Gases and Gas Exchange

Question 1

What is Boyle’s law?

Answer 1

Pressure of a gas is inversely proportional to its volume

Question 2

In a mixture of non reacting gases, what is the total pressure exerted?

Answer 2

The sum of the partial pressures of the individual gases

Question 3

Why is atmospheric pressure lower at high altidudes?

Answer 3

Amount of air pressing down is less

Question 4

What is the partial pressure of oxygen in atmospheric air at sea level?

Answer 4

21.1 kPa

Question 5

What happens to gases when they come into contact with body fluids?

Answer 5

Gas molecules will enter water to dissolve in liquid

Question 6

What do water molecules entering the air exert?

Answer 6

Vapour pressure

Question 7

What is saturated vapour pressure (SVP) at body temperature?

Answer 7

6.28 kPa

Question 8

Where does inhaled air become saturated with water?

Answer 8

In the upper respiratory tract

Question 9

How does water vapour affect the partial pressure of other gases?

Answer 9

Pressure of the rest of the gases = 101-6.28 = 94.7kPa

Still in same ratios as in dry air so eg:

pO2 = (101-6.28) x 20.9% = 19.8 kPa

Question 10

How do you calculate amount of gas dissolved?

Answer 10

Partial pressure x solubility coefficient of gas

Solubility coefficient of oxygen in plasma is 0.01/mmol/L/kPa at body temperature

Question 11

How much dissolved oxygen does plasma have per litre?

Answer 11

0.13 mmol/L

Question 12

In alveolar air, what is the partial pressure of oxygen?

Answer 12

13.3 kPa

Question 13

In alveolar air, what is the partial pressure of CO2?

Answer 13

5.3 kPa

Question 14

What factors affect the rate of diffusion in a fluid?

Answer 14

Pressure difference, solubility of the gas in solution, cross-sectional area of fluid, distance molecles must diffuse, molecular weight of gas, temperature

Question 15

True or False:

Oxygen is far more soluble than carbon dioxide so it diffuses faster in a liquid

Answer 15

False

Carbon dioxide is much more soluble than oxygen so diffuses in a liquid 20 times faster than oxygen - smaller gradient

Question 16

What is the diffusion barrier?

Answer 16

5 cell membranes
3 layers of cytoplasm
2 layers of tissue fluid
- epithelial cell of alveolus
- tissue fluid
- endothelial cell of capillary
- plasma
- red cell membrane

Question 17

Describe the PO2 in the alveoli

Answer 17

Lower in alveoli compared to external environment

• continous diffusion of oxygen across alveolar wall
• diluting effect of CO2 entering the alveoli as it travels in the opposite direction

Higher in alveoli than in capillaries, so oxygen diffuses into blood

Question 18

Why does emphysema affect the rate of diffusion?

Answer 18

Results in alveoli combining
Decreases surface area
Decreases rate of diffusion

Question 19

How does fibrosis affect rate of diffusion?

Answer 19

Decreases diffusion rate
Causes thickening of exchange surface
Larger distance

Question 20

What things can increase diffusion distance?

Answer 20

Pulmonary Oedema

Question 21

In lung diseases that interfere with the rate of diffusion, why is PO2 low but PCO2 often normal?

Answer 21

CO2 crosses alveolar membrane more easily than oxygen and is more soluble