Gas Exchange in the Lung

Question 1

What is MET?

Answer 1

1 MET is generally assumed to be 3.5 ml/min/kg resting oxygen consumption (VO2 resting)
3.5 mls O2/kg/min
metabolic equivalent (MET) units

Question 2

What happens when blood has passed through the capillary lumen?

Answer 2

Equilibrated- partial pressure in blood (oxygen) and gas equal

Question 3

Describe the structure of haemoglobin

Answer 3

Tetramer- 2 alpha and 2 beta subunits
Each subunit has a Haem group- a polyphyrin with a central Ferrous atom binds O2
Combines loosely with oxygen
Combination alters shape and charge

Question 4

What is the allosteric effect?

Answer 4

The affinity of binding O2 increases with each successively bound O2 molecule
Once bound a number of factors can change the ability of Hb to take up and liberate oxygens

Question 5

What factors shift the oxygen dissociation curve to the right?

Answer 5

• Increase CO2
• Increase (H+)
• Increase temp
• Increase 2,3 DPG

Question 6

What is shunting?

Answer 6

To move something from one place to another, usually because that thing is not wanted, without considering any unpleasant effects

Question 7

Why might the PO2 of arterial blood be lower than we might expect?

Answer 7

Anatomical shunts

Physiological shunts

Question 8

How might anatomical shunts lower PO2?

Answer 8

• A small amount of arterial blood doesn’t come from the lung (Thebesian veins)
• A small amount of blood goes through without seeing gas (bronchial circulation)

Question 9

How might physiological shunts lower PO2?

Answer 9

• Physiological shunts decrease V and alveolar dead space decrease Q
• Not all lung units have the same ratio of ventilation (V) to blood flow (Q)
• V/Q mismatch

Question 10

What can decrease the partial pressure of oxygen in the blood?

Answer 10

• Hypoventilation so less oxygen to enter blood
• Allows less air to enter and leave the alveoli and have decreased alveolar oxygen
• Decreased environmental oxygen (altitude)

Question 11

What may lead to hypoventilation?

Answer 11

• CNS= decreased central respiratory drive
• Respiratory= restrictive chest physiology, pulmonary hypertension, hypoxaemia/ hypercapnia
• Airway= potential difficult airway, obstructive sleep apnoea
• Cardiovascular= coronary artery disease, congestive heart failure
• Others= difficult vascular access, difficult positioning

Question 12

What can increase the partial pressure of oxygen?

Answer 12

• Hyperventilation

- Administration of oxygen

Question 13

How does CO2 change with hyper and hypoventilation?

Answer 13

Dynamically
Bloods holds more CO2 than O2
change of partial pressure as the blood goes through the capillaries is less (because the curve is steeper: the amount of CO2 lost is pretty much the same as the amount of oxygen gained)

Question 14

Describe the ventilation to perfusion ratio

Answer 14

V/Q
If ventilation = perfusion then will get perfect gas exchange
In the lung naturally have V/Q mismatch with less blood and air going to the top of the lung

Question 15

What is the ‘normal’ V/Q mismatch?

Answer 15

Less airflow and blood flow at the top of the lung but V>Q = V/Q
Middle of lung V/Q normal
Bottom of lung more ventilation and more blood flow but V

Question 16

What leads to an increased V/Q ratio?

Answer 16

Lots of ventilation to alveoli, not much blood
Alveoli and blood reach an equilibrium which is closer to air
PO2 is therefore higher
(and PCO2 is lower)
The region of the lung with the high V/Q ratio has a higher PO2 (and a lower PCO2) than other regions. It is more ‘like’ air. (Think of this region of lung being ‘hyperventilated’)

Question 17

What lowers the V/Q ratio?

Answer 17

Less ventilation to alveoli, lots of blood
Alveoli and blood reach an equilibrium which is closer to venous blood
PO2 is therefore lower (and PCO2 is higher)
The region of lung with the low V/Q ratio has the lower PO2 (and higher PCO2) – it is more ‘like’ mixed venous blood (Think of this region of lung being ‘hypoventilated’)

Question 18

Describe physiological dead space

Answer 18

There are some areas of high V/Q ratio and some areas of low V/Q ratio
Anatomical dead space represents the conducting airways where no gas exchange takes place
Alveolar dead space represents areas of insufficient blood supply for gas exchange and is practically non-existent in healthy young but appears with age and disease
Physiological dead space = anatomical dead space + alveolar dead space

Question 19

How can V/Q mismatch quantify gas change problems?

Answer 19

Calculate the expected alveolar PO2 (PAO2) using the alveolar gas equation
Compare with the measured arterial PO2 (PaO2)
If PAO2 = PaO2 then no mismatch

Question 20

How can A-a gradient quantify gas exchange problems?

Answer 20

Tells us the difference between alveolar and arterial oxygen level
Can help to diagnose the cause of hypoxaemia
High A-a gradient
Problem with gas diffusion
V/Q mismatch
Right to left shunt