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CPR Exam 2 > Alveolar Gas Exchange > Flashcards

Alveolar Gas Exchange Flashcards

1
Q

What is the point of the lungs?

A

To bring ventilation and perfusion together

Alveolus do this!

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2
Q

What is the anatomic dead space?

A

The airways not designed to participate in gas exchange

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3
Q

What’s happening physiologically in the conducting airways?

A

Air Flow!

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4
Q

What does bronchiole diameter control?

A

Airway resistance

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5
Q

What does airways resistance do?

A

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

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6
Q

What controls the airway resistance?

A

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

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7
Q

Why do we want to change the airway resistance?

A

Want to send air in lungs to the “right” places

For now, that means alveoli that have a good BLOOD SUPPLY

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8
Q

What are examples of airway obstruction diseases?

A

Chronic Dyspnea

Asthma
COPD
Bronchiectasis

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9
Q

If there is a High AA Gradient what are the ventilation/perfusion mismatch diseases?

A

Hypoxemia

Airway Disease (Asthma, COPD)
Vascular (PE)
Parenchymal Disease

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10
Q

What is Alveolar Ventilation?

A

volume of air reaching the alveoli

if per minute: VA(dot) = VA x F
4 L/min is average value

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11
Q

What is Perfusion (Q)?

A

Blood flow from the right ventricle to lungs

5 L blood/min

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12
Q

What is the equation for diffusion rate?

A

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

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13
Q

What is “J” (gas exchange per minute) under normal resting conditions?

A

250 ml O2
200 ml CO2

*They are NOT equal!

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14
Q

Are the diffusion of each gas (O2 and CO2) dependent on each other?

A

NO

They are independent of one another

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15
Q

What factors for diffusion rate are directly dependent on the structure of the alveoli?

A

the Surface Area and Distance (thickness of alveolar barrier)

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16
Q

What does SA correspond to?

What else affects it?

A

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

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17
Q

What does a cross section of normal lung look like in terms of alveoli?

A

Millions of small alveoli

if laid out in single layer it would be 70 sq meters (size of tennis court)

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18
Q

What does a cross section of COPD/emphysema look like?

What do these patients have a hard time doing?

A

Large alveoli, large holes

Patients have a hard time getting sufficient oxygen into their system

SA is decreased!

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19
Q

What are some diseases associated with abnormal alveoli with chronic dyspnea?

A

pneumonia
ARDS
COPD
Neoplasm

20
Q

What does the distance include in terms of the thickness of alveolar barrier in determining J?

What is the average size?

A

includes:

  • fluid layer
  • alveolar epithelium
  • interstitial space
  • blood vessel wall

average = 0.6 microns

21
Q

What can start to deposit in interstitial spaces and what is the effect?

What is it associated with?

A

Collagen can deposit
Increases the diffusion distance
Decreases diffusion of gases across barrier

Associated with interstitial lung diseases

22
Q

What are the interstitium lung diseases associated with chronic dyspnea?

A

ILD
CHF
Sarcoidosis

23
Q

What does the diffusion coefficient for each gas depend on?

A

The solubility of the gas in water

The molecular weight of the gas

24
Q

Is O2 or CO2 more soluble in water?

A

CO2 is more soluble

O2 is less soluble in water

25
Q

Does CO2 or O2 have a greater molecular weight?

A

CO2 weighs more than O2

This is a major advantage to oxygen

26
Q

Does CO2 or O2 have a greater diffusion coefficient?

By how much?

A

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.

27
Q

What is the pressure gradient of O2 and what direction does it favor the gas moving in?

A

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
28
Q

What is the pressure gradient of CO2 and what direction does it favor?

A 
PACO2 = 40 mm Hg
PvCO2 = 45 mm Hg

40 - 45 = -5 mm Hg

Favors moving OUT of capillary

29
Q

How long does a RBC spend in the pulmonary capillary under resting conditions?

A

0.75 seconds

30
Q

How long does O2 require to be in the pulmonary capillary to reach equilibrium?

A
  1. 25 seconds

* There is also a perfusion limit based on how much blood there is to take the air away

31
Q

How long does a RBS spend in a pulmonary capillary during exercise?

A

0.25 seconds

So JUST enough time to pick up the “full load” of oxygen

32
Q

Why will someone with lung disease notice problems during exercise first?

A

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.

33
Q

What is the diffusion capacity of a lung for oxygen (DLo2) in a normal person?

A

21 ml O2/min/mm HG

This is the average

This is less than the calculated 60 mmHg which is the maximum.

34
Q

Why do we use carbon monoxide to measure DLo2 (diffusion capacity)?

What is the correction factor?

A

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

35
Q

How long does CO2 take to reach equilibrium in a pulmonary capillary? What is DLco2?

A

Almost immediatly
CO2 is so soluble
DLco2 at rest is ~400 ml CO2/min/mm Hg !!!

36
Q

What are some examples of diseases caused from diffusion limitations?

A

hypoxemia

interstitial lung disease
pulmonary arterial hypertension

37
Q

What is LaPlace’s Law?

A

Pressure = 2T/r

T= Tension
r = radius
38
Q

Why does surfactant matter?

A

surface tension!

When water is exposed to air = tension

39
Q

What is the pressure in a large alveolus?

A

Since the radius is large the pressure is low

based on P = 2T/r

40
Q

What is pressure in small alveolus?

A

Pressure is high since radius is small.

based on P=2T/r

41
Q

What does the various size in alveoli mean?

A

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!

42
Q

How does surfactant combat various alveoli sizes?

A

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

43
Q

What is surfactant composed of?

A

DPPC = a phospholipid: Dipalmitoylphosphatidylcholine

Multiple proteins: SPB is particularly important for function

44
Q

Where is surfactant stored?

A

Intracellular lamellar bodies

45
Q

What secretes surfactant and into where ?

A

Type II pneumocytes

into alveolus

46
Q

Why does surfactant matter?

A

It allows little and big alveoli to coexist peacefully. Little ones don’t collapse so SA remains high and gas exchange can occur.

CPR Exam 2 (5 decks)

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