Lecture 24: Pulmonary Gas Exchange

Question 1

At sea level the partial pressure of a specific gas in air can be determined by:

Answer 1

multiplying the percent of the gas in the air mixture by 760 mmHg

Question 2

Henry’s law states that the amount of a given gas that dissolves in a given type and volume of liquid is:

Answer 2

directly proportional to the partial pressure of that gas at equilibrium.

Question 3

Dalton’s law states that the total pressure exerted by the mixture of non-reactive gasses is:

Answer 3

equal to the sum of the partial pressures of the individual gasses.

Question 4

Boyle’s law states that for a fixed amount of an ideal gas kept at a fixed temperature:

Answer 4

Pressure and volume are inversely proportional

Question 5

The addition of water vapor to the mixture of air in the alveoli

Answer 5

decreases the partial pressures of all other gasses in the mixture.

Question 6

What is the alveolar pressure at the end of a normal quiet inspiration cycle:

Answer 6

0 mmHG

Question 7

Va/Q= 0.8

Answer 7

Normally Va/Q is 0.8 (pulmonary= 5L/min; Ventilation =4 L/min).

Question 8

Va/Q=0

Answer 8

when Va=0, but there is still perfusion; this is due to airway obstruction (mucus plug). The blood gas composition remains unchanged.

Question 9

Va/Q= Infinity

Answer 9

When Q= 0, but there is still ventilation (no gas exchange) this is due to vascular obstruction (PE). Alveolar gas composition remains unchanged because there is no blood contact. This creates a physiologic shunt.

Question 10

Major components of air and relative concentrations:

Answer 10

Nitrogen 78%
Oxygen 21%
Argon 1%
CO2 0.03%

Question 11

Pressure of gas

Answer 11

Directly proportional to the concentration of the gas molecules.

Question 12

Rate of diffusion is

Answer 12

directly proportional to the pressure caused by that gas alone.

Question 13

A gas dissolved in a fluid

Answer 13

also exerts its own partial pressure against a cell membrane.

Question 14

5 Factors that affect rate of gas diffusion in a fluid:

Answer 14

1. Solubility of gas in the fluid
2. Cross-sectional area of the fluid
3. Distance through which the gas must diffuse
4. molecular weight of gas
5. Temperature of fluid (remains reasonably constant)

Question 15

What are the factors that control oxygen concentration in alveoli?

Answer 15

Rate of absorption of oxygen into the blood.

Rate of new oxygen entry into the lungs (Alveolar ventilation)

Question 16

Explain why alveolar ventilation cannot increase partial pressure of O2 above 149 mmHg under normal conditions.

Answer 16

Maximum partial pressure in the atmosphere

Question 17

What are the factors that control carbon dioxide concentration in the alveoli?

Answer 17

Rate of carbon dioxide excretion
*Alveolar PCO2 increase in direct proportion to rate of excretion.

Alveolar ventilation:
*Alveolar PCO2 decreases in inverse proportion to alveolar ventilation.

Question 18

Why alveolar ventilation cannot increase PO2 above 149 mm Hg under normal conditions.

Answer 18

Maximum partial pressure in atmosphere.

Question 19

List factors that will determine how rapidly a gas will pass through the respiratory membrane.

Answer 19

1. Membrane Thickness
2. Membrane Surface Area
3. Diffusion coefficient of gas in the substance of the membrane
4. Partial Pressure difference of gas between the two sides of the membrane.

Question 20

Va/Q Ratio represents:

Answer 20

Ventilation-perfusion ratio: Alveolar ventilation/ blood flow

Question 21

Where is gas exchange complete? for normal ventilation?

Answer 21

Gas exchange is complete in the initial 1/3 of the capillary

Question 22

Which of the following changes occurs during strenuous exercise?

A. Ventilation rate and O2 consumption increase to the same extent
B. Systemic arterial PO2 decreases to about 70 mmHG
C. Systemic arterial PCO2 increases to about 60 mmHg
D. Systemic venous PCO2 increases to about 60 mmHG
Pulmonary blood Flow decreases at the expense of systemic blood flow

Answer 22

A. Ventilation rate and O2 consumption increase to the same extent

Question 23

If an area of the lung is not ventilated because of bronchial obstruction, the pulmonary capillary blood serving that area will have a PO2 that is:
A. Equal to atmospheric PO2
B. Equal to mixed venous PO2
C. Equal to normal systemic arterial PO2
D. Higher than inspired PO2
E. Lower than mixed venous PO2

Answer 23

B. Equal to mixed venous PO2

If an area of lung is not ventilated, there can be no gas exchange in that region. The pulmonary capillary blood serving that region will not equilibrate with alveolar PO2 but will have a PO2 equal to that of mixed venous blood.