Oxygen and carbon dioxide transport in blood

Author: Scott A. Sheffield MS

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Oxygen transport from the lungs into the blood

Oxygen enters arterial blood by diffusing through the respiratory membrane in the alveoli of the lungs. This occurs because the partial pressure of oxygen (PO2) in the alveoli is higher than that in the blood. Specifically, the PO2 in the alveolus is 104 mm Hg, while in the venous capillary it is 40 mm Hg.

This gradient promotes the diffusion of oxygen into the capillaries until the pressures equalize, that is, when the capillary PO2 reaches 104 mm Hg. At this point, the deoxygenated venous blood becomes oxygenated arterial blood, which then flows to the left atrium of the heart. However, approximately 2% of the blood in the left atrium comes from the bronchial circulation, which is not exposed to air from the lungs and therefore remains deoxygenated. This mixture of oxygenated and deoxygenated blood in the left atrium is called the venous admixture of blood. It causes the PO2 in the blood to decrease from 104 to 95 mm Hg.

Oxygen has poor water solubility, so most of the oxygen is transported by hemoglobin (Hb or Hgb) in erythrocytes (97%). Each erythrocyte typically contains around 200-300 million molecules of hemoglobin, which enables 30-100 times more oxygen transport through the blood than would be possible in dissolved form in blood plasma.

image descriptionHb molecules consist of four subunits (two alpha and two beta). Each subunit comprises 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 image descriptionbond with an O2 molecule. Thus, Hb molecules can associate with four O2 molecules. Red blood cells (RBCs) are considered 100% saturated when all their heme groups are bound to O2. Hemoglobin that’s associated with oxygen is called oxyhemoglobin.

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

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Hb molecule associating with four O2 molecules.

Oxygen transport from the blood into the tissues

In the interstitial fluid surrounding body tissues, the PO2 is 40 mm Hg. Since the PO2 in the capillaries is 95 mm Hg, this difference causes oxygen to image descriptiondiffuse into the tissues until the capillary pressure drops to 40 mm Hg. As oxygen molecules dissociate from the Hb, the RBCs change color from red to purple. Simultaneously, the PCO2 in the tissues is higher than that in the capillaries, prompting CO2 to diffuse into the blood and bind to hemoglobin for transport to the lungs.

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

Carbon dioxide transport

Carbon dioxide leaves the tissues in a dissolved state, but it is transported through the blood in different chemical forms:

  • Dissolved
  • As bicarbonate ions
  • In association with hemoglobin and plasma proteins

CO2 image descriptionexits the cells in a dissolved state. As such, a portion of it is transported dissolved in the plasma. However, this accounts for only 7% of the total transported CO2.

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CO2 moving into the plasma from the tissue fluids.

The largest portion of CO2, 70% of the total amount, is transported in the form of bicarbonate ions (HCO3-) in the plasma. To reach this state, CO2 first enters erythrocytes. Erythrocytes possess an enzyme called carbonic anhydrase. This enzyme catalyzes the chemical reaction between CO2 and water in the erythrocytes, resulting in the formation of carbonic acid (H2CO3). The carbonic acid then dissociates into a hydrogen ion (H+) and a bicarbonate ion (HCO3-). The hydrogen ions increase the acidity, which is buffered by hemoglobin, which combines with these ions. Since the erythrocytes possess a special protein called the bicarbonate-chloride carrier, the HCO3- exits the cell and enters the plasma, in exchange for chloride ions that enter the cell, leaving the plasma.

The remaining 23% of total CO2 is transported by either binding to hemoglobin or plasma proteins in the blood. Even though the majority of CO2 reacts with water in the erythrocytes, a small portion also reacts with hemoglobin, resulting in the creation of carbaminohemoglobin, which is hemoglobin loosely associated with CO2. Another small portion also loosely binds to plasma proteins.

References

  • Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., & Kruse, D. H. (2022). Anatomy and Physiology (2nd ed.). OpenStax. https://openstax.org/details/books/anatomy-and-physiology-2e
  • Hall, J. E., & Guyton, A. C. (2016). Guyton and Hall Textbook of Medical Physiology (13th ed.). Elsevier, Philadelphia PA
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