Properties of gases and diffusion

Question 1

Describe the kinetic theory of gases

Answer 1

Goes are a collection of molecules moving randomly around a space.

Pressure is generated by collisions of molecules with a surface.

The more frequency and harder the collisions, the higher the pressure generated by the gas.

Question 2

What is Boyle’s law?

Answer 2

‘Pressure of a gas is inversely proportional to its volume’

Inspiration and expiration are examples of Boyle’s Law

Question 3

Whta is atmospheric pressure?

Answer 3

Pressure exerted by the weight of the air above the earth in the atmosphere.

At sea level = 101kPa = 1atm = 760mmHg

At high altitudes, atmospheric pressure is lower.

Question 4

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

Answer 4

Gas molecules will enter water to dissolve liquid

Water molecules evaporate to enter air.

Question 5

What is SVP?

Answer 5

This is the saturated vapour pressure.

This is because water molecules entering the air exert a ‘vapour pressure.’ At equilibrium, the rate of water evaporation = rate of water condensation so, altho the air is saturated with vapour, this value remains constant.

Value is: 6.28kPa

Question 6

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

Answer 6

You still have the same ratio of gases, but you have to do 101 (air pressure) - 6.28 (SVP) and then work out pressures from the remaining value.

Question 7

At what point is equilibrium reached between air and water?

Answer 7

Partial pressure of gas in liquid = partial pressure of gas in air above it

Question 8

How many moles of oxygen will dissolve in plasma per litre?

Answer 8

Solubility coefficient of O2 in plasma = 0.01 mmol

When exposed to pO2 of 13.3kPa in alveolar air

0.01x13.3 = 0.13 mmol of oxygen will dissolve

Plasma has 0.13 mmol of dissolved oxygen per litre.

Question 9

What happens to partial pressure of oxygen when Hb in blood?

Answer 9

Oxygen in the cells doesn’t contribute to partial pressure - reaction must be complete (all Hb must have four oxygen molecules bound to it) before equilibrium can be established.
Equilibrium when ppOxygen of plasma = ppOxygen of alveolar air

Question 10

Define partial pressure

Answer 10

Pressure exerted by the dissolved gas in a liquid

Question 11

Define concentration of dissolved gas

Answer 11

number of mmol of gas dissolved in a litre of liquid

Question 12

What is the solubility coefficient?

Answer 12

A constant for each gas (solute) and solvent

Question 13

What makes up total content of gas

Answer 13

dissolved gas + gas bound to or reacted with a component. (e.g. oxygen dissolved in plasma + oxygen bound to Hb)

Question 14

Why is pO2 lower and pCO2 higher in alveoli than in inhaled air?

Answer 14

Inhaled air is mixed with residual volumes

Effect of O2 diffusion acts the alveolar wall

Effect of CO2 entering the alveoli

Alveolar air composition stays constant at 13.3kPa of oxygen and 5.3kPa of carbon dioxide - blood equilibrates to this level.

Question 15

What is pO2 and pCO2 in mixed venous blood?

Answer 15

pO2 = 6.0 KPa

pCO2 = 6.1kPa

But, this varies with metabolism

Question 16

What is pO2 and pCO2 in alveolar gas?

Answer 16

pO2 - 13.3kPa

pCO2 = 5.3kPa

Question 17

What factors affect the rate of diffusion?

Answer 17

partial pressure difference across membrane

SA available

Thickness (distance traveled)

Diffusion coefficient - (solubility / √Molecular weight)

Question 18

What is the difference between the diffusion of CO2 and O2?

Answer 18

CO2 is MUCH more soluble than O2 so, despite O2 having a lower molecular weight, the CO2 diffuses 20 times faster than O2.
The large difference in partial pressure compensates for slower diffusion of oxygen.
This difference means that, in a diseased lung, oxygen gas exchange is thus more impaired than CO2.

Question 19

What must gases cross to diffuse from alveolar air to RBC in capillary?

Answer 19

Fluid film lining alveolus
Epithelial cell of alveolus
Interstitial space
Endothelial cell of capillary
Plasma
Red cell membrane 

5 Layers
2 Layers os cytoplasm
2 Layers of tissue fluid + plasma

Question 20

In what diseases is thickness of membrane increased?

Answer 20

Oedema - fluid in interstitial space and in alveoli

Lung fibrosis - increased thickness of alveolar capillary membrane

Question 21

In what diseases is surface area of membrane decreased?

Answer 21

Remove a lung

Emphysema

Question 22

How do diseases affect the diffusion coefficient of gas change?

Answer 22

carbon dioxide always diffuses much faster than oxygen so the diffusion fo oxygen is affected(low pO2) and carbon dioxide not (normal pCO2).

Question 23

What is the difference between partial pressure and amount of a dissolved gas?

Answer 23

Amount dissolved = partial pressure x solubility coefficient.