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The dynamic carrier

Posted by ubpdqn on April 1, 2011

Hemoglobin conformation change with oxygen binding

Oxygen consumption is usually reported in equivalent volume of oxygen /minute (at 1.01325 kPa and 273 K : the molar volume of a gas  is  \frac {RT}{P}=\frac{8.314 \times 273}{101.325}=22.4 mL/mmol ).

In resting state oxygen consumption  is approximately 250 mL/minute (11.2 mmol/minute).

The solubility coefficient of oxygen in blood is 0.03\text{ mL}\cdot\text{L}^{-1}\cdot\text{mm Hg}^{-1}.

At a partial pressure of  oxygen  of 100 mm Hg, the amount of dissolved oxygen per litre of blood is, therefore, 3 mL/L (0.13 mmol/L).  Just to deliver the volume of oxygen consumed would require a cardiac output of over 80 L/minute  ( with heart rate of over 1000/minute). Even more is required to achieve oxygen concentrations to allow diffusion of oxygen to the tissues.

Hemoglobin residing in red cells is the dynamic oxygen carrier that allows oxygen delivery to the tissues:

  • Hemoglobin is a globular tetrameric protein, that with heme as a cofactor, can bind four oxygen molecules per molecule of hemoglobin
  • Atomic mass: 65069 u
  • Hemoglobin concentration: 150 g/L  ( approximately 2. 3 mmol/L)
  • Fully saturated oxygen concentration:  200 mL/L ( Hufner constant 1.34 mL/g or 9.2 mmol/L \times 22.4 mL/mmol)
  • red cell count:  5 \times 10^{12}\text{/L}
  • hematocrit (fraction of blood volume due to red cells): 0.40
  • mean corpuscular volume:  hematocrit/red cell count = 8 \times 10^{-14} \text{L}
  • mean hemoglobin per cell (mean corpuscular hemoglobin) = Hb/red cell count = 3 \times 10^{-11}
  • mean concentration of hemoglobin per red cell (mean corpuscular hemoglobin concentration) = Hb/ hematocrit = 375 g/L

The hemoglobin demonstrates cooperative binding with oxygen and this explains the oxygen dissociation curve. This is explored in the post Cooperation.

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