Ventilation

Lungs

Heart

Nerves

Gonads

Muscle

Kidney

B3.1.5
Ventilation of the lungs

Breathing is the active movement of respiratory muscles that enables the passage of air into and out of the lungs

The contraction of respiratory muscles changes the volume of the thoracic cavity (i.e. the chest)
Changing chest volume creates a pressure differential between the chest and atmosphere – with air then moving to equalise

Because muscles only do work via contraction, different groups of respiratory muscles are required to expand and contract chest volume

Muscles that increase the volume of the chest cause inspiration (as chest pressure is less than atmospheric pressure)
Muscles the decrease the volume of the chest cause expiration (as chest pressure is greater than atmospheric pressure)

The core muscles involved in ventilation are the diaphragm and the intercostal muscles between the ribs (internal and external)

Additional accessory muscle groups support the act of breathing – including the abdominal muscles and pectoral muscles

Inspiration (inhaling) and expiration (exhaling) are controlled by two sets of antagonistic muscle groups

The muscles responsible for inspiration are the diaphragm and external intercostals (plus some accessory muscles)
The muscles responsible for expiration are the abdominal muscles and internal intercostals (plus some accessory muscles)

Inspiration

The diaphragm muscles contract, causing the diaphragm to flatten and increase the volume of the thoracic cavity
External intercostals contract, pulling ribs upwards and outwards (expanding chest)
As the volume of the thoracic cavity increases, the pressure within the lungs decreases below atmospheric pressure and air moves into the lungs

Expiration

The diaphragm muscles relax, causing the diaphragm to curve upwards and reduce the volume of the thoracic cavity
External intercostals contract, pulling ribs upwards and outwards (expanding chest)
Abdominal muscles contract and push the diaphragm upwards during forced exhalation
With the volume of the thoracic cavity reduced, the pressure within the lungs increases above atmospheric pressure and air is expelled from the lungs