Lecture 9 - Diffusion

Question 1

Even though the pressure gradient is smaller for CO2 than for O2, CO2 diffuses up to 20 times faster. Why?

Answer 1

It has a MUCH greater Solubility

Question 2

N2O and O2 pressures equilibrate rapidly, such that their alveolar pressure rapidly levels out with the capillary pressure (N2O a bit faster than O2). The same is NOT true for CO. Why?`

Answer 2

CO has a MUCH greater binding affinity for Hb, so the bound CO won’t contribute to the partial pressure.

Question 3

N2O is considered to be ______ limited, while CO is ______ limited. In healthy states, O2 is considered ____ limited.

Answer 3

N2O is Perfusion limited –> you need more blood flow for it to continue diffusing bc the blood gets saturated quickly.

CO is Diffusion limited –> Hb sucks it all up, so it can continue diffusing into the blood bc the partial pressure doesn’t build up in the capillary.

O2 is Perfusion limited in healthy state –> same idea as N2O

Question 4

In a healthy state, PO2 takes about ___s to equilibrate between the capillary and the alveolus. An RBC is typically within a capillary for about ____s at rest. Keep in mind when Cardiac Output increases (i.e. during exercise), the time an RBC is in the capillary decreases. Think about how this might affect someone with a disease state that damages/thickens the Alveolar/Capillary divide.

Answer 4

0. 25s
1. 75s

If the Alveolar/Capillary divide is damaged/thickened, it will take longer for PO2 to equilibrate at rest. If CO increases and the time the RBCs remain in the capillaries decreases, PO2 may not equilibrate completely –> O2 saturation will go down.

Question 5

The PO2 of venous blood is typically ___mmHg. The PO2 in the alveoli is typically ___mmHg.

Answer 5

40mmHg

100mmHg

Question 6

Alveolar PCO2 is typically about ____mmHg, while Venous PCO2 is typically ____mmHg at rest. Remember, though this is a much smaller gradient than that of O2, diffusion of CO2 is up to 20 times FASTER bc CO2 has much greater _______.

Answer 6

40mmHg

45mmHg

Solubility

Question 7

Diffusing capacity of the lung is measured using which gas?

The gas is inhaled –> patient holds breath for 10s –> amount of gas exhaled subtracted from the amount administered to obtain Vco.

Answer 7

CO, which should make sense since it is diffusion limited!

Question 8

How does anemia (low Hb) affect Diffusion capacity for the Lung?

Answer 8

It would lower the diffusion capacity bc gas will have less Hb to bind to –> higher Partial pressure of gas in the blood.