General Structure and Functions of Red Blood Cells

Author: Scott A. Sheffield MS

Last update:

20 to 30 trillion red blood cells (erythrocytes; RBCs) image descriptioncirculate in the bloodstream of an average adult. (The ring-shaped fat-filled cells in the illustration are called Adipocytes)

Cross-section of capillaries and connective tissue fibers.
Cross-section of capillaries and connective tissue fibers with erythrocytes inside a capillary.
Cross-section of capillaries and connective tissue fibers with erythrocytes inside a capillary.
Cross-section of capillaries and connective tissue fibers with erythrocytes inside a capillary.
Cross-section of capillaries and connective tissue fibers with erythrocytes inside a capillary.
Cross-section of capillaries and connective tissue fibers with erythrocytes inside a capillary.
Cross-section of capillaries and connective tissue fibers with erythrocytes inside a capillary.
Cross-section of capillaries and connective tissue fibers with erythrocytes inside a capillary.
Cross-section of capillaries and connective tissue fibers with erythrocytes inside a capillary.
Cross-section of capillaries and connective tissue fibers with erythrocytes inside a capillary.
Cross-section of capillaries and connective tissue fibers with erythrocytes inside a capillary.
Cross-section of capillaries and connective tissue fibers with erythrocytes inside a capillary.
Cross-section of capillaries and connective tissue fibers with erythrocytes inside a capillary.
Cross-section of capillaries and connective tissue fibers with erythrocytes inside a capillary.
Cross-section of capillaries and connective tissue fibers with erythrocytes inside a capillary.
Cross-section of capillaries and connective tissue fibers with an erythrocyte inside a capillary.
Cross-section of capillaries and connective tissue fibers
Red blood cells circulating in the blood stream.

RBCs are small, disc-shaped cells that measure 7 – 8 micrometers (μm) in image descriptiondiameter.

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 fill their cytoplasm with hemoglobin (Hb) molecules, which bind and transport oxygen (O2) and carbon dioxide (CO2).

Mature RBCs are also image descriptionbiconcave in shape, which means they are indented in the middle and raised along the margins.

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

Splitting RBC animation slide 1
Splitting RBC animation slide 2
Splitting RBC animation slide 3
Splitting RBC animation slide 4
Splitting RBC animation slide 5
Splitting RBC animation slide 6
The biconcavity of a red blood cell.

The thinnest area of an RBC normally image descriptionmeasures about 1 μm and the thickest area measures 2-3 μm.

Due to their shape, RBCs appear pale in the middle and darker along the edges.

The biconcave shape provides RBCs with more surface area than other spherical cells of the same diameter. The additional surface area increases the rate of gas (O2; CO2) exchange with the tissues and lungs.

The biconcave shape also makes RBCs more flexible, which helps them flow through the narrow openings of the capillaries more easily.

Splitting RBC animation slide 6
The measurements of the thickest and thinnest areas of a red blood cell
The measurements of the thinnest and thickest areas of a red blood cell.
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Number of Red Blood Cells in the Blood:

As the blood moves throughout the body, it circulates about 20 – 30 trillion red blood cells (RBCs). Even a small drop of blood (1.0 mm3) image descriptioncontains millions of RBCs:

  • 4.2 – 5.4 million RBCs/mm3 in males
  • 3.6 – 5.0 million RBCs/mm3 in females
Drop of blood on a glass slide
Animation showing magnified view of a drop of blood containing a normal RBC count.
Animation showing magnified view of a drop of blood containing a normal RBC count.
Animation showing magnified view of a drop of blood containing a normal RBC count.
Animation showing magnified view of a drop of blood containing a normal RBC count.
Animation showing magnified view of a drop of blood containing a normal RBC count.
Animation showing magnified view of a drop of blood containing a normal RBC count.
Animation showing magnified view of a drop of blood containing a normal RBC count.
Animation showing magnified view of a drop of blood containing a normal RBC count.
Animation showing magnified view of a drop of blood containing a normal RBC count.
Animation showing magnified view of a drop of blood containing a normal RBC count.
Animation showing magnified view of a drop of blood containing a normal RBC count.
Animation showing magnified view of a drop of blood containing a normal RBC count.
Magnified view of a drop of blood containing WBCs, platelets, and a normal RBC count.
Normal RBC count.

Because they lack a nucleus, RBCs stay viable for only about 120 days and must be replaced at the same rate as they disappear from the blood (= 2.5 million cell/second).

  1. image descriptionA high number of RBCs per volume of blood is referred to as polycythemia. When this abnormality occurs, it can increase blood volume, pressure, and viscosity.
  1. image descriptionA low number of RBCs of blood reduces the hemoglobin concentration, which is measured in grams Hb per deciliter (100 milliliters) of whole blood:
    • 14 – 18 grams/dL in males
    • 12 – 16 grams/dL in females

A low hemoglobin concentration, which is called anemia, decreases oxygen (O2) delivery to the tissues.

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

Magnified view of a drop of blood containing WBCs, platelets, and a normal RBC count.
Magnified view of a drop of blood containing WBCs, platelets, and a high RBC count.
Magnified view of a drop of blood containing WBCs, platelets, and a low RBC count.
High and low RBC counts.
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Introduction to the Functions of Red Blood Cells:

Red Blood Cells (RBCs) transport respiratory gases through the blood stream so they can be exchanged at the lungs and tissues.

Involved in this process is the molecule, image descriptionhemoglobin. Each RBC makes and stores about 200-300 million of these complex proteins. There are so many that they take up about a third of the cell volume.

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
Hemoglobin molecules in an RBC animation slide 7
Hemoglobin molecules in an RBC animation slide 6
Hemoglobin molecules in an RBC animation slide 5
Hemoglobin molecules in an RBC animation slide 4
Hemoglobin molecules in an RBC animation slide 3
Hemoglobin molecules in an RBC animation slide 2
Hemoglobin molecules in RBCs.

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
Hemoglobin molecules accepting CO2 and releasing O2 animation slide 10
Hemoglobin molecules accepting CO2 and releasing O2 animation slide 9
Hemoglobin molecules accepting CO2 and releasing O2 animation slide 8
Hemoglobin molecules accepting CO2 and releasing O2 animation slide 7
Hemoglobin molecules accepting CO2 and releasing O2 animation slide 6
Hemoglobin molecules accepting CO2 and releasing O2 animation slide 5
Hemoglobin molecules accepting CO2 and releasing O2 animation slide 4
Hemoglobin molecules accepting CO2 and releasing O2 animation slide 3
Hemoglobin molecules accepting CO2 and releasing O2 animation slide 2
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
The hemoglobin molecules bonding with O2 and releasing CO2 animation slide 12
The hemoglobin molecules bonding with O2 and releasing CO2 animation slide 11
The hemoglobin molecules bonding with O2 and releasing CO2 animation slide 10
The hemoglobin molecules bonding with O2 and releasing CO2 animation slide 9
The hemoglobin molecules bonding with O2 and releasing CO2 animation slide 8
The hemoglobin molecules bonding with O2 and releasing CO2 animation slide 7
The hemoglobin molecules bonding with O2 and releasing CO2 animation slide 6
The hemoglobin molecules bonding with O2 and releasing CO2 animation slide 5
The hemoglobin molecules bonding with O2 and releasing CO2 animation slide 4
The hemoglobin molecules bonding with O2 and releasing CO2 animation slide 3
The hemoglobin molecules bonding with O2 and releasing CO2 animation slide 2
Hemoglobin bonding with O2 and releasing CO2.

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

Carbonic anhydrase enzyme in RBCs animation slide 1
Carbonic anhydrase enzyme in RBCs animation slide 10
Carbonic anhydrase enzyme in RBCs animation slide 9
Carbonic anhydrase enzyme in RBCs animation slide 8
Carbonic anhydrase enzyme in RBCs animation slide 7
Carbonic anhydrase enzyme in RBCs animation slide 6
Carbonic anhydrase enzyme in RBCs animation slide 5
Carbonic anhydrase enzyme in RBCs animation slide 4
Carbonic anhydrase enzyme in RBCs animation slide 3
Carbonic anhydrase enzyme in RBCs animation slide 2
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
Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 16
Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 15
Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 14
Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 13
Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 12
Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 11
Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 10
Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 9
Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 8
Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 7
Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 6
Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 5
Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 4
Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 3
Carbonic anhydrase catalyzing a reaction converting CO2 into HCO3- animation slide 2
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
Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 16
Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 15
Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 14
Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 13
Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 12
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
Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 3
Bicarbonate ions enter the RBCs and are converted back into CO2 animation slide 2
Bicarbonate ions entering the RBCs from the plasma and converting back into CO2.