume (RV) | Volume of air always remaining in the lungs (cannot be exhaled) | ~1,500 cm³ |
| Total lung capacity | Vital capacity + residual volume | ~6,000 cm³ |
Worked example — reading a spirogram:
A student's spirogram shows that each breath moves 0.5 dm³ of air. The student breathes 16 times per minute.
- Minute ventilation = tidal volume × breathing rate
- = 0.5 dm³ × 16 = 8.0 dm³ per minute
After exercise, the trace would show an increased tidal volume and breathing rate, reflecting the greater oxygen demand of the muscles.
How does exercise affect breathing?
During exercise, muscles respire more rapidly, consuming oxygen and producing carbon dioxide faster. The body responds by:
- Increasing breathing rate (breaths per minute)
- Increasing tidal volume (more air per breath)
- Engaging internal intercostal and abdominal muscles for forced exhalation
The stimulus for increased breathing is detected by chemoreceptors in the medulla oblongata (brain stem) and in the aorta and carotid arteries. They respond primarily to rising CO₂ concentration (and the resulting fall in blood pH) rather than directly to falling oxygen levels. The medulla then sends increased nerve signals to the diaphragm and intercostal muscles.
Frequently asked questions
Why does the air pressure inside the lungs change during breathing?
Pressure and volume are inversely related (Boyle's Law): when volume increases, pressure decreases. When the diaphragm and intercostal muscles contract and enlarge the thorax, the lungs expand and lung volume increases — so pressure inside falls below atmospheric. Air then flows in from the atmosphere along the pressure gradient. When the muscles relax and volume decreases, pressure rises above atmospheric, and air flows out.
Is exhalation always a passive process?
During quiet, normal breathing at rest, exhalation is passive — the diaphragm and external intercostal muscles simply relax, and the natural elastic recoil of lung tissue and the rib cage pushes air out without any active muscle contraction. However, during exercise, deep breathing, coughing, singing, or playing a wind instrument, exhalation becomes active: the internal intercostal muscles and abdominal muscles contract to drive air out faster and more completely.
What does tidal volume mean and how does it change during exercise?
Tidal volume is the volume of air moved in or out of the lungs with each normal breath — at rest, typically about 500 cm³ in an adult. During exercise, muscles demand more oxygen, so tidal volume increases (breathing becomes deeper) as well as breathing rate increasing. Both changes increase the total volume of air entering and leaving the lungs per minute (minute ventilation), ensuring more oxygen reaches the alveoli and more carbon dioxide is removed.
What is residual volume and why can you not breathe all the air out of your lungs?
Residual volume is the amount of air (~1,500 cm³) that always remains in the lungs even after the most forceful possible exhalation. It cannot be expelled because the airways and alveoli are not rigid — if all air were forced out, they would collapse and stick together, making reinflation very difficult (as happens in some premature infants whose lungs lack surfactant). Residual volume also ensures that gas exchange does not stop completely between breaths — oxygen and carbon dioxide continue to diffuse during the pause at end-exhalation.
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