Archive for the ‘ L1-02 & CO2 Carriage ’ Category

The Respiratory Function of The Blood;O2 & CO2 Carriage

What is needed to ensure adequate O2 supply & delivery?

•Lung & pulmonary capillary exchange area to extract O2 from air

•Specialized carrier to increase O2 carrying capacity

•Circulatory system to transport O2 to tissues

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Gas Diffusion
(remember factors affecting gas diffusion)

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What are Factors Regulating O2 Delivery to Tissues?

•Main factors affecting O2 delivery to tissues:

1- O2 content in the arterial blood

2- Blood flow (Cardiac output; C.O)

Arterial blood content of O2 = physically dissolved O2 + O2 bound to haemoglobin (HbO2)
Picture3Transport of O2 (O2 Carriage)

Physically dissolved: ~1.5%

transported dissolved in plasma(0.3ml/100 ml blood)

•Hb bound: ~ 98%

transported in red blood cell in association with hemoglobin (Hb)

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Q. What is normal RBCs count?

A. 5 millions / mm3

Hemoglobin binding:

-On Heme groups:

  1. O2
  2. CO

-On globin chains:

  1. CO2
  2. H+
  3. Nitric oxide (NO)
  4. 2,3 DPG

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Hb Binding with O2

•It is reversible combination

•Each Hb molecule binds 4 O2 molecules

Hb + 4O2 ↔ Hb(O2)4;

(oxygenation not oxidation reaction)

• HHb (reduced Hb) + O2 (lungs) ↔ (tissues)  HbO2 (oxyHb) + H+

How Much O2 is bound to Hb?

•The amount of O2 attaches to 1g of Hb =1.34 ml

•Thus, in a person with normal range of Hb (15  g /100 ml blood)

The O2 transported as Hb bound (i.e. Hb carrying capacity)

= 1.34 ml O2/g Hb x 15g Hb/100ml blood

= 20.1 ml O2/100ml arterial blood

Hb saturation with O2 (SbO2)

•The extent of O2 combination with Hb is termed % saturation of HB (SbO2)

•It is fully saturated when the 4 subunits bind O2

SbO2 = O2 content in blood / Hb carrying capacity x 100

•At PO2 of 100 mm Hg at pulmonary capillaries, the SbO2 is 97.5% (because of physiologic shunt adding some venous blood)

Effect of PO2 on Hb saturation with O2

•The relation between PO2 & SbO2 is drawn producing oxyHb dissociation curve

•With ↑PO2 , the SbO2 ↑

•In the pulmonary capillaries at PO2 100 mm Hg the SbO2 is 98%

•In the tissues at PO2 40mm Hg, the SbO2 is 75%

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The OxyHb Dissociation Curve

•Sigmoidal or s-shaped curve (O2 binding enhance more binding of O2 to Hb)

•The upper part is nearly flat referring to association of O2 to Hb; loading of O2 in the lungs capillaries.

•Even when alveolar O2 is decreased from 100 to 70 mm Hg, only arterial O2 concentration is reduced by 1ml.

Picture9•The lower slope part between PO2 50-20 m Hg (Dissociation part)

•Is important in tissues capillaries for unloading of O2

•For a relatively small change in PO2, there is reduction in O2 content by ~50%

•Transition from association to dissociation parts of the curve is normally at PO2 of 60 mm Hg (above it flat, below it steep)

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Factors affecting SbO2; Shift of oxyHb curve

Picture11Factors affecting SbO2; Shift of oxyHb curve

•Shift to the right means reduced affinity of Hb to O2 (more unloading of O2; more O2 is liberated for a given decrease in PO2)

•Shift to the left means increased affinity of Hb to O2 (less O2 is freed to the tissues)

•Shifts have little effects on association part of the curve (loading of O2 in lungs) but have important effect on tissues (unloading of O2).

•A good index for shift of oxyHb dissociation curve is P50

P50 : is the PO2 at which Hb is half saturated with O2

•The greater the P50, the lower the affinity of Hb for O2 (Rh. Shift).

Factors affecting oxyHb curve; Shift of oxyHb curve to Right

•Factors that shift curve to Rh. are markers of ↑ O2 demand, therefore Hb binding to O2 is ↓

•E.g. during exercise, there is ↑ CO2 production, [H+], heat (temperature)

•O2 demand is ↑ during exercise, so less affinity between Hb & O2 is needed

Picture12•Thus, the ↑PCO2 & [H+], temperature will shift curve to Rh. Releasing more O2 from Hb to tissues.

•Such effect of PCO2 & H+ is known as the Bohr Effect (coupling of O2 & CO2 transport)

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BiphosphoGlycerate (2,3 DPG):

•An intermediate product of glycolysis pathway in RBCs that normally shift curve to Rh.

•It binds to B chain of deoxyHb;

HbO2 + 2,3 DPG ↔ Hb-2,3DPG + O2

•Its conc. is ↑during conditions of chronic hypoxia (e.g. anaemia, high altitudes) → ↑ release of O2 to tissues (important adaptive mechanism for hypoxia).

Factors affecting oxyHb curve; Shift of oxyHb curve to the left

•↓ PCO2 , ↓ H+, body temp.,

•In the lungs, there is a ↓ PCO2 (diffusion of CO2 from blood to alveolar air) & [H+]

•Thus shifting curve to Lt. (↑affinity of Hb to O2)

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Fetal Hb has higher affinity for O2 than adult Hb

•Because of poor binding between 2,3 DPG & gamma chain of Hb F

•This higher affinity of Hb F for O2 to enhance movement of O2 from maternal blood to foetus.

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CO2 Transport

About 5ml CO2 is added by tissues to each 100 ml of arterial blood

•This is termed Tidal CO2

•Tidal CO2 is transported in 3 forms

  1. 7% dissolved in Plasma
  2. 93% in RBCs;

-23% as  Hb-CO2

-70% as HCO3

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In the tissues & in the Lungs

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Haldane Effect

•States that Binding of O2 with Hb (in the lungs) promotes dissociation of CO2 from blood

•Thus, as CO2 promotes unloading of O2 in tissues (Bohr effect), O2 promotes displacement of CO2 from blood into alveoli

•This is because oxyHb is more acidic with less tendency to combine with CO2 & releases more H+ which reacts with HCO3 forming H2CO3 that dissociates again into H2O + CO2

CO2 Dissociation Curve

•Linear relationship; blood CO2 content increases with the increase in PCO2

•Reduced Hb has higher affinity to CO2 (Haldane effect)

•At any PCO2, affinity of oxyHb to CO2 is less (O2 shift CO2 curve down & to Rh)

•At PCO2 of 40 mm Hg (arterial),

CO2 content = 50 ml/100ml

•At PCO2 of 46 mm Hg (venous),

CO2 content = 55ml/100ml

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Summary of Gas Transport

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Remember Basic Definitions

•O2 Capacity or Hb carrying capacity: the maximum quantity of O2 that can bind with Hb

= 1.34 ml O2 / g Hb

•O2 Content; how much O2 in the blood

= physically dissolved+ chemically bound

•Haemoglobin oxygen saturation (SbO2);

The % of all the available heme binding sites saturated with O2

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