Ventilation and Gas Exchange

Question 1

What are important lung volumes?

Answer 1

Tidal volume - volume difference between air inhaled and exhaled at light respiratory effort.
Maximum respiratory effort - maximum amount of air inhaled (using nose and mouth)
Inspiratory reserve volume - difference between maximum of tidal volume and maximum respiratory effort
Expiratory reserve volume - difference between maximum expiratory effort and tidal volume.
Residual volume - amount of volume remaining following maximum expiratory effort as a result of lung surfactants which prevent complete collapse and alveolar surfaces sticking together.

Question 2

What are important lung capacities?

Answer 2

Total lung capacity - maximum inspiratory volume
Vital capacity - difference between maximum inspiratory volume and maximum expiratory volume.
Functional residual capacity - Baseline of tidal volume where diaphragm and lung forces are equal.
Inspiratory capacity - difference between maximum inspiratory capacity and baseline of tidal volume.

Question 3

What is minute ventilation?

Answer 3

Tidal volume (litres) x Breathing frequency (breaths/min) = Minute ventilation (litres/min)

Question 4

What is alveolar ventilation?

Answer 4

[Tidal volume (litres) - Dead space (litres)] x Breathing frequency (breaths/min) = Alveolar ventilation (litres/min)

Question 5

What factors affect lung volumes and capacities?

Answer 5

1. Height and morphology
2. Sex - men tend to have larger lung volumes
3. Disease - pulmonary/neurological, don’t always cause a decrease
4. Age - development of disease/unfit
5. Fitness - innate (athletic parents) or training

Question 6

What constitutes physiological dead space?

Answer 6

Conducting zone - 16 generations of bronchal branching, no gas exchange occurs here, typically 150ml in adults at FRC.
Respiratory zone - 7 generations, this is where gas exchange occurs, typically 350ml in adults and air reaching here is equivalent to alveolar ventilation.
However, respiratory zone can contain non-perfused parenchyma which are alveoli without a blood supply. No gas exchange occurs here and this is typically 0ml in adults - called alveolar dead space.

Physiological dead space = anatomical dead space + alevolar dead space

Question 7

How can dead space be increased or decreased?

Answer 7

Can be increased via an anaesthetic circuit or during snorkelling as everything beyond lips is usually atmospheric air but intubation changes that.
Can be decreased via shortening of the airways through a tracheostomy or crichothryrocotomy.

Question 8

What is the chest-wall relationship?

Answer 8

The chest wall has a tendency to spring outwards, and the lung has a tendency to recoil inwards. These forces are in equilibrium at end-tidal expiration (functional residual capacity; FRC), which is the ‘neutral’ position of the intact chest.
The vaccum created between the parietal pleural membrane attached to chest wall and visceral pleural membrane attached to lungs allows the two to move in unison.

Question 9

How does muscle effort bring about inspiration and expiration?

Answer 9

With no muscle effort, chest recoil = lung recoil.
With inspiratory muscle effort, chest recoil exceeds lung recoil and so lung volume increases leading to inspiration.
With expiratory muscle effort, lung recoil exceeds chest recoil and so lung volume decreases leading to expiration.

Question 10

Describe pleural membrane anatomy

Answer 10

The lungs are surrounded by a visceral pleural membrane. The inner surface of the chest wall is covered by a parietal pleural membrane. The pleural cavity (the gap between pleural membranes) is a fixed volume and contains protein-rich pleural fluid.

Question 11

How can pleural cavity integrity be breached?

Answer 11

Haemothorax - intrapleural bleeding -> results in ventilation difficulty as pressures can’t be maintained
Pneumothorax - can be caused by perforated chest wall or punctured lung

Question 12

What are the 2 breathing pressures?

Answer 12

Negative pressure breathing is where alveolar pressure is reduced below atmospheric pressure.
Positive pressure breathing is where atmospheric pressure is increased above alveolar pressure.

Question 13

What are examples of positive pressure breathing?

Answer 13

Resuscitation (CPR), Mechanical ventilation and for fighter pilots.

Question 14

What are 3 important pressures?

Answer 14

PTT = Transthoracic pressure
PTP = Transpulmonary pressure
PRS = Transrespiratory system pressure

Question 15

How is transthoracic pressure measured?

Answer 15

Intrapleural pressure (Ppl) - Atmospheric pressure (Patm)

Question 16

How is transpulmonary pressure measured?

Answer 16

Alveolar pressure (Palv) - Intrapleural pressure (Ppl)

Question 17

How is transrespiratory pressure measured?

Answer 17

Alveolar pressure (Palv) - Atmospheric pressure (Patm)
When this is negative, inhalation occurs.

Question 18

What are the different inspiratory muscle forces?

Answer 18

Diaphragm provides a pulling force in one direction while other respiratory muscles provide an upwards and outwards swinging force. At normal breathing, light diaphragmatic action occurs but when breathing at maximal rates, all the different mechanisms act.

Question 19

What is Dalton’s Law?

Answer 19

Pressure of a gas mixture is equal to the sum (Σ) of the partial pressures (P) of gases in that mixture.

Question 20

What is Fick’s Law?

Answer 20

Molecules diffuse from regions of high concentration to low concentration at a rate proportional to the concentration gradient (P1-P2), the exchange surface area (A) and the diffusion capacity (D) of the gas, and inversely proportional to the thickness of the exchange surface (T)

Question 21

What is Henry’s Law?

Answer 21

At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

Question 22

What is Boyle’s Law?

Answer 22

At a constant temperature, the volume of a gas is inversely proportional to the pressure of that gas.

Question 23

What is Charles’ Law?

Answer 23

At a constant pressure, the volume of a gas is proportional to the temperature of that gas.

Question 24

How is inspired gas modified in the airways?

Answer 24

Partial pressure of water vapour increases as air saturated with water to protect the respiratory surfaces. Air is warmed, humidified, slowed and mixed as air diffuses down the respiratory tree.

Question 25

What conditions shift the oxygen dissociation curve right?

Answer 25

Increase in temperature, Acidosis (Bohr effect), Hypercapnia, Increase in 2,3-DPG.

Question 26

What conditions shift the oxygen dissociation curve left?

Answer 26

Decrease in temperature, Alkalosis, Hypocapnia, Decrease in 2,3-DPG.

Question 27

What condition shifts the oxygen dissociation curve up and down?

Answer 27

Shifted up in polycythaemia as oxygen carrying capacity increased. Shifted down in anaemia as capacity decreased.

Question 28

What is the effect of CO on the oxygen dissociation curve?

Answer 28

Causes downward and leftward shift as allows less oxygen to bind and makes unloading more difficult.

Question 29

How do foetal haemoglobin and myoglobin shift the curve?

Answer 29

Foetal haemoglobin has a greater affinity than adult HbA to ‘extract’ oxygen from mothers blood in placenta. Hence, sharp left shift. Myoglobin has a much much greater affinity than adult HbA to ‘extract’ oxygen from circulating blood and store it.