Diffusion and Perfusion

Question 1

What is the average transit time of the RBCs in the capillaries?

Answer 1

<1 second

Question 2

How does the transfer of gas across the blood-gas barrier occur?

Answer 2

It occurs passively through diffusion

Question 3

What does Fick’s Law state?

Answer 3

• Amount of gas transferred is proportional to the area and the difference in the partial pressure
• Amount of gas transferred is indirectly proportional to the thickness

Question 4

What does Graham’s Law state?

Answer 4

• Rate of diffusion is directly proportional to the solubility coefficient of the gas
• Rate of diffusion is inversely proportional to the molecular weight of the gas

Question 5

Why does CO2 still diffuse well despite having a much lower pressure gradient than O2?

Answer 5

It diffuses well due to its high solubility coefficient

Question 6

Define perfusion limited gas transfer and an example of this biologically.

Answer 6

It is where gas transfer is limited by the flow of blood to the alveoli as the partial pressure of the gas equilibrates rapidly in the blood.

This happens with insoluble gas as like N2O as it does not combine with proteins.

Question 7

Define diffusion limited gas transfer and an example of this biologically.

Answer 7

It is where gas transfer is limited by the diffusion of the gas into the blood itself rather than the flow as the gas binds to the proteins in the blood and it does not contribute to the partial pressure at all.

This happens with soluble gases like CO which binds to hemoglobin with high affinity.

Question 8

What type of gas exchange do O2 and CO2 fall under?

Answer 8

CO2 and O2 are both soluble gases that bind to hemoglobin, although they have a lower binding affinity than CO. As a result, the partial pressure still builds up rapidly and they are PERFUSION LIMITED.

Question 9

What is the normal time for equilibration of the O2 gradient?

Answer 9

Around 0.2 seconds

Question 10

What happens to the equilibration time of the O2 gradient during exercise?

Answer 10

It will increase.

The time for equilibration increases because the transit time of the RBCs drops down to 1/3 of the normal time. This will increase the blood flow and make the perfusion limitation of the gas exchange less as more blood is flowing so the partial pressure takes longer to equilibrate.

Question 11

What happens to O2 gas exchange at high altitude?

Answer 11

At high altitude, the diffusion of the gas and intake of the air is normal. However, there is less oxygen in the air and so the pressure gradient is much less which leads to the impairment of O2 transfer.

Question 12

What is special about the pulmonary arteries?

Answer 12

They are the only arteries that carry unoxygenated blood and they have thin walls with little smooth muscle that are more compliant, making them good for low pressure circulation.

Question 13

What happens to the vessels in the lungs as the vascular pressure is increased?

Answer 13

The peripheral vascular resistance will be decreased to increase the flow. This is the opposite of the effect of auto regulation in the rest of the circulatory system. The pulmonary circuit is not auto regulated.

Question 14

Why is the high compliance of the pulmonary vessels important?

Answer 14

The small vessels and capillaries can be distended easily to accommodate more blood.

Question 15

How does the lung decrease resistance with increasing pressure?

Answer 15

Alveolar capillaries that are normally not perfused are recruited and opened.

Opened vessels are distended

Question 16

Where is the PVR the lowest in regards to the lung volume?

Answer 16

At the FRC

Question 17

What happens to the extralveolar pulmonary vascular resistance as the lung volume increases?

Answer 17

Decreases

Question 18

What happens to the alveolar pulmonary vascular resistance as the lung volume increases?

Answer 18

Increases

Question 19

What is Zone 1 in the lung?

Answer 19

It is where the alveolar pressure is greater than the arterial and the venous pressure and so no blood flows.

Question 20

What is Zone 2 in the lung?

Answer 20

It is the waterfall zone the arterial pressure is greater than the alveolar pressure, but the venous pressure is less than it. Therefore flow depends on the arterial-alveolar gradient and flow in this area is “compressive” as the alveolar pressure increases the resistance of the vessels.

Question 21

What is Zone 3 in the lung?

Answer 21

It is the normal zone at the base of the lungs where both the pulmonary and the arterial pressures exceed the alveolar pressure and so blood flows in an arterial-venous gradient.

Question 22

What is hypoxic vasoconstriction?

Answer 22

It is caused by the low PO2 in the alveolar gas without high arterial PCO2 and it shifts blood from hypoxic areas to well-perfused ones.

The process might at first seem illogical, as low oxygen levels should theoretically lead to increased blood flow to the lungs to receive increased gaseous exchange. However, it is explained by the fact that constriction leads to redistribution of bloodflow to better-ventilated areas of the lung, which increases the total area involved in gaseous exchange.

This improves ventilation/perfusion ratio and arterial oxygenation, but is less helpful in the case of long-term whole-body hypoxia. This is seen in COPD, at altitude and in heart failure.

Question 23

What are the fluid drainage rates from the lung determined by?

Answer 23

The balance between the intravascular and interstitial hydrostatic and oncotic pressures.

Question 24

What happens when the drainage rate is exceeded?

Answer 24

Pulmonary edema develops

Question 25

What can cause O2 to become diffusion limited?

Answer 25

Alveolar Hypoxia
Exercise
Thickening of the Blood-Gas Barrier