Gas exchange in the Lung

Question 1

How does oxygen get from the atmosphere to cells? (simple)

Answer 1

1. Oxygen is inhaled from atmosphere into alveoli within lungs
   
   2. O2 diffuses from alveoli into blood within pulmonary capillaries
   3. O2 transported in blood, predominantly bound to haemoglobin
   4. O2 diffuses into cells/tissues for use in aerobic respiration
   5. CO2 diffuses from respiring tissues to blood- exchanges at lungs

Question 2

How does the mixture of gases interact in the air?

Answer 2

Air consists of a mixture of gases, which behave in accordance with their partial pressure, rather than concentration
Ptotal= PH2O + SP constituent gases
Pgas= (Pbarometric- PH2O) x ngas
Pgas- partial pressure of individual constituent gas
Pbarometric- atmospheric pressure
PH20- water vapour pressure (0kPa in dry air, 6 in fully humidified air)
Partial pressure is calculated by multiplying total pressure by mole fraction

Question 3

What determines how much gas dissolves in a liquid?

Answer 3

The concentration of a gas dissolves within a liquid is determines by the partial pressure and solubility of the gas:
Concentration= partial pressure x solubility
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
Double the pressure of surrounding gas= double the concentration of dissolved gas

Question 4

How is gas exchanged through multiple structures and mediums?

Answer 4

Gas exchange involves diffusion of blood gases through multiple structures and mediums

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

Question 5

What determines the rate of diffusion?

Answer 5

Gas exchange= diffusion of gases between air and blood
So what determines the rate of diffusion?
Rate of diffusion a (surface area/ distance^2) x (PA-PC)
Surface area- alveolar surface area
Distance^2- epithelial and endothelial cell thickness+ basement membrane thickness+ fluid layer depth
(PA-PC)- partial pressure gradient between alveolar air (PA) and capillary blood (Pc)

Question 6

How can you get maximum diffusion?

Answer 6

Increased partial pressure gradient

Question 7

How are alveoli adapted for gas exchange?

Answer 7

Alveoli have intricate structures and multiple adaptations to maximise the rate of gas exchange
Large surface area (lungs have high surface area-volume ratio due to 3D structure)
Wall= one cell layer thick + basement membrane fused with blood vessel
Richly innervated by capillaries (adequate blood supply)

Question 8

How are pressure gradients maintained?

Answer 8

Pressure gradients between alveoli and blood are maintained by adequate ventilation
Increased ventilation= increased partial pressure gradient (between alveoli and blood)= increased gas exchange

Question 9

What is VQ coupling?

Answer 9

Efficient gas exchange requires matching of ventilation to perfusion (“VQ coupling”)
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 describes this relationship
1L of blood can carry around 200mL O2, 1L dry air = around 200ml O2, therefore ideally V|Q should = 1
At rest ventilation and perfusion both = around 5L/min, V|Q = around 1 to 0.8

Question 10

How is ventilation-perfusion coupling maintained?

Answer 10

Ventilation-perfusion coupling is maintained by hypoxic vasoconstriction
Homeostatic mechanisms exist to reduce ventilation-perfusion mismatching
Hypoxic vasoconstriction of capillaries diverts blood flow from poor- to well-ventilated alveoli

Question 11

What happens when there is an inequality between ventilation and perfusion?

Answer 11

Ventilation-perfusion inequality/ mismatch
In situations where ventilation and perfusion to individual alveolar units are not matched, gas exchange will be reduced
Note that we are not necessarily referring to changes in the overall level of ventilation or perfusion to the lungs, but rather situations where V/Q ratios vary substantially between alveolar units
In theory, V-Q inequality affects O2 and CO2 exchange, however in most, increased PaCO2 will induce a reflex hyperventilation that clears the excess CO2 (but doesn’t increase PaO2

Question 12

What happens during physiologic dead space?

Answer 12

Physiologic dead space (ventilation without perfusion)
Reduced perfusion of lung regions, causes an increase in V/Q ratio from:
Heart failure (cardiac arrest)
Blocked vessels (pulmonary embolism)
Loss/damage to capillaries (emphysema)
The affected alveoli= physiological dead-space, as no/reduced gas exchange occurs
O2 therapy helps

Question 13

What happens during a shunt?

Answer 13

Shunt (perfusion without ventilation)
Reduced ventilation of alveoli or limits to diffusion cause a decrease in V/Q ratio:
Cardiac shunts
Pneumonia, acute lung injury, respiratory distress syndrome, atelectasis
Blood from the right heart to the left, without taking part in gas exchange (shunt)
Response to O2 therapy is poor

Question 14

What types of O2 therapies on hypoxaemia are there?

Answer 14

The effect of O2 therapy on hypoxemia depends on the nature of the pathology (overall V/Q inequality vs. shunt)
SDQ= a method of quantifying variation between V/Q ratios in different regions of the lung
FiO2= fraction of oxygen in inspired air (0.2= 20%)
Effect of increased FiO2 on shunt is much more limited (however the modest effect produced can still be critical)