Diffusion and Gas Transport Lecture 5

Question 1

Identify factors that determine exchange rates of gases across membranes.

Answer 1

1. Pressure difference - the gas between the two sides of the membrane
2. Thickness - respiratory membrane
3. Surface area - the membrane
4. Diffusion coefficient - the gas in the substance of the membrane

Question 2

Define Fick’s Law

Answer 2

Describes how particles under random thermal motion tend to spread form a region of higher concentration to a region of lower concentration. This law describes the time course of the transfer of a solute between two compartments that are seperated by a thin membrane.

Question 3

Identify two factors which determine the concentration of a gas in a solution using Henry’s Law

Answer 3

1. Partial pressure of gas
2. Solubility coefficient

Question 4

Compare solubility coefficients for oxygen, carbon dioxide, carbon monoxide, and nitrogen

Answer 4

O₂ - 0.024

CO₂ - 0.018

CO - 0.018

N - 0.012

Question 5

Using Henry’s Law, calculate the amount of oxygen dissolved in blood for PO₂ of 50, 100, and 600 mmHg

Answer 5

Concentration of dissolved gas = Partial pressure of gas x Solubility coefficient

C_gas= K x P_gas

K of oxygen dissolved in blood = 0.003

• If PO₂ is 100 mmHg – 0.003 x 100 = 0.3 mL O₂/100 mL blood
• If PO₂ is 50 mmHg – 0.003 x 50 = 0.15 mL O₂/ 100 mL blood
• If PO₂ is 600 mmHg – 0.003 x 600 = 1.8 mL O₂/ 100 mL blood

Question 6

Identify the mechanism responsible for most carriage of oxygen in the plasma

Question 7

Describe the effects of increasing FiO₂ from 100 to 600 mmHg on oxygen carrying capacity

Answer 7

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Question 8

Identify the average capillary exposure/transit time

Answer 8

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Question 9

Identify the normal time period required for diffusion to occur

Answer 9

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Question 10

Describe the effect of lower alveolar PO₂ on diffusion and arterial PO₂

Answer 10

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Question 11

Compare arterial and tissue PO₂ and the effect on diffusion

Answer 11

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Question 12

Discuss the mechanism of diffusion of carbon dioxide across the alveolar septa

Answer 12

• Carbon dioxide crosses the alveolar septa via simple diffusion based on a concentration gradient

Question 13

Discuss the mechanism of diffusion of oxygen across the alveolar septa

Answer 13

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Question 14

Define Graham’s Law

Answer 14

When gases are dissolved in liquids, the relative rate of diffusion of a given gas (diffusion capacity) is

• Proportional to solubility in the liquid
• Inversely proportional to the suare root of its molecular weight

Question 15

Compare the diffusion capacity of oxygen relative to carbon dioxide, nitrous oxide, carbon monoxide, and nitrogen

Answer 15

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Question 16

Describe the two major elements of diffusion capacity and their components

Answer 16

1. Membrane capacity (DM)
   
   • Affected by factors involving movement of gas between alveoli and blood
2. Reaction time with hemoglobin (O - V_C)
   
   • Affected by factors involving movement into capillary and uptake by RBC

Question 17

List four conditions that decrease diffusing capacity

Answer 17

1. Thickening of the barrier
   
   • Interstitial or alveolar edema
   • Interstitial or alveolar fibrosis
     
     • Sarcoidosis
     • Scleroderma
2. Decreased surface area
   
   • Emphysema
   • Tumors
   • Low cardiac output
   • Low pulmonary capillary blood volume
3. Decreased uptake by erythrocytes
   
   • Anemia
   • Low pulmonary capillary blood volume
4. Ventilation - perfusion mismatch

Question 18

Calculate the amount of oxygen dissolved in the plasma given a PO₂

Answer 18

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Question 19

Identify the maximum amount of oxygen which may combine with a gram of hemoglobin

Answer 19

• Maximum amount is 1.34 ml/gram of hemoglobin

Question 20

Identify the determinant of the equilibrium point of reaction that determines the amount of oxygen that binds to hemoglobin in the red blood cell.

Answer 20

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Question 21

Calculate oxygen content of the blood given hemoglobin concentration and oxygen saturation

Answer 21

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Question 22

Define P₅₀ for hemoglobin saturation

Answer 22

• Point on the curve where 50% saturation occurs
• This point may move left or right, and represents changes in loading and unloading conditions
• Affected by temperature, pH, PCO₂, DPG levels and type of hemoglobin

Question 23

Calculate the oxygen carrying capacity for varying hemoglobin concentrations and PO₂

Answer 23

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Question 24

Describe the structure of hemoglobin in relationship to its oxygen carrying capacity

Answer 24

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Question 25

Calculate delivery of oxygen given P_aO₂ and mixed venous PO₂ (comparable to tissue PO₂)

Answer 25

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Question 26

Describe rationale for monitoring SVO₂

Answer 26

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Question 27

List five determinants of SVO₂

Answer 27

1. Hemoglobin concentration
2. Hemoglobin saturation
3. Cardiac output
4. Oxygen consumption
5. Oxygen utilization

Question 28

List six clinical conditions that may decrease SVO₂

Answer 28

1. Cardiac output
2. Oxygen saturation/ ventilation/ shunt, V/Q
3. Tissue metabolism
4. Anesmia/ Hypovolemia
5. Fever, shivering
6. Seizures
7. Pain/ Exercise
8. Hyperthyroidism

Question 29

List five factors taht may alter P₅₀ and how the oxygen-hemoglobin saturation curve is affected

Answer 29

1.

Question 30

Define the Bohr Effect as it relates to shifts in the oxyhemoglobin dissociation curve and oxygen loading/unloading

Answer 30

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Question 31

List four clinical conditions shifting the oxyhemoglobin dissociation curve to the left

Answer 31

1. Decrease in H⁺ concentrations
2. Decrease in PCO₂
3. Decrase in temperature
4. Decrease in 2,3 BPG
   
   • Bank blood has decreased 2,3 BPG
   • High atitude and hypoxia cause increases in 2,3 BPG
5. Fetal Hb (HbF)
6. Carboxyhemoglobin
7. Methemoglobin

Question 32

List four clinical conditions shifting the oxyhemoglobin dissociation curve to the right

Answer 32

RIGHTS

1. Rightward shift
2. Increased PCO₂
3. Increased 2,3 BPG
4. Hydrogen Ions
5. Temperature high
6. Sickle cell

Question 33

Describe the effect of carbon monoxide poisoning on oxygen carrying capacity and the oxygen dissociation curve

Answer 33

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Question 34

Discuss mechanisms for dealing with carbon monoxide poisoning

Answer 34

• Carbon monoxide interferes with O₂ transport by combining with Hb to form carboxyhemoglobin (COHb)
• Hb concentration and PO₂ are normal but O₂ content is grossly reduced
• Oxyhemoglobin dissociation curve shiftts to left
  
  • 210 x affinity for hgb as oxygen
• Normal O₂ saturation does not exclude possibility of carbon monoxide toxicity
• Increased FiO₂ promotes elimination of carbon monoxide (decreases half-life form 4 hours to < 1 hour)

Question 35

Discuss the pathophysiology of methemoglobinemia and clinical considerations.

Answer 35

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Question 36

Describe P₅₀ considerations of abberant hemoglobin’s

Answer 36

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Question 37

List six anesthetic considerations for patients with sickle cell

Answer 37

1. Avoid hypoxia and acidemia (FiO₂ > 0.5)
2. Avoid hypovolemia
3. Avoid hypothermia
4. Transfuse the preoperative PCV 30%
5. No tourniquets
6. Symptoms: Microvascular occlusions, cheast or abdominal pain, fever, tachycardia, hematuria.

Question 38

Describe how carbon dioxide is transported in the arterial and venous blood

Answer 38

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Question 39

Calculate the amount of CO₂ dissolved in the blood given a PCO₂

Answer 39

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Question 40

Compare the CO₂ carrying capacity of reduced hemoglobin compared to oxyhemoglobin

Answer 40

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Question 41

Define the Haldane effect in relationship to the ability of hemoglobin to carry CO₂

Answer 41

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Question 42

Define “Chloride shift” as it relates to RBC electrical neutrality and carbonic acid

Answer 42

• More bicarb ions leave RBC than H⁺ ions
• Hamburger shifts occurs and allows excess bicarbonate ions to diffuse out of cell in exchange for chloride ions (preserves electrical neutrality)

Question 43

Compare the carboxyhemoglobin dissociation curve to the oxyhemoglobin dissociation curve

Answer 43

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