Lecture 12: Gas Exchange And V/Q Ratio

Question 1

What is the pressure in the pulmonary circuit?

Answer 1

22/8 mmHg

Question 2

Pulmonary capillaries have ________ resistance

Answer 2

Low

Question 3

How do alveolar pressure and surface tension add to the starling equilibrium?

Answer 3

Surface tension pulls fluid out of the capillaries

Alveolar pressure forces gasses in

Question 4

What prevents pulmonary oedema?

Answer 4

Low capillary pressure

Question 5

Blood flow (Q’) _______ uniform throughout the lung. Air flow (V’) _________ uniform throughout the lung.

Answer 5

Is not

Is not

Question 6

Blood flow must match _____________

Answer 6

Air flow

Question 7

What causes pulmonary resistance to fall?

Answer 7

Increasing blood pressure

Increasing CO

Question 8

Describe the pulmonary circuit at rest:

Answer 8

Many blood vessels are closed so not all the lung is perfused, increasing CO or blood pressure opens these vessels

Question 9

What is Laplace’s Law? What are the consequences?

Answer 9

Pressure = 2(surface tension)/radius

• surfactant decreases surface tension which improves compliance
• without surfactant the pressure in small alveoli would be very high
• decreasing the surface tension helps stabilise small alveoli

Question 10

Define V, V’,Q and Q’

Answer 10

V = volume of air
V' = ventilation 
Q = volume of blood
Q' = blood flow/perfusion

Question 11

What is the difference between PAO2 and PaO2?

Answer 11

PA denotes alveolar pressure

Pa denotes arterial pressure

Question 12

What does PAO2 depend on?

Answer 12

PAO2 depends on the balance of alveolar ventilation (fresh gas flow) and O2 removal by the circulation

Question 13

When does ideal gas exchange occur?

Answer 13

When V’/Q’ = 1

Question 14

With respect to O2, Do areas with a high V’/Q’ ratio compensate for areas with a low V’/Q’ ratio?

Answer 14

No, high V’/Q’ areas are limited by Hb saturation

Question 15

With respect to CO2, Do areas with a high V’/Q’ ratio compensate for areas with a low V’/Q’ ratio?

Answer 15

Partially

Question 16

V’/Q’ mismatch can cause severe __________ without __________

Answer 16

Hypoxia

Hypercapnea

Question 17

Where is perfused and ventilated best when standing?

Answer 17

The base of the lungs

Question 18

What is the normal range for V’/Q’ ratios in the lung

Answer 18

0.5-3

Question 19

To correct V’/Q’ mismatch, Hypoxia causes ________________ of the pulmonary vessels and Hypercapnea causes ______________.

Answer 19

Vasoconstriction

Vasodilation

Question 20

Poorly ventilated alveoli are _________ and therefore get reduced perfusion

Answer 20

Hypoxic

Reduced perfusion because it’s not going to give a good V’/Q’ ratio