Decoursey Ventilation and Gas Exchange Flashcards
What are the two types of ventilation
Minute ventilation
Define minute ventiliation
Equation
What does it contribute to?
Amount of air moved in and out the lung in one minute
Ve= Frequencyx Tidal Volume
Contributes to WORK
Define alveolar ventilation
Equation?
What does it contribute to?
total air moved into the respiratory zone per minute
Va= Frequency x (Vtidal-Vdead space)
Contributes to gas exchange.
Explain the two types of dead spae
- Anatomic dead space- air that doesn’t reach alveoli so can’t exchange with blood, its composition is unchanged.
It’s the volume of the conducting airways
- Phsiologic dead space- should be the same as anatomic dead space, but in disease we add the portion of the lungs that doesn’t participate in exchange. It’s a functional measurement
Why does Bohr equation work?
What happens in disease?
We are trying to physiologic dead space. the air entering the lung has ~0CO2 while PCO2 in the alveoli is about 40.
We can measure the ratio of the two from an expired breath (has both because some if from dead space) and that’ll tell you fraction of dead space.
IN disease the ratio would increase because some of the lung acts like dead space
What is the best way to breath to get the high alveolar ventilation
Deep breaths. Dead space is constant so beyond dead space volume the rest is exchanged.
Shallow breaths would proportionally have a lot in the dead space
How can alveolar ventilation change when frequency and tidal volume are constant?
in disease there can be an increase in physiologic dead space so the patient gets less benefit from breathing
Anatomic dead space rule of thumb
It’s really hard to measure but generally the volume in ml is equal to the persons weight in pounds
Equation for work
Work= forcex distance Work= Pressure x volume
What is the a static lung working against?
Dyanmic?
faster dyanmic?
Elastic recoil. Our working lung is actually dynamic.
Dynamic we have airway resistance in addition to elastic recoil
Faster dynamic- we have increased airway resistance because were are forcing air to move faster
What happens to the respiratory muscles in a person with emphysema?
Moderate exercises increases the work done by the lungs to that of a normal marathon runner taking almost all of their respiratory muscle power
At constant alveolar ventilation decreasing frequency does what to tidal volume?
Increases the tidal volume although this increases the dead space (because the airways are stretched more), deep breathing maximizes ventilation alveolar
What are you working hard against at low frequency?
Elastic recoil. Low frequency means the tidal volume is higher so the lungs have to stretch beyond the high compliance region (to the flatter part of the Pv volume curve)
At high frequency what work is high?
Work against airway resistance.
Respiratory muscles have to contract harder to get air in and out faster… need a higher pressure gradient.
turbulece also becomes a factor at high frequency demanding even more work
Summarize effects of low, middle and high frequenecy
Low- high tidal volume- more work
MIddle- minimal work
High- high tubulence/more contraction for faster movement and gradient- more work
You can get the same alveolar ventilation with all three types of frequency but work increases when you move away from the middle breathing (15-20/min)
Why is middle breathing effective
At deep breaths (low frequency) elastic resistance is high but resistance is low
At shallow breaths (high frequency) elastic resistance is low but turbulence and airway resistance is high.
Middle breathing requires the lowest total work against both forces
What is the most efficient way to breath with emphysema? Why?
Less frequently. Getting air out is the problem so we minimize work by breathing with less frequency
Elastic recoil is reduce and airway resistance is increased during expiration due to dynamic compression
Efficient breathing with fibrosis?
Elastic recoil is increased so its harder to inflate lungs so increase frequency and reduce tidal volume
Composition of Earths atmosphere
Nitrogen- 78%
Oxygen- 21%
Argon- 1%
CO2- almost 0
Neon helium krypton Hydrogen and Xenon are in trace amounts
What does Daltons law state
the total pressure exerted by a mxture is equal to the sum of all the pressures that would be exerted if the gases were alone.
Why is partial pressure important?
because it applies to gases in liquid and in gases and gases always go down their pressure gradient
What is the definition of partial pressure
Bolded-
At equilibrium gas dissolves into the liquid from gas at the same rate that gas evaporates from liquid to gas
Explain how to find parial pressure
total pressure x fraction that has occupies
What two factors determine the amount of a gas dissvoled
Solubilty x Partial pressure= Concentration of gas=Henrys Law
Solve for Partial pressure of O2 and Nitrogen
760 x .21= 160 torr= PO2
760x .78= 600 torr= Pnitrogen
Partial pressure is not equivalent to content.. why?
The partial pressure is an indiction of how much gas is present but you have to consider solubility of the gas.
Henry law suggest Concentration= Partial pressure x solubility
Explain what happens to partial pressure throughout the system
The total pressure remains constant throughout the respiratory system. Nitrogen remains fairly constant
CO2- Significantly higher throughout the body then ambient
O2- decreases from ambient to the tissues
H20- ambient (5.7) reaches 47 almost immediately in the trachea
Explain what happens to H20 when it enters the body
- Body is 100% humidity (47 torr water vapor at 37C)
- Ambient air has less (dependent on humidity)
- When we inspire air we humidify the air and water vapordisplaces other gases… mainly Nitrogen but also about 10 torr of O2
Remember we’re staying at 760 total
Why does fog condense when we breath out and it’s cold
When we exhale the breath is warmer and has 47 torr of water vapor and the windshield is much lower temp. As temp of the air decreases and it’s capacity to contain water vapor decreases so it makes fog.
What does the air blood barrier consist of>
Alveolar epithelial cells
CApillary endothlial cells
Interstitial space
All < 1um
This structure is key to diffusion
What Ficks Diffusion Law
Jgas(diffusion)= (Dgas x Areax concentration gradient)/ distance
For gas we have to consider concentration in terms of partial pressure but we need to consider solubility.
So J=D x A X solubilty x delta Partial Pressure/ distance
What type of O2 contributes to the PP?
ONly DISSOLVED.
Once O2 binds to Hb it no longer contributes to the PP values
Dicuss diffusion and equilibrium of O2
This happens almost immediatley and in a normal subject isn’t a problem.
takes blood PO2 about .25 seconds to equilibriate with that of air.
In disease (pulmonary edema etc) equilibrium takes much long or may never happen
What happens in exercise to the diffusion of O2?
CO ouput increases by upto 5x so blood is flowing past the alveolar capillary in about .25 seconds.
Even still in a healthy lung, this is enough time because of recruitment and dilation of pulmonary vessels.
Why do CO2 and O2 equilibriate at about the same rate but have such different pressure gradients?
CO2 gradient is about 5
O2 is about 60
We’re considering the gradient between tissue and alveolar
So O2 tissue- 40 Alveolar- 100
CO2 tissue- 46
Alveolar- 40
But we have to consider solubility. CO2 is 25x more soluble so that makes up for the small driving force.
Why doesn’t alveolar PO2 and PCO2 change dramatically with each breath
There are changes but they are small because the FRC (and even RV) act as buffer that prevent large changes to the alveolar gases
Also staying constant meanst hat there is relatively constant/large pressure gradient
What does Dl measure?
How well diffusion it occurring b/w alevolar air and the blood
Its the rate that a lung takes up O2 corrected for driving force.
Equation for Dl
Dl= Flow of O2/ (PAo2-PvO2)
The driving force DOESN”T determine/change the DL. If you half the gradient then the Flow halves and the DL is constant
Why is CO ideal for testing of how well diffusion is working>
It transfer from air to blood. venous CO is generally zero because it immediately binds to Hb.
Bad for a patient because it has higher affinity than O2 but good for test because nothing is the way of duiffusion
Name a couple things that DON”T affect Dl
breathing faster
Larger tidal volumes
HIgher FiO2
The pressure gradient
Alevolar Gas Equation
PAO2= Pinspired airO2- (Parterial CO2/RQ) + Correction factor
What determines alveolar gas content
- Ventilation (increase O2 and decrease CO2)
- Metabolim (decrease O2 and add CO2)
Balance results in alveolar PO2=100 and PCO2= 40
What determines Dl
Everything in Ficks except the driving force
Why does increasing ventilation affect CO2 more? (metabolism is constant)
Alveolar O2 starts at 100 and CO2 starts at 40. Atms O2 is 150 and CO2 is 0.
When you double ventilation you can half CO2 but it’s impossible to double O2 becuase you’re starting at 100 and the air is only 150.
What does alveolar PO2 fluctuate between
98-100. Very small changes because of FRC
