Red blood cells (RBCs)


Red blood cells (RBCs) are the most common type of blood cell in the human body. They are sometimes called erythrocytes. Under the microscope, red blood cells look like small, round, disc-shaped cells with a pale center. Their main job is to carry oxygen from the lungs to the rest of the body and to carry carbon dioxide back to the lungs so it can be exhaled.

Normal red blood cells

What do red blood cells do?

Red blood cells contain a protein called hemoglobin. Hemoglobin attaches to oxygen in the lungs and releases it into the body’s tissues where it is needed. Hemoglobin also picks up carbon dioxide, a waste product, and carries it back to the lungs. The continuous delivery of oxygen and removal of carbon dioxide are essential for keeping the body’s organs and tissues alive and functioning properly.

Where are red blood cells found?

Red blood cells are found in the bloodstream, where they make up about 40 to 45 percent of the blood’s volume. They are produced in the bone marrow, the soft tissue inside bones. The body constantly makes new red blood cells, and each one normally lives for about 120 days before being removed from circulation by the spleen and liver.

Common medical conditions associated with changes in red blood cells

Changes in the number, size, or shape of red blood cells can be a sign of disease. Pathologists and doctors often look at red blood cells in blood tests and under the microscope to help diagnose health problems. Common conditions include:

  • Anemia: This means there are not enough red blood cells or not enough hemoglobin. Anemia can cause fatigue, weakness, and shortness of breath. It has many possible causes, including iron deficiency, chronic disease, blood loss, or bone marrow problems.

  • Polycythemia: This means there are too many red blood cells. It can make the blood thicker and increase the risk of clots. Polycythemia may be caused by bone marrow disorders, chronic lung or heart disease, or living at high altitude.

  • Sickle cell disease: An inherited condition where red blood cells take on an abnormal crescent or “sickle” shape. These cells can break apart easily and block blood flow, leading to pain, infections, or organ damage.

  • Thalassemia: An inherited condition that affects hemoglobin production, leading to anemia and abnormal red blood cell shapes.

  • Vitamin deficiencies: Low levels of vitamin B12 or folate can lead to larger than normal red blood cells, a condition called megaloblastic anemia.

  • Hemolysis: This means red blood cells are breaking down too quickly. Hemolysis can be caused by autoimmune diseases, infections, medications, or inherited conditions.

Questions to ask your doctor

  • What did my test show about my red blood cells?

  • Are the changes in my red blood cells related to anemia or another condition?

  • What is the cause of the abnormal red blood cell count or appearance?

  • Do I need more tests, such as iron studies, vitamin levels, or a bone marrow biopsy?

  • How will these findings affect my treatment or follow-up care?

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