Diffusion & Perfusion

Question 1

True or False: O₂ is highly soluble in blood.

Answer 1

False. O₂ is poorly soluble in blood.

Question 2

Arterial O₂ content is the sum of what two things?

Answer 2

Hb-bound O₂ and freely-dissolved O₂

Question 3

What are the typical values for the following components:

1. Arterial O₂ content
2. Hb-bound O₂
3. Freely-dissolved O₂

Answer 3

1. Arterial O₂ content = 20.7 ml O₂/100 ml blood
2. Hb-bound O₂ = 20.4 ml O₂​/100 ml blood
3. Freely-dissolved O₂ = 0.3 ml O₂​/100 ml blood

This demonstrates that O₂ is poorly solube in blood. Most of arterial O₂ content is from Hb-bound O₂.

Question 4

Does oxygen or carbon dioxide dissolve more easily in blood?

Answer 4

CO₂. By about 23 times.

Question 5

True or False: In normal conditions, O₂ binding sites on hemoglobin are mostly vacant.

Answer 5

False. They are mostly saturated.

The diagram below demonstrates how at a normal P_aO2 of 100 torr, saturation of hemoglobin binding sites is about 98%

Question 6

How does P_aO2 (partial pressure of arterial oxygen) relate to C_aO2 (total arterial oxygen content)?

Answer 6

P_aO2, the partial pressure of arterial oxygen, only measures the freely-dissolved O₂. Remember, out of the total arterial oxygen content of 20.7 mlO₂/100mlblood, only 0.3 of it is freely-dissolved O₂ in the blood.

If freely-dissolved O₂ only makes up a small part of the total arterial oxygen content, why is P_aO2 so important? Because, P_aO2 dictates the binding of oxygen to hemoglobin (the part which makes up the majority of the arterial oxygen content). The relationship between P_aO2 and saturation of hemoglobin is demonstrates by the oxyhemoglobin dissociation curve.

In other words, the partial pressure of arterial oxygen is directly related to the freely-dissolved O₂ but also affects the hemoglobin-bound O₂.

Question 7

What 3 factors affect the rate of gas diffusion and how do they effect gas diffusion?

Answer 7

1. Pressure gradient (the greater the pressure gradient, the greater the diffusion)
2. Cross-sectional Area (the greater the area, the greater the diffusion)
3. Thickness (the greater the thickness between the alveoli and capillaries, the less the diffusion)

Question 8

How is the O₂ pressure gradient between alveoli and blood maintained?

Answer 8

When O₂ enters the capillaries, they are quickly bound by Hb. This preserves the alveolar-blood O₂ pressure gradient because it is only the free O₂ level that affects the gradient.

Question 9

Our lungs are set up to facilitate diffusion very well. About how far does deoxygenated blood arriving from the right ventricle have to go (% capillary bed passed) to become fully oxygenated?

Answer 9

About one third (33%).

The diagram shows how blood arriving from the right ventricle is has a partial pressure of oxygen of about 40 torr and it is increased to a partial pressure of 100 torr by the time it passes through about a third of the pulmonary capillary beds.

Question 10

Explain how you might get some of these abnormal curves shown on the diagram

Answer 10

In a normal patient, demonstrated by the black line, the mixed venous blood only has to travel through about a third of the pulmonary capillary beds until it is fully oxygenated.

The pink line shows what the curve may look like in a patient with moderate disease or a patient during exercise. In moderate disease, the diffusion rate may be slowed down but the patient could still oxygenate the blood to 100 torr. During exercise, the diffusion rate is also decreased because the blood is moving faster and has less time to oxygenate.

The red line shows what the curve may look like in a patient with severe disease. These patients have a high deficiency in their diffusion which results in arterial blood that has insufficient partial pressure of oxygen. This could also happen in patients with moderate disease during exercise.

Question 11

Name two diffusion disorders and explain how they decrease diffusion

Answer 11

1. Interstitial disease. This decreases diffusion by increasing the distance between alveoli and the capillaries (thickness).
2. Emphysema. This decreases diffusion by decreasing the surface area participating in diffusion. In emphysema, there is a destruction in the integrity of elastin which ends up destroying many of the alveoli and bronchioles. This is why there is reduced surface area.

Question 12

True or False: CO₂ diffusion is affected much less by disease. Even in the presence of severe disease, the P_CO2 is able to drop to the desired level of 40 torr throughout the course of the capillary beds.

Answer 12

True

CO₂ is about 23 times more soluble than O₂ so it moves readily and is less susceptible to negative diffusion effects of diseases.

Question 13

What is perfusion?

Answer 13

In physiology, perfusion is the process of a body delivering blood to a capillary bed in its biological tissue

Question 14

What is minute perfusion (Q)?

Answer 14

The amount of blood flow in the lung in one minute

Question 15

What is an average minute perfusion for an adult?

Answer 15

5L (same as typical cardiac output)

Question 16

What factors affect perfusion?

Answer 16

1. O₂ tension (hypoxic pulmonary vasoconstriction)
2. Capillary recruitment
3. Gravity

Question 17

What is hypoxic pulmonary vasoconstriction and how is it advantageous?

Answer 17

Hypoxic pulmonary vasoconstrictoin is when blood vessels in areas of the lung with less O₂ are narrower because of vasoconstriction. This is advantageous because when vessels in areas with less O₂ constrict, blood is diverted to areas in the lungs that have more O₂.

Question 18

What is capillary recruitment?

Answer 18

Normally, there are many collapsed capillaries throughout the lung that are not in use. During exercise, cardiac output rises which causes more blood to go through the lungs and the increase in blood causes an increase in pressure which pulls the collapsed capillaries open.

Question 19

How does gravity affect perfusion?

Answer 19

When a person is standing upright, blood pools towards the bottom of the lung due to gravity. The higher volume of blood at the bottom of the lungs increases pressure which opens more vessels.

Perfusion at the bottom of the lungs is about 6 times greater than the top of the lungs.

Question 20

What is V/Q mismatch?

Answer 20

This is a mismatch between ventilation (air flow) and perfusion (blood flow) in specific parts of the lungs. This can cause problems in oxyygenation even if the total ventilation and perfusion are normal.

Question 21

What can cause V/Q mismatch? (name 2 main things)

Answer 21

1. Resistance/Compliance Problems
2. Gravity

There can be a low V/Q mismatch ratio if there is an occlusion in an airway to an alveolus. There can be a high V/Q mismatch ratio if there is an occlusion in a capillary associated to an alveolus. Check out this diagram:

Question 22

How can areas of the lung with low V/Q ratios cause other areas to have high V/Q ratios?

Answer 22

The body is very interested in maintaining a near-normal P_aCO2 in order to maintain normal pH. If you remember the alveolar ventilation equation, P_aCO2 is inversely related to V_A (alveolar ventilation). In areas with a low V/Q ratio, there is less alveolar ventilation which results in an increase in P_aCO2 that the body doesn’t want. In order to counteract this, the body is able to increase the ventilation to other areas of the lung, resulting in a high V/Q ratio at those sites. This helps the body to maintain P_aCO2 levels.

Question 23

What is a typical C_VO2 value?

Answer 23

15.7 ml O₂/100 ml blood

Question 24

True or False: Areas of the lung with low V/Q are compensated well by areas of the lung that have high V/Q.

Answer 24

False. Areas of the lung with high V/Q ratio try to compensate for areas of the lung with low V/Q ratio but it ultimately doesn’t work no matter how high the ventilation because the hemoglobin that flows through is getting fully saturated and cannot bind more oxygen.

Look at the attached diagram, you can see that while the V/Q ratio of the top unit is 10 times that of a normal V/Q ratio, the oxygen content resulting from that is not substantially higher.

Question 25

True or False: V/Q mismatch affects both arterial O₂ and arterial CO₂ levels.

Answer 25

False. V/Q mismatch affects arterial O₂ levels but generally not arterial CO₂ levels.

As long as alveolar ventilation is compensated, arterial CO₂ levels are compensated (as seen in the alveolar ventilation equation). O₂ levels are not adequately compensated just by increasing ventilation because the hemoglobin flowing through the areas with high V/Q ratio (areas trying to compensate) get fully saturated.

If the total ventilation is being brought down by a disease process, then your CO₂ levels are affected. But, if it is only a localized part of your lung that has low V/Q ratio, other parts of the lung compensate by having high V/Q ratio.

Question 26

True or False: V/Q mismatch only happens in moderate-severe disease conditions.

Answer 26

False. V/Q mismatch can happen in mild, moderate, and severe disease conditions.

This is different from situations like hypoventilation when total ventilation goes down.

Question 27

Is ventilation greater in areas at the bottom or top of the lung? How about perfusion? How a bout V/Q ratio?

Answer 27

Ventilation and perfusion are both greater at the bottom of the lung than the top of the lung due to gravity. V/Q ratio is greater on the top of the lung than the bottom of the lung. This is because gravity increases the Q at the bottom of the lungs by a factor of 6 while only increasing V by a factor of 2.5. So, for the V/Q ratio, Q is increased more by gravity than V at the bottom of the lung, so the bottom of the lungs have a smaller V/Q ratio compared to the top.

V_bottom/V_top = 2.5

Q_bottom/Q_top = 6

Question 28

Why is there a difference between P_AO2 (normally 100 torr) and P_aO2 (normally 90-100 torr)?

Answer 28

V/Q mismatch resulting from gravity is the main contributor to the 5-10 torr difference between P_AO2 and P_aO2.