Surfactant and Compliance

Question 1

What is surfactant?

Answer 1

It is a detergent like fluid produced by type II alveolar cells (neumocytes)

Question 2

What is the function of the surfactant?

Answer 2

• it reduces surface tension and reduces the tendency for the alveoli to collapse.
• increases lung compliance (distensibility - how easy it is to stretch the lung open)
• Reduces the lung’s tendency to recoil
• Makes the work of breathing easier

Question 3

What is surface tension?

Answer 3

It occurs where ever there is an air-water interface and refers to the attraction between water molecules. (e.g rain droplets falling down a window pane)

Question 4

How does an air water interface relate to the alveoli?

Answer 4

There is air in the middle of the alveoli and this air needs to be saturated before it can diffuse and so this happens on the surface of the alveoli and so there is an air-water interface.

Question 5

What effect do the water droplets on the surface of the alveoli have on the alveoli?

Answer 5

The water droplets create an inwardly directed pressure that could result in the collapse of the alveoli.

Question 6

Where does surfactant come into play with these water molecules?

Answer 6

It reduces the force of attraction between the water molecules and thus reduce the inwardly directed force.

Question 7

Is surfactant more effective in small or larger alveoli?

Answer 7

In smaller molecules because the alveoli are more close together and so they are therefore more concentrated.

Question 8

State the law of Laplace.

Answer 8

The pressure required to keep an alveoli open is equal to 2T/r ( 2 x surface tension /radius)

Question 9

Why is it better to have one medium alveoli and a small alveoli rather than just one big one?

Answer 9

The surface area available for gas exchange of the two alveoli is bigger than just one alveoli.

Question 10

Why is the surface tension lower in a smaller alveoli?

Answer 10

The surfactant is more closely packed together and so it is more concentrated.

Question 11

During pregnancy, when does surfactant production start?

Answer 11

25 week and it ends at 36 weeks gestation

Question 12

. What stimulates the growth of surfactant?

Answer 12

Thyroid hormones and cortisol which are increased in production towards end of pregnancy.

Question 13

What disease do premature babies suffer from?

Answer 13

Infant Respiratory Distress Syndrome

Question 14

Why is it so difficult for babies that are born prematurely to breathe without struggling?

Answer 14

They haven’t been in the womb for long enough to develop surfactant but now there are new aerosol cans that can provide babies with surfactant.

Question 15

What requires more pressure to get into the lungs, saline or air?

Answer 15

Air

Question 16

What is compliance?

Answer 16

How much does volume change for any given change in pressure.

Question 17

What does compliance represent?

Answer 17

It represents the stretchability of the lungs (not the elasticity), how easy it is to get air into the lungs, doesnt tell us anything about getting air out of the lungs

Question 18

What does high compliance?

Answer 18

Large increase in lung volume for small decrease in ip pressure

Question 19

What is low compliance?

Answer 19

Small increase in lung volume for large decrease in ip pressure

Question 20

What can change compliance?

Answer 20

Disease states, age and action of surfactant.

Question 21

What is emphysema?

Answer 21

High compliance but low elasticity.

Question 22

What force has to be overcome during inspiration?

Answer 22

Inward force caused by the elasticity of the lungs - they are stretched slightly open even at the end of a relaxed expiration - natural tendency to recoil (contract inwards)

Question 23

What type of alveoli has the greatest pressure?

Answer 23

The smallest one

Question 24

Why is less change in pressure required to inflate lungs in utero?

Answer 24

Don’t need to overcome surface tension (no air-water interface)

Question 25

It requires a greater change in pressure to reach a particular lung volume during inspiration, than to maintain that volume during expiration. Why?

Answer 25

Normally effort (work) of inspiration is recovered as elastic recoil during expiration (hence expiration is passive).

Question 26

What does fibrosis do to the work of breathing?

Answer 26

Inert fibrous tissue means an effort of inspiration increases

Question 27

Why does compliance decrease with height?

Answer 27

Alveoli at the apex are more inflated at FRC. At the base, the alveoli are compressed between the weight of the lung above and the diaphragm below and hence more compliant on inspiration. Alveoli at the top are further inflated than the alveoli at the base of the lung for the duration of the breathing cycle - hence they can inflate less