Diffusion

Question 1

What variables affect gas exchange ?

Answer 1

• Partial pressure difference of the gas of interest on either side of the membrane
• Thickness of membrane
• Area of membrane being crossed
• Solubility of the gas of interest
• Diffusion constant of the gas of interest

Question 2

IN comparison to oxygen, CO2

Answer 2

has a higher solubility AND diffusion constant than oxygen

Question 3

Most of the oxygen carried in the blood is

Answer 3

bound to hemoglobin

Question 4

Where the cross-sectional area of the bronchial tree the smallest?

What does this mean?

Answer 4

Medium-sized bronchioles

This is where air will be move the fastest (bulk flow) and has the greatest resistance to flow

Question 5

After passing through the medium sized bronchioles, air slows to a virtual standstill due to the exponential increase in cross-sectional area so that

Answer 5

Diffusion occurs; it is the only force that allows gas concentrations to equilibrate between the blood & alveolar gases

Question 6

The CO2 gradient from blood to alveolar air: __ –> ___ mmHg

The O2 gradient from alveolar air to blood: __ –> ___ mmHg

Answer 6

CO₂: 45 –> 0 mmHg

O₂: 100 –> 30 - 40 mmHg

Question 7

Many disease states will impact what two variables in Fick’s law of diffusion?

Answer 7

May decrease surface area for gas exchange

May thicken the alveolar-capillary membrane

Question 8

When carbon monoxide (CO) is inhaled, it crosses the alveolar-capillary membrane and what happens?

What is this an example of?

Answer 8

It is taken up by hemoglobin on RBCs so quickly that the pCO in the plasma hardly changes (thus the straight line), making it easier for more CO is diffuse across the alveolar wall.

–> Diffusion limitation: the amt of CO that enters the blood is limited ONLY by the the diffusion properties of the blood-gas barrier - not the amt of blood available.

Question 9

Nitrous oxide (N₂O) is an example of?

Answer 9

N₂O diffuses easily across and doesn’t bind hemoglobin –> pN₂O in the blood rapidly equilibrates with pN₂O in the alveoli –> no more N₂O is transferred until new blood flows in.

Perfusion limitation: the amt of N₂O taken up by the blood is limited only by the amount of blood perfusing the alveoli

Question 10

Under normal circumstances, oxygen is __-limited

Answer 10

Perfusion-limited

_{(Doesn’t diffuse <em>as</em> rapidly as N2​O tho)}

Question 11

____ limits our maximal exercise capacity

Answer 11

The heart’s ability to push blood

Becuase oxygen is perfusion limited - the amt of oxygen we’re getting depends on how much blood we can get through the alveoli capillaries

Question 12

Capillary pO2 virtually reaches that of alveolar gas when the RBC is about ~ 1/3 of the way along the capillary.

On the average, blood spends about ___s in the capillaries under resting conditions.

Answer 12

0.75 s

Even during exercise, 0.75s is more than enough time for hemoglobin to pick up oxygen and for carbon dioxide to diffuse out of the blood into the alveolar air space.

Question 13

Disease states (Ex. pulmonary edema, thickening of the alveolar-capilary membrane, pneumonia) can cause the equilibration of oxygen to become

Answer 13

diffusion limited, in addition to perfusion-limited

Severe exercise reducing the time available for oxygenation makes this much worse.

Question 14

At high altitudes, you have alveolar hypoxia: the alveolar air has a lower P_O2.

What will happen?

Answer 14

You’ve lost the big pressure gradient difference to drive oxygenation –> rise in PO2 along the capillary slows and failure to reach alveolar PO2 is more likely.

If diffusion is even 1/4 normal, the blood won’t be fully oxygenated by the time it leaves the alveolar capillaries.

Question 15

Why is CO used to measure DLCO (diffusing capcity of the lungs for carbon monoxide)?

Answer 15

CO never equilibrates with the plasma because it’s so rapidly removed from it by hemoglobin, so the amt of CO in th eblood depends only on how quickly it diffuses across the alveolar capillary barrier.

Thus, if you give a pt CO, the amt that is taken up vs the amt that is exhaled reflects how much successfully diffused across.

Question 16

Resistance =

Answer 16

R = 1 / D

Where D= diffusibility of the gas

Question 17

Not only does air have to diffuse across the alveolar capillary membrane, but it also has to get through the plasma to the hemoglobin within the RBC.

What factors limit the diffusion from the alveolar air into the blood?

Answer 17

• Diffusion across the membrane (see Fick’s law)
• Reaction rate with hemoglobin (theta)
• Amount of hemoglobin in blood

Question 18

Percent hemoglobin saturation (SpO2) is measured using

Partial pressure of oxygen in the plasma is measured using

Answer 18

SpO_2, AKA the amt of oxyhemoglobin in blood - Pulse oximeter

P_O2 in plasma - arterial blood gas, ABG

Question 19

The amt of oxygen chemically bound to hemoglobin is ___

The amt of oxygen dissolved in plasma is ___

Add these together to get the total amt of oxygen in blood

Answer 19

Note: giving a hypoxic patient hemoglobin would be more effective than it would be to give them oxygen.

Question 20

Becuase CO2 is more soluble than oxygen and diffuses more easily across biological membranes, what can you say about its equilibration with alveolar air?

Answer 20

It equilibrates quickly, so rate ventilation will determine the amt of CO2 in alveolar air (more you ventilate, more CO2 removed from blood)

Question 21

Pulmonary arterial blood (AKA mixed venous blood) as a P_CO2 of __ mmHg at the start of the alveolar capillary and __ mmHg about halfway to the end of the alveolar capillary

Answer 21

Drops from 45 at the start to 40 ~halfway through.

This means there’s plenty of time to remove CO2 from the blood if cardiac output increases from exercise or illness.

Question 22

If CO2 diffusion is impaired

Answer 22

there won’t be enough time for it to be unloaded into the alveolar air, so it will build up in the blood –> clinical probs

Question 23

Answer 23

Person w pulmonary fibrosis

Question 24

According to Henry’s law, the amount of dissolved gas is proportional to

Answer 24

its partial pressure in the gas phase

Question 25

Reductions in hemoglobin has what impact on the diffusion capacity of the lung?

Answer 25

It decreases it!

Question 26

Explain the limitation of CO₂ removal from the blood in the lungs under normal and abnormal (diseased) conditions

Answer 26

• Normal conditions: Blood CO₂ equalizes w the alveolar gas well before the blood leaves the capillary (perfusion limitation)
• Diseased: CO₂ does not completely equalize with the alveolar gas prior to leaving the capillary (diffusion limitation)