Lung Mechanics

Question 1

Define the pleural seal, what will happen if it is broken?

Answer 1

The thin layer of fluid that holds the outer surface of the lungs to the thorax (visceral and parietal pleura). The pleural seal expands the lungs so that they occupy the entire volume of the thorax (a space much larger than themselves), so if the pleural seal were to break the lungs would recoil

Question 2

What is pneumothorax?

Answer 2

Air in the pleural seal, causing the part of lung around the seal or the whole lung to shrink back

Question 3

Explain the mechanisms of breathing in and breathing out normally

Answer 3

Breathing in: contracts the diaphragm and flattens it so lungs are able to expand, use the external intercostal muscles. This creates -ve pressure in the pleural cavity

Breathing out is passive and is initiated by stopping breathing in

Question 4

Which muscles are involved in forced expiration? Why can you never fully empty the lungs?

Answer 4

Abdominal + internal intercostal muscles push the diaphragm up. However, the lungs are still never fully emptied as the more you expirate, the narrower small airways become - until resistance is so high air is trapped and cannot escape.

Question 5

What 3 situations might make inspiration more difficult?

Answer 5

When there is resistance against the diaphragm contracting: in pregnancy, obesity or corsets

Question 6

What is lung compliance, and what does it tell us?

Answer 6

Relationship between pressure being applied and change in volume of lungs that’s produced: tells you how hard it is to stretch the lungs

Question 7

Name a disease where you would see lower and higher compliance, how would this affect the ‘compliance slope’ on the axes of volume and pressure

Answer 7

Lower: stiff lung, shallow slope in fibrosis

Higher: easier to stretch, steep slope in emphysema

Question 8

What makes lungs elastic?

Answer 8

Elastin

Question 9

Why does air inside the lungs make them stiffer?

What can help reduce this?

Answer 9

Due to surface tension: interactions between molecules at the surface of a liquid makes the surface resist stretching

Surfactant disrupts the interactions between surface molecules and reduces the surface tension when lungs are deflated (less air in them)- so they are easy to re-inflate.
Secreted by type 2 alveolar cells: cuboidal surfactant secreting cells

Question 10

Explain Laplace’s law and how it relates to the law of bubbles. If the law of bubbles were true, what would this mean in terms of our alveoli?

Answer 10

Laplace’s law: pressure in a bubble = 2X the surface tension of the fluid wall/radius; so big bubbles have low pressure, and small bubbles have high pressure

Air moves from high –> low pressure, so theoretically (law of bubbles), the big bubbles would consume the small bubbles. If this were true it would mean we would have one large alveolus in each lung

Question 11

What prevents the law of bubbles from being true?

Answer 11

Surfactant: As bubbles become larger, surfactant becomes less effective and the surface tension increases - so the pressure goes up and matches the pressure within the smaller bubbles (with high levels of surfactant). Therefore, larger bubbles are unable to consume smaller bubbles as they have equal pressures.

Question 12

What happens in respiratory distress syndrome? How would you treat it?

Can these symptoms be mimicked in an adult?

Answer 12

prematurely born babies have underdeveloped lungs that lack surfactant. Their lungs are stiff and contain few and large alveoli, and breathing and gas exchange is compromised. Treat by giving mum steroids to accelerate the development of baby’s lungs, or giving baby artificial surfactant until they’re able to make it

Adults can develop similar symptoms after trauma as their lungs can stop producing surfactant

Question 13

How much energy is used to stretch the liquid in our lungs (fight surface tension) and how much is used to stretch our actual lung?

Answer 13

50/50

Question 14

What is surfactant composed of?

Answer 14

Phospholipids and proteins with detergent properties

Question 15

How does poiseulles law relate to resistance in large and small airways? What fixes this so we’re able to attain the resistance we have?

Answer 15

Poiseulles law states small tubes have a high flow resistance, BUT since our smaller airways are in parallel, the resistance decreases. So larger airways (like trachea) have high resistance and smaller airways have very low resistance. Therefore as you move down the lungs, the more parallel airways exist and the easier it is for air to move down

Question 16

What’s the core difference between obstructive and restrictive airway disease? Name 3 obstructive diseases

Answer 16

In obstructive: smaller airways are narrowed by disease and resistance increases earlier in expiration.

In restrictive, it is harder to inhale and expand the lungs
COPD, chronic bronchitis, asthma

Question 17

How are the FVC and FEV1 influenced in..
Obstructive disease
Restrictive disease

Answer 17

Obstructive: Reduced FEV1, same FVC

Restrictive: Reduced FVC, FEV1 is reduced but remains proportional to the now reduced FVC - so can say it’s the same