Hb Structure, Function and Gas Transport

Question 1

What determines how much oxygen can be carried in the blood (5)?

Answer 1

Question 2

What can causethe amount of O2 dissolved in the blood to increase?

Answer 2

With high levels of inhaled oxygen the amount dissolved in the blood can become more significant.

Question 3

What drives the movement of gases between the alveoli and the blood?

Answer 3

Partial pressures of CO₂ and O₂ in the respective environments. The gases will move from one environment to the other based on their respective pressure gradients.

Question 4

When can perfusion limited gas exchange be transformed to diffusion limited?

Answer 4

During heavy exercise (with increase in cardiac output) or with a diffusion barrier the normal perfusion limitation can transform to diffusion limitation.

Question 5

What are the ways that CO₂ can be carried in the blood?

Answer 5

CO₂ leaves tissues and enters blood. 5% of it remains dissolved and unchanged in the blood. 95% of it is taken up into the RBC and then 5% of it binds to Hb in the RBC. The remaining 90% of it is converted to H₂CO₃, which then rapidly decomposes to H⁺ + HCO₃^-. The proton can bind to Hb, making it have lower affinity for O₂ so it will release more O₂ to the tissues. The HCO₃^- is then pumped out of the RBC by a transporter that uses Cl^-.

Question 6

What does a pulse oximeter measure?

Answer 6

Whether or not hemoglobin has something bound to it. It DOES NOT distinguish between what is bound to hemoglobin. Thus, if a person has things other than oxygen bound to Hb, their pulse ox can appear normal and their arterial blood gas can reveal hypoxemia.

Question 7

Concentration of a gas in solution depends on its […] in the solution

Answer 7

Partial pressure and solubility

Question 8

What is the Bohr Effect?

Answer 8

Shift in oxyhemoglobin dissociation curve to left or right

Question 9

What is the haldane effect?

Answer 9

The less O₂ that is bound to Hb, the higher the affinity the Hb molecule has for CO₂ and vice versa.

Question 10

CO₂ is able to diffuse across the alveolar/capillary membrane 20x faster than O₂. However, these gases take the same amount of time to reach equilibrium with regards to gas exchange across this membrane. Why is this?

Answer 10

Although CO₂ diffuses faster, the pressure gradient for O₂ is larger so there is a larger driving force for O₂ to cross the membrane. As such, they diffuse in about the same amount of time due to different driving factors.

Question 11

If CO enters the blood, will it expert a partial pressure?

Answer 11

NO - Carbon monoxide has very high affinity for hemoglobin. If CO is introduced into the blood, very little exists in the plasma exerting a partial pressure and virtually all of the CO is bound to the hemoglobin.

Question 13

How do you determine the partial pressure of a gas?

Answer 13

Each gas contributes to the total pressure in proportion to its concentration. I.e. if the total pressure of the gas is 760mmHg and the gas is 21% oxygen, then oxygen’s partial pressure is (760*0.21) = ~160mmHg

Question 14

What does the CO₂ dissociation curve look like?

Answer 14

Question 15

What are the various forms that a gas can take in solution?

Answer 15

Dissolved gas

Bound to carrier (hemoglobin)

Chemically modified (CO₂ being converted to HCO₃^-)

Question 16

CO₂ is […] more soluble in aqueous phase then O₂

Answer 16

20 x

Question 17

How many mL of O₂ are “unloaded” to tissues during gas exchange in capillary beds of tissues?

Answer 17

5mL

In fully oxygenated blood there is about 20mL of O2 per 100mL of blood, in fully deoxygenated blood there is about 15mL of O2 per 100mL of blood

Question 18

How do oxygen and carbon dioxide move from the alveoli into the blood?

Answer 18

Via diffusion - they are freely soluble in membranes

Question 20

What things decrease the P50 of the oxyhemoglobin dissociation curve (i.e. increase affinity of hemoglobin for oxygen)?

Answer 20

Decreasing temp

Decreasing partial pressure CO2

Decreasing concentration of 2,3 BPG

Decreasing [H+]

Question 21

What is the equation that describes alveolar respiratory membrane diffusion?

Answer 21

Question 22

Why is an O2 sat of 90, 91, 92 concerning?

Answer 22

A patient with those levels of % oxygen has a partial pressure of about 60mmHg in their blood. If their blood saturation drops further, they are approaching an area of the dissociation curve where further deoxygenation can trigger a massive loss of oxygen binding due to the cooperative nature of oxygen binding to hemoglobin. This would put them at risk for becoming hypoxic and suffering tissue damage.

Question 23

What is the PO₂ and PCO₂ in tissues before gas exchange occurs with oxygenated blood?

Answer 23

23mmHg

46mmHg

Question 24

Why is the partial pressure of oxygen in the systemic arterial blood supply 100mmHg and the partial pressure of CO2 in the systemic arterial blood is 40mmHg?

Answer 24

Because these are the partial pressures of oxygen and carbon dioxide respectively in the alveoli. Gas exchange occurs until the partial pressures in the alveoli and the capillaries are equal, so that’s why the pressures are the same

Question 25

Pressure of a gas acting on the surface of blood vessels is proportional to the […] of the gas

Answer 25

Concentration (in moles)

Question 26

What volume of O₂ is transferred from the lungs to the tissues?

Answer 26

5mL

Question 27

• How much O₂ can be carried per gram of Hb?
• How much O₂ is dissolved in the blood and not bound to Hb?
• What equation describe the relationship between the [O₂], the amount of O₂ bound to Hb and the amount of O₂ dissolved in the blood?

Answer 27

• 1.34 mL O₂ / 1 gram Hb
• 0.003mL O₂ / 100mL of blood / mmHg
• CaO2 = (SaO2 % x Hb g/dl x 1.34 ml O2/ g Hb) + (0.003 ml O2/100ml/1 mm Hg x Pao2 mmHg)

Takeaway: 98% of O₂ in blood is bound to Hb

Question 28

What is meant by the term “perfusion limited gas exchange”?

Answer 28

As RBCs travel through the capillary beds of the lungs from the venous side to the arterial side, they reach full oxygenation (i.e. partial pressure O₂ of 100mmHg) fairly early in their path of travel. Thus, the limitation on how much oxygen can be delivered to the body is not on the rate of diffusion of the gases but rather on the amount of blood that perfuses through the capillaries. To increase oxygen delivery to the periphery, cardiac output must be increased.

In fact, this equilibrium occurs in about one third the time (0.25 seconds) it takes for the blood to move through the capillary (0.75 seconds).

Question 29

What things increase the P50 of the oxyhemoglobin dissociation curve (i.e. decrease affinity of hemoglobin for oxygen)?

Answer 29

Increasing temperature

Increasing partial pressure CO2

Increasing [2,3 BPG]

Increasing [H+]

Question 30

What factors affect the rate at which oxygen and carbon dioxide will diffuse from the alveoli into the blood?

Answer 30

Partial pressure of gasses in each environment

Area across which they are diffusing

Distance across which they must diffuse

Diffusion coefficient

V = (delta_P)(A)(diffusion constant) / (distance)

Question 31

In the alveoli, the partial pressure of oxygen is 100mmHg and the partial pressure of carbon dioxide is 40mmHg. In venous blood, the partial pressure of oxygen is 40mmHg and the partial pressure of carbon dioxide is 46mmHg. The pressure differential for oxygen is 60mmHg whereas carbon dioxide it is only 6mmHg. This would lead to the assumption that oxygen diffuses faster due to the larger concentration gradient. However this is not the case. Why?

Answer 31

CO₂ is 20x more soluble than oxygen so it can still diffuse well even with a small driving force of only 6mmHg pressure differential

Question 32

What is the respiratory quotient and where does it derive from?

Answer 32

R = CO₂ exhaled / O₂ consumed = 0.8

Derived from looking at CO₂ dissociation curve and oxygen dissociation curves. For every 5 molecules of oxygen that are consumed, 4 molecules of CO2 are exhaled, thus giving 4/5 or 0.8.

Question 33

Answer 33

Question 34

What determines the partial pressure of oxygen in the blood?

Answer 34

Determined by the oxygen that is dissolved in the blood NOT the oxygen that is bound to hemoglobin

Question 35

What is meant by the term “diffusion limited gas exchange” and when is this seen clinically?

Answer 35

This term means that the oxygenation level of the blood is limited by the diffusion of the gas itself. This is seen in people who have fibrosis or scar tissue in the interstitial space between the alveoli epithelial cells and the capillary endothelial cells or in the fused basement membrane. The build up of this tissue increases the distance the gas must diffuse across and increases the resistance of the gas to diffusion, so the blood is never able to completely equilibrate with the alveoli and the partial pressure of oxygen in arterial blood is decreased.

Question 36

Fill in the missing information indicated by the arrows as well as the information in the table.

Answer 36

Question 37

• What is methemoglobin?
• How does oxygen bind to methemoglobin?
• What impact does methemoglobin have on the oxyhemoglobin dissociation curve?
• Why can methemoglobinemia lead to hypoxia?

Answer 37

• The form of hemoglobin with ferric iron is termed methemoglobin.
• If iron is in the ferric form (Fe3+ ) it cannot bind oxygen and the other oxygen binding sites in the molecule demonstrate higher affinity for oxygen than normal-driving the oxyhemoglobin dissociation curve to the left.
• There is reduced circulating oxyhemoglobin and an increased affinity of the bound oxygen in methemoclobin for Hb so less is released to the tissues from these molecules.