Anatomy Of Pleural Cavity, Mechanics Of Breathing, Surfactant And Compliance

Question 1

What is the total lung capacity?

Answer 1

Vital capacity + residual volume (6L)

Question 2

What is the tidal volume?

Answer 2

Volume you breathe in and out at rest (500ml in 500ml out)

Question 3

What is the functional residual capacity

Answer 3

Volume of air still in the lungs after breathing out the tidal volume (normal relaxed expiration) (2.3L)

Question 4

What is the inspiratory reserve volume?

Answer 4

The maximum volume of air we can breath in at the end of normal inspiration (an extra 3L) to fill up lung capacity

Question 5

What is the expiratory reserve volume?

Answer 5

The maximum volume of air which can be expelled at the end of normal expiration (1L)

Question 6

What is the residual volume?

Answer 6

The volume of air that cant be voluntarily expired (1-1.2L)

Important to prevent alveoli collapsing

Question 7

What would happen if alveoli collapsed at end of expiration

Answer 7

Would need much greater energy to inflate alveoli on next inspiratory breath (e.g. blowing up a balloon the initial effort is the most difficult)

Provides a volume of air which allows gas exchange to occur between breaths

Question 8

What is the vital capacity?

Answer 8

Tidal volume + inspiratory reserve volume + expiratory reserve volume

(Maximum inhalation followed by maximum expiration, total amount of air expired = vital capacity)

Question 9

How many pleural cavities within thoracic cavity?

Answer 9

2 (one around each lung)

Question 10

How much fluid is in the pleural cavity?

Answer 10

3ml

Question 11

What are the membranes of the pleura cavity? Which one against ribs and which one against lungs?

Answer 11

Visceral membrane = stuck to lungs

Parietal membrane = stuck to ribs (inferiorly stuck to diaphragm)

Question 12

Why is the pleural membrane important for respiration?

Answer 12

Allows lungs to be stuck to rib cage and diaphragm and follow whatever they do
Otherwise lungs would hang as independent entity and when ribs and diaphragm move lungs wouldn’t do anything

Question 13

Why is it important that the parietal membrane and visceral membrane has fluid between them?

Answer 13

Allows membranes to glide across one another
As we breath lungs have to glide across the surface of ribs and diaphragm - pleural fluid allows for friction free movement

Question 14

How does the pleural fluid prevent lungs from recoiling?

Answer 14

At end of expiration the elastic fibres are still slightly stretched - they want to recoil more but this is prevented by cohesive force of pleural fluid

Question 15

What happens to pleural cavity in stab wounds?

Answer 15

Air introduced into pleural cavity causes membranes to be forced apart
Therefore nothing preventing the lung recoiling so it collapses (no longer follow movements of diaphragm and ribs)

Question 16

What is boyle’s law?

Answer 16

The pressure exerted by a gas is inversely proportional to its volume
E.g. the bigger the volume the less pressure the gas will exert

Gases move from areas of high pressure to low pressure

Question 17

How does changing the volume of the thoracic cavity impact pressure and therefore breathing?

Answer 17

Increase the volume = decrease pressure (gas will move in as area of low pressure inside cavity compared with outside)
Decrease volume = increase pressure (gas will move out as pressure outside cavity lower than inside)

Question 18

Which muscles are used to increase thoracic volume during inspiration?

Answer 18

External intercostal muscles and the diaphragm

Main muscle is diaphragm (70%)
When external intercostal muscles contract they lift ribs upwards and outwards
Under heavy respiratory load scalene and sternocleidomastoid muscles are recruited (act on upper ribs and clavicle to lift them)

Question 19

Which muscles are used to decrease thoracic volume during expiration?

Answer 19

Expiration is passive at rest
Speed & force of expiration can be increased by the internal intercostal muscles and abdominal muscles

Pull rib cage downward and inward & pushes abdominal cavity up against diaphragm = decreased volume of thoracic cavity

Question 20

Which nerve innervates the diaphragm causing it to contract? What happens during expiration

Answer 20

The phrenic nerve

Expiration: stops stimulating diaphragm so relaxes to dome shape (reducing volume of thoracic cavity)

Question 21

Why is intrapleural pressure always less than alveolar pressure?

Answer 21

The alveolar pressure can be negative or positive compared to atmospheric pressure
- pressure in lungs > atmospheric pressure = positive
- pressure in lungs < atmospheric = negative
Intra-pleural pressure is always negative in healthy lungs - becomes even more negative during inspiration (end of expiration lungs still have elastic recoil & want to contract further, at same time ribs wan to expand - pleural membrane holds together so is slightly pulled apart = more neg. Pressure)

Question 22

What is the role of surfactant?

Answer 22

Surfactant increases lung compliance (makes it easier for air to get into lung)
Makes it less likely for the lungs to recoil (by reducing the surface tension)

Question 23

When does surfactant production start in gestation?

Answer 23

Starts at 25 weeks and isn’t complete until 36 weeks
Born before 36 weeks is at risk of infant respiratory distress syndrome (difficulty breathing as every breath needs to overcome surface tension to inflate alveoli)

Question 24

What is compliance?

Answer 24

The change in volume relative to change in pressure (how much does volume change for any given change in pressure)

Represents how easy it is to inflate the lungs

Question 25

What does a high or low compliance mean? Do healthy lungs have high or low compliance?

Answer 25

High compliance: large increase in lung volume for small decrease in intrapleural pressure
Low compliance: small increase in lung volume for large decrease in intrapleural pressure

Healthy lungs have high compliance (don’t need much energy to inflate)

Question 26

Why may high compliance not always be good?

Answer 26

Conditions such as emphysema
- loss of elasticity of lungs - don’t need to overcome the elastic tension on inspiration (very difficult to get the air out though)

Question 27

Why is low compliance never good? What causes low compliance?

Answer 27

Always have to work hard to get adequate air into lungs

Conditions such as fibrosis - fibrous tissue alongside elastic tissue (resists stretch)
Natural loss in compliance with ageing

Question 28

How does compliance change from base to apex of lungs?

Answer 28

Compliance greatest at the base of lungs

Lungs hang within the body so alveoli at the apex are already stretched open
Alveoli at the base are slightly compressed by diaphragm therefore more compliant on inspiration

Question 29

What is the volume of the anatomical dead space?

Answer 29

150ml