Diffusion and perfusion

Question 1

Rate of gas transfer from alveoli into arterial blood depends on what?

Answer 1

Diffusion from alveolar membrane into pulm capillaries and perfusion (extent of blood flow into ventilated regions of lung)

Question 2

Arterial oxygen content (CaO2)

Answer 2

CaO2 = hemoglobin-bound O2 concentration + freely-dissolved O2 concentration. Total concentration of O2 in blood. Most O2 is hemoglobin bound, but it is free O2 that is measured in partial pressure values

Question 3

Define solubility coefficient and describe how they differ for oxygen and CO2.

Answer 3

The tendency of any molecule to dissolve in a liquid is given by its solubility coefficient. aO2= 0.0013mM/Torr. aCO2= 0.03mM/Torr. This means CO2 is about 20 times more soluble in blood than O2

Question 4

Describe the basic properties of the oxy-hemoglobin dissociation curve (ODC).

Answer 4

Curve that relates oxygen saturation of hemoglobin (SaO2) at equilibrium to PO2 in the blood.

Question 5

What is hemoglobin saturation at normal arterial oxygen pressure?

Answer 5

For PO2 = 100 Torr, which is normal arterial oxygen, SaO2 = 97.5%. This means that in arterial blood, hemoglobin is very close to full saturation

Question 6

What effect does increasing arterial PO2 have on SO2

Answer 6

IT will not substantially increase amount of O2 carried by hemoglobin because hemoglobin is already near saturation at normal blood oxygen pressures.

Question 7

What is hemoglobin saturation in mixed venous blood

Answer 7

For PO2=40Torr, SaO2= 75%

Question 8

What factors affect rate of gas transfer across tissue plane

Answer 8

difference in partial pressure of the gas on two sides of membrane, tissue plane area, tissue thickness, tissue solubility and molecular weight of gas

Question 9

Equation for gas transfer rate (Ficks law)

Answer 9

Vg (gas transfer rate) = (P1-P2) x (A/d) x k. Where P is pressure, A is area, d is tissue thickness, and k is constant

Question 10

equation for diffusing capacity

Answer 10

Dm (diffusing capacity) = (A/d) x k, where A is area, d is thickness, k is constant

Question 11

Describe factors that promote rapid oxygen diffusion between alveoli and pulmonary pulmonary capillaries in healthy individual

Answer 11

Large surface area of alveolar membrane, thin membrane and the large oxygen pressure gradient btw alveoli and capillaries (due to low solubility of O2 in blood and tendency for O2 to bind hemoglobin which both keep O2 pressure in the capillary low)

Question 12

Describe oxygen diffusion btw alveoli and pulm capillaries in interstitial disease

Answer 12

During interstitial disease, when there is thickening of the alveolar walls (increased d), the rate of diffusion is slowed, sometimes to the point where arterial oxygen levels do not reach alveolar levels

Question 13

Describe oxygen diffusion btw alveoli and pulm capillaries in emphysema

Answer 13

breakdown in lung tissue decreases the surface area for diffusion (decreased A), which decreases overall diffusion

Question 14

Describe oxygen diffusion btw alveoli and pulm capillaries in polycythemia and anemia

Answer 14

Diffusion increases in polycythemia and decreases in anemia due to abnormally high or low hemoglobin levels. But note that changes in hemoglobin conc usually alters oxygen delivery more than oxygen diffusion

Question 15

Compare diffusion of CO2 vs O2

Answer 15

CO2 is 20 times more soluble in blood than O2, so O2 diffusion is somewhat slowed by its poor solubility and thus is susceptible to disease. CO2 diffusion is NOT limited by solubility, thus it is less affected by disease

Question 16

Perfusion limited O2 transfer rate

Answer 16

The only factor limiting the rate of oxygen uptake and CO2 excretion is pulmonary blood flow. There is rapid equilibration of blood with alveolar air (PaO2= PAO2, and PaCO2=PACO2). This is normal, even during exercise

Question 17

Diffusion limited O2 transfer rate

Answer 17

Diffusion rate limits the O2 transfer rate - there is not complete equilibration of alveolar air and blood. PO2 from pulmonary blood < alveolar PO2

Question 18

Define perfusion

Answer 18

Perfusion (Q) is the blood flow of the pulmonary circulation available for gas exchange (in one minute).

Question 19

Perfusion = ?

Answer 19

Cardiac output (5L/min)

Question 20

Describe pulmonary circulation pressures and why

Answer 20

Very low blood pressure- due to vast number of vessels and normally dilated state. Walls of pulmonary artery are thin

Question 21

Factors that regulate perfusion

Answer 21

O2 tension, chemical agents, capillary recruitment, gravity

Question 22

How does O2 tension affect perfusion

Answer 22

Alveolar O2 tension (PAO2) is most important in regulating pulm circulation. Low PAO2 results in constriction of arterioles (hypoxic pulmonary vasoconstriction) which decreases local blood flow and shifts it to other regions of lung.

Question 23

How do chemical agents affect perfusion

Answer 23

Thromboxane A2 is the most important
vasoconstrictor, and, as with O2, its effect is mainly localized to the region of release. Prostacyclin (prostaglandin I2) is a potent vasodilator.

Question 24

How does capillary recruitment affect perfusion

Answer 24

During moderate exercise, CO increases and is accomodated by recruitment of new capillaries and distension of previously open microvessels.

Question 25

How does gravity affect perfusion

Answer 25

Because of gravity, the pulmonary blood pressure is low at the apex of the lung. At the base of the lung, the blood pressure is higher, allowing more capillaries to open and higher blood flow. Thus, perfusion increases as we go from the apex to the bottom of the lung.

Question 26

Describe the mechanisms by which dead-space impacst gas exchange.

Answer 26

Alveolar dead space occurs when ventilation occurs, but no perfusion (ie. If patient has a blocked pulmonary capillary). Thus they do not engage in gas exchange

Question 27

Describe the mechanisms by which shunts impact gas exchange.

Answer 27

Blood perfusion without any ventilation results in a shunt. This can reduce arterial oxygenation significantly when well ventilated blood mixes with shunted blood. PCO2 in arteries is NOT raised b/c increased ventilation will prevent that.

Question 28

Describe the mechanisms by which V/Q mismatch impact gas exchange.

Answer 28

V/Q mismatch is an unequal ratio of ventilation to perfusion, as can happen in obstructive lung disease and from gravity. When V/Q is low (due to alveolar blockage), V/Q in other alveoli may increase, but since hemoglobin is already at near saturation, increased V/Q doesn’t fully compensate for the low V/Q. PCO2 does NOT increase PCO2 overall b/c any increases in PCO2 in parts of the lung with low V/Q will be countered by increased ventilation (high V/Q) in other parts of the lung

Question 29

Describe how gravity leads to regional variations in ventilation and perfusion in an upright person.

Answer 29

V/Q is higher at the apex due to gravity which causes decreased perfusion at apex.