Gas Exchange

Question 1

What is the formula for henry’s law for concentrations of dissolved gasses?

Answer 1

[x] = P_X * Solubility_X

Question 2

It is important to remember that the concentration of a gas in solution applies only to:

Answer 2

A dissolved gas that is free in solution, and does not include any gas that is present in bound form. (i.e. hemoglobin)

Question 3

What is the driving force for diffusion of gas?

Answer 3

The partial pressure difference of the gas across a membrane.

NOT the concentration difference

Question 4

What does the diffusion coefficient of a gas depend upon?

Answer 4

The solubility of a gas and it’s molecular weight.

Question 5

Lung diffusing capacity D_L combines the diffusion coefficient of a gas, surface area of a membrane, thickness of a membrane and the time required for the gas to combine with proteins in pulmonary capillary blood. We can measure D_L with CO. Why?

Answer 5

Since CO transfer across the alveolar /pulmonary capillary barrier is limited exlusively by the diffusion process.

Question 6

What happens to D_L in emphysema? Why?

Answer 6

D_L decreases due to destruction of alveoli, resulting in a reduction in surface area available for gas exchange.

Question 7

What happens to D_Lin pulmonary fibrosis or pulmonary edema?

Answer 7

The fibrosis causes thickening of the membrane across which gas must diffuse. Leading to a reduction in D_L

Question 8

What impact does anemia have on D_L?

Answer 8

Since the amount of hemoglobin in the blood is decreased (D_L includes the protein binding component of O₂ exchange), lung diffusing capacity is decreased.

Question 9

What impact does exercise have on D_L?

Answer 9

Exercise increases D_L, because additional capillaries are perfused with blood, which increases the surface area for gas exchange.

Question 10

What is the total concentration of a gas in solution based on?

Answer 10

The sum of the dissolved gas, bound gas and chemically modified gas present in solution.

Question 11

What is Pa_O2?

Answer 11

Measure of dissolved oxygen molecules in the blood. (Not bound to Hb)

Question 12

What is Sa_O2?

Answer 12

The percentage of all Hb binding sites that are occupied by oxygen (saturated with)

Question 13

What is an example of a chemically modified gas in the human respiratory chain?

Answer 13

CO₂, which is converted to bicarbonate in RBC’s by carbonic anhydrase. Most CO₂ is carried in blood in this form rather than as dissolved or bound CO₂

Question 14

What is a physiological shunt?

Answer 14

When a small fraction of pulmonary blood flow bypasses the alveoli and therefore is not arterialized (oxygenated). This accounts for the small discrepancy between alveolar air and systemic arterial blood)

Question 15

What are the two sources of physiological shunt?

Answer 15

1. Bronchial blood flow
2. a small portion of coronary venous blood that drains directly into the left ventricle rather than going to the lungs to be oxygenated

Question 16

The physiological shunt may be increased in several pathologic conditions. What is this called? What is the outcome?

Answer 16

This is called a ventilation/perfusion defect. When the size of a shunt increases, equilibration between the alveolar gas and pulmonary capillary blood cannot adequately occur, and pulmonary capillary blood is not fully arterialized.

Question 17

What is the A-a difference? Describe it under physiological and pathological shunting conditions.

Answer 17

This is the difference in P_AO2 and P_aO2

If the shunt is small “physiologic” then the A-a difference is small or negligable

If the shunt is larger - pathological - then the A-a difference increases to the extent that equilibration fails to occur.

Question 18

Describe diffusion-limited gas exchange. Under what onditions will diffusion continue?

Answer 18

This means that the total amount of a gas that is transported across an alveolar-capillary barrier is limited by the diffusion process.

As long as the partial pressure gradient for the gas is maintained, diffusion will continue along the length of the capillary.

Question 19

Describe perfusion-limited gas exchange. In this situation, how can you increase gas transport?

Answer 19

This means that the total amount of gas transported across the alveolar-capillary barrieris limited by blood flow through the pulmonary capillaries.

In perfusion limited exchange, the partial pressure gradient is not maintained, and the only way to increase gas transport is to increase blood flow.

Question 20

Describe the gas trasport model for Oxygen.

Answer 20

Normally Oxygen is a perfusion limited gas. Under certain circumstances… strenuous exercise, emphysema or fibrosis…

Oxygen becomes a diffusion limited gas.

Question 21

Normally, O2 equilibration occurs at what distance down the capillary?

Answer 21

1/3 the total distance.

Question 22

In fibrosis, how is the gas exchange modality for oxygen changed?

Answer 22

The alveolar wall thickens, increasing the diffusion distance and decreasing D_L. This slows the rate of diffusion of oxygen and prevents equilibration of oxygen between alveolar air and pulmonary capillary blood.

In this case the partial pressure gradient of oxygen is maintained across the entire length of the capillary, converting it to a diffusion-limited process.

Question 23

It might seem as though the total amount of oxygen transferred would be greater in a person with fibrosis than in a person with normal lungs, this is not correct. Why?

Answer 23

Although the oxygen partial pressure gradient is maintained across a longer distance of capillary, the total transfer is greatly decreased due to the substantially reduced D_L

Question 24

Why does O2 exchange become diffusion limited during strenuous exercise?

Answer 24

Due to excessively rapid blood flow through the capillaries. Not enough time for O2 to bind.

Question 25

What is the formula for O₂ delivery?

Answer 25

oxygen delivery = (cardiac output) x (oxygen content of blood)

Question 26

What is the percent saturation of hemoglobin a function of?

Answer 26

the partial pressure of oxygen in blood

Question 27

If there is a right-ward shift of the oxygen hemoglobin dissociation curve, what does this mean?

Answer 27

Indicates tbere is a decreased affinity of Hb for Oxygen.

Question 28

What impact do increases in P_CO2 and decreases in pH do to the oxygen-Hb dissociation curve?

What is this mechanism called?

Answer 28

Causes a rightward shift.

When metabolic activity of tissues increases the production of CO2 increases, leading to an increased [H+].

This shifts the curve right - facilitating the unloading of oxygen from Hb in tissues. This mechanism ensures that O2 delivery can meet O2 demand.

The mechanism is called the Bohr effect

Question 29

What impact does increased temperature have on the O2-Hb dissociation curve?

Answer 29

Right-ward shift. As in exercise, the heat generated by working muscle pushes the curve to the right, favoring offloading of O2 to the working muscles.

Question 30

Increases in 2,3-DPG cause a rightward shift in the O2-Hb dissociation curve. Why? When do we see this upregulated as a compensatory mechanism?

Answer 30

2,3-DPG is a by product of RBC glycolysis that binds to the beta chains of deoxyhemoglobin and reduces their affinity for O2.

production of 2,3-DPG increases under hypoxic conditions, such as living at high altitude, where hypoxemia occurs. This upregulation of 2,3-DPG facilitates delivery of O2 to tissues as an adaptive mechanism.

Question 31

What does a shift of the O2-Hb curve to the left indicate?

Answer 31

An increased affinity for O2

Question 32

What are four things that produce a leftward O2-Hb dissociation curve shift?

Answer 32

1. Decreased P_CO2, and increased pH
2. Decreased temperature
3. decreased 2,3-DPG concentration
4. Hemoglobin F

Question 33

What impact does CO have on the dissociation of oxygen from Hb?

How does it accomplish this?

Answer 33

CO decreases the O2 bound to Hb, and produces a left shift in the O2-Hb dissociation curve.

CO binds to Hb with an affinity 250x that of O2, forming carboxyhemoglobin. This decreases the # of binding sites abailable to O2 on Hb. This reduces O2 content of blood and delivery to tissues. In causing the left shift, those heme groups not bound to CO have an increased affinity for O₂ This decreases offloading of the little O2 that is bound to Hb to be even further depressed.

Question 34

In what forms will you find carbon dioxide in blood?

Answer 34

1. dissolved
2. carbaminohemoglobin
3. bicarbonate

Question 35

How does O2 bound to Hb change its affinity for CO2?

Answer 35

More O2 bound to CO2 decreases the affinity for CO2, and conversely, low O2 increases the affinity of Hb for CO2