Transport of O2 and CO2

From Iusmphysiology

Contents 1 Transport of O2 and CO2 1.1 Henry's law of dissolved gases 1.2 Oxygen delivery and consumption 1.3 Hb 1.4 O2 dissociation curve 1.5 FActors decrease Hb O2 affinity 1.6 Arterial vs venous blood 1.7 Calculation of O2 content 1.8 Effects of blood O2 capacity 1.9 CO2 transport 1.10 CO2 Transaport - tissue 1.11 CO2 Transport - lungs 1.12 Carbonic anhydrase reaction 1.13 CO2 and Control of ventilation 1.14 Acid-base balance 1.15 Acid-base baance 1.16 Base excess 1.17 Questions

[edit] Transport of O2 and CO2

[edit] Henry's law of dissolved gases

• The amount of gas dissolved int he liquid is proportional to the partial pressure of the gas in the liquid.
• So 0.3 percent of PO2 is physically dissolved in plasma.
  • Must be dissolved for cells to uptake the O
• PO2 determines how much is dissolved and how much is dissolved dtermines how much cells can take up.
• If you double PO2, the amount of O in plasma doubles.

[edit] Oxygen delivery and consumption

• Normally, about 250 ml O per minute is consumed.
• IN the absence of Hb (no oxygen carrier), the P02
• 5L is a normal cardiac output; 20 is (textbook) maximum.
• 83.8 L / minute of CO would be needed if we had no Hb.

[edit] Hb

• PO2 of 100 with no carrier for O is death.
• Hb increases O content of the blood for a given PO2.
• There are four binding sites.
• OxyHb is Hb with oxygen bound.

[edit] O2 dissociation curve

• Oxygen saturation on the y
  • This is how much Hb has O on it.
• PO2 on the X
• The saturation of Hb is directly dependent on PO2!
• As PO2 goes up, there is an initial exponential increase in saturaiton.
• Eventually, as PO2 goes up, the curve levels off.
  • This means that PO2 decreases faster than saturation, initially.
  • P50 = the PO2 at which Hb is half-saturated.
  • P50 is 27mmHg.
• Recall that PO2 is a measure of the blood.

[edit] FActors decrease Hb O2 affinity

• Hb's affinity for Oxygen is changing as we shift left or right.
• Metabolic biproducts:
  • High levels of PCO2
    • decreases low pH
    • Leads to Bohr effect
    • Shifts dissociation curve right
    • At a given PO2, a lower percent of the Hb is saturated.
    • Bohr's effect occurs whenver there is low pH (respiratory or metabolic)
    • Hb is holding on less well to O
  • Low PCO2:
    • increases pH
    • Shifts curve to the left
    • SAturation is higher at a given PO2
    • Hb is binding better
    • Left shift reduces the P50 (decreases the pressurea t which 50% saturation occurs)

[edit] Arterial vs venous blood

• Oxygen leaves and CO2 enters as blood moves from arterial blood to venous blood
• So pH goes up and p50 goes up
• As oxygen leaves the blood, we work our way down the curve.
  • PO2 is going down.
• CO2 is going up
  • PCO2 is going up.
• So the curve ends up shifting to the right
  • This is important because tissue wants oxygen and Hb's affinity for O is going down.
• Recall that normal venous PO2 is 40.
• This is the bohr effect: decreasing pH maps with decreasing affinity for oxygen.

[edit] Calculation of O2 content

• O2 content is the sum of the oxygen on the Hb and the oxygen in the plasma:
• O2 carrying capacity of Hb is 1.34 ml of O / gm of Hb
• Healthy patient example:
  • O2 content = 19.35 ml O2 / dL
• Anemic patient example:
  • Note that anemics have normal PO2 and have a normal O2 saturation, they just don't have much Hb.
  • O2 content = 6.5 ml O2 / dL

[edit] Effects of blood O2 capacity

• O content! is on the y axis
• PO2 is the x axis

• The anemic pt
  • has a lower oxygen content.
  • PaO2 is 100
  • Saturation is 100
  • Oxygen content is lower
  • Venous PO2 is lower (20s when 40 is lower) because the same amount of O gets used by the tissues but we started with less.

• CO poisoning pt:
  • Hb affinity for CO >>>>> Hb affinity for O
  • Similar to anemia
  • Overall oxygen content levels go down
  • Normal PO2 and normal Hb concentration
  • P50 decreases; the oxygen that IS bound to Hb is bound more tightly
    • This causes less release at the tissue.
  • Left shift of curve with CO

[edit] CO2 transport

• CO2 must get out at the lungs.
• ABout 200 ml of CO2 produced per minute
• 8% of CO2 is dissolved in plasma
• Most of CO2 travels as HCO3- (bicarb)
  • Some of this is in the plasma, some in the Hb
• 11% travels as CO2 on Hb

[edit] CO2 Transaport - tissue

• At the tissue, O2 is on RBC
• O2 comes off, dissolves
• O2 gets picked up by cells.
• CO2 is in the cells
  • Released and dissolve dinto plasma
  • Combines with water to form carbonic acid (H2CO3)
  • Breaks down into H+ and HCO3-
    • Impt for pH
• Bicarb will leave the RBC
  • Cloride is taken in to balance the loss of HCO3- form the RBC.
  • This is called the chloride shift.

[edit] CO2 Transport - lungs

• Run everything in the shift
• Pressures cause O2 and CO2 to move, recall.
• PACO2 = 40, PaCO2 = 46.
• Cl goes out, bicarb comes in, joins H+, turns into CO2.
• Most of the CO2 is carried as bicarb

[edit] Carbonic anhydrase reaction

Any diseases of CA?  Do pt's live?

• Recall bohr: increase CO2 promotes loss of Hb.
• Haldane effect: opposite of Bohr; a lower PO2 carries more CO2
  • At a given PCO2, deoxygenated blood is carrying more CO2 on Hb than does oxygenated blood (which has a higher PO2).
• This matters at the lungs because PO2 is going up so Hb's affinity for CO2 is going down.
  • In fact, all three ways to carry CO2 are decreased (because they are all related through the carbonic anhydrase rxn).

[edit] CO2 and Control of ventilation

What is VA?

• PaCO2 direclty affects pH because CO2 will go through the rxn.
• pH = 6.1 + log HCO3 / 0.03 PaCO2

[edit] Acid-base balance

• Dataport diagram demonstrates relationship between
• Normal pH = 7.4
• EAch line represents a different PCO2
• What regulates pH?
  • PaCO2: as they go up, the pH goes down.
  • Bicarb: acts as a buffer
• Hypoventilate: Point B is respiratory acidosis: PCO2 is going up, pH down
• Hyperventilate: Point C is respiratory alkalosis: PCO2 is low, pH is up

How does bicarb buffer?

[edit] Acid-base baance

• Same points A (normal), B (acidosis) and C (alkolosis).
• B: Hypoventilation
  • High CO2
  • Acidoti
  • Kidneys can retain HCO3- to buffer
  • This causes pH to normalize
  • We call this respiratory acidosis with metabolic
• C: Hyperventilation
  • Low CO2
• Metabolic alkalosis
  • Kidney retians too much HCO3-
  • PCO2 will be normal,

Review this; reread CPS paper.

• This is called a "Davenport diagram"

[edit] Base excess

• Metabolic acidosis: there is not excess bases so it is negative
• etc.

[edit] Questions

• Compensatory mechanisms are never perfect.
• So when doing acid-base problems:
  • First look at the pH (high, low, normal).
  • Now to determine cause:
    • respiratory is CO2
    • metabolic is HCO3-
    • Ask which one is going in the direction of the problem.
    • If CO2 is low the pH goes up, so in an alkalotic pt, if CO2 is low we would call it respiratory alkalosis.
    • Whichever one is going in the direction of the pH problem is causative and whichever is going in the opposite direction is the compensating mechanism.

• stopped here on 02/16/11 at 12PM.