2.1.4

Explain the mechanics of ventilation in the human lungs.

Inhalation: 

• The external intercostal muscles contract. This moves the ribcage up & out. 
• The diaphragm contracts (moves down & becomes relatively flat). 
• Both of these muscle contractions result in an increase in the volume of the thorax, which in turn results in a drop in pressure inside the thorax.
• Pressure eventually drops below atmospheric pressure. 
• Air then flow into the lungs from outside the body, through the mouth or nose, trachea, bronchi, bronchioles, and alveoli. 
• Air continues to enter the lungs until the pressure inside the lungs rises to the atmospheric pressure.

Exhalation: 

• The internal intercostal muscles contract. This moves the ribcage down & in. 
• The diaphragm relaxed (passive)- abdominal muscles contract
• This results in a decrease in the volume of the thorax. 
• As a result of the decrease in volume of the thorax, the pressure increases. 
• Eventually the pressure rises above atmospheric pressure.
• Air flows out of the lungs to outside of the body through the nose/mouth. 
• Air continues to flow out of the lungs until the pressure in the lungs has fallen back to atmospheric pressure.

During exercise there is a need for additional oxygen and a need to get ride of additional carbon dioxide being produced.  The rate of breathing will increase. In addition, muscle of the crest wall, abdomen and shoulders will assist in increasing the area (volume) of the chest cavity to bring in more air.  They will also assist in compressing the chest cavity to decrease the area and increase the pressure so more air can be exhaled.