L09 - Alveolar gases and diffusion

Question 1

What is partial pressure?

Answer 1

The force exerted by a gas (the pressure that would be exerted by one of the gases in a mixture if it occupied the same volume on its own

Question 2

What is approx percentage of oxygen in dry air?

Answer 2

20%

Question 3

What is the partial pressure of inspired oxygen in the mouth

Answer 3

~20kPa (150mmHg)

Question 4

What is the partial pressure of inspired carbon dioxide in the mouth?

Answer 4

~0kPa as there is virtually no CO2 in dry air

Question 5

What is the partial pressure of CO2 in the conducting zone?

Answer 5

5kPa from CO2 prod

Breathe out = conducting zone fill with expired air from alveoli

Question 6

What is the partial pressure of oxygen after expiration in the conducting zone?

Answer 6

13kPa as O2 has been removed from the air inspired

Question 7

Why is the alveolar partial pressure important?

Answer 7

It sets the blood gas tensions (partial pressure of gases in the blood)

Question 8

What is the alveolar gas equation?

Answer 8

PAO2 = PIO2 - (PACO2/R)

where R = V.CO2/V.O2

Question 9

What happens to alveolar O2/ CO2 and so arterial O2/ CO2 when don’t breathe enough?

Answer 9

A O2 falls –> a O2 falls

A CO2 inc –> a CO2 inc

Question 10

What happens to alveolar O2/ CO2 and so arterial O2/ CO2 when breathe too much?

Answer 10

A O2 inc –> a O2 inc

A CO2 falls –> a CO2 falls

Question 11

What determines PaCO2 tensions?

Answer 11

INCREASING = Inc met or CO2 prod ---> con PACO2
DECREASING = Inc vent --> dilute PACO2 as breathing more

Question 12

What is the relationship in terms of proportions between PACO2 and V.CO2?

Answer 12

PACO2 a V.CO2

Question 13

What is the relationship in terms of proportions between PACO2 and V.A?

Answer 13

PACO2 a (1/V.A)

Question 14

What equation do you use to calculate PACO2 if V.CO2 and V.A is known?

Answer 14

PACO2 = 115 x (V.CO2/V.A)

Question 15

What structures make up the alveolar-cap memb?

Answer 15

1. Alveolar epithelium
2. Basal lamina
3. Cap endothelium

Question 16

What does diffusion of a gas in a liquid depend on and according to which law?

Answer 16

Henry’s law
- Conc diff (prop to change in partial pressure in gas)
Therefore, more soluble gas maintains higher conc diff and so DIFFUSES EASIER

Question 17

Equation for Henry’s law?

Answer 17

Conc of gas = P x solubility coeff (a)

Question 18

What is the solubility coeff of CO2?

Answer 18

5.3 mlL^(-1)kPa^(-1)

Question 19

What is the solubility coeff of O2?

Answer 19

0.23 mlL^(-1)kPa^(-1)

Question 20

What does diffusion of a gas in a liquid depend on and according to which law?

Answer 20

Graham’s law
- diff in partial pressure
- Inversely propor to 1/ sqrt MW
Therefore, bigger mlcs diff slower than smaller ones

Question 21

How to work out the proportion of diffusibility of CO2 to diffusibility of O2?

Answer 21

Diff ratio in air x diff ratio in liquid
= answer
CO2 diff (answer) times more easily than O2

Question 22

Why can more CO2 be transferred per min than O2 despite a smaller alveolar- cap diff?

Answer 22

CO2 can diffuse 20 times more easily than O2, therefore it is faster at diff across the a-c memb

Question 23

What is the uptake of a gas into blood dependent on?

Answer 23

Uptake of gas into blood dependent on the partial press diff which depends on:

• solubility
• chemical combination

Question 24

What is pulmonary diffusing capacity red by>

Answer 24

1. Red effective SA

2. Inc diff path length