Breathing air into the lungs
Ventilation describes the physical replacement of air in the lungs during breathing. During inhalation, contraction of respiratory muscles changes thoracic volume and pressure so that air is drawn in.
- The diaphragm contracts and changes from a domed shape towards a flatter one.
- The external intercostal muscles contract, while the internal intercostal muscles relax.
- The rib cage moves upwards and outwards.
- The space within the thorax becomes larger.
- Increasing thoracic volume causes the pressure within the lungs to fall.
- Atmospheric air then enters down the resulting pressure gradient.
Because the diaphragm and external intercostal muscles must contract, inhalation is an active process requiring energy.
muscle contraction → thoracic volume increases → pressure decreases → air enters
Exam Tip: Do not use ventilation and respiration as interchangeable terms. Ventilation concerns the physical movement of air, whereas respiration refers to the energy-releasing chemical reactions occurring in cells.