Gas Laws & Gas Transfer Flashcards
Explain the difference in movement of gas by mechanical force or diffusion.
Mechanical force (convective transport): bulk flow of matter down a pressure gradient via muscular work in + elastic recoil out (in large airways + circulation)
Diffusion: Gas movement down concentration gradient of each individual gas i.e. no movement of matter (from terminal bronchioles to mitochondria of metabolising tissues)
What are the 4 main functions of the cardio-respiratory system?
- Transport O2 + nutrients to tissues
- Remove CO2 + other waste products from tissues
- Mobility of cell-mediated + humoral immunity
- Transport of hormones, cytokines + other mediators
What is the kinetic theory of gases?
Posits that gases are composed of a collection of submicroscopic molecules in constant random motion proportional to their energy - these molecules only form a tiny proportion of total gas volume, the majority is empty space
Aids understanding of macroscopic properties of gas
How is pressure generated in terms of the kinetic theory of gases?
By force of the impacts of the molecules in the gas with the container walls/other molecules so more frequent/harder collision generates higher pressure
How is pressure (P) defined?
As force (F) per unit area (A) - P = F/A
Unit is in Pascal (Pa)
What is Boyle’s law?
Pressure (P) inversely proportional to volume (V) if temperature + amount of gas constant (c) -> P = c/V meaning rearranged:
PV = c so P1V1 = P2V2 = constant
What is Charles’s law?
Volume (V) is directly proportional to temperate (T) on absolute temperate scale if pressure + amount of gas are constant (c) -> T = cV meaning rearranged:
T/V = c so T1/V1 = T2/V2 = constant
How does the universal gas law come about?
Rearranging the equations of Boyle’s + Charles’s law gives you:
B: P = 1/V and V = 1/P
C: V = T
So V = T/P -> T = V x P thus, pressure and volume are both proportional to temperature
Can be rearranged to PV/T = constant (R) where R = gas constant
Final equation: PV = nRT where n is no. of moles of gas
What does the universal gas law allow for?
Calculation of one change in a parameter when the others are changed but are know i.e. need to know all of the values of the equation except one
What are partial pressures in terms of Daltons law?
When non-reactive gases (e.g. O2, N + CO2) mix, molecules of each type till behave independently + exert their own partial pressure which is a fraction of the total pressure
Total pressure = sum of partial pressures of individual gases
Same fraction of total pressure = fraction of total volume of gas in mixture so can work out partial pressure by multiplying fraction of volume by total pressure
If there is 2 gases with the same pressure + amount in 2 separate boxes, and then the partition is removed allowing the gases to mix, what will be the partial pressure of the gases?
The volume of the box has doubled and each gas will take up half of the box therefore, the partial pressure of both gases will be 50%
Using Daltons law, what would be the equation for atmospheric pressure?
= Sum of partial pressures of nitrogen, oxygen, water vapour + any other gases in the air
In Dalton’s law, in a mixture of gases, each gas exerts the same pressure that it would exert if it ___ __ __ ___ __.
Occupied the same volume alone
In biological systems gas mixtures are always in contact with water (major constituent of tissues + blood), what happens at this contact point?
Water molecules evaporate + gas molecules dissolve but there is no chemical reaction between them
Equilibrium is reached when partial pressures of each substance on either side of interface are equal
How can equilibrium between water and gas be attained? When is it impossible to reach equilibrium?
Water molecules enter gas exerting a vapour pressure
If they enter a closed volume of gas (e.g. in alveolus) equilibrium is reached typical of biological systems -> equilibrium pressure for water vapour = saturated vapour pressure (SVP)
If gas volume is unlimited, there will be fluid evaporation to dryness as no equilibrium can be reached
What is Saturated Vapour Pressure (SVP) and what does it depend on?
SVP = equilibrium pressure for water vapour = 6.28 kPa at 37 degrees (good approximation of temperature in biological systems)
SVP depends only on temperature of system as SVP increases with temperature because increased kinetic energy allows more vapour to escape liquid surface
The temperature at which the Saturated Vapour Pressure (SVP) is equal to the atmospheric pressure is called the ___ __.
Boiling point
What is Henry’s law?
The physical amount of gas (θ) dissolving in a solvent is directly proportional to the pressure of the gas (δ) at the solvent’s surface (if temperature is constant)
Constant of proportionality (c) of this relationship is an expression of solubility of gas in the liquid
So θ = c x δ
What gas movement is occurring at the alveolar-blood interface? Why?
O2 is stored in the blood in the lungs and will come in from bronchi and diffuse down its concentration gradient into the blood
CO2 is higher in the blood as it is stored in the blood of tissues for transport so it will diffuse down its concentration gradient into the alveoli
Why is CO2 higher in the tissues/blood leaving the tissues?
Because it is a metabolism waste product produced at the tissues
What is gas solubility in a liquid? What factors can alter it?
Represents the gas’ ability to dissolve in that liquid and is constant for the specific gas + liquid depending on their physical properties - it also depends on temperature, pressure + pH of solution
The amount of gas which enters the liquid to establish a particular tension is determined by its ____.
Solubility
How is gas solubility in a liquid expressed?
As amount of solute per amount of solvent
Measured at saturation concentration where adding further solute results in its precipitation e.g. adding sugar to water until the sugar starts to precipitate = solubility
What happens when a gas dissolved in water?
Gas molecules exert ‘tension’ i.e. a pressure within the liquid
At equilibrium, the tension of each constituent gas is proportional to its partial pressure in the adjacent gas mixture (Henry’s law)
What does gas tension in liquids indicate?
How readily the gas will leave the liquid
Measure of partial pressure of solute in solvent (kilopascals) e.g. oxygen in blood so does not indicate physical amount of gas in liquid
What is the gas content in a liquid? What does it depend upon?
Physical amount of given gas in a liquid e.g. mls O2 per 100ml blood
Changes with temperature + gas pressure above it
Proportional to its solubility in that liquid + gas tension at equilibrium
How is gas exchanged facilitated in the lungs?
At rest, 5L of blood must pick up 12 mmol of O2 per minute so exchange needs a very large SA ~ 70 square metres to expose enough blood to gas coming from the atmosphere
Requires large no. of small compartments = 300 million alveoli (thin hexagonal structure surrounded by capillary plexus + fed through tiny bronchiole called alveolar duct)
What is molecular diffusion?
Principal mechanism of gas mixing in lung periphery
Requires presence of molecular concentration gradient
Due to random motion of individual gas moelcules -> net transfer of a given gas from a region of high to low concentration
What is the role of blood?
Picks up O2 in lungs + deposits it in tissues & picks up CO2 in tissues + deposits it in lungs due to properties of Hb carried in RBCs
Gas transfer occurs because blood is exposed to appropriate gaseous environments by the pulmonary system + tissues
What will happen if a gas enters a liquid and then reacts with?
Gas may react with component of liquid (e.g. RBC) - reaction must be complete + reached equilibrium itself before tension within fluid reaches its equilibrium value
Total content of gas in liquid = reacted gas (with for e.g. Hb) + dissolved gas in liquid (e.g. plasma)
What 3 factors is the final gas content at equilibrium in a liquid dependent on?
- Gas tension
- Solubility of gas in liquid
- Affinity of Hb for O2
This occurs in order as equilibria for each reaction must occur in sequence
How does gas exchange occur between the air and the blood?
Via diffusion across the alveolar membrane
Alveolar gas has different composition to atmosphere - less O2 + more CO2 because exchange occurs between alveolar gas + adjacent gases carried in blood
What are the normal partial pressures of atmospheric air + alveolar air?
Atmospheric: pO2 = 21.1 kPa, pCO2 = minimal + vapour pressure depends on humidity
Alveolar: pO2 = 13.3 kPa, pCO2 = 5.3 kPa + saturated with water vapour
What is mixed venous blood? What are the typical partial pressures within it?
Returns to lungs from systemic circulation
pO2 = 6.0 kPa + pCO2 = 6.5 kPa but varies with metabolism
What are the gradients of partial pressure of O2 and CO2 at the alveolar membrane + how does this influence diffusion of gases?
pO2 in alveolar gas > pO2 in returning mixed venous blood
pCO2 in alveolar gas < pCO2 in returning mixed venous blood
= O2 will diffuse into blood + CO2 will diffuse out
What is proportional + inversely proportional to diffusion?
P: SA of alveoli + gradient down which individual gas diffuses caused by partial pressure differences
IP: Resistance to diffusion of alveolar capillary membrane to gas (affected in disease states)
What are the 3 factors that affect the diffusion rate of a gas?
- Nature of barrier
- Nature of gas
- Interaction between the gas + barrier i.e. solubility of gas in tissue/fluid
What 5 components make up the diffusion barrier of the alveoli that gases must get through?
- Epithelial cell of alveolus
- Tissue fluid + connective tissue
- Endothelial cell of capillary
- Plasma
- RBC membrane + cytoplasm
(a lot can therefore go wrong here to resist/limit diffusion rate)
In terms of the nature of the gas, how does this affect the diffusion rate?
Gases diffuse at rate inversely proportional to square root of their MW - big MW = slower diffusion so CO2 diffuses a little slower than O2
In terms of gas solubility, how does this affect diffusion rate?
Gases diffuse at rate proportional to solubility - CO2 has higher tissue solubility than O2 so diffuses 21x faster overall even though O2 is smaller
Define solubility.
The amount of solvent that can dissolve in unit volume of solute e.g. ml of O2/L of water
Why is O2 transfer to tissues perfusion limited under normal circumstances? What can happen in disease states?
As O2 has fully saturated Hb by the time it is 25% along the capillary length
However, in disease states, transfer may become DIFFUSION LIMITED with full equilibration not occurring by the end of the capillary
What is the equation for the diffusion rate (DR)?
DR = A/TD X (P1-P2)
A = Area
T = Thickness
D = diffusion constant = Solubility (Sol)/√MW
P - Partial pressure
