Ventilation, Perfusion and Gas Exchange

Question 1

What are factors that affect the Diffusion of Gases? (Fick’s Law)

Answer 1

1. Partial pressure of gases
2. Area of lungs available for diff.
3. Thickness of alveoli epithelium
4. Diffusion constant
5. Matching of ventilation and perfusion

Question 2

Diagram of Pressure Gradients

Answer 2

Question 3

What is Dalton’s Law

Answer 3

In a mixture of non-reacting gases, the total pressure exerted is equal to the
sum of the partial pressures of the individual gases

Question 4

What is Henry’s Law?

Answer 4

Amount [concentration] of gas in solution depends on the atmospheric pressure

Question 5

Describe 2 ways in which the Alveolar Membrane assists diffusion

Answer 5

1. Thin membrane – small distance for gases to travel to reach RBC’s
2. Large surface area = more diffusion. Stretches open on inhalation

Question 6

What are some conditions that affect the thickness of the alveolar membrane?

Answer 6

Fibrosis (dev. of fibrous connective tissue) and Edema (swelling) increase the thickness of the membrane and decrease diffusion rate

Question 7

What is a condition that affects the area of the alveolar membrane?

Answer 7

Emphysema -> walls of alveoli are damaged leading them to rupture -> creates one larger air space as opposed to many small ones

Question 8

How do we measure gas diffusion?

Answer 8

Oximeter
- Normal PO2 = 100mmHg
- Normal PCO = 40mmHg

Question 9

What is a DLCO Test?

Answer 9

= a lung function test
1. Patient takes in single maximal breath from the
bag and holds their breath for 10s
2. After 10s, patient exhales into a separate bag.
3. Analyzed for remaining CO

Question 10

Why do we use CO in a DLCO test to measure diffusion? (3 reasons)

Answer 10

1. Diffuses across alveolar surface easily
2. Highly soluble in blood
3. Not normally present in plasma -> so easy to measure how much is transferred from inhaled air to plasma

Question 11

What is Ventilation (V)?

Answer 11

Refers to the flow of air into and out of the alveoli

Question 12

What is Perfusion?

Answer 12

refers to flow of blood to alveolar capillaries

Question 13

What is a normal V/Q ratio?

Answer 13

0.8

Question 14

What is indicated when V/Q ratio defers from the normal?

Answer 14

• V/Q ratio < 0.8 = shunting - less ventilation (e.g Pneumonia, Asthma)
• V/Q ratio > 0.8 = dead space - reduced perfusion (e.g Pulmonary Embolism)

Question 15

What are the steps of V/Q Matching?

Answer 15

1. Normally ventilation and alveolar blood
   flow are matched
2. Local regulatory mechanisms in place to
   facilitate adequate matching
3. Capillaries collapse if PO2 falls in alveolus and
   blood is diverted away to other areas of the lung
   where PO2 is higher
4. Bronchiolar diameter is regulated by PCO2
   levels in exhaled air. Increase PCO2 leads to
   bronchiolar dilation.