L38: Ventilation/perfusion Relationship

Question 1

What is the approximate pressure of the systemic circulation?

Answer 1

100mgHg

Question 2

What is the approximate pressure of the pulmonary circulation?

Answer 2

15mmHg

Question 3

What regulated pulmonary blood flow?

Answer 3

• Major factor is the PAO2
• alterations in the resistance of arterioles
• This changes the tone of the arterioles and is mediated by local vasoactive substances

Question 4

What is hypoxic vasoconstriction?

Answer 4

• A decrease in PAO2
• An adaptive mechanism to reduce Blood flow to poorly ventilated regions. This is good otherwise blood flow would be wasted
• Assists in directing pulmonary blood flow to other regions of the lung where gas exchange can occur

Question 5

How does hypoxic vasoconstriction happen?

Answer 5

When PAO2 is decreased, vascular smooth muscle cell senses the hypoxia and the arterioles vasoconstricts and there is decreased blood flow to that area.
- needs to inhibit NO production from endothelial cells of the vessel wall

Question 6

What is the MOA of NO?

Answer 6

• Endothelial derived relaxing factor (EDRF) Produced from L-Arginine
• Activated Guanylyl cyclase to produce cGMP
• CGMP directly acts on vascular smooth muscle producing relaxation and vasodilation

*therefore, inhaled NO reverses hypoxic vasoconstriction and NOS inhibitors enhance the vasoconstriction.

Question 7

What is Global Hypoxic vasocontriction?

Answer 7

• Results from high altitude or breathing low O2 mix.
• Decreased PAO2 throughout lung which produces global vasoconstriction of pulmonary arterioles and increased pulmonary vascular resistance. This leads to increased pulmonary arterial pressure, which leads to hypertrophy of the right ventricle.

Question 8

Where does Thromboxane A2 come from?

Answer 8

From Arachidonic acid metabolism by the cyclooxygenase pathway

• Comes from macrophages, leukocytes, and endothelial cells
• produced in response to lung injury

Question 9

What does thomboxane A2 do to pulmonary blood flow?

Answer 9

Vasoconstriction of arterioles and venules

Question 10

Where does prostacyclin (PGI2) come from?

Answer 10

• Arachidonic acid metabolism by the cyclooxygenase pathway

- Produced by the lung endothelial cells

Question 11

What was prostacyclin do to the lung tissue?

Answer 11

Local vasodilation

Question 12

Where do leukotrienes come from?

Answer 12

-Arachidonic acid metabolism by the lipoxygenase pathway

Question 13

What do leukotrienes do to lung tissue?

Answer 13

Vasconstriction of blood vessels and airway

Question 14

Why is the distribution of blood flow in the lung uneven when standing?

Answer 14

Gravity- Zone 3(the bottom of the lung) gets the most blood flow. Zone 1 (The top) gets the least.

Question 15

What is the V/Q ratio?

Answer 15

the ratio of alveolar ventilation (VA) to pulmonary blood flow (Q)

Normal is 0.8 when tidal volume, breathing frequency, and cardiac output are normal.

Question 16

When is dead space seen within the lungs?

Answer 16

PE - when blood flow to part or all of the lung is occluded.

-Dead space alveolar gas has the same composition as humidified inspired air (PACO2= 0, PAO2= 150)

Question 17

What happens in a high V/Q ratio?

Answer 17

• There is high ventilation relative to perfusion (Blood flow is low, but they still have some perfusion)
• Pulmonary capillary blood from these regions have high PO2 and low PCO2

Question 18

Which lung zone has highest PaO2 and which has high PaCO2?

Answer 18

Zone 1 has high PaO2

Zone 3 has high PaCO2

Question 19

What happens in a low V/Q ratio?

Answer 19

• There is low ventilation relative to perfusion (ventilation is decreased, but there is still some ventilation)
• Pulmonary capillary blood from these regions have low PO2 and high PCO2 (like in zone 3)

Question 20

What happens in SHUNT V/Q=0?

Answer 20

• Perfusion of lungs that are NOT ventilated, no gas exchange in possible because there is no ventilation to deliver O2 to blood or carry CO2 from blood.
• Seen in airway obstruction and 100% O2 supplementation will not improve PO2.
• Pulmonary capillary blood (shunted blood only) has the same composition of mixed venous blood (PaO2=40, PaCO2 =46)