Red blood cells (erythrocytes)

Author: Kevin Kuschel MD

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Erythrocytes (red blood cells or RBC) are blood cells whose primary function is to transport oxygen and carbon dioxide through the blood.

Structure

Erythrocytes have a biconcave disc shape, which means they are indented in the middle and raised along the margins. Due to their shape, RBCs appear pale in the middle and darker along the edges. Their image descriptiondiameter is 7.8 µm, with a peripheral thickness of 2.6 µm and a central thickness of 0.8 µm. The biconcave shape provides RBCs with more surface area than other spherical cells of the same diameter. The biconcave shape also makes RBCs more flexible, which helps them flow through the narrow openings of the capillaries more easily. A number of cytoskeletal proteins, called integral and peripheral proteins, help maintain the shape of erythrocytes.

Unlabelled image of a red blood cells
Diameter of a red blood cell
The diameter of a red blood cell.
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As they mature, RBCs extrude their nucleus and other organelles. Mature erythrocytes do not possess nuclei or any other cell organelles. Instead, they are filled with a protein called hemoglobin (Hb or Hgb), which is specialized for transporting oxygen and carbon dioxide.

The average lifespan of an erythrocyte is around 120 days. Aged cells are mostly removed from the bloodstream by macrophages in the spleen, bone marrow, and liver. This process removes 90% of aged erythrocytes from circulation. The remaining 10% break down within the circulation (hemolysis), releasing their contents (e.g., hemoglobin) into the bloodstream.

The production of new erythrocytes occurs in the red bone marrow through a process called erythropoiesis. This process is primarily stimulated by the hormone erythropoietin, which is released from the kidneys and helps maintain the optimal number of erythrocytes.

Learn to identify different cells under the microscope with these interactive quizzes and labelling diagrams.

Functions

The main function of erythrocytes is to transport oxygen and carbon dioxide through the bloodstream. In addition, they play an important role in maintaining the body’s optimal acid-base balance because the hemoglobin in erythrocytes can buffer excess hydrogen ions (H+) and contribute to counteracting acidosis.

As blood flows through the image descriptiontissues, hemoglobin accepts CO2 from surrounding cells and releases O2.

Hemoglobin molecules accepting CO2 and releasing O2 animation slide 1
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Hemoglobin accepting CO2 and releasing O2.

The loss of O2 causes the colour of hemoglobin (and RBCs) to change from red to purple. The situation rapidly reverses in the image descriptionlungs, where the hemoglobin bonds to O2 and releases CO2.

Learn more about the histology of blood with this comprehensive guide.

The hemoglobin molecules bonding with O2 and releasing CO2 animation slide 1
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The hemoglobin molecules bonding with O2 and releasing CO2 animation slide 10
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Hemoglobin bonding with O2 and releasing CO2.
Red Blood Cells transport respiratory gases through the blood stream. Review the functions of red blood cells in this interactive tutorial.
Hemoglobin molecules in an RBC animation slide 8
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Hemoglobin molecules in RBCs.

An enzyme called image descriptioncarbonic anhydrase is also made and stored by RBCs.

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Carbonic anhydrase enzyme stored in RBCs

In the image descriptiontissues, carbonic anhydrase catalyzes (accelerates) a reversible reaction that converts CO2 to HCO3- (bicarbonate ions).

Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 1
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Carbonic anhydrase catalyzing a reversible reaction converting CO2 into HCO3-.

Most of the CO2 that enters the RBCs is converted to this water-soluble ion and released into the plasma. In the image descriptionlungs, bicarbonate ions enter the RBCs from the plasma and are converted back to CO2, which is exhaled.

Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 1
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Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 11
Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 10
Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 9
Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 8
Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 7
Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 6
Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 5
Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 4
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Bicarbonate ions entering the RBCs from the plasma and converting back into CO2.

RBC Count and other measurements

The RBC count is one of the tests performed within the complete blood count (CBC). Besides the RBC count, this test also includes counts of white blood cells and platelets, as well as a series of tests related to erythrocytes: measurements of hemoglobin, hematocrit, mean corpuscular volume (MCV), mean corpuscular hemoglobin concentration (MCHC), and mean cell hemoglobin (MCH). The latter three tests (MCV, MCHC, and MCH) are called the RBC indices. Now let’s review the RBC count and other tests related to erythrocytes:

The RBC count provides information about the number of erythrocytes in the blood. The normal values differ between males and females:

  • Male: 4.3-5.9 x 10^12/L
  • Female: 3.5-5.5 x 10^12/L

A decreased RBC count (and/or reduced hemoglobin concentration) is a state called anemia. An elevated RBC count is called polycythemia. When this abnormality occurs, it can increase blood volume, pressure, and viscosity.

The hemoglobin test is used to determine the amount of hemoglobin per deciliter of blood:

  • Male: 13.5 to 18.0 g/dL
  • Female: 12.0 to 15.0 g/dL

The hematocrit test (Hct) measures the percentage of whole blood that is made up of red blood cells (RBCs). Physiological values are:

  • Male: 40%-54%
  • Female: 36%-48%

MCV (Mean Corpuscular Volume) measures the average size of red blood cells and is expressed in femtoliters, while MCHC (Mean Corpuscular Hemoglobin Concentration) measures the average concentration of hemoglobin in red blood cells and is measured in grams per deciliter. The physiological range of MCV is 87 ± 7 fl, while the range of MCHC is 34 ± 2 g/dl. MCH (Mean Corpuscular Hemoglobin) measures the amount of hemoglobin per red blood cell and is expressed in picograms. The normal values for MCH are 29 ± 2 pg per cell.

Learn how to identify different cells under the microscope with these interactive histology quizzes and labelling exercises.

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
  • Mescher, A. L. (2013). Junqueira’s Basic Histology (13th ed.). McGraw Hill.
  • Pawlina, W. (2016). Histology Text and Atlas (7th ed.). Wolters Kluwer.
  • National Center for Biotechnology Information. (2005). Table 1, Complete blood count. In A. D. Baxevanis & B. F. Francis Ouellette (Eds.), Bioinformatics: A Practical Guide to the Analysis of Genes and Proteins (3rd ed.). NCBI Bookshelf. Retrieved April 20, 2023, from https://www.ncbi.nlm.nih.gov/books/NBK2263/table/ch1.T1/
  • Grossman, S. C., & Porth, C. M. (2014). Porth’s Pathophysiology (9th ed.). Wolters Kluwer.
  • National Center for Biotechnology Information. (2021). Hematocrit. In StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. Available from: https://www.ncbi.nlm.nih.gov/books/NBK542276/
  • National Center for Biotechnology Information. (2021). Anemia. In StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. Available from: https://www.ncbi.nlm.nih.gov/books/NBK499994/
  • National Center for Biotechnology Information. (2021). Physiology, Blood Volume. In StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing. Available from: https://www.ncbi.nlm.nih.gov/books/NBK260/
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