Diffusion Flashcards
What variables affect gas exchange ?
- Partial pressure difference of the gas of interest on either side of the membrane
- Thickness of membrane
- Area of membrane being crossed
- Solubility of the gas of interest
- Diffusion constant of the gas of interest
IN comparison to oxygen, CO2
has a higher solubility AND diffusion constant than oxygen
Most of the oxygen carried in the blood is
bound to hemoglobin
Where the cross-sectional area of the bronchial tree the smallest?
What does this mean?
Medium-sized bronchioles
This is where air will be move the fastest (bulk flow) and has the greatest resistance to flow
After passing through the medium sized bronchioles, air slows to a virtual standstill due to the exponential increase in cross-sectional area so that
Diffusion occurs; it is the only force that allows gas concentrations to equilibrate between the blood & alveolar gases
The CO2 gradient from blood to alveolar air: __ –> ___ mmHg
The O2 gradient from alveolar air to blood: __ –> ___ mmHg
CO2: 45 –> 0 mmHg
O2: 100 –> 30 - 40 mmHg
Many disease states will impact what two variables in Fick’s law of diffusion?
May decrease surface area for gas exchange
May thicken the alveolar-capillary membrane
When carbon monoxide (CO) is inhaled, it crosses the alveolar-capillary membrane and what happens?
What is this an example of?
It is taken up by hemoglobin on RBCs so quickly that the pCO in the plasma hardly changes (thus the straight line), making it easier for more CO is diffuse across the alveolar wall.
–> Diffusion limitation: the amt of CO that enters the blood is limited ONLY by the the diffusion properties of the blood-gas barrier - not the amt of blood available.
Nitrous oxide (N2O) is an example of?
N2O diffuses easily across and doesn’t bind hemoglobin –> pN2O in the blood rapidly equilibrates with pN2O in the alveoli –> no more N2O is transferred until new blood flows in.
Perfusion limitation: the amt of N2O taken up by the blood is limited only by the amount of blood perfusing the alveoli
Under normal circumstances, oxygen is __-limited
Perfusion-limited
(Doesn’t diffuse <em>as</em> rapidly as N2O tho)
____ limits our maximal exercise capacity
The heart’s ability to push blood
Becuase oxygen is perfusion limited - the amt of oxygen we’re getting depends on how much blood we can get through the alveoli capillaries
Capillary pO2 virtually reaches that of alveolar gas when the RBC is about ~ 1/3 of the way along the capillary.
On the average, blood spends about ___s in the capillaries under resting conditions.
0.75 s
Even during exercise, 0.75s is more than enough time for hemoglobin to pick up oxygen and for carbon dioxide to diffuse out of the blood into the alveolar air space.
Disease states (Ex. pulmonary edema, thickening of the alveolar-capilary membrane, pneumonia) can cause the equilibration of oxygen to become
diffusion limited, in addition to perfusion-limited
Severe exercise reducing the time available for oxygenation makes this much worse.
At high altitudes, you have alveolar hypoxia: the alveolar air has a lower PO2.
What will happen?
You’ve lost the big pressure gradient difference to drive oxygenation –> rise in PO2 along the capillary slows and failure to reach alveolar PO2 is more likely.
If diffusion is even 1/4 normal, the blood won’t be fully oxygenated by the time it leaves the alveolar capillaries.
Why is CO used to measure DLCO (diffusing capcity of the lungs for carbon monoxide)?
CO never equilibrates with the plasma because it’s so rapidly removed from it by hemoglobin, so the amt of CO in th eblood depends only on how quickly it diffuses across the alveolar capillary barrier.
Thus, if you give a pt CO, the amt that is taken up vs the amt that is exhaled reflects how much successfully diffused across.
Resistance =
R = 1 / D
Where D= diffusibility of the gas
Not only does air have to diffuse across the alveolar capillary membrane, but it also has to get through the plasma to the hemoglobin within the RBC.
What factors limit the diffusion from the alveolar air into the blood?
- Diffusion across the membrane (see Fick’s law)
- Reaction rate with hemoglobin (theta)
- Amount of hemoglobin in blood
Percent hemoglobin saturation (SpO2) is measured using
Partial pressure of oxygen in the plasma is measured using
SpO2, AKA the amt of oxyhemoglobin in blood - Pulse oximeter
PO2 in plasma - arterial blood gas, ABG
The amt of oxygen chemically bound to hemoglobin is ___
The amt of oxygen dissolved in plasma is ___
Add these together to get the total amt of oxygen in blood
Note: giving a hypoxic patient hemoglobin would be more effective than it would be to give them oxygen.
Becuase CO2 is more soluble than oxygen and diffuses more easily across biological membranes, what can you say about its equilibration with alveolar air?
It equilibrates quickly, so rate ventilation will determine the amt of CO2 in alveolar air (more you ventilate, more CO2 removed from blood)
Pulmonary arterial blood (AKA mixed venous blood) as a PCO2 of __ mmHg at the start of the alveolar capillary and __ mmHg about halfway to the end of the alveolar capillary
Drops from 45 at the start to 40 ~halfway through.
This means there’s plenty of time to remove CO2 from the blood if cardiac output increases from exercise or illness.
If CO2 diffusion is impaired
there won’t be enough time for it to be unloaded into the alveolar air, so it will build up in the blood –> clinical probs
Person w pulmonary fibrosis
According to Henry’s law, the amount of dissolved gas is proportional to
its partial pressure in the gas phase
Reductions in hemoglobin has what impact on the diffusion capacity of the lung?
It decreases it!
Explain the limitation of CO2 removal from the blood in the lungs under normal and abnormal (diseased) conditions
- Normal conditions: Blood CO2 equalizes w the alveolar gas well before the blood leaves the capillary (perfusion limitation)
- Diseased: CO2 does not completely equalize with the alveolar gas prior to leaving the capillary (diffusion limitation)
