Respiratory Mechanics

Question 1

intrapleural space

Answer 1

Both lungs are surrounded by a closed chamber, the “pleural sac” or intrapleural space.

Question 2

Alveolar surface tension

Answer 2

 Alveolar surface tension is responsible for the transmural pressure gradient and the at rest lower-than-atmospheric
intrapleural pressure.

• pressure in lungs must be equal to atmospheric pressure

Question 3

transmural pressure gradient across lung wall

Answer 3

inta-alveolar pressure minus intrapleural pressure

- about 4mm of mercury

Question 4

transmural pressure gradient across thoracic wall

Answer 4

atmospheric pressure minus intrapleural pressure

Question 5

The transmural pressure gradient

Answer 5

The transmural pressure gradient prevents the lungs from collapsing. If the intrapleural pressure is allowed to equalise with the atmospheric pressure, visceral and parietal pleurae separate and the lung collapses resulting in a pneumothorax.

Question 6

Quiet breathing

 Inspiration:

Answer 6

lowering the diaphragm lowers the intrapleural pressure which in turn inflates the lungs. Intra-alveolar pressure falls below atmospheric pressure, and air flows into the lungs.

Question 7

Quiet breathing

 Expiration:

Answer 7

upon relaxation of the diaphragm, intrapleural pressure returns to the normal lower-than- atmospheric pressure. The recoil of the lung creates a slightly higher than atmospheric intra-alveolar pressure, and air moves out of the lungs.

Question 8

The airflow rate

Answer 8

not only depends on the pressure gradient but also on the airway resistance.

Airflow rate = pressure gradient / airways resistance

Question 9

Airway resistance

Answer 9

is due to friction along the wall of the airway, and depends mostly on the diameter of the airway.

Question 10

Increased ventilation

- Inspiration

Answer 10

contraction of the diaphragm and external intercostal muscles expand the thoracic cavity and lowers the intrapleural pressure which in turn inflates the lungs. Intra-alveolar pressure falls below atmospheric pressure, and air flows into the lungs.

Question 11

Increased ventilation

expiration

Answer 11

contraction of the internal intercostal muscles flattens the ribcage; contraction of the abdominal muscles pushes the diaphragm upwards. This markedly increases the intrapleural pressure. Together with the recoil of the lung this creates an even higher intra-alveolar pressure, and air moves out of the lungs.

Question 12

Dynamic airway closure

Answer 12

When frictional losses cause the airway pressure to fall below the surrounding elevated intrapleural pressure, the small non-rigid airways are compressed closed, blocking further expiration and trapping air within the alveoli. The amount of air left in the alveoli after dynamic airway closure is called residual volume.

Question 13

What if pressure changes the diameter of the conduit?

Answer 13

• In a rigid tube flow is dependent on the pressure difference between each end and the diameter of the tube.
• In a tube with compliant walls flow is also dependent on absolute pressure because pressure between lumen and outside will change the cross section area (diameter) of the conduit.

Question 14

Thickness of airway wall

Answer 14

Smaller airways have thinner walls and are more compliant

Question 15

emphysema,

Answer 15

loses alveloi valves and decreased recoil of the lung = at rest ransmural pressure
gradient will not be as low and lung won’t shrink back as much anymore so intrapleural
pressure will be higher than in a healthy person. WHen person forecfully exhales and
puts pressure on the chest using muscles force, as pressure was higher initially it will also be
higher during force expiration. If intrapleural pressure if higher in forced expiration, the intrapressure
point is reached at a time when you have much more air left at your lungs and get dynamic small
airway closure over time when have much more air left in lungs and air traping

Question 16

Opposing Forces Acting on Lung

- Forces Keeping the Alveoli Open

Answer 16

 Transmural pressure gradient
 Pulmonarysurfactant
Alveolar interdependence

Question 17

Opposing Forces Acting on Lung

- Forces Promoting Alveolar Collapse

Answer 17

 Elasticity of stretched pulmonary connective tissue

 Alveolar surface tension

Question 18

Prematurely born babies

Answer 18

often present with Respiratory Distress Syndrome due to insufficient pulmonary surfactant production

Question 19

Pulmonary surfactant deficiency leads to

Answer 19

 increased alveolar surface tension thus increased recoil of the lung
 increased transmural pressure gradient and decreased intrapleural pressure
 late small airway closure with reduced residual volume
 increased breathing effort due to increased recoil of the lung and decreased compliance of the lung.