SM 156a - Diffusion and Gas Transport

Question 1

Under normal conditions, at what partial pressure of oxygen is hemoglobin in the blood 50% saturated with oxygen?

(What is the P₅₀ for oxygen bound to hemoglobin under normal conditions?)

Answer 1

27 mmHg

Question 2

What causes anemic hypoxia?

Answer 2

Low O₂ capacity due to low hemoglobin

Question 3

What is the equation for oxygen saturation (S_O2)?

Answer 3

S_O2 = O₂ content / O₂ capacity * 100

Question 4

Which gas is perfusion limited?

How can you tell?

Answer 4

Gas X is perfusion limited

P_c’X reaches P_AX in <0.75 seconds; this means that blood gets across the alveolar-capillary membrane with enough time to spare, and perfusion must keep up in order to maintain the concentration gradient

Question 5

What causes circulatory hypoxia?

Answer 5

Shock or hypofusion of tissues

Question 6

In the pulmonary capillary bed, is oxygen usually perfusion or diffusion limited?

Answer 6

Perfusion limited

In healthy lungs, diffusion occurs quickly; gas exchange depends on the rate of blood flow through the pulmonary capillary bed

Question 7

Hypoxia due to shock or hypofusion of tissues is called ___________ hypoxia

Answer 7

Hypoxia due to shock or hypofusion of tissues is called circulatory hypoxia

Question 8

How does gas content vary between the gas phase and the liquid phase?

Answer 8

• Gas phase
  
  • Content: n = PV/RT
  • Governed by the ideal gas law
• Liquid phase
  
  • Content: C = solubility * Partial pressure
  • Governed by Henry’s law

Question 9

Which gas is diffusion limited?

How can you tell?

Answer 9

Gas Y is diffusion limited

In 0.75 seconds, P_c’Y has not reached P_AY; Diffusion across the alveolar-capillary membrane is too slow for the partial pressures to equilibrate in the time that blood is in the pulmonary capillary bed

Question 10

What happens to diffusing capacity when the individual alveolar air spaces become large?

Why?

Answer 10

Diffusing capacity decreases

• Surface area for diffusion decreases
• Distance that gas must diffuse increases

Question 11

In which 3 forms is CO₂ transported in the blood?

Answer 11

1. Dissolved CO2
2. Bicarbonate (hydrated CO2)
3. Carbamino compounds (bound to hemoglobin)

Question 12

Dissolved CO2 makes up ___% of the total CO2 in venous blood, and ___% of the CO2 released to the lungs

Answer 12

Dissolved CO2 makes up 5 % of the total CO2 in venous blood, and 10% of the CO2 released to the lungs

Question 13

What is the normal difference in oxygen content between arterial and mixed venous blood?

Answer 13

20.1 vol% - 15.1 vol% = 5 vol% O₂

Question 14

Which enzyme catalyzes the rate limiting reaction in the formation of HCO₃^-?

Answer 14

Carbonic anhydrase

Question 15

Describe the Bohr effect

Answer 15

H⁺ + HbO₂ -> HHb + O₂ -> HbNH₂ + CO₂ -> HbNH-COOH

When H+ is elevated in tissus, oxygen is displaced from hemoglobin by CO₂, creating a carbamino compound. This causes a right shift in the oxygen-hemoglobin binding curve.

Occurs in the tissues

Question 16

What causes histotoxic hypoxia?

Answer 16

The available oxygen cannot be utilized and oxygen extraction is lowered (ex: cyanide poisoning)

Question 17

Oxygen capaity is a direct function of the amount of _________ present in the blood

Answer 17

Oxygen capaity is a direct function of the amount of hemoglobin present in the blood

Question 18

What is the equation for blood O2 content?

Answer 18

C_O2 = S_O2 * [Hb] * O₂ capacity

Question 19

What are the two factors that contribute to resistance to oxygen diffusion into the pulmonary capillaries?

Answer 19

1. Resistance to diffusion thought the blood-gas barrier (1/D_m)
2. Chemical reaction rates of O₂ and CO₂ ( 1/(θ*V_c) )
   
   • θ = Volume of gas contributed by RBCs in 1 mL of blood per mmHg of pressure gradient between plasma and RBC
   • V_c = Pulmonary capillary blood volume

Question 20

Is carbon dioxide usually perfusion or diffusion limited?

Answer 20

Perfusion limited

Question 21

Suppose a tissue is not adequately perfused. What is likely to occur first: toxic CO₂ levels or hypoxia?

Answer 21

Hypoxia

CO₂ can typically be cleared away becaue it diffuses across membranes 20x faster than oxygen

Question 22

In which two forms is oxygen transported in the blood?

Answer 22

Dissolved O2

Chemically bound to hemoglobin

Question 23

For a gas (X) that is perfusion limited, a small increase in the cardiac output will…

1. Increase P_c’X
2. Have no effect on P_c’X
3. Decrease P_c’X

Answer 23

b. Have no effect on P_c’X

Blood will spend less time in the pulmonary capillary, but perfusion limited gases typically have a large “safety margin;” their partial pressure equilibrates with P_A long before blood leaves the pulmonary capillary

Question 24

What is the equation for Henry’s law?

What does this equation describe?

Answer 24

C_g = K * P_g

• C_g = concentration of the dissolved gas (g)
• K = solubility coefficient
• P_g = partial pressure of the gas

Henry’s law describes the concentration of dissolved gas in a solvent

Question 25

What is the chloride shift?

Answer 25

A rate-limiting chemical reaction that slows CO₂ equillibration at the lung * Increased HCO₃^- in the RBC activates the Cl^-/HCO₃^- exchanger * Cl^- comes into teh cell * H⁺ ions stay in the RBC, buffered by hemoglobin * **This slows down CO₂ offloading to the lungs**

Question 26

Why is carbon monoxide dangerous when inhaled?

Answer 26

Carbon monoxide binds very stongly with hemoglobin, and only desaturates at very low P_CO Carbon monoxide causes poisoning because oxygen cannot bind hemoglobin, and therefore cannot be delivered to tissues

Question 27

Which form of O₂ in the blood contributes to P_aO2? 1. Dissolved O₂ 2. O₂ Bound to hemoglobin 3. Both a and b 4. Neither a or b

Answer 27

a. Dissolved O₂ =\> Oxygen bound to hemoglobin does not contribute to P_aO2 =\> Oxygen must continue to bind to hemoglobin to maintain the concentration gradient

Question 28

What is the equation for Fick's law of diffusion?

Answer 28

Question 29

What is the O₂ uptake by the body per minute?

Answer 29

250 mL O₂/min Arterial O₂ content = 20.1 vol% Mixed venous O₂ content = 15.1 vol% Difference = 5 vol% Cardiac output = 5000mL blood/min 5 mL O₂ /100 mL blood \* 5000 mL blood/min = **250 mL O₂/min** vol% = mL O₂ per 100 mL blood

Question 30

Hypoxia caused by low PO₂ in arterial blood is called ________ hypoxia

Answer 30

Low PO₂ in arterial blood is called **_hypoxic_** hypoxia

Question 31

Carbamino compounds make up **\_\_\_%** of the total CO2 in venous blood, and **\_\_\_%** of the CO2 released to the lungs

Answer 31

Carbamino compounds make up **_5_** **_%_** of the total CO2 in venous blood, and **_30_****_%_** of the CO2 released to the lungs (High proportion of CO2 released to lungs due to the Bohr/Haldane effect)

Question 32

What is the normal value of mixed venous O2 content?

Answer 32

C_O2 = S_O2 \* [Hb] \* O₂ capacity Normal mixed venous: 0.75 \* 15 \* 1.34 = 15.1 mL O₂ / 100 mL blood

Question 33

What binds more CO₂: reduced hemoglobin or oxyhemoglobin?

Answer 33

Reduced hemoglobin (Deoxyhemoglobin) This means that deoxyhemoglobin is a weaker acid than oxyhemoglobin

Question 34

CO₂ diffuses across the alveolar-capillary membrane 20x faster than oxygen. Why then, is equillibration of CO₂ only slightly faster than equillibration of O₂?

Answer 34

Chemical reactions limit the equillibration of CO₂ * Formation of HCO₃ * Formation of carbamino compounds with blood proteins (hemoglobin) * Chloride shift

Question 35

Bicarbonate makes up **\_\_\_%** of the total CO2 in venous blood, and **\_\_\_%** of the CO2 released to the lungs

Answer 35

Bicarbonate makes up **_90_** **_%_** of the total CO2 in venous blood, and **_60_****_%_** of the CO2 released to the lungs Release to the lungs is slowed by chemical reactions (ex: chloride shift)

Question 36

Which gas represents the dissociation of CO2 in the blood?

Answer 36

Gas A

Question 37

For a gas (Y) that is diffusion limited, increasing the cardiac output will... 1. Increase P_c’Y 2. Have no effect on P_c’Y 3. Decrease P_c’Y

Answer 37

c. decrease P_c’Y Blood will spend less time in the pulmonary capillary, thus decreasing P_c’Y

Question 38

Hypoxia caused by low oxygen capacity due to low hemoglobin is called __________ hypoxia

Answer 38

Hypoxia caused by low oxygen capacity due to low hemoglobin is called **_anemic_** hypoxia

Question 39

What is the normal value of arterial O2 content?

Answer 39

C_O2 = S_O2 \* [Hb] \* O₂ capacity Normal: 0.975 \* 15 \* 1.34 = 19.6 mL O₂ / 100 mL blood If you ignore dissolved oxygen: 1.00 \* 15 \* 1.34 = 20.1 mL O₂ / 100 mL blood

Question 40

Which reaction is rate limiting for oxygen transfer from gas to blood?

Answer 40

Oxygen combining with hemoglobin

Question 41

Which gas represents the dissociaton of oxygen in the blood?

Answer 41

Curve B

Question 42

What causes hypoxic hypoxia?

Answer 42

Low PO₂ in arterial blood

Question 43

List the barriers that oxygen must cross between inspiration and the mitochondria of a tissue cell

Answer 43

* Gas space within the alveolus * Alveolar-capillary membrane * Plasma capillary * Into and out of the RBC * Includes hemoglobin binding and unbinding * Plasma capillary * Interstitial fluid * Plasma membrane * Mitochondrial membrane

Question 44

In the blood, 1g of hemoglobin binds \_\_\_mL of O₂

Answer 44

In the blood, 1g of hemoglobin binds **_1.34_** mL of O₂​

Question 45

What is oxygen capacity?

Answer 45

The amount of oxygen bound to hemoglobin when hemoglobin is saturated

Question 46

Describe the Haldane effect

Answer 46

HHb + O₂ -\> HbO₂ + H⁺ Increased free H+ pushes this equation to the right: H⁺ + HCO₃^- -\> H₂O + CO₂ As a result, H⁺ decreases, facilitating: HbNH-COOH -\> HbNH₂ + CO₂ This occurs in the lungs (lots of O₂ around) and allows CO₂ to be offloaded from the blood to the lungs