VPB 121: Veterinary Physiology - II (2+1)

OXY HEMOGLOBIN DISSOCIATION CURVE

• The Loading and unloading of O₂ ability of the Hb in the form of a graph referred as oxy – hemoglobin dissociation curve.
• This graph is obtained when the Hb is allowed to equilibrate with various PO₂ and the values of percentages of Hb saturation (X axis) against the PO₂ (Y axis), are plotted. For oxy Hb the curve is “S” or sigmoid shaped and for myoglobin the dissociation curve is rectangular hyperbola in shape.
• Under normal conditions, at a PO₂ of 100mm. Hg, blood leaving the lungs is 95 – 98% saturated with O₂. Further increase in PO₂ do not increase O₂ carrying capacity of blood but increase the amount of O₂ in physical solution according to Henry’s law.
• One gram of Hb can transport 1.34 ml of O_2. The average hemoglobin concentration is 15 g/ 100 ml of blood. In atrial blood at a PO₂ of 100 mm.Hg, Hb is 97.5% saturated with O₂ and transport 15 x 1.34 x 97.5/100 = 19.6 ml / 100 ml or 19.6 volumes percent.

• 100 ml of blood carries 19.9 ml of O₂ of which 19.6 ml in combination with hemoglobin and remaining 0.3 ml in physical solution As the arterial blood reaches the tissue, O₂ is unloaded from the blood to tissues, the O₂ saturation falls to about 72% in venous blood.
• At 72% saturation of O₂, the blood will have 14.5 ml O₂. Hence each 100 ml blood unloads approximately 5 volume percent of O₂ to tissues. PO₂ of venous blood is 40 mm Hg.
• Shift of dissociation curve to right results is greater release of O₂ from oxy Hb due to decreased affinity of Hb to O₂. A shift to left increases the affinity of Hb to O₂ and decreases O₂ released from Hb. Variation occur in O₂ dissociation of foetus and adults.
• The extent of oxygen dissociation from the oxy hemoglobin dissociation curve is influences by.
  • PO₂
  • PCO₂
  • H⁺ ion concentration
  • Temperature
  • Concentration of DPG (2 – 3 diphosphoglycerate) in erythrocytes.
• Increase in H⁺ ion concentration and CO₂ level shifts the curve to down the right as also increase in temperature and 2 – 3 DPG. The 2-3 DPG level increases in hypoxia. Shifting the curve to down and right causes increase in release of O₂ from the Hb.
• The shift of oxy-Hb dissociation curve to down and right by increased CO₂ tension is termed as Bohr’s effect. In tissues as O₂ is unloaded, CO₂ is simultaneously taken up by blood and this enhances unloading of O₂.