Unit 1 Lecture 2: Respiratory Mechanics

Question 1

What pressures are affected when inspiration needs to take place?

Answer 1

Alveolar pressure (inside the lungs) needs to be less than atmospheric pressure (external environment)

Note: drop in alveolar pressure is due to inspiratory muscles lowering pleural pressure

Question 2

What is recoil pressure?

Answer 2

Refers to the lungs recoiling back to it’s normal shape after being inflated; the difference between pleural and alveolar pressure

• Formula: Recoil pressure = alveolar pressure - pleural pressure

Note: Recoil pressure is also known as transpulmonary pressure

Question 3

What is the pleura and what is it mainly known for?

Answer 3

Pleura is this double spaced sac that sits in between the chest wall and lungs and separates them; it produces lubricating fluid in the cavity to let the membranes slide off of one another

Question 4

What is the secondary function of the pleura that is related to pressure?

Answer 4

The pleura helps keep the lungs from deflating too much and the chest wall from expanding too much but there is a space between them which allows for volume change

Note: The pleural space will always have a negative pressure

Question 5

Where do the Visceral and Parietal pleura attach to?

Answer 5

Visceral attaches from the lung tissue to the pleura and Parietal attaches from the pleural sac to the chest wall

Question 6

Hydrostatic pressure of the pleural fluid is also known as…?

Answer 6

Intrapleural pressure

This is negative compared to the external environment

Question 7

During what point of both inspiration and expiration does the alveolar pressure turn into atmospheric pressure/go back to 0?

Answer 7

At the end of inspiration it goes from -1 to 0 and at the end of expiration where it goes from +1 back to 0

Note: When we stop breathing in and out the alveolar pressure is equivalent to the atmospheric pressure which is why we say it goes to “0” because they cancel out and flow of air stops

Question 8

Why does the lung tend to recoil?

Answer 8

Elasticity and surface tension

Question 9

What is transmural pressure?

Answer 9

• It is the same value as intra-pleural pressure except it is a +ve value
• Alveolar pressure - Pleural pressure

Pressure acting on a membrane

Question 10

When someone says the pressure is atmospheric what can you always assume?

Answer 10

The pressure is 0 because it is equal to the atmospheric pressure in the external environment

Note: If atmospheric pressure is 760mmHg outside then it is equal inside unless there is a change (i.e. breathing in and out)

Question 11

What would it mean if the alveolar pressure became greater than atmospheric pressure?

Answer 11

This would mean according to pressure gradients, we would see the flow of air from the alveoli to the atmospheric pressure in the external environment (A.K.A. Expiration)

Question 12

The greater the lung expands the greater the storage of what?

Answer 12

The greater the storage of the lung as recoil tendency

Question 13

Which pressure makes the lungs inflate/expand?

Answer 13

Transpulmonary pressure because since the lungs have the tendency to recoil (transpulmonary), we increase the transpulmonary to increase the size of the lung itself

Note: Don’t confuse with alveolar pressure because that is the pressure for air to flow into the lung, not make it expand

Question 14

When measuring transmural pressure what must be always done when calculating?

Answer 14

Measure from inside to outside

I.e, Transpulmonary pressure would be alveolar (most inner) - pleural (outer)

Question 15

How can the esophagus be used to estimate intrapleural pressure?

Answer 15

Because it is closely connected to the trachea, a balloon attached to a catheter can be sent down the esophagus to check the pressure

Question 16

What does flow being negative mean?

Answer 16

That the air is flowing into the lungs

Positive flow means expired flow; air moving out

• Also because intrapleural pressure becomes more negative/decreases

Question 17

Explain what a pneumothorax is? What does it mean for pressure?

Answer 17

A pneumothorax is a collapsed lung from some sort of puncture letting air into the pleural space The pressure in the pleural space is equivalent to the atmospheric pressure

Question 18

Explain how the different types of pressure are at play in this image

Answer 18

• In a. Before inspiration, the alveolar pressure is equivalent to atmospheric pressure and the transpulmonary and pleural pressure are at +5 and -5 respectively
• Once the alveoli start to fill with air, there’s a drop in pressure in the alveoli because the pressure gradient is going from high to low from the external environment to the inside (alveoli). At this time, the intrapleural pressure decreases while the transpulmonary pressure increases.
• At the end of inspiration, the changes in pressure stop. Alveolar pressure is equal to atmospheric pressure and the transpulmonary and intrapleural pressure are +8 and -8 respectively.
• At the beginning of expiration, the pressure gradient is going from high to low (in the alveoli to the outside), and intrapleural presssure increases while transpulmonary pressure decreases. Volume in the lungs start to decrease as Ap + Ipp are increasing causing collapse

Remember: Transpulmonary pressure which is the difference between alveolar and intrapleural is the greatest during inhalation when the elastic recoil is at it’s farthest point. The pressure decreases during expiration while the alveolar and intrapleural increases in pressure