Alveolar Gas Exchange Flashcards
What is the point of the lungs?
To bring ventilation and perfusion together
Alveolus do this!
What is the anatomic dead space?
The airways not designed to participate in gas exchange
What’s happening physiologically in the conducting airways?
Air Flow!
What does bronchiole diameter control?
Airway resistance
What does airways resistance do?
Makes airflow more difficult
If airway resistance is High, airflow slows down, and takes more muscle effort to produce
If airway resistance is Low, airflow is fast and easy
What controls the airway resistance?
R = 8nL/r^4
Air viscosity doesn’t change to affect R
radius of tube is biggest determinant
Radius changes due to contracting/relaxing SMOOTH MUSCLE in airway walls
Why do we want to change the airway resistance?
Want to send air in lungs to the “right” places
For now, that means alveoli that have a good BLOOD SUPPLY
What are examples of airway obstruction diseases?
Chronic Dyspnea
Asthma
COPD
Bronchiectasis
If there is a High AA Gradient what are the ventilation/perfusion mismatch diseases?
Hypoxemia
Airway Disease (Asthma, COPD)
Vascular (PE)
Parenchymal Disease
What is Alveolar Ventilation?
volume of air reaching the alveoli
if per minute: VA(dot) = VA x F
4 L/min is average value
What is Perfusion (Q)?
Blood flow from the right ventricle to lungs
5 L blood/min
What is the equation for diffusion rate?
J = [(SA) x D x (P1-P2)] / distance
J: diffusion rate in ml/min
D: Diffusion coefficient for each gas (O2 and CO2)
P1-P2: Pressure gradient across alveolar membrane
SA: Surface area available for diffusion
distance: Diffusion distance (thickness of alveolar barrier)
This equation is solved for each gas individually
What is “J” (gas exchange per minute) under normal resting conditions?
250 ml O2
200 ml CO2
*They are NOT equal!
Are the diffusion of each gas (O2 and CO2) dependent on each other?
NO
They are independent of one another
What factors for diffusion rate are directly dependent on the structure of the alveoli?
the Surface Area and Distance (thickness of alveolar barrier)
What does SA correspond to?
What else affects it?
The number of alveoli in the lungs / SA available for diffusion
As SA increases, J increases.
SA also depends on the number of “open” pulmonary capillaries
- number varies with demand
~70 ml of blood in pulmonary capillaries at rest
as much as 200 ml during exercise
What does a cross section of normal lung look like in terms of alveoli?
Millions of small alveoli
if laid out in single layer it would be 70 sq meters (size of tennis court)
What does a cross section of COPD/emphysema look like?
What do these patients have a hard time doing?
Large alveoli, large holes
Patients have a hard time getting sufficient oxygen into their system
SA is decreased!
What are some diseases associated with abnormal alveoli with chronic dyspnea?
pneumonia
ARDS
COPD
Neoplasm
What does the distance include in terms of the thickness of alveolar barrier in determining J?
What is the average size?
includes:
- fluid layer
- alveolar epithelium
- interstitial space
- blood vessel wall
average = 0.6 microns
What can start to deposit in interstitial spaces and what is the effect?
What is it associated with?
Collagen can deposit
Increases the diffusion distance
Decreases diffusion of gases across barrier
Associated with interstitial lung diseases
What are the interstitium lung diseases associated with chronic dyspnea?
ILD
CHF
Sarcoidosis
What does the diffusion coefficient for each gas depend on?
The solubility of the gas in water
The molecular weight of the gas
Is O2 or CO2 more soluble in water?
CO2 is more soluble
O2 is less soluble in water
Does CO2 or O2 have a greater molecular weight?
CO2 weighs more than O2
This is a major advantage to oxygen
Does CO2 or O2 have a greater diffusion coefficient?
By how much?
Dco2 is 20X the Do2
The solubility of CO2 more than counters the difference in molecular weight.
CO2 diffuses much faster! Can cross even when it is very difficult for O2 to cross.
What is the pressure gradient of O2 and what direction does it favor the gas moving in?
At the start of capillary:
PAo2 aka P1 = 104 mm Hg
Pvo2 aka P2 = 40 mm Hg
a - alveolar space
v = vascular / capillary
Gradient = 104 - 40 = ~60 mm Hg
Favors moving INTO capillary
- as travel length of capillary the pressure gradient gets smaller
What is the pressure gradient of CO2 and what direction does it favor?
PACO2 = 40 mm Hg PvCO2 = 45 mm Hg
40 - 45 = -5 mm Hg
Favors moving OUT of capillary
How long does a RBC spend in the pulmonary capillary under resting conditions?
0.75 seconds
How long does O2 require to be in the pulmonary capillary to reach equilibrium?
- 25 seconds
* There is also a perfusion limit based on how much blood there is to take the air away
How long does a RBS spend in a pulmonary capillary during exercise?
0.25 seconds
So JUST enough time to pick up the “full load” of oxygen
Why will someone with lung disease notice problems during exercise first?
Because during exercise the RBC only spends 0.25 seconds in the pulmonary capillary which is just enough time for O2 to reach equilibrium and be picked up by the RBC.
So if there is any type of problem in the lungs the RBC might not be able to pick up the full load of oxygen in such short amount of time.
During resting, the RBC spends 0.75 in the cap so it has extra time to pick up oxygen which allows for a problem to be masked and RBC to still get the full load.
Changes in PaO2 will occur earlier in the disease due to limitation in oxygen diffusion.
What is the diffusion capacity of a lung for oxygen (DLo2) in a normal person?
21 ml O2/min/mm HG
This is the average
This is less than the calculated 60 mmHg which is the maximum.
Why do we use carbon monoxide to measure DLo2 (diffusion capacity)?
What is the correction factor?
CO binds to Hb so avidly that it doesn’t dissolve in plasma. The Paco is 0 mmHg
You have patient inhale single breath of air with small percentage CO added.
Use correction factor: DLo2 = 1.23 x DLco
How long does CO2 take to reach equilibrium in a pulmonary capillary? What is DLco2?
Almost immediatly
CO2 is so soluble
DLco2 at rest is ~400 ml CO2/min/mm Hg !!!
What are some examples of diseases caused from diffusion limitations?
hypoxemia
interstitial lung disease
pulmonary arterial hypertension
What is LaPlace’s Law?
Pressure = 2T/r
T= Tension r = radius
Why does surfactant matter?
surface tension!
When water is exposed to air = tension
What is the pressure in a large alveolus?
Since the radius is large the pressure is low
based on P = 2T/r
What is pressure in small alveolus?
Pressure is high since radius is small.
based on P=2T/r
What does the various size in alveoli mean?
Without surfactant
The air would go from high to low pressure so the small alveoli would collapse into the large alveoli and this would reduce SA in the lungs and therefore decrease gas exchange!
How does surfactant combat various alveoli sizes?
It reduces the T in the smallest alveoli more than the larger alveoli
This reduces the P, so even with a small r, the pressure is lower. This equalizes the pressure among various size alveoli so they do not collapse.
Now there is NO gradient for air to move down.
Based on P = 2T/r
What is surfactant composed of?
DPPC = a phospholipid: Dipalmitoylphosphatidylcholine
Multiple proteins: SPB is particularly important for function
Where is surfactant stored?
Intracellular lamellar bodies
What secretes surfactant and into where ?
Type II pneumocytes
into alveolus
Why does surfactant matter?
It allows little and big alveoli to coexist peacefully. Little ones don’t collapse so SA remains high and gas exchange can occur.
