Alveolar ventilation
From Iusmphysiology
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(→Partial pressure of gases) |
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===Partial pressureo of respiratory gases=== | ===Partial pressureo of respiratory gases=== | ||
| - | *Barometric PN = | + | *Barometric PN = 600 mmHg, PO2 = 160 mmHg |
*Brought into lungs | *Brought into lungs | ||
*Saturated with lungs 47 mmHg. | *Saturated with lungs 47 mmHg. | ||
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**So 760-47 * 0.79 = 563 = PN | **So 760-47 * 0.79 = 563 = PN | ||
*Alveolar air | *Alveolar air | ||
| - | **PO2 = | + | **PO2 = 760-47 * 0.21 |
===Definitionns=== | ===Definitionns=== | ||
Current revision as of 03:18, 17 February 2011
- started here on 02/14/11 at 11AM.
[edit] Alveolar ventilation
[edit] Dalton's law of partial pressure
- Partial pressure is pressure exerted by an individual gas in a gaseous mixture
- Dalton's law states total pressure in a gas mixture is equal tothe sum of the partial pressures of all the gases in the mixture.
[edit] Partial pressure of gases
- We need oxygen for life.
- Gases in the air are nitrogen and oxygen.
- Negligible CO2 and argon
- Barometric pressure = air pressure; at sea level = 760 mmHg
- Air: 78% N, 21% O, 0.03 CO2, 0.93% argon
- At Mt. Everest, no change in percentage but decreased pressure.
- Finding partial pressure:
- Take the barometric prssure and multiply by the percentage of the gas.
- At sea: 760 * 0.2 = 160 mmHg = PO2
[edit] Partial pressureo of respiratory gases
- Barometric PN = 600 mmHg, PO2 = 160 mmHg
- Brought into lungs
- Saturated with lungs 47 mmHg.
- So we subtract 47 from 760.
- So 760-47 * 0.79 = 563 = PN
- Alveolar air
- PO2 = 760-47 * 0.21
[edit] Definitionns
- Total (minute) ventilation:
- tidal volume * respiratory rate
- similar to stroke volume (how often moving blood * how much blood moved)
- Usually about 7.5 liters / minute
- Alveolar ventilation:
- (tidal volume - anatomic dead space) * respiratory rate
- Takes into acount the dead space volume
- 5.25 l / minute
- High minute ventilation
[edit] Alveolar ventilation
- PA = partial pressure alveolar
- Pa = partial pressure arterial
- We assume these are equal unless some imparement to diffusion
- If you breath more (alveiolar vent high) you blow off more CO2.
- The higher your ventilation the more CO2 goes out and the lower your PCO2.
- As alveolar vent goes up, more O gomes in so PO2 goes up.
- PaCO2 = VCO2 / Va * K
- Va = alveolar ventilation; volume / minute
- VCO2 = amount of CO2 made by body; volume / minute
- K takes care of the units
- So when hypoventilating
[edit] Alveolar ventilation equation
- Clinically hypoventilation is a measure of PCO2
[edit] Distribution of ventilation
- Ventilation of the lung differs from base to apex.
- Sensors can measure ventilation at apex, middle and base.
- Ventilation is highest at the base and lowest at the apex.
- Gravity pulls lungs down such that complance is lower at apex than base so more air goes tot he base than to the apex.
[edit] Alveolar gas exchange
- Normally, the PaCO2 = PACO2 (unless told otherwise on an exam question).
- Note the values of partial pressures at teach location
Work through this slide
[edit] Partial pressures of alveolar gas
- Partial pressures int he alveoli stay relatively constant.
- During normal tidal volume exchange, about 500 ml are brought in and mixed with 2.5 L of not fresh air.
- So dilution levels off huge fluctuations of partial pressures in the alveoli.
- That would be a "bad day".
- Decrease in PO2 in the alveoli relative to atmosphere is due to three things:
- saturation of water removes some pressure
- inspired air is mixed with FRC
- Recall that the end of the expiration is the same as alveolar partial pressure.
**expire...?
[edit] Alveolar gas equation
- This tells us the determinants of alveolar PO2:
- Atmospheric PO2 (PIO2 = Partial pressure of oxygen in inspired air)
- Level of ventilation (controlled by PaCO2)
- R = respiratory quotient; oxygen consumed and CO2 produced
- Second term gives estimate of overall ventilation
- PIO2 knwon by barometric pressure - the water vapor pressure tiems the fractionn of O2
- R = VCO2 / VO2
- Usually about 0.8 (80%)
- Diet can change this quotient
- Assume 0.8 unless told otherwise
Will definitely have to calculate this on a test
- Normal PACO2 = 40
- Emphysema:
- PACO2 = 64 mmHg
- COPD:
Figure these out.
[edit] Diffusion of gas
- Factors affecting diffusion across membrane: ATDP
- Surface area:
- Usually as much as a tennis cort
- Thickness of membrane:
- Thicker = longer time of difusion
- Diffusion constant:
- Pressure gradient
- Affects spead of diffusion
- Emphysema
- decreases SA
- thicker membrane decreases diffusion
[edit] Diffusion of oxygen
- In mixed venous blood, PO2 = 40
- This is as it enters the alveolar blood.
- PO2 in alveolar is 100.
- So oxygen moves from alveolus to the RBC.
- Within 1/4 of one second, the blood will have equilibriated with the alveolar pressure.
- RBC is usually in the cap bed for 3/4 of a second.
- Lag time is good because during fibrosis, O moves from alveolar air to RBC more slowly.
- During exercise, CO goes up and time in the cap bed goes down to like 1/4 second so that'd convenient.
- Now if alveolar PO2 were 50:
- Equilibriation takes a longer time: 1/2 second.
- This is because the pressure gradient is lower.
- So if something equilibriates quickly, it is called "perfusion limited" or "the amoutn of oxygen delivered to tissue is determined primarily by the overall level of perfusion".
- So there isn't usually a diffusion problem when you're not getting enough O (because the pressure gradient is usually high) but an issue of how much blood perfusionn is occuring.
[edit] Diffusion of CO2
- CO2 is going out, remember.
- PCO2 in mixed venous blood is 46.
- PCO2 in alveolar gas is 40.
- Equilibriates pretty close to O, even though the pressure gradient is much smaller.
- This is becasue CO2 diffuses much faster because it is much more soluble.
- Also, CO2 has to be made from HCO3-.
- CO2 is also considered perfusion limited.
- Note that both CO2 and O2 are ventilation limited, too, but that should go without saying.
[edit] Diffustion limitations
- N2O = nitrous oxide
- Also perfusion limited
- Equilibriates very quickly.
- So amount delivered to tissue is determined by perfusion (which is affected by CO).
- Diffusion limited:
- CO is diffusion limited.
- CO rapidly, quickly, tightly binds to Hb.
- So PCO in plamsa stays low; never equilibriates with PCO in the air.
- So when pressure changes never really changes the only other factor affecting how much is taken in is the diffusion gradient.
[edit] Measurement of diffusion capacity
- CO is unique, so it can be used to measure some diffusion capacity.
- So if you give a small, known amount of CO to the pt, and measure how much comes out, you can calcualte the DLCO = diffusion limitation of CO.
- This will tell you how well CO diffuses.
- This can tell you if the diffusion is the problem.
- Normally about 25 ml / min / mmHg.
- In emphysema goes down because of less surface area.
[edit] P(A-a)O2 Gradient
Will definitely being on the test!
- Clinical measurement of diffusion.
- PaCO2 and PACO2 should be equal if everything is ok.
- So the gradient, when not 13(ish), identifies disease.
- Measure PaO2 and PaCO2 and calcualte the PAO2.
- Normal gradient is no higher than 12-15.
- The gradient can be increased by diffustion impairment, anatomical shunt (venous blood is shunted into blood that does have oxygen), imbalance between ventilation and perfusion.
[edit] Question
- Everything is good
[edit] Question
Know the A-a gradient.
[edit] Test question from last year
- stopped here on 02/14/11 at 12PM.
