Inhalation and exhalation

Where it happens

Breathing movements take place in the thorax, driven by the diaphragm below the lungs and the intercostal muscles between the ribs. This mechanical process is called ventilation, and it is separate from respiration, which is the release of energy from food inside cells.

The lungs themselves contain no muscle. They are elastic, spongy organs that change size only because the airtight thorax around them changes size.

Each lung is enclosed by two pleural membranes with a thin film of pleural fluid between them, which reduces friction and keeps the lung surface pressed against the thoracic wall. The airway runs from the nasal cavity through the pharynx, larynx and trachea, then branches into two bronchi, smaller bronchioles and finally the alveoli, where gas exchange occurs.

Inputs and outputs

• Input: atmospheric air, drawn through the nose or mouth, trachea, bronchi and bronchioles into the alveoli during inhalation.
• Input: energy from respiration in the diaphragm and intercostal muscles, because muscle contraction needs ATP.
• Input: the nerve impulses from the respiratory centre in the medulla oblongata that set the rhythm of contraction.
• Output: exhaled air, which contains less oxygen, more carbon dioxide and more water vapour than inhaled air.
• Output: a change in thoracic volume and lung pressure that reverses at each breath, alternating between a pressure below and above atmospheric pressure.
• Output: fresh air in the alveoli, which maintains the concentration gradients for oxygen and carbon dioxide across the alveolar wall.

The steps

1. Inhalation begins when the respiratory centre in the medulla oblongata sends impulses to the diaphragm and the external intercostal muscles.
2. The diaphragm contracts and flattens, and the external intercostal muscles contract, pulling the ribcage up and outward.
3. The volume of the thorax increases, so the pressure inside the lungs falls below atmospheric pressure.
4. Air moves from the higher pressure outside into the lungs, down the pressure gradient, until the two pressures are equal.
5. Exhalation begins when the diaphragm relaxes and curves upward, and the external intercostal muscles relax so the ribcage moves down and inward under its own weight; during forced exhalation the internal intercostal muscles contract to pull the ribs down further.
6. The volume of the thorax decreases, so the pressure inside the lungs rises above atmospheric pressure.
7. Air is pushed out of the lungs down the pressure gradient, and the elastic recoil of the stretched lung tissue helps to squeeze it out.

Why it matters and how it is controlled

Breathing keeps a fresh supply of oxygen moving into the alveoli and removes carbon dioxide, maintaining the concentration gradients that gas exchange by diffusion depends on. Without continuous ventilation, oxygen levels in the blood would fall and carbon dioxide would build up.

Breathing is controlled by the respiratory centre in the medulla oblongata, which does not need conscious thought. Chemoreceptors in the medulla and in the walls of the aorta and carotid arteries detect a rise in carbon dioxide concentration, which lowers blood pH.

The respiratory centre then sends more frequent impulses to the diaphragm and intercostal muscles, so breathing becomes faster and deeper. During exercise, muscle cells respire more and produce more carbon dioxide, and this feedback loop raises the breathing rate until the extra carbon dioxide is removed.

When the carbon dioxide level falls, the stimulation decreases and breathing slows again, which is an example of negative feedback.

How it is examined

You may be asked to compare the position of the diaphragm and ribcage, and the pressure and volume of the thorax, during inhalation and exhalation, often using a table or diagram. You may also be asked to link breathing rate to activity level or to explain why breathing rate increases during exercise.

A common structured question shows a bell-jar model: a glass jar with a rubber sheet at the base representing the diaphragm, a Y-shaped tube for the trachea and bronchi, and two balloons for the lungs. You are expected to match each part to the real structure, explain what happens to the balloons when the rubber sheet is pulled down, and state one limitation of the model, such as the jar being rigid while the real ribcage moves.

Compare-and-contrast tables are also common, so practise filling a four-column table of diaphragm, ribcage, thoracic volume and lung pressure for each phase.

Common misconceptions

Worked exam-style question

Question. A student built a model of the human breathing mechanism using a bell jar, a Y-shaped glass tube, two balloons and a rubber sheet stretched across the open base. (a) State which structure of the human body is represented by the rubber sheet and by the balloons.

(b) Describe and explain what happens to the balloons when the rubber sheet is pulled downward. (c) Explain why the model does not fully represent inhalation in a human.

Model answer. (a) The rubber sheet represents the diaphragm and the balloons represent the lungs. (b) Pulling the sheet down increases the volume inside the jar, so the air pressure inside the jar decreases below atmospheric pressure.

Air from outside moves down the pressure gradient through the glass tube into the balloons, so the balloons inflate. This is the same principle as inhalation, when the diaphragm contracts and flattens.

(c) The glass jar is rigid, so it cannot show the ribcage moving up and outward when the external intercostal muscles contract; in a human, both the diaphragm and the ribcage contribute to the increase in thoracic volume. The balloons also lack elastic recoil and the millions of alveoli of real lungs.

Source:SRC-DSKP-EN

Frequently asked questions