Lec 3 - Lung mechanics and ventilation of the lungs

Question 1

What is ventilation?

Answer 1

It is the process of inspiration and expiration.

- The physical action of breathing and moving air into and out of the lung.

Question 2

What is tidal volume?

Answer 2

It is the volume of air being moved.
-It represents the volume of air entering and leaving the lungs in a single breath - during quiet inspiration and expiration.

Question 3

What happens during quiet inspiration?

Answer 3

• diaphragm flattens.
• the external intercostal muscles expand.
• -> this expansion is to lower pressure, drops intrapulmonary pressure to below atmospheric pressure so air is forced into the lungs.
• the volume in the thoracic cavity increases and in turn the volume of the lungs also increases.

Question 4

What keeps the lungs against the chest wall?

Answer 4

• there is pleural fluid found between visceral and parietal pleura (in the intrapleural space) which forms a seal between the lung and thoracic wall.
• -> so the lungs expand with the thoracic cavity.

Question 5

How is the pleural seal created?

Answer 5

-surface tension between the pleural surfaces is created by the presence of thin film of pleural fluid, which holds the outer surface of the lung to inner surface of the chest wall.

Question 6

What happens during the resting expiratory level - the bit in between quiet inspiration and expiration. i.e the state of equilibrium)?

Answer 6

1. the lungs pull in and up.
2. the chest wall pulls out –> elastic recoil.
3. the diaphragm pulls down due to passive stretch not active contraction.
4. Forces are equal and opposite so balance out meaning no movement of chest wall in between inspiration and expiration.
5. There is also a tendency to always want to return to this resting state like an elastic band or spring.

Question 7

What disturbs the equilibrium between quiet inspiration and quiet expiration?

Answer 7

Ventilation.

Question 8

What happens during inspiration?

Answer 8

• muscles contract to allow the chest wall and diaphragm to overcome the inward pull of the lung recoil.

Question 9

What happens during expiration?

Answer 9

• The muscles stop contracting.
• The chest wall and the diaphragm no longer overcome the inward pull of lung recoil.
• Returns to resting expiratory level.

Question 10

What are the volume changes in inspiration?

Answer 10

• There is the expansion of thoracic cavity and lungs.
• There is an increase in volume and decrease in pressure.
• This moves air into the airways.
• It is active as it requires contraction of external intercostals and diaphragm.

Question 11

What are the volume changes in expiration?

Answer 11

• There is the reduction in size of thoracic cavity and lungs.
• There is a decrease in volume and an increase in pressure..
• This moves air out of the airway.
• There is the passive relaxation of muscles and natural elastic recoil of lung tissue.

Question 12

What creates the negative pressure between the pleura?

Answer 12

The inward pull of the lung.

-The elastic recoil of the lung pulls visceral pleura inward and chest wall pulls the parietal pleura outward.

Question 13

What happens if the integrity of the pleural seal is broken?

Answer 13

The negative pressure in the intrapleural space will draw air in from the outside chest wall into the pleural space.
-this causes the lung to collapse.

Question 14

What are the accessory muscles of inspiration used in forced inspiration?

Answer 14

• sternocleidomastoid.
• scalene muscles
• serratus anterior
• pectoralis major

Question 15

What are the accessory muscles of expiration used in forced expiration?

Answer 15

• Internal intercostals - pull down on ribs

- abdominal wall muscles - pull down on ribs.

Question 16

What is compliance of the lung determined by?

Answer 16

• elastic tissue in lung
• surface tension forces of fluid lining the alveoli.

–> The higher the compliance the easier it is to stretch.

Question 17

What does surface tension do?

Answer 17

Surface tension of fluid limits expansion of the alveoli.
—> This decreases compliance making it difficult for alveoli (and therefore lungs) to expand.

Surfactant – it is found in the wall of the alveoli.

• –> It is secreted by type II pneumocytes in lungs.
• –> It has detergent properties –> It breaks up H20 on the surface of alveoli, which breaks up surface tension created by H20 molecules around the alveoli.

Question 18

When is surfactant most effective at disrupting surface tension of fluid?

Answer 18

Around smaller alveoli, because the force is required to expand small alveoli is less than required for large alveoli.

Question 19

What happens to surfactant in larger alveoli?

Answer 19

• Surfactant molecules spread further apart.
• Less effective at disrupting the surface tension.
• Surface tension increases as lungs expand, hence forced inspiration is harder than quiet inspiration.

Question 20

What happens to surfactant in smaller alveoli?

Answer 20

• Surfactant molecules closer together .
• so they are more effective at disrupting surface tension of fluid.
• So there is reduced surface tension.

Question 21

What is law of Laplace?

Answer 21

Pressure = 2 x surface tension divided by radius.

Question 22

What else does surfactant allow?

Answer 22

• it also prevents small alveoli collapsing into big alveoli.
• It also allows different sized alveoli to have the same pressure within them.

Question 23

What happens to the alveoli when there is no surfactant?

Answer 23

• The small and big alveoli have equal surface tension.
• The small alveoli has a higher pressure due to there being a smaller radius.
• The small alveoli are more likely to collapse and be harder to inflate.
• The small alveoli would collapse into the big one.
• There is no inspiration as don’t have enough to overcome the surface tension.

Question 24

What happens to the alveoli when there is surfactant?

Answer 24

• The small alveoli has less surface tension, so more surfactant per area.
• The small and big alveoli have equal pressure due to surfactant.
• The small alveoli will inflate at a faster rate than the bigger alveoli.

–Pressure goes up as the alveoli gets bigger, so excess of surfactant and interference of surface tension means no matter the size of the alveoli will both have same pressure.

Question 25

Define surfactant in the big alveoli.

Answer 25

1. Due to surfactant, bigger alveoli have greater surface tension than smaller alveoli.
2. Thus, pressure inside the bigger alveoli stays high despite it being bigger
   - –> For example, pressure does not drop despite increased ‘volume’ of the alveolus as the increased pull inwards from the surface tension counters this.
3. If pressure remains high in bigger alveoli, keeps pressures equal to smaller alveoli, which prevents collapsing of small alveoli into big alveoli.

Question 26

What are the differences in resistance in normal lungs?

Answer 26

• Highest resistance is in upper respiratory tree (trachea and large bronchi)
• Lowest resistance in smaller airways except when these become compressed during forced expiration.