Diffusion & Perfusion

Question 1

What’s the solubility coefficient?

Answer 1

It is the value assigned to different molecules that describes how soluble they are (how well they dissolve) in blood.

Question 2

Does oxygen or carbon dioxide have a greater solubility coefficient in blood?

What are their values?

Answer 2

Carbon dioxide has a greater solubility coefficient, meaning it’s more soluble in blood.

CO2= 0.03 mM/Torr

O2=0.0013 mM/Torr

Question 3

What do aO2 and aCO2 stand for?

Answer 3

Those are the symbol for solubility coefficients.

Question 4

T or F?

The PaO2 (the partial pressure of arterial oxygen) is based on the amount of oxygen bound the hemoglobin.

Answer 4

False.

This PaO2 is actually found from the freely dissolved oxygen in blood, which is really low (due to the low solubility of oxygen in blood).

T

Question 5

A patient comes in with expected carbon monoxide poisoning. Ironically, you notice that their PaO2 levels are normal. How do you explain this discrepancy?

Answer 5

This isn’t a discrepancy at all. Carbon monoxide binds to hemoglobin, so it competes with oxygen bound to hgb. It has no effect on the amount of oxygen dissolved in the blood, so the PaO2 levels will look normal during carbon monoxide poisoning.

Question 6

T or F?

The PaO2 values correlate to the hemoglobin saturation values on the oxyhemoglobin dissociation curve?

(in other words, when O2 bound Hgb is low, PaO2 sat is low as well)

Answer 6

True

This isn’t the case in carbon monoxide poisioning, but in general when PaO2 is low, the % of hemoglobin saturation is also low.

Question 7

Why does the hemoglobin saturation drop under environments of low partial pressure of oxygen?

Answer 7

The affinity between hemoglobin and oxygen isn’t as high in such environments. This allows oxygen to disassociate and move into tissue that is at lower levels of oxygen pressures.

Question 8

According to Fick’s Law, what factors promote greater oxygen diffusion?

Answer 8

1) Increased surface area
2) Decreased thickness of the vessel

Question 9

In a healthy person how long does it take for the blood to get oxygenated to its ideal level?

Answer 9

It takes about 1/3rd of the time it takes for the blood to pass through the capillary bed.

You have a buffer time (this allows for blood to still get oxygenated during exercise where blood moves through the capillary bed faster)

Question 10

I have a moderate disease that slows down diffusion in the capillary bed. How will this effect the ability for my blood to reach ideal oxygenation?

Answer 10

If the disease is truly moderate, your blood should still reach ideal levels of oxygenation. Remember that normally diffusion occurs in 1/3rd the allotted time, so even if diffusion takes longer than normal, you still have enough time in the capillary to get oxygenated. (similar to exercise)

Question 11

In relation to the Fick equation, what effect does interstitial disease have to lower diffusion?

Answer 11

It increases the thickness (D variable)

Question 12

In relation to the Fick equation, what effect does emphysema have to lower diffusion?

Answer 12

It decreases the surface area (the A variable)

Question 13

T or F?

CO2 diffusion is much more effected by disease

Answer 13

False.

Even with severe diseases, CO2 diffusion is very minimally impacted. This is because CO2 is 25x more soluble.

O2 has poor solubility which slows diffusion, which disease can exacerbate. That doesn’t happen with CO2 b/c it dissolves so readily.

Question 14

In terms of the lungs, what does perfusion refer to?

What about minute perfusion?

Answer 14

Blood flow in the lung.

Minute perfusion is blood flow in the lung in a minute. This is usually equal to Q (Cardiac Output) which is 5000 ml/min.

Question 15

What effect does hypoxia in the alvelous have on the blood vessels?

Answer 15

Vasoconstriction.

The vessels in hypoxic conditions vasoconstrict to lower perfusion to those alveoli so that not as much blood can’t reach its optimal oxygen state.

Question 16

What does exercise cause to happen to capillary recruitment?

Answer 16

Exercise induces increased capillary recruitment.

Question 17

You go on the magic school bus into someone’s lung and realize that the bottom of the lung is experiencing 6x the perfusion as the top of the lung. What pathophysiological process is occuring?

Answer 17

No pathophysiological process, this is normal.

Gravity causes more blood to pool in the bottom of the lung than the top, so more vessels open at the bottom. This results in about 6x more perfusion to the bottom of the lung than the top.

Question 18

What is the general explanation behind V/Q mismatch?

Answer 18

In the lung you may have an increased V/Q ratio in one area (more ventilation than perfusion), whereas in a different part of the lung you may have a decreased V/Q ratio. Overall however, you will usually have normal TOTAL ventilation and perfusion in the lung.

(You will still have gas exhange problems in these scenarios though)

Question 19

What would the effect of the following abnormalities cause on V/Q ratio?

Blocked alveolus

Blocked blood vessel

A lung that is diseased in one portion and not diseased in another part.

Answer 19

Blocked alvelous=Decreased V/Q ratio

Blocked blood vessel=Increased V/Q ratio.

Diseased Lung=Increased V/Q ratio in the non diseased part of the lung (to maintain the PaO2.)

Question 20

T or F?

Compared to a normal V/Q ratio, an increased V/Q ratio has a greater impact on arterial O2 than a decreased V/Q ratio.

Answer 20

False

A higher V/Q ratio actually has a very small impact on arterial oxygen whereas a decreased V/Q ratio has a significant impact on arterial oxygen (lowers it).

As an explanation, think of the oxyhemoglobin saturation curve.

Question 21

T or F?

V/Q mismatch will lower arterial oxygen content but will have no effect on the PaO2.

Answer 21

False.

V/Q mismatch will lower both arterial oxygen content and PaO2

Question 22

T/F?

V/Q mismatch does not effect the CO2 diffusion

Answer 22

True.

CO2 is only effected if total ventilation goes down (in V/Q mismatch, TOTAL ventilation and perfusion are normal).

Question 23

List some major causes of V/Q mismatch

Answer 23

1) Resistance/Compliance problems
2) Gravity

*Mild/moderate and severe disease can lead to V/Q mismatch

Question 24

OK, so the magic school bus taught as that perfusion is 6 higher at the bottom of the lung compared to the top. What effect does gravity have on ventilation?

Answer 24

Ventilation is 2.5x higher in the bottom than at the top. This also leads to V/Q mismatch due to regional variations.

The V/Q ratio is 2.4 times higher at the top than at the bottom.

Question 25

In terms of Torr, what effect does V/Q mismatch cause on the difference between PAO2 and PaO2?

Answer 25

5-10 Torr difference.

PAO2=100 Torr

PaO2=90-95 Torr.

Question 26

How does alveolar dead space occur and how does it impact gas exchange?

Answer 26

Alveolar dead space occurs when you have a ventilated alveolus that is not perfused. This can be due to a capillary blockage.

It results in less gas exchange that would be expected in an ideal situation.

Question 27

How do shunts in the pulmonary system work and how does it effect gas exchange?

Answer 27

Shunts cause blood perfusion where there is no ventilation, typically formed by anastomoses in the pulmonary system.

It results in about 1-2% of Cardiac Output consisting of deoxygenated blood (b/c the deoxygnated blood goes through a shunt and is dumped into the left atrium).

Question 28

T or F?

A shunt usually results in a raised arterial PCO2?

Answer 28

False.

The body will compensate for the deoxygenated blood in the arteries by increasing ventilation to keep PCO2 where it should be.