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• The oxygen hemoglobin dissociation curve describes the relation between partial pressure of oxygen (X axis) & oxygen saturation (Y axis).

• The oxyhemoglobin curve is either sigmoid or 'S' shaped curve.

• A hemoglobin molecule can bind to four oxygen molecule ; the binding of 1^st molecule is difficult but 2^nd & 3^rd molecule is facilitated & gain binding of the 4^th molecule is difficult as the tendency is to dissociate.

• At pressure of 60mmHg the curve becomes flat, which means O₂ content doesn't change significantly with large increase in PO_2.

• When the alveolar Po2 is decreased to as low as 60mmHg, arterial hemoglobin is still 89% saturated with oxygen.

• The tissues still remove 5ml of oxygen from 100ml of blood passing through the tissue; to remove this oxygen, the pO2 of venous blood falls to 35mmHg from normal level of 40mmHg.Thus the Po2 hardly changes despite of drop in alveolar Po2.

What is P50?

• It is the partial pressure of oxygen in blood at which Hb is 50% saturated. Typically it is 26.6mmHg,

• An increase in P50 indicates shift to right which means larger PO2 is needed to attain 50% oxygen saturation , indicating decreased affinity.



What is Bohr effect ?

• A shift of the oxygen-hemoglobin dissociation curve to right in response to increase in blood carbondioxide and hydrogen ions has a significant effect by enhancing the release of oxygen from blood in tissues and enhancing oxygenation of the blood in lungs. This is called bhor effect.

   

• As the blood passes through tissues the carbondioxide diffuses from the tissue cells into blood. This increases blood PCo2, which in turn raises the blood carbonic acid level and hydrogen ion concentration.

• These effect shift the oxygen-hemoglobin dissociation curve to right and downward forcing oxygen away from the hemoglobin and therefore delivering increased amounts of oxygen to the tissues.

• In the lungs , carbondioxide diffuses from blood into alveoli and reduces blood PCo2 and decreases hydrogen ion concentration, shifting the oxygen-hemoglobin dissociation curve to the left upward.

• Therefore the quantity of oxygen that binds with hemoglobin at any given alveolar Po2 becomes considerably increased, thus allowing greater oxygen transport to the tissues.

FACTORS AFFECTING SHIFT OF OXYGEN DISSOSCIATION CURVE

Transport of carbondioxide

• The dissolved carbondioxide in the blood reacts with water to from carbonic acid inside the RBC catalysed by the enzyme carbonic anhydrase. This accounts for 70% of transportation of carbondioxide from tissues to lungs.

• The carbonic acid dissociates into hydrogen ions and bicarbonate immediately.

• Most of the hydrogen ions combine with hemoglobin in the RBC as hemoglobin is a powerful acid base buffer.

• Large amount of bicarbonate ions diffuse from red cells into plasma, while chloride diffuse into the red cells by special bicarbonate chloride carrier protein in the RBC membrane.

• Thus the chloride content in the venous blood is greater than that of arterial red cells, a phenomenon called chloride shift.

CO₂ + H₂0 ↔ H₂CO₃ ↔ H + H⁺ CO₃^-

• Carbondioxide reacts with amine radicals of hemoglobin molecule to form carbaminohemoglobin, this is a reversible reaction and carbondioxide is released easily into alveoli. About 20 -30% is transported by this mechanism.

What is Haldane effect ?

• The binding of oxygen with hemoglobin tends to displace carbondioxide from the blood . This effect is called haldane effect.

• The haldane effect results from single fact that combination of oxygen with hemoglobin in the lungs causes the hemoglobin to become a stronger acid. This causes displacement of carbondioxide from blood and into alveoli by

  • Acidic hemoglobin has less tendency to combine with carbondioxide to form carboxyhemoglobin, thus displacing much of carbondioxide from the blood.

  • Increased acidity of hemoglobin also causes it to release an excess of hydrogen ions and these bind with bicarbonate ions to from carbonic acid; this dissociates into water and carbondioxide, and the carbondioxide is released from blood into the alveoli and finally into air.

Oxygen Transport

The concentration of oxygen in blood, also called O2 content is the summed contribution of O2 that is bound to hemoglobin and O₂ is dissolved in plasma.

Hemoglobin bound O₂

HbO₂ =1.34 * Hb * SO₂

• Hb is hemoglobin concentration in blood

• 1.34 is oxygen binding capacity of hemoglobin, that means each gram of hemoglobin will bind with 1.34ml of oxygen.

Dissolved O₂

• Concentration of dissolved oxygen in plasma is determined by the solubility of oxygen in water and the partial pressure of oxygen in blood.

• Dissolved oxygen = 0.003 * PO₂

Arterial Oxygen Content

• CaO₂ = (1.34 * Hb * SaO₂) + 0.003 * PaO₂

• Oxygen content is the sum of hemoglobin bound oxygen and dissolved oxygen. The concentration of dissolved oxygen in plasma is so small that is usually eliminated from the oxygen content equation. So the simplified Oxygen content equation is

  • O₂ content =1.34 * Hb * SO₂

Venous Oxygen Content

• Concentration of the oxygen is also calculated in a similar fashion.

• CvO₂ = (1.34 * Hb * SvO₂) + (0.003 * PvO₂)

Oxygen Delivery

• Oxygen that enters the bloodstream in the lungs is carried to the vital organs by the cardiac output. The rate at which this occurs is called the oxygen delivery (DO₂). It describes the volume of oxygen that reaches the systemic capillaries each minute (ml/min).

• DO₂ = Q * CaCO₂ * 10

• Where Q is CO.

• 10 is used to convert the CaO₂ from ml/dL to ml/L; Do2 will be expressed in ml/min.

Oxygen Uptake

• When blood reaches the systemic capillaries, oxygen dissociates from hemoglobin and moves into tissues. The rate at which this occurs is called the oxygen uptake.

• VO₂ = Q * (CaO₂ – CvO₂) * 10

• This method of measuring VO₂ is called reverse Fick method.

• This equation can be modified into VO₂ = Q * 1.34* Hb*(SaO₂ – SvO₂).

Oxygen Extraction Ratio

• It is the fractional uptake of oxygen from systemic microcirculation and its equivalent to the ratio of oxygen uptake to oxygen delivery.

• OE = VO2/D02 (Multiplying with 100 will express it as percent)

What is SO₂ ?

• SO₂ = It is the ratio of oxygenated hemoglobin to total hemoglobin in blood (HbO₂/ total Hb).

What is respiratory quotient ?

• The respiratory exchange ratio is the ratio of carbondioxide output to oxygen uptake .

• RQ in person with normal diet is 0.825.

• Whereas in person consumes only carbohydrate diet RQ is 1.o & Only fat RQ is 0.70.

Updated on 26/6/2013.

Reference

• Guyton & Hall

• The ICU ; By Paul marino.

• Wikipedia