Lec 4 - Properties of Gases and Diffusion

Question 1

What is Boyle’s law and give an example of Boyle’s law?

Answer 1

This is the idea that the pressure of gas is inversely proportional to its volume.
- This only applies if temperature and number of gas molecules remains constant in a closed system.

• Inspiration and expiration are an example of Boyle’ law.

Question 2

What is atmospheric pressure?

Answer 2

This is the pressure exerted by the weight of the air above the earth in the atmosphere.

Question 3

What is atmospheric pressure at sea level?

Answer 3

At sea level - 101 kPa = 1 atmosphere = 760 mmHg.

Question 4

Why is atmospheric pressure low at high altitudes?

Answer 4

This is because the air molecules are dispersed.

• means the air is thin.
• This means less air molecules are available when you breathe.

Question 5

Describe the composition of atmospheric air.

Answer 5

Oxygen = 20.9%
Nitrogen = 78%
Carbon dioxide = 0.03%

Question 6

What are the partial pressure of gases in atmospheric pressure?

Answer 6

partial pressure of O2 = 101 x 20.9% = 21.1 kPa.

partial pressure of N2 = 101 x 78% = 78.7 kPa

partial pressure of CO2 = 101 x 0.03% = 0.03 kPa

Question 7

What happens when inspired gases come into contact with body fluids?

Answer 7

• Gas molecules will enter water to dissolve in liquid.

- Water molecules evaporate to enter air.

Question 8

When is air saturated with vapour and what is saturated vapour pressure?

Answer 8

Air is saturated with vapour when the rate pf H20 evaporation is equal to the rate of H20 condensation.

• Saturated Vapour Pressure (SVP) = 6.28 kPa at body temp.

Question 9

When does air become saturated with water?

Answer 9

-Inhaled air becomes saturated with water, in the upper respiratory tract.

Question 10

Describe partial pressures at equilibrium.

Answer 10

• At elm, the partial pressure of the dissolved gas in the liquid = the partial pressure of the gas it is exposed to.

Question 11

Define the term amount dissolved?

Answer 11

Amount dissolved = partial pressure x solubility coefficient of gas.

• calculated in mmol/L

Question 12

What does solubility coefficient mean ?

Answer 12

It is a constant for the individual gas and the solvent.

Question 13

What is the solubility coefficient of O2 in plasma?

Answer 13

The solubility coefficient of O2 in plasma = 0.01 mmol/L/kPa at 37 degrees celsius.

Question 14

How much dissolved oxygen is in plasma?

Answer 14

• when exposed to a pO2 of 13.3 kPa (as in alveolar air)

- –> 0.01 x 13.3 = 0.13 mmol of O2 will dissolve.

Question 15

Describe the process for the binding of Oxygen to Hb.

Answer 15

1. Oxygen enters the plasma and dissolves into it.
2. The dissolved oxygen enters the RBC to bind to Hb.
3. The process continues till Hb is fully saturated - means each Hb binds to 4 oxygens.
4. After Hb is fully saturated, oxygen continues to dissolve until eqm is reached.
5. At eqm, pO2 of plasma = pO2 of alveolar air.
6. Blood contains both dissolved and Hb bound oxygen.
7. The pO2 is a measure of dissolved O2 in the blood.
8. Dissolved O2 is available to diffuse into tissues down its partial pressure gradient.
9. As dissolved O2 leaves the blood , it will be replaced by O2 bound to Hb.
10. In this way the O2 bound to Hb will be downloaded and diffuse into tissues.

Question 16

Define the term Concentration of dissolved gas.

Answer 16

Number of mmol of gas, dissolved in a litre of liquid.

Question 17

Define the term total content of gas.

Answer 17

Total content of gas = dissolved gas + gas bound to or reacted with a component.

Question 18

What are the partial pressures of O2 and CO2 in the alveolar air and why is it like this?

Answer 18

pO2 = 13.3 kPa - lower than inhaled air.
pCO2 = 5.3 kPa - higher than inhaled air.

• This is because:
  1. inhaled air mixes with residual volume.
  2. Effect of O2 diffusing across the alveolar wall.
  3. effect of CO2 entering the alveoli.

Question 19

What are the partial pressures of O2 and CO2 in mixed venous blood?

Answer 19

PO2 = 6.0 kPa
PCO2 = 6.1 kPa

Question 20

Compare alveolar partial pressures with mixed venous blood partial pressures and what does this mean?

Answer 20

• Alveolar PO2 > PO2 in mixed venous blood.
• Alveolar PCO2 < PCO2 in mixed venous blood.

—> So oxygen will diffuse into blood and carbon dioxide out.

Question 21

What are factors that affect the rate of diffusion?

Answer 21

1. partial pressure difference across the membrane (P1 - P2)
2. The surface area available for diffusion (A)
3. Thickness - distance molecules must diffuse (T)
4. Diffusion co-efficient of the individual gas

Question 22

What is the equation to calculate the rate of diffusion?

Answer 22

Rate of diffusion is proportional to (A x D x (P1-P2)/ T

Question 23

What properties of the individual gas does diffusion also depend on?

Answer 23

1. The solubility of the gas in the liquid - The greater the solubility, the faster the rate of diffusion.
2. The molecular weight of gas - The higher the molecular weight, the slower the rate of diffusion.

Question 24

What is the equation of the diffusion coefficient and what is it used for?

Answer 24

D is proportional to solubility/ square root molecular weight.

• It is used to determine the relative rates at which different gases will diffuse across the same membrane at the same pressures.

Question 25

Compare the solubility and molecular weight of oxygen and carbon dioxide and what this means?

Answer 25

Solubility
- CO2 is much more soluble than O2, so CO2 diffuses faster than O2

Molecular weight
- CO2 > O2 so CO2 moves slower than O2

—> It means overall the effect of solubility on CO2 is greater so CO2 diffuses 20 times faster than O2.

Question 26

What must diffusion from alveolar air to RBC in capillary cross?

Answer 26

1. Fluid film lining alveolus
2. epithelial cell of alveolus
3. Interstitial space
4. Endothelial cell of capillary
5. Plasma
6. Red cell membrane
7. 5 cell membranes
8. 3 layers of cytoplasm
9. 2 layers of tissue fluid + plasma

Question 27

What factors affect the rate of gas diffusion in disease?

Answer 27

1. Thickness of the membrane
   - –> Increases as a result of oedema fluid in the interstitial space and in the alveoli.
   - –> Lung fibrosis - increased thickness of alveolar capillary membrane.
2. Surface area of the membrane
   - –> decreased by removal of an entire lung
   - –>emphysema - decreased surface area.
3. Diffusion coefficient of the gas
   - –> CO2 always diffuses much faster than O2.
   - –> so diffusion of O2 is affected as the PO2 is low
   - –> Diffusion of CO2 is not affected as the PCO2 is normal.

Question 28

What happens in fibrotic lung disease?

Answer 28

• the thickened alveolar membrane slows down gas exchange.

- The PO2 is low and PCO2 is normal , so CO2 crosses the alveolar membrane more easily than oxygen.

Question 29

What happens in emphysema?

Answer 29

• There is destruction of alveoli, which reduces the surface area for gas exchange.
• PO2 is low and PCO2 is normal so CO2 crosses the alveolar membrane more easily than oxygen.

Question 30

What happens in pulmonary oedema?

Answer 30

• There is fluid in the interstitial space which increases diffusion distance.
• the exchange surface is normal.
• PO2 is normal
• Arterial PO2 is normal due to higher CO2 solubility in water.