Lecture 4: Properties of gases and gas exchange in the lung Flashcards
What is Boyle’s Law?
The pressure of a fixed quantity of gas at a constant temperature is inversely proportional to its volume
- as volume increases, pressure decreases
- as volume decreases, pressure increases
What is Dalton’s law?
In a mixture of gases, each component gas exerts a ‘partial pressure’ in proportion to its volume percentage in the mixture
-pressure is expressed as kPa
What does it mean when a system is in equilibrium for a gas and liquid?
Gas molecules enter the liquid to dissolve, at the same time some molecules which entered the liquid will leave to return to the gas phase
-when the rate of gas coming out of the solution is equal to the rate at which it enters the liquid,the system is in equilibrium for that gas and liquid
What is the partial pressure of a gas in a liquid?
When a gas is dissolved in a liquid, the collision of gas molecules within the walls of the container generates a pressure in the liquid.
At equilibrium, the partial pressure of the gas in the liquid is equal to the partial pressure of that gas in the gas phase
e.g. if partial pressure of oxygen in alveolar air is 13.3 kPa and this air is allowed to equilibriate with the blood in the pulmonary capillaries, the partial pressure of oxygen in the capillar is also 13.3 kPa
Is partial pressure of a gas and the content of gas dissolved in a liquid the same parameter?
No
Henry’s Law
-the amount/content of gas that dissolves in a specific volume of liquid, is proportional to the partial pressure of that gas in the gas phase AND its soluability coefficient
What is the soluability coefficient?
Amount (mmol) of a gas that will dissolve in a litre of plasma at 37 degrees, when exposed to a given partial pressure
What if the gas dissolved combines chemically with the liquid?
The content of gas= amount of gas chemically bound + amount of gas free in solution
e.g. amount of dossolve O2 in plasma is 0.13 mmol/l, and the amount bound to Hb is 8.8 mmol/l, total O2 content is 8.93 mmol/l
What is the saturated vapour pressure?
When a gas is in contact with water, water molecules evaporate to enter the gas phase, while some water vapour condenses to return to the liquid phase.
Equilibrium is reached when rate of evaporation=rate of condensation
-at equilibrium the gas mixture is saturated with water vapour, and the pressure it exerts is called the saturated vapour pressure
What factors does saturated vapour pressure (SVP) depend on?
Only temperature
What is the atmospheric pressure at sea level?
101 kPa
How much pressure does water vapour contribute to the pressure of the air in the airways?
6.28 kPa, while the other gases account for the remainder of 94.72 kPa of pressure
(since the other gases remain in the same proportion as in the dry air)
What is partial pressure of a gas?
The pressure exerted by an individual gas in a mixture of gases
- gases diffuse down a partial pressure gradient from high to low partial pressure
- use partial pressures of gas to calculate the amount of gas dissolved in a liquid
How do you calculate the partial pressure of a gas?
Multiply the fractional % (converted to a decimal) of the individual gas by the total pressure exerted by the mixture of gases
(assumes there are no chemical reactions between the gases)
-application of Dalton’s law
What is atmospheric pressure?
Pressure exerted by the weight of the air above the earth in the atmosphere
- atmospheric pressure changes with altitude (at high altitudes, atmospheric pressure is lower)
- at sea level 101 kilopascals/1 atm
What is the composition of atmospheric air?
78% nitrogen
21% oxygen
What is the partial pressure of oxygen in the atmospheric air?
101kPa x 0.21= 21.21 kPa
What happens to partial pressure of gases as we breath in?
-When we breath in the air is moistened-water vapour added and this exerts a pressure (SATURATED VAPOUR PRESSURE)
-saturated vapour pressure is dependent on body temp (37 degrees)
-we need to subtract the water vapour pressure from the total pressure of the dry gas mixture in a moist environment
: 101kPa - 6.28 kPa= 94.7 kPa = total pressure of gas mixture in moist environment
(other gases are still in the same ratios as in dry air)
What is the saturated vapour pressure?
Water vapour pressure= 6.28 kPa at body temp
-displaces a proportion of the total pressure of the mixture of gases
What is the partial pressure nomenclature?
pCO2/pO2: generic partial pressure
PaCO2/PaO2: partial pressure in arterial blood
PACO2/PAO2: partial pressure in alveoli
PvCO2/PvO2: partial pressure in venous blood
Why is there a difference in partial pressure of oxygen in upper resp tract compared to alveolar partial pressure?
Upper resp tract: 19.8 kPa
Alevolar: 13.3 kPa
Oxygen is constantly being extracted from alveolar air into pulmonary circulation
-not all the alveolar air is replaced (some stays in there)
How does alveolar ventilation differ from pulmonary ventilation?
Total amount of inhaled air is 450ml (tidal volume)
- 30% of tidal volume never reaches the alveoli as it stays in conducting portion of the resp tract= ANATOMICAL DEAD SPACE (150ml)
- 300ml of air therefore reaches the alveoli= ALVEOLAR VENTILATION
What is pulmonary minute ventilation and alveolar minute ventilation?
PMV: how much air is moved into your lungs per min
e.g. 12 breaths per min, 450 ml per breath= 450 x 12= 5400 ml/min
AMV: air reaching alveoli in a minute
e.g. 12 breaths per min, 300 ml reaches alveoli= 12 x 300= 3600 ml/min
What happens when inspired gases come into contact with body fluids?
Gas molecules will enter fluid and dissolve in the liquid
- amount of gas that dissolves is directionally proportional to the partial pressure of that gas
e. g.higher PP of that gas, the more gas will dissolve into that liquid - gas diffuses down a PP gradient
- diffusion will therefore continue until equilibrium is reached: when number of molecules of gas from air entering the fluid is matched by the number of molecules of air leaving the fluid: NO NET MOVEMENT
-once equilibrium is reached the partial pressure in the liquid is the same as in the air even though their concentrations aren’t the same
At equilibrium, how is the partial pressure of a gas in the air equal to the partial pressure of the gas in liquid when their concentrations aren’t the same?
At a low soluability, because these molecules leave the water much readily, equilibrium will be reached when there is a much lower conc of gas dissolved in the water