Oxygen Transport

Question 1

Dalton’s Law of Partial Pressures

Answer 1

Partial Pressure=total pressure x Fractional Gas concentration
pO2=760mmHg X 0.21=160mmHg
In humidified air, must correct for H2O fraction
pO2=(760-47)X0.21
=150mmHg

Question 2

Partial Pressures of Oxygen and CO2

Answer 2

Dry inspired air: pO2=160 pCO2=0
Humidified air: pO2=150 pCO2=0
Alveolar air: pO2=100 pCO2=40
-O2 diffused from alveoli to pulmonary capillaries
-CO2 added from pulmonary capillaries into alveoli
Systemic arterial blood: pO2=100 pCO2=40
-blood equilibrated w/ alveolar air
Mixed Venous Blood: pO2=40 pCO2=46
-O2 diffused from arterial blood into tissues
-CO2 diffused into blood from tissues

Question 3

Determinants of gas diffusion

Answer 3

• The rate of diffusion is determined by the partial pressure differential across the membranes
  
  • Also area affects rate

Question 4

Perfusion limited and diffusion limited exchange

Answer 4

• Perfusion limited exchange
  
  • N2O and O2 under normal conditions
  • gas equilibrates early in the length of the capillary
  • partial pressure equilibrates b/t artery and alveoli
  • Diffusion of gas can only increase if blood flow increases
• Diffusion limited exchange
  
  • CO and O2 in strenuous exercise
  • Fibrosis: diffusion limited by thickened alveolar membrane
  • Emphysema: diffusion decreased b/c surface area decrease
  • Gas doesn’t equilibrate by the end of the capillary

Question 5

Transport of O2

Answer 5

• Carried in 2 forms
  
  1. Dissolved in blood
  2. Bound to hemoglobin (most important)
• Hb increases blood O2 carrying 70-fold

Question 6

Hemoglobin characteristics

Answer 6

• Globular protein w/ 4 subunits
• Each subunit has a heme moiety: Iron containing porphyrin
• Iron is in the ferrous state (Fe2+) and binds O2
  
  • Ferric state (Fe3+) is methemoglobin-doesn’t bind O2
• Each subunit has a polypeptide chain
  
  • Two have alpha chains
  • Two have beta chains

Question 7

O2 content of blood

Answer 7

• Total O2 carried in blood including bound and dissolved O2
• Depends on Hb concentration, pO2, and p50 of Hb
• O2 content=(O2 binding capacity X % saturation)+dissolved O2

Question 7

Hemoglobin-O2 dissociation Curve

Answer 7

• Hb combines rapidly and reversibly w/ O2: forms oxyhemoglobin
• Plot of Hb % saturation as function of pO2
• At pO2 100mmHg: arterial blood
  
  • Hb 100% saturated- O2 bound to all 4 heme groups
• At pO2 40mmHg: mixed venous blood
  
  • Hb 75% saturated-O2 bound to 3 of 4 heme groups
• At pO2 25mmHg:
  
  • Hb is 50% saturated-O2 bound to 2 of 4 heme groups
• Curve has a sigmoid shape:
  
  • Due to change in affinity of Hb for each added O2
  • positive cooperativity
  • Adding 1st makes 2nd easier to add: 4th has highest
  • Facilitates loading O2 in the lungs and unloading in tissue

Question 8

Physiological implications of the Hb-O2 dissociation curve in the lung and peripheral tissues

Answer 8

• In the lungs:
  
  • Alveolar gas pO2=100
  • pulmonary capillary blood “arterialized” by O2 diffusion
  • Hb high affinity for O2 at pO2 100 facilitates diffusion
  • By tightly binding O2, pO2 kept low, maintaining gradient
  • Curve is almost flat b/t pO2 60 and 100
  • Tolerate changes in ATM pressure w/o losing O2 carrying
• In peripheral tissues
  
  • O2 diffuses from arterial blood to the cells
  • O2 gradient maintained b/c cells consume O2
  • Hb has lower affinity for O2 in steep part of curve
    
    • facilitates unloading of O2 to tissues

Question 9

Changes in Hb-O2 dissociation Curve

Answer 9

• Shifts to the right: Hb affinity for O2 is decreased
  
  • p50 increased and unloading of blood O2 is facilitated
  • Increases in pCO2 or pH
    
    • Bohr Effect
    • during exercise: increase CO2-> decrease H+
    • Stimulates O2 delivery to exercising muscle
  • Increases in temperature (also in exercise)
  • Increases in 2,3-DPG concentration
    
    • binds beta chains of deoxy-Hb, decreasing affinity
    • adaptation to chronic hypoxemia
• Shifts to the left: Hb affinity for O2 increased
  
  • p50 decreased and blood O2 unloading more difficult
  • decreased pCO2
  • increased pH
  • decreased temp
  • decreased 2,3-DPG

Question 10

Effects of Carbon Monoxide on Hb-O2 dissociation curve

Answer 10

• Competes w/ O2 for binding sites on Hb
• Hb affinity for CO is 200x greater thanO2
• CO occupies O2-binding sites, decreasing O2 content of blood
• Binding of CO increases affinity of remaining sites for O2
  
  • Shifts curve to the left