Pressures in Ventilation

Question 1

What is the atmospheric pressure

Answer 1

760mmHg at sea level

Question 2

Define intra-alveolar pressure

Answer 2

Pressure within the alveoli

Question 3

What is the intra-alveolar pressure?

Answer 3

760mmHg (same as atmosphere)

Question 4

Why is the intra-alveolar pressure same as the atmosphere?

Answer 4

• Atmosphere and alveoli are linked by conducting airways
• Air quickly diffuses down the pressure gradient whenever the intra-alveolar pressure differs from atmospheric pressure until pressures become equal

Question 5

Define intrapleural pressure

Answer 5

Pressure within the pleural sac, exerted outside the lungs within the thoracic cavity

Question 6

What is the intrapleural pressure?

Answer 6

756mmHg

Question 7

Why is the intrapleural pressure 756mmHg (below atmospheric pressure)?

Answer 7

• Elastic lungs try to pull inward from the thoracic wall as they are stretched to fill the larger thoracic cavity
• The transmural pressure gradient prevents the lungs from pulling away
• As a result, the slight expansion of pleural cavity causes the pressure to drop.
• The pleural cavity is filled with fluid which cannot expand to fill the increased volume
• This results in a vacuum, causing a small drop in pressure.

Question 8

Define transmural pressure gradient

Answer 8

Pressure difference between the lungs and the pleural cavity

Question 9

What is the function of the transmural pressure gradient?

Answer 9

• Holds the lungs and thoracic wall as close as possible, stretching the lungs to fill the larger thoracic cavity
• There is a greater pressure pushing outwards from the alveoli.
• This pushes the lungs out toward the thoracic wall