gas exchange in lungs Flashcards
Why is pressure used rather than concentraion when quantifying gases?
2 main reasons?
This is because gases react, dissolve, and diffuse more in accordance with their pressure than concentration.
This is because pressure takes into account other factors that affect the properties/behaviour of a gas, such as temperature.
what is partial pressure?
partial pressure describes the degree to which an individual gas in a mixture of gases contribute to total pressure
total partial pressure calculation
what do you take into account?
ππππ‘ππ = π(π»2 π) + βπ(πΆπππ π‘ππ‘π’πππ‘ πππ ππ )
partial pressure of individual gas
What do you take into account?
How do you calculate it?
ππππ = (ππππππππ‘πππ β π(π»2 π)) Γ π_πππ
Partial pressures for gas phases can be calculated by subtracting water vapour pressure (which also contributes to total pressure and varies from 0kPa in dry air to approximately 6kPa in fully warmed, humidified air within the lung) from the total pressure (atmospheric) , and then multiplying the resulting value by the individual gasβ mole fraction
(the fraction of total moles represented by the individual gas, e.g. oxygen at sea level = 0.21 or 21%).
water vapour pressure
contributes to total pressure and varies from 0kPa in dry air to approximately 6kPa in fully warmed, humidified air within the lung
what is PO2 in alveolar (humidified) air at sea level, if PBarometric = 100kPa?
(100 - 6) x 0.21 = 19.74
What happens if barometric pressure decreases (e.g. altitude)
Less total/atmospheric pressure but same mole fraction of o2 hence less o2 in the air
What determines how much gas dissolves in a liquid?
equation?
what does the partial pressure of gas dissolved in a liquid reflect?
The concentration of a gas dissolved within a liquid is determined by the partial pressure and solubility of the gas:
πΆππππππ‘πππ‘πππ=ππππ‘πππ ππππ π π’ππ Γππππ’πππππ‘π¦
The partial pressure of gas dissolved in a liquid reflects the amount of gas that would dissolve (at equilibrium) if the liquid was placed in contact with a gas phase of equivalent partial pressure.
why canβt partial pressures for different gases in liquid be compared?
comparisons cannot be made between different gases (e.g. oxygen and carbon dioxide) as the solubility will vary.
co2 solubility - 5.0 mL.L-1.kPa-1
if PaCO2 = 5.0 kPa, the concentration of CO2 dissolved in the plasma will be
carbon dioxide has a water solubility of approximately 5.0 mL.L-1.kPa-1.
This means there will be 5.0 mL of CO2 per litre of blood for each kPa of pressure.
Therefore, if PaCO2 = 5.0 kPa, the concentration of CO2 dissolved in the plasma will be:
[CO2] = 5.0 mL.L-1.kPa-1 x 5 kPa = 25 mL
properties of alveoli to allow good gas exchange to occur
3 factors
a large surface area (both individually and cumulatively),
thin outer structure (typically one cell thick),
richly innervated by capillaries
structures and mediums blood gases need to bypass
what is the jounrey of o2 to transporter?
1) O2 enters the alveolar airspace from the atmosphere.
2) O2 dissolves in ALF. (alveolar lining fluid)
3) O2 diffuses through alveolar epithelium, basement membrane, & capillary endothelial cells.
4) O2 dissolves in blood plasma
5) O2 binds Hb molecule
any kind of change in these structures like depth or consistency will affect rate
why must diffusion be rapid to allow adequeate oxygenation of blood?
how long does it take for rbc to pass capillary in tyoical conditions?
how does this chnage during exercise?
In typical conditions, it takes approximately 0.75 seconds for a red blood cell to pass through a pulmonary capillary, during which time oxygenation must occur.
During intensive exercise, where pulmonary blood flow is increased, this time may even decrease to as little as 0.25 seconds - not an issue with healthy individuals
consequence of thickening of blood gas barrier and further problems during exercise
reduce diffuision rate hence the rate of gas exchnage + rate at which o2 is oxygenated so by 3/4 secs, full oxidation doesnβt take place
problem is further worsened when blood flow is increased -> during exercise, CO increaes and rbc flow through pulmonary capillaries at increased speed hence takes 0.25s than 0.75s to flow through which means a lot less diffusion takes place
rate of diffusion equation for gas exchange
equation?
π ππ‘π ππ πππππ’π πππβ(ππ’πππππ ππππ)/γπ·ππ π‘ππππγ^2 Γ (π_π΄ β π_πΆ)
The rate of diffusion (on which gas exchange depends) is determined by the partial pressure gradient between the two areas, the size of the diffusion distance, and the surface area.
defects in partial pressure gradient
pathology and problems with it?
hypoventilation (type II respiratory failure)
donβt breathe in a sufficient rate to bring fresh air into body + cope with demands
therefore decreased pressure gradient
defects in surface area
pathology?
emphysema will reduce SA
defects in distance hence barrier thickness
2 pathology?
Fibrosis = β basement membrane thickness (scarring)
Pulmonary oedema (e.g. pneumonia) = β thickness of fluid layer/oedema
The role of ventilation in determining the level of gas exchange
what is changed and what for?
changing the rate of alveolar ventilation (the volume of fresh air reaching alveoli per unit of time), in order to modulate the partial pressure gradients between the alveoli and blood
to meet the o2/co2 demands of the body
ventilation/perfusion ratio
why does this need to be matched?
Blood flow through pulmonary capillaries (perfusion, πΜ) needs to be matched to alveolar ventilation (πΜ_π΄) to enable efficient gas exchange, as there is a maximum amount of O2 each unit of blood can carry.
v/q ratios
patholgies of over and under 1?
v/q cause
>1 hypoperfusion (βdead space effectβ)
=1 normal
<1 hypoventilation (βshuntβ)
How is v/q mismatch reduced?
what maintains ventilation-perfusion coupling?
how does it do this? detector and effect?
what is the net effect?
Ventilation-perfusion coupling is maintained by hypoxic vasoconstriction which diverts blood flow from poor to well ventilated alveoli
the way it does this is that within the wall of endothelium, there are receptors that can detect or respond to level of o2, relatively when it becomes hypoxic (less o2)
in response to less o2, the smooth muscle in the wall of the capillary contracts and vasoconstricts hence it is harder for blood to flow through (increased R)
as it is harder for blood to flow through that vessel, blood is diverted to a different vessel that isnβt constricted and this is because, this new vessel hasnβt been exposed to hypoxia so wonβt be constricted
the net effetc is that blood is diverted from hypoxic alveoli to well ventilated alveoli therefore help match ventilation to perfusion (VQ coupling)
how does v/q ineqality affect co2 and o2?
In theory, V-Q inequality affects both O2 and CO2 exchange, however in most cases, βPaCO2 will induce a reflex hyperventilation that clears the excess CO2 (but doesnβt βPaO2) because of v:q mismatch)
v/q mismatch doesnt refer to chnages in overall level of ventilation or perfusion to the lungs
not necessarily referring to changes in the overall level of ventilation or perfusion to the lungs, but rather situations where V/Q ratios vary substantially between alveolar units
