Red Blood Cells (RBC)

Red Blood Cells (RBC)Synonyms: erythrocytes, RBC count

What is RBC?

Erythrocytes are red blood cells delivering oxygen to the body tissues and maintaining biological oxidation processes occurring in the body.

Red blood cells are the most common blood cells in the body. They account for 40-45% of the total blood volume. RBCs are produced in the bone marrow under the influence of the renal hormone erythropoietin. The body produces about two million red blood cells every second. The life span of red blood cells is about 120 days. The destruction (hemolysis) of RBCs occurs in the liver and spleen.

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The mature red blood cell is bright red and has the shape of a biconcave disc with a flattened center. Unlike other cells, red blood cells do not have a nucleus, they are covered with a membrane of lipids and proteins, they can easily change shape, which helps them to penetrate through various blood vessels in the body. Changes in the appearance (form) of red blood cells are rare, but they can indicate different pathologies. On the surface of red blood cells are antigens that can be used to determine the blood group of people.

The main function of RBC is to transport oxygen. Moreover, red blood cells return carbon dioxide to the lungs, which is subsequently exhaled. This is because RBCs contain hemoglobin (protein), which easily comes into contact with oxygen and carbon dioxide. Red blood cells are the main carrier of drugs to vital organs.

Red blood cells are involved in the regulation of acid-base balance in the body and ionic equilibrium. They also participate in immune processes, absorbing toxins and harmful substances, which are then destroyed. Red blood cells play a significant role in the regulation of blood coagulation system (thromboplastin formation).

Scientists managed to grow red blood cells from human stem cells in the lab. This research could help to create blood suitable for transfusions.

RBCs in Pregnancy

During pregnancy, the blood volume increases. But the fluid portion of blood increases faster than the red blood cell number. As a result, there is a relative decrease in the number of red blood cells, while their absolute number remains within the normal range.

RBCs in Children

RBC’s reference range in children depends on age. Newborns have a higher number of RBCs because they have higher oxygen levels in the womb and therefore, they need more RBCs to transport the oxygen. In several weeks the RBCs number decreases and gradually rises to the adult level by puberty.

Units of Measurement

The number of red blood cells can be expressed in international units:

  • 1012 cells/liter
  • T/L – tera per liter (Tera = 1 trillion)

Conventional units:

  • millions per cubic millimeter (1000000/mm3, 106/mm3)
  • millions per microliter (1000000/µL, 106/µL)
  • M/mm3 (Mega = 1 million = 106)
  • M/µL
  • cells/mm3
  • cells/µL

Conversion factors:

Cubic millimeter (mm3) = microliter (mcL, µL)1012 cells/liter = T/L = 106/mm3 = 106/µL = M/mm3 = M/µL

Reference Range

One adult has about 5.2 million red blood cells per cubic millimeter of blood (40-45% of the total volume). A red blood cell count is included in a complete blood count (CBC).

RBC count may vary in different laboratories, but usually, RBC normal values fall into the following ranges:

  • Men – 4.7 to 6.1 million cells per microliter (mcL)
  • Women – 4.2 to 5.4 million/mcL
  • Children – 4.0 to 5.5 million/mcL

RBC Normal Range in Pregnancy

Normal ranges for women who are pregnant differ from those who are not pregnant. The reason is that during pregnancy, the volume of circulating blood increases due to the liquid part of the blood, while the number of RBCs grows more slowly. Therefore, the following RBC ranges are considered normal during pregnancy:

  • First Trimester – 3.42 to 4.55 million cells per microliter (106/mcL)
  • Second Trimester – 2.81 to 4.49 million/mcL
  • Third Trimester – 2.72 to 4.43 million/mcL

RBC Normal Range in Children

Red blood cells normal range differ depending on the age of a child. RBCs level in the newborn is increased.

According to a study conducted on the basis of extensive data from the National Health and Nutrition Examination Survey, NHANES, the level of red blood cells in children and adolescents is as follows:

Red Blood Cells
Reference interval
(million cells/mcL)

High RBC Count

High red blood cell count (erythrocytosis, polycythemia) is generally caused by the thickening of the blood as a result of a deficiency of fluid in the body (dehydration), and is also associated with some rare diseases of the hematopoietic system.

An increase in red blood cell count sometimes may be caused by physiological factors (e.g. severe stress). Also, high RBC count may be seen in people living in a high altitude (there is not enough oxygen), which is considered normal for them. Red blood cells in the blood of pilots, climbers, and athletes are physiologically elevated. RBC count returns to normal if a person changes his/her living conditions and lifestyle. An increase in RBC volume may be caused by certain medications (anabolic steroids, erythropoietin, antibiotics, iron supplements).

An increase in red blood cell levels may occur due to the problems with the production of red blood cells in the bone marrow – primary polycythemia, and due to other conditions or diseases that increase the number of red blood cells in the blood – secondary polycythemia (erythrocytosis). In most cases, polycythemia is “secondary”. Primary polycythemia is relatively rare.

Primary polycythemia occurs as a result of acquired or inherited genetic mutations that cause increased production of red blood cells.

Secondary polycythemia is usually caused by an increased level of circulating erythropoietin (EPO) in the blood. The causes of secondary polycythemia usually include:

  • chronic obstructive pulmonary disease
  • pulmonary hypertension
  • chronic heart disease
  • sleep apnea
  • poor blood supply to the kidneys
  • some types of tumors
  • smoking


Symptoms of increased red blood cell count are quite extensive. Some people with polycythemia may not have any symptoms at all. Most symptoms of secondary polycythemia are associated with the underlying disease.

Symptoms of high RBC levels:

  • fatigue
  • headaches
  • shortness of breath
  • itching skin, especially after a shower
  • joint pain
  • sleep disturbance

Low RBC Count

Low red blood cell count (erythropenia) could be a symptom of anemia due to blood loss, hemolysis, or vitamin B9 (folic acid) and B12 (cyanocobalamin) deficiency.

The physiological cause of decreased RBC count can be hyperhydration (excess water in the body), pregnancy.


Low red blood cell count (anemia) can cause the following symptoms:

  • fatigue
  • dizziness, weakness
  • pale skin
  • shortness of breath
  • increased heart rate

Possible causes of low RBC count are as follows:

  • anemia
  • nutritional deficiencies (a lack of iron, folic acid, vitamins B6 or B12)
  • hemolysis (destruction of red blood cells)
  • bleeding (internal or external)
  • erythropoietin deficiency (the main cause of anemia in chronic kidney disease)
  • bone marrow failure

Some drugs can reduce red blood cell count (e.g., phenobarbital, chemotherapy drugs, chloramphenicol, hydantoins, quinidine).

Low RBC in Pregnancy

Mild anemia (low RBC count) is common during pregnancy. In pregnancy, the volume of blood increases, and blood becomes more diluted. Therefore, in comparison to increased plasma volume (the liquid part of the blood), it seems that there are too few red blood cells.

But significant deviations from the normal range can indicate serious complications or diseases. Reduced red blood cell count may indicate the development of anemia.

Complete Blood Count (CBC) Interpretation