Pulmonary ventilation

Question 1

What do lung volumes and capacities depend on?

Answer 1

Factors such as:

• Age
• Sex
• Height
• Lung properties (E.g. Compliance, obstruction/damage due to disease)

Question 2

What is ventilation and how can volume of it be calculated?

Answer 2

Ventilation - Supply of fresh air to lungs

V = Vt x f

Vt = tidal volume, the volume of air inhaled in each breath

f = frequency, the number of breaths per minute

Question 3

What is alveolar minute volume and how can it be calculated?

Answer 3

Alveolar minute volume (mL) - Total volume of fresh air entering the alveoli across all breaths over one minute

VA = (Vt - Vd) x f

Vt = Tidal volume
Vd = Dead space volume, the volume of air remaining in the respiratory system at the end of expiration

Vt-Vd = The volume of fresh air entering the alveoli in each breath

f = frequency

Question 4

What happens when the rate of ventilation increases?

Answer 4

Increases alveolar oxygen partial pressure and decreases alveolar carbon dioxide partial pressure.

Decreasing the rate of ventilation has opposite effects

Question 5

What is the primary function of the respiratory system?

Answer 5

To supply respiring tissues with oxygen and remove excess carbon dioxide by ensuring sufficient levels of gas exchange take place within the lungs

Question 6

Define pulmonary ventilation

Answer 6

Movement of air from the atmosphere to gas exchange surfaces (alveoli) within the lung

Question 7

Why is pulmonary ventilation important?

Answer 7

Required to maintain O2 and CO2 gradients between alveolar air and arterial blood

This enables a sufficient level of gas exchange to take place, ensuring adequate O2 supply/CO2 removal to/from respiring tissues (via blood).

Question 8

What are hypoventilation and hyperventilation defined as?

Answer 8

Insufficient (hypo) or excessive (hyper) levels of breathing relative to that required to meet the metabolic demands of the body, and can be identified by the level of CO2 present within the blood.

Hypoventilation results in excessive levels of CO2within arterial blood (PaCO2>6.0 kPa)

Hyperventilation results inreducedlevels of CO2within arterial blood (PaCO2< 4.9 kPa).

Question 9

How are pressure gradients between alveoli and blood maintained?

Answer 9

By adequate ventilation

Increased ventilation = increased partial pressure gradient (b/w alveoli and blood), therefore increases gas exchange

Question 10

Where does gas exchange only take place?

Answer 10

Alveoli, but air must pass through the airways (airways = anatomic dead space)

Question 11

Why is the respiratory system 2 way?

Answer 11

Air enters and leaves via the same path. A residual volume of air remains in the airway + lungs at the end of expiration.

This means that the final 150ml (dead space volume) of each inspiration never reaches the alveoli or takes place in gas exchange

Question 12

What does it mean if air is ‘fresh’ or ‘stale’?

Answer 12

Fresh - Air that has just entered the respiratory system from the atmosphere

Stale - Air that entered the lungs during a previous breath and which has already participated in gas exchange.

Question 13

How does the respiratory system achieve movement of air? Discuss how gases move and Boyle’s law in relation to the ideal gas equation

Answer 13

Gases naturally move from (connected) areas of higher pressure to lower pressure, until an equilibrium is re-established.

Boyle’s law is directly derived from the ideal gas law, and this means that the pressure within a contained space can be changed by altering the volume of the space.

Ideal gas law = PV=nRT, so Boyle’s law states the pressure is directly proportional to n/V (n=moles, V= volume)

Any pressure gradient b/w this container and those it is connected to will induce movement of gas molecules from high to low pressure areas until an equal level of pressure in each area has been re-established
Movement of air b/w the atmosphere and lungs must be achieved solely by changing alveolar pressure. To move air into the lungs during inspiration, alveolar pressure must fall below atmospheric pressure (so air moves down the concentration gradient), and during expiration, alveolar pressure must rise above atmospheric pressure.

These pressure changes are achieved by contraction/relaxation of respiratory muscles. This alters the volume of the thoracic cavity (with the opposite effect on alveolar pressure).

Question 14

What do changes in lung volume induce? Describe the changes in terms of what happens during both inspiration and expiration

Answer 14

Changes in alveolar pressure, which generate pressure gradients between alveoli and atmosphere, causing air to flow.

During inspiration, air flows into the alveoli from the atmosphere as the atmosphere is at higher pressure, so the outer surfaces of the lung are pulled outwards, leading to expansion. The diaphragm contracts, and the thoracic cavity expands. This increases the volume of the lungs, therefore decreasing alveolar pressure.

During expiration, there’s now a higher pressure in the alveoli than the atmosphere, so air flows out into the atmosphere, and this generates a collapsing force, causing volume to decrease, and therefore alveolar pressure increases. The diaphragm relaxes (lung recoils), and thoracic cavity volume decreases.

Question 15

What happens when there’s lower alveolar pressure than atmospheric pressure?

Answer 15

Outer surface of lung pulled outwards (expansion)

⬆️Volume = ⬇️Alveolar pressure

Air flow from high (atmosphere) to low (alveoli) pressure

Question 16

What happens when there’s higher alveolar pressure than atmospheric pressure?

Answer 16

Air within the lung is compressed.

⬇️LungVolume = ⬆️Alveolar pressure

Air flows from high (alveoli) to low (atmosphere) pressure)

Question 17

What is pleurae?

Answer 17

Serous membranes separating the lungs and chest wall.

The lungs are not directly attached to the wall of the thoracic cavity.

Respiratory muscles are not attached to or pull on the lungs directly.

The (inner) visceral pleura lines each lung, whereas the (outer) parietal pleural lines the thoracic cavity, surrounding the chest, diaphragm + mediastinum. B/w the pleura is the pleural cavity

Question 18

What is the pleural cavity?

Answer 18

Fluid filled space between membranes (pleura) that line the chest wall and each lung - its presence reduces friction between lungs and chest during breathing

Question 19

What do the properties of the pleural cavity mean for volume?

Answer 19

The pleural cavity is sealed and fluid-filled. This means that it resists changes in volume. Thus, changes in the volume of the thoracic cavity (due to respiration, muscle activity) result in changes in lung volume (rather than pleural cavity volume).

Question 20

What is the significance of the elastic properties of pleura?

Answer 20

Tissues attached to each pleura recoil in opposite directions, stretching the sealed pleural cavity b/w them very slightly and resulting in the pressure within the cavity being naturally sub-atmospheric

Question 21

What does the opposing elastic recoil of the chest wall and the lungs result in?

Answer 21

Chest wall = Outwards

Lungs = Inwards

This results in the pressure within the pleural cavity being sub-atmospheric (under ‘negative pressure’)

Question 22

In the absence of sufficient opposing force, how is equilibrium re-established?

Answer 22

Via either movement of liquid/gas, or collapse/expansion of volume (at the expense of surrounding structures)

Question 23

Define negative pressure

Answer 23

Lower number of molecules per volume (relative to surroundings) → generates collapsing force (pulls surfaces of contained space together)

There’s a greater pressure within the atmosphere than within respiratory surfaces, so air moves from the atmosphere to the alveoli, and flows into the system

Question 24

Define positive pressure

Answer 24

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

There’s a greater pressure within the system, and a lower pressure in the environment, so air moves from within the system to the environment

Question 25

How is air movement into/out of the lungs achieved?

Answer 25

By changing the volume of the thoracic cavity

Question 26

Describe inspiration

Answer 26

Respiratory muscles (e.g. diaphragm) contract

Volume of thoracic cavity increases

Intrapleural pressure becomes more negative

Lungs expand, increasing volume

Alveolar pressure decreases below atmospheric pressure

Air moves down pressure gradient, through airways into alveoli, expanding the lungs

Question 27

Describe expiration

Answer 27

Respiratory muscles (e.g. diaphragm) relax, lungs recoil due to elastic fibres

Volume of thoracic cavity decreases

Intrapleural pressure increases

Lungs compressed, volume decreases

• Compression of the lungs is due to the increased intrapleural pressure and only occurs during forced expiration. In quiet breathing, elastic recoil is sufficient to decrease lung volume.

Alveolar pressure increases above atmospheric pressure

Air moves down pressure gradient, into atmosphere, deflating lungs

Question 28

What happens to intrapleural pressure during inspiration?

Answer 28

As lung volume increases during inspiration, intrapleural pressure becomes more negative due to the elastic properties of the lung generating increasing recoiling force

Question 29

What happens to volume as the lungs expand?

Answer 29

Volume increases, causing a decrease in alveolar pressure. As air enters the lungs, the pressure re-equilibrates as the increased concentration of gas molecules compensates for the increased volume (P= n/V)

Question 30

What happens when alveolar pressure is less than atmospheric pressure?

Answer 30

The pressure gradient causes air to move into the lungs. This leads to inflation and increased volume, which is reversed during expiration

Question 31

What happens when alveolar pressure is more than atmospheric pressure?

Answer 31

Air moves out

Question 32

What does speed of airflow depend on?

Answer 32

Pressure gradient and level of airway resistance

Question 33

What does pneumothorax involve?

Answer 33

Entry of air into the pleural cavity, loss of negative intrapleural pressure, and collapse of lung tissue.

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

Question 34

What happens if either pleural membrane is ruptured?

Answer 34

The pressure gradient between the pleural cavity and surrounding environment will cause air to enter (pneumothorax) until intrapleural pressure = atmospheric pressure

Question 35

What happens to pleural cavity volume as air enters?

Answer 35

It increases (at the expense of the lung volume, which decreases). Elastic recoil of lung tissue, and expansion of the chest during inspiration can then potentially draw further air into the pleural space.