Properties of Gases Flashcards
State the ideal gas equation and Boyles Law
- Ideal gas equation - PV = nRT
- Boyles Law - P1V1 = P2V2
Explain the concept of the ‘partial pressure’ of an individual gas in a gas mixture
- In a mixture of gases, the total pressure exerted = the sum of the partial pressure of the individual gases
- Each gas exert a partial pressure equal to the pressure it would exert if it was the only gas present
- Gases dissolve and diffuse according to their partial pressure
- In the body, gases diffuse down their partial pressure gradient
- Eg. Movement of oxygen from alveolar air to blood
How do you calculate partial pressure from a gas mixture
101kPa x percentage of gas in mixture
Outline the effect of altitude on partial pressure
- At high altitudes, atmospheric pressure is lower - amount of air pressing down is less
- The same tidal volume of nspired air will have fewer oxygen molecules at high altitude
- Partial pressure of oxygen will decrease at high altitude
Outline the effect of diving on partial pressure
- When diving, pressure around is much higher than atmospheric pressure
- Nitrogen moves from high pressure in the lungs into the blood
- A slow return to the surface lets the nitrogen return to the lungs where it is breathed out
- Swimming up too quickly doesn’t give the nitrogen enough time to leave the blood
- Can form painful bubbles and lead to air embolism
Explain the concept of saturated vapour pressure and how to calculate partial pressure to account for this
- Water molecules entering the air exert vapour pressure
- When water molecules leave and enter water at same rate, the air is saturated with vapour
- Saturated vapour pressure = 6.28kPa at body temperature
- Inhaled air becomes saturated with water in the upper respiratory tract
- Since saturated vapour pressure = 6.28 kPa at body temperature, pressure of the rest of the gases = 101-6.28 = 94.7 kPa
- Still in the same ratio as dry air
Explain what is meant by ‘partial pressure’ of oxygen in blood, and how it is different from the ‘content’ of oxygen in the blood
- Partial pressure is a measure of pressure and has units kPa
- Content or amount is a measure of concentration and has units mmol/L
- Partial pressure = pressure exerted by the dissolved gas in a liquid
- Concentration of dissolved gas = number of mmol of gas, dissolved in a litre of liquid
Calculate the content of oxygen and carbon dioxide in plasma using their solubility coefficients, and partial pressure
- Amount of gas dissolved = partial pressure x solubility coefficient of gas
- Eg. When the pO2 of blood is 13.3 kPa, 0.01 mmol/L.kPa x 13.3 kPa = 0.13 mmol/L
- Plasma has 0.13 mmol/L of dissolved oxygen
- Eg. When the pO2 of blood is 13.3 kPa, 0.01 mmol/L.kPa x 13.3 kPa = 0.13 mmol/L
Explain the effect of partial pressure in oxygen entering the blood in the lungs
- Oxygen enters plasma from alveoli and dissolves in it as lower partial pressure in plasma
- Dissolved oxygen enters RBC and binds to haemoglobin
- Process continues until haemoglobin fully saturated (4 oxygen molecules bound to haemoglobin)
- After haemoglobin saturated, oxygen continues to dissolve until equilibrium is reached
- At equilibrium, pO2 of plasma = pO2 of alveolar air
State the normal PO2 and PCO2 in alveolar air, arterial blood and mixed venous blood
- Alveolar air - PO2 = 13kPa, PCO2 = 5.3kPa
- Arterial blood - same as alveolar air as equilibrates
- Venous blood - pO2 = 6kPa, pCO2 = 6kPa
Describe the layers making up the diffusion barrier at the air-blood interphase
- Epithelial cell of alveolus
- Tissue fluid
- Endothelial cell of capillary
- Plasma
- Red cell membrane
Describe factors affecting the rate of diffusion across the air blood interphase
- Rate of diffusion proportional to (AD(P1-P2)/T
- P1 - P2 is the partial pressure difference
- A - the surface area available for diffusion
- T - the distance the molecules must diffuse (thickness)
- D - diffusion coefficient for the gas = S/sqrt(MW)
- S - solubility of gas in solution
- MW - molecular weight of gas
- Temperature of the fluid assumed to be constant at body temperature
State and explain the difference in the diffusion rates of O2 and CO2
- Carbon dioxide diffuses more easily than oxygen through air blood interphase
- Carbon dioxide has higher solubility than oxygen
- Therefore problems with oxygen in terms of gas exchange more serious and happen earlier than carbon dioxide
Give examples of diseases which affect rate of diffusion in the alveoli
- Fibrotic lung disease - thickened alveolar membrane
- Pulmonary oedema - fluid in interstitial space increases diffusion distance
- Emphysema - destruction of alveoli reduces surface area for gas exchange
