Lecture 5

Question 1

What is the way air gets into lungs?

Answer 1

The alveolar pressure needs to be smaller than the barometric pressure (atmospheric pressure).

Question 2

How do we achieve air into the lungs?

Answer 2

Increase the volume of the thorax through the exercise of voluntary muscles. Decent of diaphragm at rest and elevation of thorax during exercise.

Question 3

What is Functional Residual Capacity?

Answer 3

Single unique lung volume.

Question 4

What is the intrapleural pressure?

Answer 4

Resident within the thorax and lies between parietal pleura, which adhere to the inside of chest wall and the visceral pleura which covers the lung. There is a potential space between these two - pressure can be measured and called intrapleural pressure. It has a thin film of fluid which allows the pleura to slide with respect to one another so that the lung can inflate.

Question 5

What is in the intrapleural space?

Answer 5

Thin film of fluid which allows the lungs to inflate when the chest expands.

Question 6

What is the lung?

Answer 6

Highly elastic tissue. Left to its own it will collapse down to zero volume.

Question 7

What happens when the lung collapses?

Answer 7

Pulls in toward it the visceral pleura, which pulls in toward it the parietal pleura which pulls in toward it the chest wall. Eventually the chest wall will recoil outward.

Question 8

What does it mean when the intra-pleural pressure is sub-atmospheric?

Answer 8

The intrapleural pressure is commonly always negative, below the barometric pressure. Sub-atmospheric.

Question 9

What is a pneumothorax?

Answer 9

Air in the thorax, and can enter through a wound. It breaks the adherence between two plural surfaces and allows lung to collapse. And the lung is no longer in virtual contact with the chest wall, so the chest wall ill expand outwards.

Question 10

What is Hooke’s Law?

Answer 10

The change of volume is proportional to the change of pressure.

Question 11

What is elastance?

Answer 11

This is the inverse of compliance.

Question 12

What are the components of lung compliance?

Answer 12

1. Tissue compliance (conferred primarily by elastic fibres).
2. Air-water surface tension.
3. Atelectasis - collapsed individual alveoli.

Question 13

Describe compliance of the lung?

Answer 13

Change of volume for a change of pressure.

Question 14

Describe a hysteresis loop?

Answer 14

When we reduce the pressure around the lung (Pip) the lung volume increases up to its maximum value. When we release the pressure, the volume does not come down the same pathway it comes up.

Question 15

Describe the contribution of air-water surface tension to lung compliance?

Answer 15

When you fill the lungs with saline the volume increases and there is no hysteresis loop. When you inflate with air you get the hysteresis loop. The air-water surface tension that exists in the air condition has been removed in the saline condition.

Question 16

What is the difference between saline-filled lungs in terms of compliance?

Answer 16

Saline-filled lungs are more compliant because there is no air-water surface tension. Thus there is no hysteresis loop.

Question 17

What is atelectasis?

Answer 17

Where the alveoli have shrunk down to the point where they are empty and with all sides touching. Have to get up to a pressure that is at least as high than we achieve in a saline-filled lung to get an increase in volume to open atelectic alveoli.

Question 18

How do we measure surface tension?

Answer 18

Can change the area exposed to force transducer by moving barrier back and forth. There is a narrow piece of cardboard/plywood and the only requirement is that water will adhere to it. Can pull on the device and see how much force it takes to break away from the film on fluid. If it is longer there will be more surface tension.

Question 19

What is Laplace’s Law?

Answer 19

The pressure inside a soap bubble is given by the tension in the wall (surface tension). The pressure is inversely dependent on the radius. The alveoli are thin skinned, but within an acinus are tethered together and this prevents them from collapsing. If they do collapse it takes a huge pressure to inflate them again.

Question 20

What is the resting pressure of the chest wall?

Answer 20

When the airway pressure is 0cm in water. And it is around 80% of vital capacity in the lung.

Question 21

What is the topological variation of compliance?

Answer 21

This is due to the influence of gravity. When the lung is empty a given change in pressure produces a large change in volume. The compliance of the lung increases progressively from the top to the bottom. The region at the bottom of the lung is pretty compliant. This is due to the influence of gravity.

Question 22

What is the topological variation of ventilation?

Answer 22

Ventilation is higher (the number of litres of air per minute) in the lower zone than the upper zone.

Question 23

What are the pathological contributors to decreased pulmonary compliance?

Answer 23

Lung:

1. Pulmonary fibrosis.
2. Alveolar oedema.
3. Elevated pulmonary venous blood pressure.
4. Atelectasis.
5. Deficiency of pulmonary surfactant.

Chest Wall:
1. Anything that contributes to increased stiffness of the thorax.

Question 24

What happens when the airway collapses?

Answer 24

The rate of expiratory flow becomes independent of effort. This is reflected in a flat region of the flow-pressure relation.

Question 25

What occurs during pre-inspiration?

Answer 25

No air-flow or pressure being developed, but static intra-pleural pressure. The transmural pressure is +5, thus we hold the airway open.

Question 26

What occurs during inspiration?

Answer 26

Reduce the pressure, so reduce Pip and alveolar pressure, we get sub-atmospheric pressure so air flows in. There is an increase in transmural pressure thus can hold airway open.

Question 27

What occurs during end-inspiration?

Answer 27

No air-flow, very high transmural pressure, so no problem holding it open.

Question 28

What occurs during forced expiration?

Answer 28

Increased intra-pleural pressure, so we get high value of alveolar pressure. Now have negative transmural pressure, so tendency to collapse the airways. The higher the effort you use the increase in tendency to collapse.

Question 29

Describe airway collapse during forced expiration?

Answer 29

Effort-independent expiratory flow rates (at all but high lung volumes) - reflect airway compression when transmural pressure becomes negative.