External Respiration

The processes of breathing or 'respiration' are often described in two parts:

External Respiration and Internal Respiration.

This page is about external respiration, which is the processes by which external air is drawn into the body in order to supply the lungs with oxygen, and (used) air is expelled from the lungs in order to remove carbon dioxide from to body.

(The processes of internal respiration concern the exchange of gases in the lungs with those in the tissues - next page).

Summary of Processes:

Inspiration (= inhalation)

  1. Intercostal muscles contract.
  2. Sternum moves upwards and outwards.
  3. Ribs move upwards and outwards.
  4. Diaphragm flattens.
  5. Increase in volume of thoracic cavity.
  6. Pressure in thoracic cavity decreases.
  7. Air is drawn into thoracic cavity.

Expiration (= exhalation)

  1. Intercostal muscles relax.
  2. Sternum moves downwards and inwards.
  3. Ribs move downwards and inwards.
  4. Diaphragm relaxes - forming a bell-shape.
  5. Volume in thoracic cavity decreases.
  6. Pressure in thoracic cavity increases.
  7. Air is expelled from thoracic cavity.

Concentration of Gases:








Carbon Dioxide






Inert Gases



Water Vapour



In order to describe how a person's respiratory system is (or could be) functioning some special terms are used to refer to the quantities of gas that pass in and out of the structures used for external respiration.

Minute Volume is the amount of air drawn into the lungs during one minute while the person is at rest.
(Typically due to around 12-16 breaths/minute, so approx. 6-8 litres/minute.)

Residual Volume is the air that remains in the lungs after expiration.
(Typically approx. 0.35 litres.)

Tidal Volume is the amount of air taken into the lungs during one breath when the person is at reast.
(Typically approx. 0.5 litres.)

Vital Capacity is the maximum volume of inspired breath, following maximum expiration.
(Typically approx. 3.5 - 4.5 litres.)

Next: See the page about Internal Respiration.

More about respiration ...

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