10. CP Gas Exchange

Question 1

What is the average value for perfusion?

Answer 1

5L / min

Question 2

What disease can increase the distance factor in the perfusion equation?

What does this do to perfusion?

Answer 2

Interstitial lung disease

Question 3

Is the diffusion coefficient for CO2 higher or lower than O2?

Answer 3

CO2’s diffusion coefficient is much higher (20x) than that of O2

Question 4

What is the pressure difference between alveolar O2 and the O2 in the capillaries?

Answer 4

60 mmHg

Question 5

Why do you see exercise intolerance in lung disease before you see resting hypoxia?

Answer 5

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.

Question 6

What is the average diffusion capacity of the lung for oxygen?

Answer 6

21ml O2 / min / mmHg at rest

Question 7

What is the correction factor for the diffusion constant of O2 vs that of CO?

Answer 7

DL_O2 = 1.23 DL_CO

Question 8

What is the purpose of surfactant?

How does it account for alveoli of different sizes?

Answer 8

Reduces surface tension to prevent collapse of the alveoli

It reduces tension in small alveoli more than in larger alveoli

Question 9

What is surfactant made of?

Answer 9

A phospholipid (Dipalmitoylphosphatidylcholine)

Surfactant proteins (SPA, SPB, SPC, SPD)

Question 10

Describe how the resistance of the airways is controlled and the consequences of the changes in airway resistance

Answer 10

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

Question 11

The purpose of the lungs is two fold:

Answer 11

perfusion (bringing blood)

ventilation (bringing air)

so that they can mix and exchange

Question 12

How do we calculate alveolar ventilation?

Answer 12

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

Question 13

How do we understand diffusion of gases?

Answer 13

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

Question 14

What is J under normal resting conditions

for O2

for CO2

Answer 14

O2=250ml

CO2=200ml

Question 15

Which factors of the diffusion equation depend directly on the structure of the alveolus?

Answer 15

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)

Question 16

What does the Diffusion coeffecient depend on in the “J” equation?

Answer 16

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

Question 17

how does the Pressure Gradient relate to the “J” equation?

Answer 17

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

Question 18

When a RBC is in the pulmonary capillary at rest, how long does it need to reach equilibrium?

Answer 18

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

Question 19

During exercise, the RBC spends about .25 sec in the capillary. What happens?

Answer 19

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

Question 20

What is the DLO2?

Answer 20

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

Question 21

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?

Answer 21

A person has to have extensive lung dz before they start to retain CO2

DLCO2 at rest is about 400ml

Question 22

Two key points to keep in mind on DLO2

Answer 22

DLCO2>>>>>DLO2

DLO2 = 1.23 x DLCO

Question 23

What is surface tension and how does surfactant and LaPLace’s law play a role in keeping the lungs from collapsing?

Answer 23

• 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

Question 24

How does surfactant affect surface tension?

Answer 24

reduces T in the smallest alveoli more than in larger ones

exact mech. is not known

Pressure in alv. = 2T/r