S3) Properties of Gases Flashcards
What is Boyle’s Law?
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
Provide an example of Boyle’s Law in our bodies
Mechanism of inspiration & expiration
Define partial pressure
Partial pressure is the pressure exerted by a dissolved gas in a liquid
Explain the concept of partial pressure
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.
What is atmospheric pressure?
Atmospheric pressure is the pressure exerted by the weight of the air above the earth in the atmosphere
State the value of atmospheric pressure at sea level
101 kilopascals (kPa) = 1 atmosphere = 760 mmHg
Describe the composition of air
How can one calculate the partial pressure of a gas at sea level?
Partial pressure = atmospheric pressure x %gas
E.g. Partial pressure of O2 = 101 x 20.9% = 21.1 kPa
What effect do partial pressures have on gases in the body?
- Gases dissolve and diffuse according to their partial pressure
- Gases diffuse down their partial pressure gradient (high to low partial pressure)
What happens when inspired gases come in contact with body fluids?
- Gas molecules dissolve to enter liquid
- Water molecules evaporate to enter gas
How does saturation occur in the body?
- Water molecules entering the air exert vapour pressure
- When water molecules leave & enter water at same rate, the air is saturated with vapour
Provide an example of where saturation occurs in the body
Inhaled air in upper respiratory tract
State the value of Saturated Vapour Pressure (SVP) in the body
SVP = 6.28kPa at body temperature
How does the water vapour affect the partial pressure of the other gases?
- 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*
For gases dissolved in a liquid, when is equilibrium reached?
When the rate of gas molecules entering water = rate of gas molecules leaving water
For gases dissolved in a liquid, what happens to partial pressures when equilibrium is reached?
Partial pressure of the gas in the liquid = partial pressure of the gas in the air above it
Identify another term for the pressure of gas in a liquid
Tension
Use 2 equations to show how partial pressure is different from the amount of a dissolved gas
Partial pressure = atmospheric pressure x %gas
Amount of gas dissolved = partial pressure x solubility coefficient of gas
State the value of the solubility coefficient of O2 in plasma
0.01 /mmol.L-1 /kPa at 37°C
In three steps, explain happens when O2 enters the alveoli
⇒ O2 enters plasma & dissolves
⇒ Dissolved O2 enters RBC to bind to Hb
⇒ Process continues till Hb fully saturated
What happens after haemoglobin has been saturated?
- O2 continues to dissolve till equilibrium is reached
- At equilibrium, pO2 of plasma = pO2 of alveolar air
What kind of oxygen is found in blood?
Blood contains both dissolved and Hb bound oxygen
What does pO2 measure in the body?
pO2 is a measure of dissolved O2 in the blood
What is the role of dissolved oxygen in the body?
- Dissolved O2 is available to diffuse into tissues
- As dissolved O2 leaves the blood, it is replaced by O2 bound to Hb
What is the alveolar air composition?
- Alveolar pO2 = 13.3 kPa
- Alveolar pCO2 = 5.3 kPa
What is the composition of air in the blood?
The blood equilibrates with alveolar air:
- Arterial pO2 = 13.3 kPa
- Arterial pCO2= 5.3 kPa
Why is pO2 lower at high altitudes?
- Atmospheric pressure is lower
- Air is thinner and molecules are dispersed
- Hence, less air molecules available when breathing
The air inspired from a scuba tank is at higher pressure than on dry land.
Why?
Pressure below sea level = atmospheric pressure + weight of water
