10. CP Gas Exchange Flashcards
What is the average value for perfusion?
5L / min
What disease can increase the distance factor in the perfusion equation?
What does this do to perfusion?
Interstitial lung disease
Is the diffusion coefficient for CO2 higher or lower than O2?
CO2’s diffusion coefficient is much higher (20x) than that of O2
What is the pressure difference between alveolar O2 and the O2 in the capillaries?
60 mmHg
Why do you see exercise intolerance in lung disease before you see resting hypoxia?
Because a given RBC spends exactly as much time in the lungs during exercise as it needs to pick up its full complement of oxygen and no more. Any damage to the perfusion means that the RBC cannot fully absorb O2 in the lungs before returning to the periphery.
What is the average diffusion capacity of the lung for oxygen?
21ml O2 / min / mmHg at rest
What is the correction factor for the diffusion constant of O2 vs that of CO?
DLO2 = 1.23 DLCO
What is the purpose of surfactant?
How does it account for alveoli of different sizes?
Reduces surface tension to prevent collapse of the alveoli
It reduces tension in small alveoli more than in larger alveoli
What is surfactant made of?
A phospholipid (Dipalmitoylphosphatidylcholine)
Surfactant proteins (SPA, SPB, SPC, SPD)
Describe how the resistance of the airways is controlled and the consequences of the changes in airway resistance
R=8nl/r^4
changing the radius of the airways has the greatest impact on the resistance to airflow
the radius can be changed by contracting/relaxing smooth muscles
can be used to direct air to alveoli with lots of blood
The purpose of the lungs is two fold:
perfusion (bringing blood)
ventilation (bringing air)
so that they can mix and exchange
How do we calculate alveolar ventilation?
volume of air reaching alveoli
if per minute volume: Va(dot) = Va x F (F being breaths per min, Va being alveolar ventilation)
avg. is 4L/min
How do we understand diffusion of gases?
J=SA x D x (P1-P2) / distance
J= diffusion rate
D=diffusion coefficient for each gas
P1-P2= pressure gradient across alveolar membrane
SA= surface area available
distance- diffusion distance (thickness of alv. barrier)
*gases are done separately
What is J under normal resting conditions
for O2
for CO2
O2=250ml
CO2=200ml
Which factors of the diffusion equation depend directly on the structure of the alveolus?
SA and distance
SA-surrface area available for diffusion, corresponding with # of alveoli and capillaries in the lungs as well as how many of the capillaries are “open” (more duringe exercise than at rest)
As SA increases, J increases (inverse is true, seen in COPD)
Distance is the thickness between alveoli (fluid layer, epithelium, instl. space, bv wall)
As distance increases, J decreases (seen in age with collagen deposition or in interst. lung disease)
What does the Diffusion coeffecient depend on in the “J” equation?
depends on the solubility of the gas in water (in the body) and molecular weight of the molecule.
O2 is less soluble in water than CO2
CO2 weighs more than O2
DCO2 is 20x the DO2
how does the Pressure Gradient relate to the “J” equation?
pressure gradient across the alveolar membrane
have to consider each gas separately
PaO2 ~100mmHg
PvO2 ~ 40mmHg
P1-P2
~60mmHg within alveolar space
do same with Co2
~ -5mmHg within alveolar space
When a RBC is in the pulmonary capillary at rest, how long does it need to reach equilibrium?
it needs 0.25seconds but it is present in the capillary for about .75 seconds
there is a large “safety” net of time before the RBC leaves the capillary
this changes during exercise
During exercise, the RBC spends about .25 sec in the capillary. What happens?
This is just enough time to pick up full load of O2
however, someone with lung dz will notice dyspnea on exertion before any other sx because of this process
What is the DLO2?
Diffusion capacity of the lung for oxygen
about 21 ml/o2/min/mmhg at rest
how is this tested?
CO is inhaled (tiny bit) in a single breath of air and can be calculated
DL)2=1.23 x DLCO
When it comes to CO2, the RBC in the pulmonary capillary equilibrates almost immediately. This leaves a huge (.75ish) seconds of margin. What happens if there are lung issues related to this?
A person has to have extensive lung dz before they start to retain CO2
DLCO2 at rest is about 400ml
Two key points to keep in mind on DLO2
DLCO2>>>>>DLO2
DLO2 = 1.23 x DLCO
What is surface tension and how does surfactant and LaPLace’s law play a role in keeping the lungs from collapsing?
- tension created when air molecules come into contact with water and water wants to clump together
- surface tension causes the pressure (=2T/r) in small alveoli to be greater than in large alveoli, so the small alveoli collapse into bigger ones
- this decreases sruface area available for diffusion
- surfactant decrease the surface tension in the smaller alveoli, reducing the surface tension more than big alveoli
How does surfactant affect surface tension?
reduces T in the smallest alveoli more than in larger ones
exact mech. is not known
Pressure in alv. = 2T/r
