Also See Blood Gas Transport and Disorders

O₂ Transport

·         Two ways oxygen is carried in the blood

o    Dissolved in the plasma

§  PO₂ x .003 = ml O₂ dissolved per 100 cc blood

§  Above expressed in vol % (ml/100 cc blood)

§  For arterial blood = 90 torr x .003 = .3 vol%

§  For venous blood = 40 torr x .003 = .1 vol%

§  Amount of O₂ carried in blood this way actually quite small

§  Even with a PaO₂ of 650(highest PO₂ possible on 100% O₂) the amount dissolved is only 2.0 vol%

§  Need at least 5.0 vol% to survive

§  Express all O₂ content values to the nearest tenth

o    Carried by the Hemoglobin

§  Hemoglobin

·         Oxygen carrying molecules found in the RBC

·         Types

o    Adult

o    Fetal-carries more O₂ at a given PO₂ than adult HGB

o    Methemoglobin-can't carry O₂

§  Each HGB molecule can carry 4 molecules of O₂

§  When it carries O₂, it is called oxyhemoglobin

§  When it is not carrying O₂ it is said to be reduced or desaturated

§  normal Hgb levels

·         male - 14 to 16 grams% (grams per 100 cc of blood)

·         female - 12 to 14 grams%

·         fetal - 14 to 20 grams%

o    Each gram of Hgb can carry 1.34 ml of O₂ if maximally saturated

o    O₂ capacity--the amount of O₂ which can be carried by the Hgb if it is maximally Saturated

§  Hgb x 1.34 ml O₂/gm Hgb = O₂ capacity

§  15 gm% x 1.34 = 20.1 vol %.

o    Saturation (SO₂)

§  The percentage of Hgb which is actually carrying O₂

§  The % Saturation primarily a function of PO₂

o    Amount of O₂ actually carried by the Hgb:

§  Hgb (grams%) x 1.34 ml 02/gm Hgb x S O₂

§  In arterial blood: 15 gms% x 1.34 x .97 = 19.5 vol%

§  Note that the amount carried by Hgb is > 60 times than the amount of O₂ which is carried in the plasma

 

 

·         O₂ content (CO₂)

o    Total amount of O₂ (in ml) which is actually carried in 100 cc of blood

o    Amount dissolved + amount carried by the Hgb.

o    CO₂ = PO₂ x .003 + (Hgb x 1.34 x SO₂)

o    Arterial O₂ content (CaO₂)

§  CaO₂ = PaO₂ x .003 + (Hgb x 1.34 x SaO₂)

§  CaO₂ = 90 x .003 + (15 x 1.34 x .97)      

§  = .3             + 19.5                  = 19.8 vol %

o    Venous O₂ content (CvO₂) Obtained in a pulmonary artery)

§  CvO₂ = PvO₂ x .003 + (Hgb x 1.34 x SvO₂)

§  CvO₂ = 40 x .003 + (15 x 1.34 x .73)

§  =      .1 +              14.7              = 14.8 vol %

o    Since .003 and 1.34 are constants: 

§  The only factors which can increase Ca02 are

·         Higher PaO₂

·         Higher Hgb level

·         Higher % Saturation (which is primarily a function of PaO₂)

·         C a-v O₂

o    The difference between arterial O₂ content and mixed venous O₂ content

o    The amount of O₂ that is actually taken up by the body tissues (O₂ uptake)

o    Normally 19.8 vol% - 14.8 vol% = 5 vol%

o    Normal range 4.5 to 5 vol%

o    Oxygen consumption = cardiac output x C a-v O₂ x 10: 5 liters/min x 5 ml 02/100 cc blood x 10 dl/l  = 250 cc.

o    Amount of O₂ taken up by the tissues will decrease if cardiac output decreases or Ca-v O₂ decreases.

o    Ca-v O₂ < 3 vol% may indicate poor O₂ extraction and hypoxia (not enough O₂ at the tissues)

o    Ca-v O₂ > 6 vol% indicates low cardiac output which also can cause hypoxia

 

·         Oxygen Dissociation Curve - Also see Our Oxyhemoglobin Dissociation Curve Page

o    Plots the relationship between PO₂ and % Sat or SO₂.

o    PO₂ is primarily responsible for SO₂ and is plotted on the x or horizontal axis.

o    SO₂ is plotted on the y or vertical axis.

o    Graph plots what the % Sat of O₂ will be at a given PO₂.

o    Some points to remember to construct an O₂ dissociation curve from memory:

·         Significance of the S shape of the Curve

o    Flat from 70 up. Allows for good Saturations and good O₂ pickup and transport at the lungs

o    Steep from 20 to 60--allows for excellent unloading of O₂ from the capillaries to the tissues

·         Shift of the O₂ dissociation curve to the right

o    The Hgb molecule will have a lower affinity for O₂

o    The SO₂ will be lower at a given PO₂.

o    A person with right shifted curve will have a lower CaO₂, O₂ will unload better in the tissues facilitating better O₂ uptake.         

o    Factors which shift the curve to the right

§  Low Ph (blood more acidic)

§  High PCO₂

§  High temperature

§  High 2,3, D.P.G. (increases in chronically low CaO₂)

·         Shift of the O₂ dissociation curve to shift to the left

o    The Hgb molecule will have a higher affinity for O₂

o    The SO₂ will be higher at a given PO₂.

o    Though person with left shifted curve will have a higher CaO₂, O₂ will not unload as well in the tissues potentially causing hypoxia  

o    Factors which shift the curve to the left

§  High Ph (blood more alkalotic)

§  Low PCO₂

§  Low temperature

§  Low 2,3, D.P.G. (stored blood and fetal Hgb)

§  Carbon monoxide poisoning (increased carboxyhemoglogin levels)

§  Fetal Hgb

Learning Objectives:

• List the three forms of carbon dioxide carried by the blood and how they interact to form the total CO₂ dissociation curve.

• Diagram using pertinent chemical reactions how CO₂ is processed by red blood cells traversing tissue and lung capillaries.

• Specify how hemoglobin functions as a hydrogen ion buffer in venous blood, contrasting principles of reduction versus titration.

• Contrast the changes in blood gas contents and partial pressures of oxygen and carbon dioxide during hyperventilation and hypoventilation.

Carbon Dioxide Dissociation Curve

1. Three Forms of Carbon Dioxide in the Blood

   • Physically dissolved CO₂ (10%)

     • dissolved CO₂ increases linearly with increases in PCO₂ (obeys Henry's law)

     • CO₂ solubility = 0.06 mL CO₂/dLblood per mm Hg (20 times higher than O₂ solubility)

     • dissolved CO₂ fraction cannot be neglected

   • Carbamino compounds (22%)

     • CO₂ joins reversibly with non-ionized terminal amino groups (-NH2) of blood borne proteins

     • primary sites are 4 terminal amino groups of hemoglobin which are not ionized at pH = 7.40

     • 44 other terminal amino groups of hemoglobin are ionized and unusable (-NH3+)

   • Bicarbonate ion formation (68%)

     • most CO₂in the blood is transported in the bicarbonate ion form

     • bicarbonate ion is formed from the CO₂ hydration reaction accelerated by carbonic anhydrase: CO₂ + H₂O H2CO3 H+ + HCO3-

     • red blood cells are crucial for production of HCO3- from CO₂ because of the presence of carbonic anhydrase

2. Total CO₂ Dissociation Curve

• total CO₂ content depends on the summation of the three CO₂ components

• total CO₂ content in blood is a hyperbolic function of PCO₂

3. Shifts in the CO₂ Dissociation Curve

• oxygen shifts the carbon dioxide dissociation curve to the right (Haldane effect)

  • presence of O₂ decreases the affinity of hemoglobin for CO₂

  • at PaO₂ = 95 mm Hg, CO₂ curve is shifted down and to the right (lower curve)

  • at PvO₂ = 40 mm Hg, CO₂ curve is shifted up and to the left (upper curve)

• total CO2 content in the blood must be read from proper curve

  • at PaCO₂ = 40 mm Hg; CaCO₂ = 50 mL CO₂/dLarterial blood (point a)

  • at PvCO₂ = 46 mm Hg; CvCO₂ = 54 mL CO₂/dLvenous blood (point v)

• concurrent changes in PO₂ and PCO₂ form a physiological dissociation curve (dashed line)

  • in the tissues, CO₂ content moves up from point a to point v
    low PO₂ in the tissues facilitates CO₂ loading (reverse Haldane effect)
    high PCO₂ in the tissues facilitates O₂ unloading (Bohr effect)

  • in the lungs, CO₂ content moves down from point v to point a
    high PO₂ in the lungs facilitates CO₂ unloading (Haldane effect)
    low PCO₂ in the lungs facilitates O₂ loading (reverse Bohr effect)

C0₂ Processing by Red Blood Cells

1. Carbon Dioxide Reactions in the Blood

• plasma reactions

  • 10% CO₂ processing

  • lack of carbonic anhydrase

• red blood cells

  • 90% CO₂ processing

  • presence of carbonic anhydrase

2. Gradients Established Across the Red Blood Cell Membrane

   • diffusion gradient

     • CO₂ is produced by tissue metabolism

     • CO₂ moves down its partial pressure gradient from tissue space to capillary blood

   • concentration gradient

     • rapid hydration of CO₂ produces a high concentration of HCO3- within RBCs

     • HCO3- diffuses out of RBCs down its concentration gradient

   • electrical gradient

     • HCO3- movement builds up a net positive charge within RBCs

     • H+ cannot move across membrane (cation barrier)

     • Cl- from the plasma moves into RBCs down its electrical charge gradient

     • a Donnan equilibrium is established between HCO3- and Cl- across RBC membrane

   • pH gradient

     • not entirely buffered, H+ ions accumulate within RBCs

     • increase in intracellular [H+] leads to a fall in pH below that of plasma

   • osmotic gradient

     • summed reactions leads to increased RBC osmolarity

     • water moves into RBCs down its osmotic gradient

     • RBCs swell on the venous side of the circulation

   • diffusion gradient

     • partial intracellular buffering of H+ drives O₂ from hemoglobin (Bohr effect)

     • O₂ moves down its partial pressure gradient from capillary blood to tissue space

Hemoglobin as a Hydrogen Ion Buffer

• pH Gradient Across RBC Membrane and Respiratory Quotient

  • respiratory quotient (RQ) = mmol CO₂ produced/mmol O₂ consumed (in tissues)

  • respiratory gas exchange ratio (R) = mmol CO₂ exhaled/mmol O₂ inhaled (in lungs)

  • RQ = R in steady state (ventilation matched to metabolic demands)

• • HHB in venous blood is a weaker acid (higher affinity for H+)

  • HHBO2 in arterial blood is a stronger acid (lower affinity for H+)

  • hemoglobin reduction pathway (point A to B)

    • for RQ = 0.7, H+ produced perfectly buffered without fall in pH

  • hemoglobin titration pathway (point B to C)

    • for RQ > 0.7, H+ produced partially buffered with fall in pH

  • slope of titration curve depends directly upon hemoglobin concentration

Blood Gas Contents and Ventilation

• Superimposition of Dissociation Curves

• points N and N': normal operating points during eupnea

• points I and I': shifted points during increased ventilation (hyperventilation)

• points D and D': shifted points during decreased ventilation (hypoventilation)

• Hyperventilation and Hypoventilation Maneuvers

  • changes in V_A cause large changes in PaO₂ and PaCO₂

  • large changes in PaO₂ result in small changes in CaO₂ (flatness of O₂ curve)

  • large changes in PaCO₂ result in large changes in CaCO₂ (steepness of CO₂ curve)

• Significance

  • it is possible to effect changes in blood CO₂ levels without jeopardizing oxygen content of the blood

  • this is important for acid base regulation in health and disease