Oxygen transport in blood

Last update: Oct 26th, 2022

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 contains approximately 200-300 million molecules of Hb.

Hemoglobin molecule inside a red blood cell.

Hemoglobin molecules in a red blood cell.

Hb molecules are composed 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 associate with four O2 molecules. RBCs are considered to be 100% saturated when all their heme groups are bound to O2.

Learn all about cells using this step-by-step guide to tissue identification.

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Hb molecule associating with four O2 molecules.

As RBCs circulate 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 diffuse 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 diffuse out of the plasma and enter the tissue cells. As oxygen molecules dissociate from the Hb, the RBCs change color from red to purple.