Mechanics of Breathing

Question 1

State Boyle’s law.

Answer 1

The pressure exerted by a gas is inversely proportional to its volume.
e.g. the bigger the volume, the less the pressure

Question 2

What is the composition of the air that we breathe in?

Answer 2

It is 79% nitrogen, 21% oxygen and a tiny fraction of carbon dioxide.

Question 3

How do gases move?

Answer 3

Gases move from areas of high pressure to areas of low pressure.

Question 4

State Daltons law.

Answer 4

The total pressure of a gas mixture is the sum of the pressures of the individual gases.

Question 5

State Charles Law.

Answer 5

The volume occupied by a gas is directly related to the absolute temperature.

Question 6

What will a given amount of gas measure in a cold room?

Answer 6

It will measure a smaller amount because gas particles have less energy and so therefore they cant move about as much.

Question 7

What will a given amount of gas measure in a warmer room?

Answer 7

It will measure a larger amount because gas particles are more excited when they are hot and so they move around more.

Question 8

State Henry’s Law

Answer 8

The amount of a gas that is dissolved in water is determined by the solubility of the gas and the pressure of the gas.

Question 9

Why does breathing happen?

Answer 9

It happens because the thoracic cavity changes volume.

Question 10

How does Boyle’s law link to the mechanics of breathing?

Answer 10

An increased volume means a decreased pressure, and a decreased volume means an increased pressure.

Question 11

What muscles does inspiration use?

Answer 11

It uses the external intercostal muscles and the diaphragm.

Question 12

What muscles does expiration use?

Answer 12

Expiration is passive at rest, but uses internal intercostal and abdominal muscles during severe respiratory load.

Question 13

What percentage of muscular activity during inspiration is the diaphragm responsible for?

Answer 13

70%

Question 14

What are intercostal muscles?

Answer 14

They are muscles that run between the ribs.

Question 15

What movement do the intercostal muscles and the diaphragm allow during inspiration?

Answer 15

When these muscles contract they lift the ribcage upwards and outwards.

Question 16

What muscles are recruited during heavy respiratory load (inspiration) and what do they do?

Answer 16

The scalene and the sternocleidomastoid muscles are recruited and they act on the upper ribs and the clavicle to lift them up.

Question 17

What is a difference between the internal and external intercostals muscles?

Answer 17

They run at different angles to each other.

Question 18

What is the job of the internal intercostal muscles?

Answer 18

They pull the ribcage downwards and inwards.

Question 19

What happens when the abdominal muscles contract?

Answer 19

They don’t directly act on the thoracic cavity, they reduce the volume of the abdominal cavity and the organs within the cavity need to go someplace, and so they gut pushed up against the diaphragm and thus get pushed into the thoracic cavity and so we reduce the volume of thoracic cavity.

Question 20

What shape is the diaphragm?

Answer 20

It is dome shaped muscle.

Question 21

What is the shape of the diaphragm when it is relaxed?

Answer 21

It is sticking up to the thoracic cavity and takes up some space in the thoracic cavity.

Question 22

What happens when we inspire (phrenic nerve)?

Answer 22

The phrenic nerve innervates the diaphragm and it contracts. It flattens down and increases the volume of the thoracic cavity in a superior, inferior dimensions. Therefore since there is a larger volume, the pressure decreases and air flows from high pressure to low pressure and so air enters the lungs.

Question 23

What happens when we expire with regards to phrenic nerve?

Answer 23

The phrenic nerve stops innervating the diaphragm, so the diaphragm stops contracting and it relaxes back to its normal dome shape. This reduces the thoracic cavity volume and so the pressure increases and air flow out of the lungs.

Question 24

What is the result of the two movements of the external intercostal muscles?

Answer 24

It increases the volume of the thoracic cavity in a anterior posterior dimensions (pump handle) and it increases the volume of the thoracic cavity in a lateral dimension. (Bucket handle)

Question 25

in how many dimensions doe the diaphragm and the external intercostal muscles increase the thoracic cavity volume?

Answer 25

3 Dimensions

Question 26

What happens to the airways during inspiration?

Answer 26

The chest wall is expanding and because the lungs are stuck to the chest wall because of the pleural fluid, the airways that form the lungs and so there is less resistance to airflow.

Question 27

What happens to the airways during expiration?

Answer 27

Airways are compressed because when the diaphragm relaxes, it sticks up into the thoracic region and so it compresses the region which results in a compression of the airways.

Question 28

What two pressures are found in the lungs?

Answer 28

Intra thoracic pressure (alveolar pressure)

Intra-pleural pressure.

Question 29

What is intra-thoracic pressure (alveolar pressure) (P)a?

Answer 29

It is pressure inside the thoracic cavity (especially pressure inside the lungs). It can be positive or negative depending on the atmospheric pressure. During rest it’s zero.

Question 30

What is intra-pleural pressure (P)ip?

Answer 30

It is pressure inside the pleural cavity, typically negative compared to atmospheric pressure.

Question 31

What is Transpulmonary pressure (P)t?

Answer 31

It is the difference between the alveolar pressure and the intra-pleural pressure. Almost always positive because Pip is negative.

Question 32

What does 0, +ve and -ve mean on an inspiration-expiration graph?

Answer 32

0 = no difference in pressure and atmospheric pressure
\+ve = greater than atmospheric pressure
-ve = less than atmospheric pressure

Question 33

During inspiration, is alveolar pressure less or more than atmospheric pressure?

Answer 33

Less than.

Question 34

During expiration, is alveolar pressure less or more than atmospheric pressure?

Answer 34

More

Question 35

When the alveolar pressure is the same as the atmospheric pressure, how does this affect the movement of air.

Answer 35

When the alveolar pressure is the same as the atmospheric pressure then there is no movement of air because we lose that pressure gradient

Question 36

How does intra-pleural pressure vary with expiration and inspiration?

Answer 36

It gets less negative when we expire, and gets more negative when we inspire.

Question 37

Why is intra-pleural pressure always negative?

Answer 37

This is because after a normal expiration, the elastic recoil will make the lungs want to contract further and at the same time the ribs want to expand further. This results in the pleural membrane being pulled apart ever so slightly and this means that the volume is increasing which also increases the pressure.

Question 38

What is the normal value for the intra-pleural pressure?

Answer 38

3mmHg less than atmospheric pressure.

Question 39

Why is the intra-pleural pressure negative during inspiration?

Answer 39

During inspiration the chest wall is leading the expansion of the thoracic cavity, it moves first and then it pulls the parietal pleura away from the visceral pleura, but due to the cohesive force of the pleural fluid, the visceral pleura and the he visceral pleura and the lungs follow, but the chest wall is always leading that expansion. And so leading this constant effort to try and increase the volume of the intrapleural cavity. And by increasing that volume ever so slightly, we decrease the pressure. And that’s what leads to this increased negativity, decreased pressure, during our inspiratory phase.

Question 40

Why is the intra-pleural pressure always negative during expiration?

Answer 40

The chest wall is contracting because it’s following the elastic recoil of the lungs and therefore the two get compressed and we have a slight reduction in pleural cavity volume, which then leads to an increase in pressure.

Question 41

When are inward and outward forces of the lungs equal?

Answer 41

At the end of an expiration.

Question 42

What is the movement of the sternum during inspiration?

Answer 42

Moves outwards and forwards - pump handle movement

Question 43

What is the movement of the ribs during inspiration?

Answer 43

Moves outwards - bucket handle movement

Question 44

What forces keep the airways open during inspiration?

Answer 44

Physical forces of inspiration holds them open

Question 45

What happens to the airways during expiration?

Answer 45

Compressed by physical forces of expiration

Question 46

What factors influence bulk flow of air between the atmosphere and the alveoli?

Answer 46

The difference between atmospheric pressure and alveolar pressure Airway resistance

Question 47

What are the features of the lungs during the end of an unforced expiration?

Answer 47

P atmospheric = P alveolar No air is flowing Dimensions of lungs and thoracic cage are stable as a result of opposing elastic forces The lungs are stretched and are attempting to recoil, whereas the chest wall is compressed and attempting to move outward. This creates a subatmospheric intrapleural pressure and hence a transpulmonary pressure that opposes the forces of elastic recoil.

Question 48

What is the major determinant of airway resistance?

Answer 48

The radii of the airways.