The Mechanism of Breathing: Inhalation and Exhalation

Breathing, or ventilation, is the movement of air into and out of the lungs, achieved by changing the volume of the chest cavity so that air pressure inside the lungs becomes lower or higher than the pressure outside. In brief: during inhalation the diaphragm contracts and flattens and the external intercostal muscles raise the ribs, which increases chest volume and lowers pressure so air flows in. During exhalation, at rest, these muscles relax, chest volume decreases, pressure rises and air flows out.

Breathing is controlled automatically, but it relies on simple physics: gases flow from regions of higher pressure to regions of lower pressure. This guide explains the pressure-volume principle, the steps of inhalation and exhalation, forced breathing, how breathing is controlled, a worked example with Boyle’s law and common mistakes.

What is the basic principle of breathing?

Air moves from high pressure to low pressure. The lungs themselves cannot move air. Muscles change the volume of the thorax (chest cavity), and the lungs, which are attached to the chest wall by the thin film of pleural fluid, expand and contract with it. According to Boyle’s law, for a fixed amount of gas at constant temperature, pressure is inversely proportional to volume. When volume increases, pressure falls, and when volume decreases, pressure rises.

Atmospheric pressure at sea level is about 101 kilopascals (kPa). During breathing, the pressure inside the lungs changes by only a very small amount, about 0.1 to 0.2 kPa (1 to 2 mmHg) below or above atmospheric, which is enough to move air.

What happens during inhalation?

Inhalation (inspiration) is an active process that uses muscle energy.

1. The diaphragm contracts and moves downward, changing from a dome to a flatter shape.
2. The external intercostal muscles contract, pulling the ribs upward and outward.
3. The volume of the thorax increases in both the vertical and horizontal directions.
4. The pressure in the lungs falls below atmospheric pressure.
5. Air flows in through the airways until the pressures are equal.

What happens during exhalation?

At rest, exhalation (expiration) is mostly passive, which means that muscle contraction is not required.

1. The diaphragm and external intercostal muscles relax.
2. The diaphragm returns to its dome shape, and the ribs move down and inward under gravity and the elastic recoil of the lung tissue.
3. The volume of the thorax decreases.
4. The pressure in the lungs rises above atmospheric pressure.
5. Air flows out until the pressures are equal.

Inhalation and exhalation compared

What is forced breathing?

During exercise, coughing or singing, exhalation becomes active. The internal intercostal muscles contract and pull the ribs down and in, and the abdominal muscles contract and push the diaphragm upward. This reduces chest volume more rapidly and by a greater amount, expelling extra air. Deep inhalation recruits additional neck and chest muscles.

Typical lung volumes illustrate the range. A resting breath (tidal volume) is about 500 mL. A person can inhale an extra 3 000 mL or so and exhale an extra 1 000 mL or so. The total volume of air that can be exhaled after a deep breath, the vital capacity, is about 3 500 to 5 000 mL for adults. Even after full exhalation about 1 200 mL remains in the lungs (residual volume), which keeps the alveoli open.

How is breathing controlled?

Breathing is controlled automatically by centers in the medulla of the brain stem, though we can also control it voluntarily for short periods. The main stimulus for changing the rate is the concentration of carbon dioxide in the blood. When it rises, it lowers the pH of the blood, and chemoreceptors detect the change. The brain then increases the breathing rate and depth to remove more carbon dioxide. This is a negative feedback loop.

Low oxygen levels also have an effect, but a smaller one in healthy people at sea level. This explains why holding the breath is uncomfortable because of high carbon dioxide, not low oxygen.

Worked example: pressure change in the lungs

Use Boyle’s law, P₁V₁ = P₂V₂, to model inhalation. Suppose the lungs hold 2 400 mL of air at the end of a quiet exhalation, at atmospheric pressure of 101.3 kPa. During inhalation the volume increases by the tidal volume, 500 mL, before air has had time to enter.

1. Initial values: P₁ = 101.3 kPa, V₁ = 2 400 mL.
2. New volume: V₂ = 2 400 + 500 = 2 900 mL.
3. New pressure: P₂ = P₁V₁ ÷ V₂ = 101.3 × 2 400 ÷ 2 900 ≈ 83.8 kPa.

The model suggests a pressure drop of about 17 kPa, much larger than the 0.1 to 0.2 kPa seen in reality. The difference arises because air flows in as the volume increases, so the pressure never falls far. The calculation shows the direction of the effect: a larger volume gives a lower pressure, and this difference drives the flow of air. It is a simplified model that treats the volume as increasing before any air enters.

How was breathing explained?

Galen, in the second century, believed that the diaphragm and the rib muscles moved the chest and that air was drawn into the lungs. Boyle’s law was published by Robert Boyle in 1662 and was later applied to respiration. In the nineteenth century, physiologists such as Donders showed experimentally how pressure changes in the chest cavity move air. Modern respirators (ventilators) used in intensive care work by pushing air in under pressure instead of lowering chest pressure, though earlier “iron lung” machines used negative pressure around the chest.

Which mistakes are common?

Mistake 1: Saying air is “sucked” in because the lungs expand on their own

Muscles expand the chest, and the lungs follow. Lower pressure inside then allows air to flow in.

Mistake 2: Saying that the diaphragm moves up during inhalation

It contracts and moves down. When it relaxes, it returns upward.

Mistake 3: Thinking that exhalation always needs muscle energy

Quiet exhalation is passive. Only forced exhalation uses muscles.

Mistake 4: Confusing breathing with respiration

Breathing moves air. Respiration releases energy in cells.