S3) Properties of Gases

Question 1

What is Boyle’s Law?

Answer 1

Boyles Law: pressure (P) of a gas is inversely proportional to its volume (V) if temperature (T) and number of gas molecules remains constant in a closed system

Question 2

Provide an example of Boyle’s Law in our bodies

Answer 2

Mechanism of inspiration & expiration

Question 3

Define partial pressure

Answer 3

Partial pressure is the pressure exerted by a dissolved gas in a liquid

Question 4

Explain the concept of partial pressure

Answer 4

In a mixture of gases, each gas has a partial pressure which is the pressure of that gas if it alone occupied the entire volume of the original mixture at the same temperature.

Question 5

What is atmospheric pressure?

Answer 5

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

Question 6

State the value of atmospheric pressure at sea level

Answer 6

101 kilopascals (kPa) = 1 atmosphere = 760 mmHg

Question 7

Describe the composition of air

Answer 7

Question 8

How can one calculate the partial pressure of a gas at sea level?

Answer 8

Partial pressure = atmospheric pressure x %gas

E.g. Partial pressure of O₂ = 101 x 20.9% = 21.1 kPa

Question 9

What effect do partial pressures have on gases in the body?

Answer 9

• Gases dissolve and diffuse according to their partial pressure
• Gases diffuse down their partial pressure gradient (high to low partial pressure)

Question 10

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

Answer 10

• Gas molecules dissolve to enter liquid
• Water molecules evaporate to enter gas

Question 11

How does saturation occur in the body?

Answer 11

• Water molecules entering the air exert vapour pressure
• When water molecules leave & enter water at same rate, the air is saturated with vapour

Question 12

Provide an example of where saturation occurs in the body

Answer 12

Inhaled air in upper respiratory tract

Question 13

State the value of Saturated Vapour Pressure (SVP) in the body

Answer 13

SVP = 6.28kPa at body temperature

Question 14

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

Answer 14

• Pressure of the rest of the gases = 101 – 6.28 = 94.7 kPa
• Same ratios as in dry air
• Eg. pO2 = (101 - 6.28) x 20.9% = 19.8 kPa*

Question 15

For gases dissolved in a liquid, when is equilibrium reached?

Answer 15

When the rate of gas molecules entering water = rate of gas molecules leaving water

Question 16

For gases dissolved in a liquid, what happens to partial pressures when equilibrium is reached?

Answer 16

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

Question 17

Identify another term for the pressure of gas in a liquid

Answer 17

Tension

Question 18

Use 2 equations to show how partial pressure is different from the amount of a dissolved gas

Answer 18

Partial pressure = atmospheric pressure x %gas

Amount of gas dissolved = partial pressure x solubility coefficient of gas

Question 19

State the value of the solubility coefficient of O₂ in plasma

Answer 19

0.01 /mmol.L^-1 /kPa at 37°C

Question 20

In three steps, explain happens when O₂ enters the alveoli

Answer 20

⇒ O₂ enters plasma & dissolves

⇒ Dissolved O₂ enters RBC to bind to Hb

⇒ Process continues till Hb fully saturated

Question 21

What happens after haemoglobin has been saturated?

Answer 21

• O₂ continues to dissolve till equilibrium is reached
• At equilibrium, pO₂ of plasma = pO₂ of alveolar air

Question 22

What kind of oxygen is found in blood?

Answer 22

Blood contains both dissolved and Hb bound oxygen

Question 23

What does pO₂ measure in the body?

Answer 23

pO₂ is a measure of dissolved O₂ in the blood

Question 24

What is the role of dissolved oxygen in the body?

Answer 24

• Dissolved O₂ is available to diffuse into tissues
• As dissolved O₂ leaves the blood, it is replaced by O₂ bound to Hb

Question 25

What is the alveolar air composition?

Answer 25

• Alveolar pO₂ = 13.3 kPa
• Alveolar pCO₂ = 5.3 kPa

Question 26

What is the composition of air in the blood?

Answer 26

The blood equilibrates with alveolar air:

• Arterial pO₂ = 13.3 kPa
• Arterial pCO₂= 5.3 kPa

Question 27

Why is pO₂ lower at high altitudes?

Answer 27

• Atmospheric pressure is lower
• Air is thinner and molecules are dispersed
• Hence, less air molecules available when breathing

Question 28

The air inspired from a scuba tank is at higher pressure than on dry land.

Why?

Answer 28

Pressure below sea level = atmospheric pressure + weight of water