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Transport of Oxygen in the Blood

Introduction to the Transport of Oxygen in the Blood:

Even though plasma makes up most of the blood volume, it transports a small number (2-3%) of oxygen molecules (O2). The carrying capacity of plasma is limited by the poor solubility of O2 in water.

Most of the O2 (97-98%) is transported by hemoglobin molecules (Hb or Hgb) in red blood cells (RBCs). Each RBC normally image descriptioncontains approximately 200-300 million molecules of Hb.

Hemoglobin molecules in a red blood cell.

Hb molecules are image descriptioncomposed of four subunits (2 alpha; 2 beta). Each subunit is made up of a large, folded polypeptide called globin and a small heme group.

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The four subunits of a Hb molecule.

An iron atom in each heme group can reversibly bond with an O2. Therefore, Hb molecules can image descriptionassociate with four O2 molecules. RBCs are considered to be 100% saturated when all their heme groups are bound to O2.

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Hb molecule associating with four O2 molecules.

As RBCs image descriptioncirculate in the blood stream, Hb molecules bind and release O2. The interaction between O2 and Hb molecules is greatly affected by the partial pressure of oxygen (pO2) in the surrounding fluids.

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RBCs circulating in the blood stream.

In the lung alveoli, the pO2 is about 100 mmHg and in the blood plasma it is about 40 mmHg. The pressure gradient causes O2 to image descriptiondiffuse out the lungs and enter the blood, where it binds to Hb in the RBCs. In response, the RBCs change colour from purple to red.

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O2 diffusing out of the lungs and entering the blood.

In the tissues, the pO2 pressure gradient reverses. The pO2 in the blood plasma is about 100 mmHg and in tissue cells it is about 40 mmHg. This causes O2 to image descriptiondiffuse out of the plasma and enter the tissue cells. As oxygen molecules dissociate from the Hb, the RBCs change color from red to purple.

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O2 diffusing out of the plasma and entering the tissue cells.