Gas exchange

Question 1

What is the kinetic theory of gases?

Answer 1

Kinetic theory explains the macroscopic properties of gases in terms of movement of molecules
It posits that molecules are in constant random motion, proportional to their energy
Molecules form only a tiny proportion of the total gas volume, the majority is empty space
Pressure is generated by the force of the impacts of molecules with the container walls

Question 2

What are partial pressures?

Answer 2

If two gases are allowed to mix, molecules of each type still behave independently, each gas exerts the same pressure as it would exert if it occupied the same volume alone.
Each gas exerts a partial pressure, which is a fraction of the total pressure.
The sum of the partial pressures equals the total pressure of the mixture.

Question 3

How does the composition of alveolar gas differ from atmospheric gas?

Answer 3

Alveolar gas has a different composition to the atmosphere:
-Less O2
-More CO2
This is because exchange occurs between alveolar gas and the gases carried in blood.
Gas exchange occurs by diffusion down the partial pressure gradient for each gas.

Question 4

What is Fick’s Law?

Answer 4

The rate of diffusion is proportional to both the partial pressure difference and surface area and is inversely proportional to the thickness of the membrane.

Question 5

According to Fick’s law, what is diffusion directly proportional to?

Answer 5

1) Partial pressure difference
2) Surface area of alveoli
3) Solubility of the gas

Question 6

According to Fick’s law, what is diffusion inversely proportional to?

Answer 6

1) Resistance of the alveolar membrane

2) Molecular weight of the gas

Question 7

What features of the alveoli aid diffusion?

Answer 7

1) Large surface area

2) Thin walls

Question 8

What must oxygen diffuse through to reach the blood plasma?

Answer 8

1) Epithelial cells lining the alveolar space
2) Basement membrane and small interstitial space
3) Endothelial cells lining the capillary

Question 9

Define solubility

Answer 9

The maximum amount of solute that can dissolve in a volume of solvent, e.g. mL O2 per litre water.

The solubility of a gas in a given liquid is constant for the specific gas and liquid and depends on the chemical properties of both.

Solubility varies with changes in temperature, pressure and pH

Question 10

How does the amount of gas dissolved in a liquid vary with pressure?

Answer 10

The amount of dissolved gas is proportional to its partial pressure in the adjacent gas.

The solubility of the gas in the liquid is given by the constant of proportionality

Volume = partial pressure x solubility