Gas exchange in the lungs

Question 1

What way do gases in air behave in?

Answer 1

Air consists of a mixture of gases, which behave in
accordance with their partial pressure, rather than
concentration.

Question 2

what is the equation for total parital pressure?

Answer 2

Ptotal= PH20 + sum of Pconstituent gases

Question 3

What is the equation for the partial pressure of individual constituent gas?

Answer 3

Pgas= (Pbarometric-PH20) x n(gas)

Question 4

What is Pgas?

Answer 4

Partial pressure of
individual constituent gas

Question 5

What is P(barometric)?

Answer 5

Atmospheric pressure

Question 6

What is PH20?

Answer 6

Water vapour pressure
(0 kPa in dry air, 6 kPa in
fully humidified air)

Question 7

What is n(gas)?

Answer 7

Mole fraction: the % of total moles represented by the individual gas

Question 8

What is the concentration of a gas dissolved within a liquid determined by?

Answer 8

The concentration of a gas dissolved within a liquid is determined by
the partial pressure and solubility of the gas:
Concentration=Partial pressure x solubility

Question 9

What does the partial pressure of a gas dissolved in a liquid reflect?

Answer 9

The partial pressure of gas dissolved in a liquid reflects the amount of
gas that would dissolve (at equilibrium) if the liquid was placed in
contact with a gas phase of equivalent partial pressure.

Question 10

What are the multiple structures and mediums that gas exchange involves?

Answer 10

1) O2 enters the alveolar
airspace from the
atmosphere.
2) O2 dissolves in ALF (alveolar lining fluid)
3) O2 diffuses through
alveolar epithelium,
basement membrane,
& capillary endothelial
cells.
4) O2 dissolves in
blood plasma
5) O2 binds Hb
molecule

Question 11

When must oxygenation of blood must occur?

Answer 11

Oxygenation of blood must occur during the brief time
taken for RBCs to flow through pulmonary capillaries

Question 12

What can cause abnormal(reduced) diffusion in pulmonary capillaries?

Answer 12

Abnormal (reduced) diffusion due to
thickening of blood-gas barrier

Question 13

What conditions do you require for maximum diffusion?

Answer 13

• ↑ partial pressure gradient
• ↑ surface area
• ↓ distance (barrier thickness)

Question 14

What defect impacts partial pressure gradient?

Answer 14

Hypoventilation
-Type 2 resp failure

Question 15

What defect impacts surface area?

Answer 15

Emphysema reduces surface area

Question 16

What defect impacts distance?

Answer 16

Fibrosis
-Increases basement membrane thickness
Pulmonary oedema(pneumonia)
-Increases thickness of fluid layer/oedema

Question 17

What are the intricate structures and multiple adaptations that alveoli have that maximise the rate of gas exchange?

Answer 17

o Large surface area (lungs have high surface area-volume ratio
due to 3D structure)
o Wall = one cell layer thick + basement membrane fused with
blood vessel
o Richly innervated by capillaries (adequate blood supply)

Question 18

What are pressure gradients between alveoli and blood maintained by?

Answer 18

Pressure gradients between alveoli and blood
are maintained by adequate ventilation

Question 19

What coupling does efficient gas exchange require?

Answer 19

Blood flow through pulmonary capillaries(Perfusion, Q) needs to be matched to alveolar ventilation(VA) to enable efficient gas exchange, as there is a maximum amount of O2 each unit of blood can carry. V/Q ratio.

Question 20

What does a V/Q ratio >1 mean?

Answer 20

Means Hypoperfusion(‘dead space effect’)

Question 21

What does a V/Q ratio =1 mean?

Answer 21

Normal

Question 22

What does a V/Q ratio <1 mean?

Answer 22

Hypoventilation(‘shunt’)

Question 23

What is ventilation perfusion coupling maintained by?

Answer 23

Ventilation-perfusion coupling is
maintained by hypoxic vasoconstriction

Question 24

What is the mechanism to reduce ventilation-perfusion mismatching?

Answer 24

1. Under normal conditions, blood
   flow and ventilation are matched.
2. If ventilation of specific alveoli decreases,
   PACO2 will rise + PAO2 will fall ∴ ↓ oxygenation of blood flowing through innervating capillaries.
3. Blood flow diverted to alveoli with ↑ ventilation
   ↓PaO2 induces vasoconstriction, = ↓ blood flow

Question 25

What happens in a pulmonary embolism in relation to ventilation-perfusion inequality?

Answer 25

1. Embolism occludes pulmonary artery supplying a region of the lung
2. The ‘cut off’ alveoli will be under perfused and become a physiologic dead space
3. The Cardiac output is therefore diverted to alveoli that is well perfused and this increases perfusion in this region.
4. Unless ventilation of these alveoli increases to match perfusion, hypoxaemia and hypercapnia will occur

Question 26

What can reduced perfusion cause?

Answer 26

-Increased V/Q ratio
-Heart failure(cardiac arrest)
-Blocked vessels(pulmonary embolism)
-Loss/damage to capillaries(emphysema)

Question 27

What are the impacts to VA, Q and V/Q from physiologic dead space and what is the response of dead space effect to O2 therapy?

Answer 27

VA is normal
Q drops
V/Q increases
O2 therapy helps

Question 28

What can cause reduced ventilation of alveoli or limits to diffusion cause a decrease in V/Q ratio?

Answer 28

-Cardiac shunts
-Pneumonia, acute lung injury, respiratory distress syndrome, atelectasis

Question 29

What are the impacts on VA, Q, V/Q and what is the response like to O2 therapy from a shunt?

Answer 29

VA drops
Q is normal
V/Q drops
The response to O2 therapy is poor