GAS TRANSPORT IN THE BLOOD

 

The gases are carried in the blood in two forms: dissolved in the plasma or combined with HEMOGLOBIN. 

Hemoglobin within red blood cells is able to combine rapidly and reversibly with oxygen to dramatically increase the solubility of oxygen in blood.  Consider that the typical cardiac output is 5L blood/min and resting oxygen consumption is 250 mL/min.  Because of the low solubility of oxygen in aqueous solution, only 3 mL of O2 will dissolve in the plasma fraction of 1 L of arterial blood.  Therefore, only 15mL of dissolved O2 reaches the systemic circulation each minute – not nearly enough to meet normal metabolic demands.  Under normal circumstances, more than 98% of the oxygen in a given volume of blood is transported in RBCs, bound to hemoglobin.  The amount of oxygen that binds to hemoglobin depends on the PO2 of the plasma surrounding the RBCs and the number of available binding sites within the RBC.  The number of potential binding sites depends largely on the total number of hemoglobin molecules in the blood.

Oxygen (O2) forms a rapid and reversible combination with hemoglobin (Hb) to give oxyhemoglobin (HbO2).  This relationship is represented by the Oxygen-Hemoglobin dissociation curve .  Note that the amount of oxygen carried by hemoglobin rapidly increases to a PO2 of 50 mm Hg, but above that the curve flattens out.  This represents the oxygen capacity, the maximum amount of oxygen that can be combined with hemoglobin.

 

The curved shape of the oxygen dissociation curve confers several physiologic advantages.   The flat upper portion of the curve means that even if the PO2 in alveolar gas drops somewhat, loading of O2 onto Hb will be little affected.  In other words, the system can continue to operate normally with slight drops in available oxygen.   Conversely, the steep lower part of of the curve means that the peripheral tissues can extract large amounts of oxygen of hemoglobin for only minute drops in capillary PO2, facilitating diffusion of oxygen into the tissues that need it.

 

The oxygen dissociation curve is shifted to the right (oxygen affinity for hemoglobin is REDUCED – harder to load oxygen, easier to unload for a given PO2) by an increase in:

  • Hydrogen concentration (decrease in pH)
  • PaCO2 (Bohr effect)
  • temperature
  • concentration of 2,3-diphosphoglycerate (2,3-DPG) in the red blood cells.

Opposite changes cause the curve to shift to the left.

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