lecture 16 Flashcards
Exam 3 content
Does venous blood have more or less bicarb than arterial blood?
Less bicarb
what is the CO2 content per dL of arterial blood if PCO2 is 40mmHg?
48mL of CO2/ dL of blood
What is the CO2 content per dL of venous blood if PCO2 is 40mmHg?
52.5 mL of CO2/ dL of blood
The difference between the CO2 content in venous blood and arterial blood corresponds to what?
52.5mL - 48mL = 4.5mL corresponds to the amount of CO2 dropped off at the lungs
Recall that 5mL of O2 is picked up by the lungs
Deoxyhemoglobin is different from oxyhemoglobin with regards to CO2 transport how? What is the name for this?
There is more room for CO2 to bind in deoxyhemoglobin than oxyhemoglobin.
This is called the Haldane Effect.
Deoxyhemoglobin is more willing to accept a proton than oxyhemoglobin, what does that make deoxyhemoglobin?
a weak acid (a good buffer!)
How long does it take for gas exchange to occur in the pulmonary capillaries?
It takes 0.25 sec under normal conditions
How long does the blood hang out in the pulmonary capillary of the alveolus?
about 0.75 sec under resting conditions
If we increase our CO, what happens to the blood in the pulmonary capillary? Does this pose a problem for gas exchange?
blood speeds up d/t high CO and the blood only hangs out in the pulmonary capillary for 0.25 sec. This isn’t a problem for gas exchange unless the person has unhealthy lungs.
What gas equilibrates between the pulmonary capillary blood and the alveolus faster than oxygen? Why?
nitrous oxide, because it is less soluble than oxygen (onsets and offsets quickly)
What gas is used as a diagnostic gas to mirror oxygen in measuring the diffusing capacity of the lungs?
carbon monoxide
What is a perfusion-limited gas?
A gas that has equilibration between the air in the alveolus and the blood in the pulmonary capillary –> oxygen (oxygen absorption depends on how much blood is moving through the lungs)
What is a diffusion-limited gas?
No equilibration between the pulmonary capillary blood and the alveolar air. Oxygen absorption is limited by the rate of diffusion.
Give an example of a diffusion-limited gas…
CO
Example of perfusion-limited gasses…
N2O and O2
According to Fick’s Law what is directly proportional to how fast gas can move across a barrier?
surface area, diffusivity, and delta P
What is diffusivity?
D ~ solubility / square root (molecular weight)
According to Fick’s Law what is inversely proportional to how fast gas can move across a barrier?
thickness of the barrier
How much more soluble is CO2 than O2? Which gas diffuses faster?
CO2 is 24x more soluble than O2, so even though it is larger than O2 it will diffuse faster (it’s diffusivity is 20x O2)
What is a normal V/Q ratio in a healthy person?
4200/5000mL (0.8)
If your patient has a V/Q ratio greater than 0.9, what does this indicate?
a perfusion problem (increased alveolar dead space)
If your patient has a V/Q ratio less than 0.8, what does this indicate?
a ventilation problem (shunting)
The V/Q ratio is higher at the____ of the lung.
The V/Q ratio is lower at the_____ of the lung.
Where is there more O2 coming from? CO2 from?
Higher V/Q: apex (top) (more O2 and less CO2)
Lower V/Q: base (more CO2 and less O2)
What happens to V/Q when you give someone anesthesia? How can you overcome this?
atelectasis occurs, and small airways collapse. Give PEEP!
What does LaPlace’s Law state?
air is going to go from a sphere with a smaller radius to the sphere with a larger radius. If these two spheres share a common airway, air will go to the sphere with a larger radius (where there is less pressure)
Does LaPlace’s Law hold up with normal healthy lungs? Why?
No, because of surfactant! (There is relatively more surfactant in the smaller sphere if the larger sphere is more stretched out)
When do we see uneven ventilation (what LaPlace would predict)?
any condition where there is a deficiency in surfactant (according to Schmidt this is every lung disorder ever studied)
How much anatomical dead space do we have?
1mL per pound of IBW
What is PEO2 or PMEO2?
PO2 of expired air or mixed expired air. This air comes from the lungs, alveoli and dead space.
What are the partial pressures of the different gasses in mixed expired air at standard barometric pressure of a 500cc TV?
Know how to calculate PEO2 and PECO2…
PEO2: 120mmHg
PECO2: 27mmHg
PEN2: 566mmHg
PH2O: 47mmHg
How do you figure a PEO2 of 120mmHg in mixed expired air?
150 mL of dead space air (PO2 of 150mmHg)
350 mL of alveolar air (PO2 of 100mmHg)
How do you calculated PECO2? PEO2?
partial pressure of CO2 = [CO2] x total pressure
to find the concentration. Then multiply the concentration by the volume of the mixed expired air breath (500mL usually or TV)
The total pressure is 760 mmHg (if at sea level/ 1 atm)
PEO2 is the same using the partial pressure and concentration of O2
