Mechanics of Breathing I

Question 1

what is the primary function of the respiratory system?

Answer 1

to ventilate gas exchange surfaces by moving air (a mixture of gases) between the alveoli and the atmosphere via the airways.

Question 2

what type of gradient do gases naturally move down?

Answer 2

a pressure gradient

-gases naturally move from areas of higher pressure to lower pressure, until an equilibrium is re-established

Question 3

The total volume of air that an individual can move in/out of the lungs during inspiration/expiration is important to what?

Answer 3

important to the quantity of oxygen that reaches alveolar surfaces

Question 4

name some factors that cause total lung volumes to vary between individuals?

Answer 4

• height
• sex
• age
• the extent of specific respiratory diseases.

Question 5

the ideal gas law

Answer 5

PV=nRT
pressure x volume = number of moles x gas constant x temperature

pressure in kPa
volume in Litres
Temperature in Kelvins

Question 6

boyle’s law

Answer 6

Pressure = the number of gas molecules ‘n’ within a given volume
If ‘n’ remains constant, ↑ Volume = ↓ Pressure

Question 7

essentially how does the respiratory system like to move gases?

Answer 7

manipulate the space they’re in in order to move them

Question 8

what is quiet breathing?

Answer 8

breathing at rest

Question 9

changes in lung volume induce changes in what?

Answer 9

changes in lung volume induce changes in alveolar pressure

-this generates pressure gradients between alveoli & atmosphere, causing air to flow

Question 10

if you want air to come from the atmosphere into the lungs where does the pressure have to be higher?

Answer 10

If you want air to come from atmosphere into the lungs the pressure in the atmosphere has to be higher than that in the lungs, so it will travel down a pressure gradient into the lungs

Question 11

how do you create the pressure gradient?

Answer 11

in order to create the gradient, the pressure of the gas in the atmosphere or the pressure of the gas in the lungs must be changed:

• given you can’t control the pressure in the atmosphere, its only the volume and pressure of the lungs that can be controlled
• the pressure in the lungs is manipulated relative to the atmosphere in order to get a pressure gradient both ways.

Question 12

what is the main respiratory muscle involved in passive/relaxed breathing?

Answer 12

the diaphragm

Question 13

what happens in inspiration?

Answer 13

-the outer surfaces of the lung are pulled outwards (expansion)
the diaphragm contracts
-thoracic cavity expands, increasing volume
-alveolar pressure decreases
-air enters down a pressure gradient, from high (atmosphere) to low (alveoli) pressure

Question 14

what happens in expiration?

Answer 14

-air within the lung is compressed
diaphragm relaxes and lung recoils
-thoracic cavity volume decreases
-alveolar pressure increases (becomes higher than pressure in atmosphere)
-air leaves down a pressure gradient

Question 15

at the end of expiration why is there no movement of air?

Answer 15

because at the end of expiration, the pressure in the alveoli = the pressure in the atmosphere

Question 16

what structure indirectly attaches the lungs and chest wall?

Answer 16

the pleural cavity

Question 17

what is the pleural cavity?

Answer 17

fluid filled space between the membranes (pleura) that line the chest wall and each lung

Question 18

what is the role of the pleura?

Answer 18

• reduce friction between the lungs and the chest wall during breathing
• ensures that movements of the chest wall result in movements of the lungs, so when the chest wall expands, the lungs also expand.

Question 19

when there is a change is pressure how is equilibrium re-established?

Answer 19

1) either movement of liquid/gas

2) collapse/expansion of volume (at the expense of surrounding structures)

Question 20

what is negative pressure?

Answer 20

when there is a lower number of molecules per volume (relative to surroundings)
→ generates collapsing force (pulls surfaces of contained space together)

Question 21

how is negative pressure generated in the body?

Answer 21

The opposing elastic recoil of the chest wall (wants to recoil outwards) and lungs (wants to recoil inwards) generates negative pressure within the pleural cavity (sealed cavity + increased volume = decreased pressure).

Question 22

what is the aim of negative pressure?

Answer 22

Negative pressure acts to pull the two membranes together (more negative pressure = more force pulling pleurae together)

Question 23

what are the names of the 2 pleura?

Answer 23

• visceral pleura, closest to lungs

- parietal pleura (a serous membrane), closest to chest wall

Question 24

what is positive pressure?

Answer 24

-increased number of molecules per volume (relative to surroundings)
→ generates expanding force (pushes surfaces of contained space apart)

Question 25

explain the effect of negative pressure:

Answer 25

• pleural pressure decreases, means its slightly unstable and the structures want to collapse in
• so, lung volume increases because the pleural membranes have essentially collapsed and in order to do that it has pulled the lungs open. Increase in volume means decrease in pressure, so air comes in.

Question 26

explain the effect of positive pressure?

Answer 26

• increase pressure in the pleural space so that it’s greater than the lungs, by compressing it slightly.
• the increase in pressure means it wants to exert an expanding force that pushes the 2 membranes apart, decreasing lung volume

Question 27

what is the expansion and retraction of the lung determined by?

Answer 27

the relative size of opposing forces

Question 28

when the will lung volume remain constant?

Answer 28

when the force pulling the visceral pleura inwards (lung recoil) = the force pulling the visceral pleura outwards (negative intrapleural pressure)

Question 29

what directions do the lungs and chest cavity have a tendency to move in?

Answer 29

• On the inside, the lung has elastic qualities- want to coil inwards
• On the outside, the chest cavity wants to extend outwards

Question 30

what qualities do the lungs have that make them want to coil inwards?

Answer 30

elastic qualities (elastic recoil)

Question 31

during inspiration, what does muscular contraction do?

Answer 31

muscular contraction pulls the parietal pleura outwards, stretching the cavity, decreasing interpleural pressure (making it more negative)

Question 32

what does the decrease in intrapleural pressure create?

Answer 32

the decreased intrapleural pressure creates a greater level of force acting to pull the pleura together

Question 33

during inspiration, why is the visceral pleura pulled outwards, expanding the lungs?

Answer 33

the visceral pleura is pulled outwards when the force generated by negative intrapleural pressure becomes greater then the inward force generated by lung recoil

Question 34

during expiration, what does relaxation of respiratory muscles do?

Answer 34

reduces the outward force acting on the parietal pleura, reducing the degree to which the cavity is stretched, increasing intrapelural pressure

Question 35

what happens when the intrapleural pressure increases so much that it no longer generates sufficient force to overcome the elastic recoil of the lung?

Answer 35

the visceral pleura will be pulled inwards (along with the pleural cavity and parietal pleura), decreasing lung volume

Question 36

what happens in a forced expiration, what is the role of contraction of other respiratory muscles?

Answer 36

• contraction of other respiratory muscles (abdominals, internal intercostals) provide further inward pressure on the parietal pleura
• this compresses the pleural cavity, further increasing intrapleural pressure
• forcing an increased and more rapid decline in lung volume

Question 37

changing the volume of the thoracic cavity causes what?

Answer 37

Movement of air into/out of the lungs

Question 38

what does expiration involve?

Answer 38

• elastic recoil of the lungs

- relaxation of respiratory muscles

Question 39

does compression of the lungs due to increased intrapleural pressure occur in both forced expiration and quiet breathing?

Answer 39

NO, compression of the lungs only occurs during forced expiration

Question 40

summary of inspiration:

Answer 40

Respiratory muscles (e.g. diaphragm) contract
↓
Volume of thoracic cavity increases
↓
Intrapleural pressure becomes more negative
↓
Outward force exerted on visceral pleura becomes greater than inward recoil force
↓
Lungs expand, increasing volume
↓
PAlv (alveolar pressure) decreases below PAtm (atmospheric pressure)
↓
Air moves down pressure gradient, through airways into alveoli, expanding the lungs

Question 41

summary of expiration:

Answer 41

Respiratory muscles (e.g. diaphragm) relax, lungs recoil due to elastic fibres
↓
Volume of thoracic cavity decreases
↓
Intrapleural pressure increases
↓
Lungs compressed*, volume decreases
↓
PAlv increases above PAtm
↓
Air moves down pressure gradient, into atmosphere, deflating lungs

Question 42

what is the speed of airflow is dependent on?

Answer 42

the pressure gradient and the level of airway resistance present.

Question 43

give an example of when the negative intrapleural pressure is lost?

Answer 43

an open pneumothorax

Question 44

what happens in an open pneumothorax?

Answer 44

• in one of the pleural membranes (e.g. a stab wound), air will enter the pleural cavity (down its pressure gradient) equalising intrapleural pressure with the atmosphere.
• no longer any negative interpleural pressure pulling the lungs towards the chest wall
• the entry of air causes an increase in pleural cavity volume, reducing intrapleural pressure generated during inspiration, preventing lungs from expanding properly
• as a result, the lungs can recoil to a low volume-lung collapse.

Question 45

what is a closed pneumothorax?

Answer 45

Closed is whenair getsin thepleural space without any outside wound- sometimes happens when the lung is already injured somehow eg. from diseases such as cancer or cystic fibrosis

Question 46

so what does a pneumothorax involve?

Answer 46

involves an entry of air into the pleural cavity, loss of negative intrapleural pressure, and collapse of lung tissue

Question 47

what does a pneumothorax cause?

Answer 47

Pneumothorax causes affected parts of the lung to collapse due to elastic recoil

Question 48

In a healthy respiratory system, the visceral and parietal pleura (effectively the lungs and chest wall) are prevented from separating by what?

Answer 48

prevented from separating by the existence of negative pressure within the pleural cavity.

Question 49

what issues will a collapsed lung create?

Answer 49

-ability to breathe and ventilate gas exchange structures sufficiently in alveoli