Lecture 9/10: Gas Exchange Flashcards
What is the point of the lungs?
The point of the lungs is to bring ventilation and perfusion together.
Does the anatomic deadspace participate in gas exchange?
No.
What do conducting airways do?
- Humidify the air 2. Conduct air
How can we alter airway resistance?
Bronchoconstrict Bronchodilate.
If resistance is ____, airflow slows down and takes more muscle effort to do.
high
If resistance is ____, airflow is fast and easy.
low
What controls the airway resistance?
Resistance
Directly related to:
- viscosity of whatever
- length of the tube -
Inversely related to:
- Radius of the tube (4)
R= 8nL/r4
Changes in _____ can have the biggest effects on resistance?
Radius of the tube
What controls the dilation and constriction of our bronchioles?
Smooth muscle
Why do we want to change airway resistance?
So that we can send air to alveoli with a good blood supply
What is alveolar ventilation?
The amount of air that reaches the alveoli.
Va= TD-VDS
=tidal volume - volume of the dead space
Minute alveolar ventilation
What is the average?
Amount of air that reaches the alveoli per minute
Va*Frequency
4L/ minute is the average.
What is perfusion (Q)?
Perfusion (Q) is the amount of blood delivered to the tissue/min.
Average is 5L blood/minute.
Alveolar gas exchange depends on what?
J= SA*D* (P1-P2)/distance
Diffusion of gases:
Increases as surface area, diffusion coefficient and pressure gradient increases.
Decreases as the thickness of the wall increases.
Are gas exchange of O2 and CO2 dependent on one another?
No.
They are independent of one another.
In the lungs, does O2 trade places with CO2?
NOOOO.
How much gas is exchanged (J) every minute?
Of O2 and CO2
250 mL of O2 every minute.
200 mL of CO2 every minute.
thus, they are not equal and not dependent on one another in the LUNGS. In the tissue, yes; they are.
What factors the affect the diffusion of gas directly involve the structure of the alveolus?
1. Surface area
2. Distance (thickness of the alveolar wall)
They can be altered due to pathology.
What is surface area referring to?
It corresponds with the number of alveoli in the lungs and the number of “open” pulmonary capillaries.
Pt presents with COPD. What is causing this?
A decrease in surface area of the alveoli.
What does the thickness of the alveolar barrier include?
1. Fluid layer
2. Alveolar epithelium
3. Interstitital space
4. Wall of the BV
Usually, how big is the alveolar barrier?
0.6 microns.
Very small.
Does diffusion distance change in normal conditions? Support your answer.
No, it does not.
It does change in pathological conditions.
For example, people who have interstitial lung disease have a increase in diffusion distance because collagen is depositied in the interstitial space.
What does the diffusion coefficient (D) for each gas depend on?
- Solubility of gas in water (CO2 is more soluble in water than O2)
- Molecular weight of the gas (CO2 weighs more than O2)
Which moves slower: CO2 or O2?
CO2.
Its solubility counters its molecular weight, causing its diffusion coefficient to be 20x higher than O2.
Venous pressure of O2 and CO2 at the lungs:
PvO2= 40 mmHg
PvCO2= 45 mmHg
Partial pressure of O2 and CO2 in the alveoli
PAO2= 104 mmHg
PACO2= 40 mmHg
Pressure gradient of O2 at the lungs
P1-P2=
104-40
=60mmHg
Thus, this favors the flow of oxygen into the capillaries,
O2 needs a big pressure gradient because the diffusion coefficient is soooo low.
Pressure gradient of CO2 at the lungs
P1-P2=
40-45
=-5 mmHg
What happens during oxygen exchange at the alveolar capillary?
At the alveolar capillar, venous pressure of O2 is about 40 mmHg. Once the blood leaves the lungs (in the pulmonary vein) PO2 will be about 100 mmHg.
Every RBC spends 0.75 seconds in a pulmonary capillary. It only takes .25 of a second for oxygen to go from 40 mmHg–> 100 mmHg. Thus, iif something occured, the RBC would have a 0.5second safety margin in the capillary to reach equillibrium, before the RBC would leave.
How does heavy excercise affect oxygen exchange at the alveoli?
However, during heavy excercise, a RBC spends 0.25 seconds in a pulmonary capillary (LESS time than a normal person)
Thus, there is no safety margin and if something is wrong with the patient, the RBC may not spend enough time in the capillary to come to equillibrium.
Thus, someone with lung disease will have difficulty working out.
Will someone with lung disease have a difficult time doing normal activities?
No. Because the RBC in the alveolar capillary long enough to pick up one load of oxygen, allowing them to do normal things,
How do we determine the diffusion capacity of the lung for oxygen (DLO2)?
It is 21 mmO2/min/mmHg.
It decreases as we go along the capillary because oxygen enters our capillaries.
How can we measure the diffusion capacity in the lung for oxygen?
We can use small [] of carbon monoxide (CO).
CO binds to Hb so tightly that it does not allow O2 to bind. Thus, the arterial Paco is 0mm Hg.
Have the person breath in a single breath of air with a small amount of CO. We know the amount of CO inhaled. Measure CO that is in the breath, when they exhale. The difference is what diffused across the lung.
There is a correction factor
DLO2= 1.23 * DLCO
Recognize what is normal or low. If low, SA is decreased, diffusion distance increased or the gradient is fucked up.
What happens during CO2 exchange at the lungs?
PCO2 in the pulmary capillaries is 45mmHg.
PCO2 in the alveoli is 40 mmHg.
PCO2 will come to equillibrium almost instantly. That being said, we have a HUGH “safety” margin or time before it leaves the RBC. Therefore, a person has to have an extensive lung disease before they have a problem with CO2 retention.
What is DLCO2 at rest
At rest, DLCO2 at rest is aboout ~400ml CO2/min/mmHg.
This is the 20 fold difference we were talking about!
How does the diffusion capacity for the lung for CO2 differ from O2?
DLCO2 >>>>>>>> DLO2
How do we figure out DLO2?
DLO2= 1.23 DLCO
What is a consequence of the diffusion capacities of CO2 and O2?
A patient with lung disease will notice limitition to exercise/exertion first because their need for oxygen will exceed the ability of the lungs to allow for diffusion of oxygen.
Change in PaO2 will occur earlier in the disease process because of its limitations to oxygen diffusion.
What makes surfactant?
Type 2 pneumocytes
Where does surface tension occur?
Air/water interface
Note that the tissue is water.
What does LaPlace’s law say?
Pressure= 2T/r
T=tension
r= radius
Alveoli are different sizes. In a large alveolus, the radius is large. Thus, what is the pressure?
Low.
Alveoli are different sizes. In a small alveolus, the radius is small. Thus, what is the pressure?
High.
Are the differences in pressure between alveoli bad, if there is no surfactant?
Yes. this would not be a problem if all alveoli were the same size. But they are not.
What happens due to the difference in alveoli sizes if surfactant is absent?
Big alveolus=big radius= small pressure
Little alveolus=little radius= high pressure.
They share the same airway to air moves from high–> low pressure. This is bad because the small alveoli gets smaller and the big alveoli gets bigger. No good news for the SA.
As a result, we have poor lung inflation and our alveoli collapse.
What will happen to a baby with collapsed alveoli?
Cyanotic
Need to be put on 40% O2.
WHAT DOES SURFACTANT DO?
Surfactant reduces surface tension in proportion to radius, decreasing the pressure in the small alveoli
*** more so in the smallest aveoli than in the larger ones.
What is surfactant made up of?
- Phospholipids
- Dipalitorylphosphatidylcholine
- Surfactant protein- A, B*, C, D
B is the most important.
Surfactant is stored in _______ but secreted into ______
intracellular lamellar bodies
alveolus
Small alveoli + surfactant=
Little tension
Little radius
=SMALL PRESSURE
Big alveolus=
big radius= small pressure.
Thus, surfactant eliminates the gradient for air to move down. and prevents the collapse.
Thus, little and big alveoli can exist peacefully.
