Overview of Anemia

ByGloria F. Gerber, MD, Johns Hopkins School of Medicine, Division of Hematology
Reviewed/Revised Apr 2024

Anemia is a condition in which the number of red blood cells or the level of hemoglobin is low.

Red blood cells contain hemoglobin, a protein that enables them to carry oxygen from the lungs and deliver it to all parts of the body. When the number of red blood cells or the hemoglobin level is reduced, the blood cannot carry an adequate supply of oxygen. An inadequate supply of oxygen in the tissues causes the symptoms of anemia.

Causes of Anemia

The causes of anemia are numerous, but most can be grouped within 3 major mechanisms that produce anemia:

  • Blood loss (excessive bleeding)

  • Inadequate production of red blood cells

  • Excessive destruction of red blood cells


Anemia caused by excessive bleeding

Anemia may be caused by excessive bleeding. Bleeding may be sudden, as may occur as a result of an injury or during surgery. Often, bleeding is gradual and repetitive (chronic bleeding), typically due to abnormalities in the digestive or urinary tract or heavy menstrual periods. Chronic bleeding typically leads to low levels of iron, which leads to worsening anemia (see Iron Deficiency Anemia). Bleeding from the gastrointestinal tract may be slow and gradual (eg. polyps or cancers) or sudden and massive (eg, ruptured artery in an ulcer or diverticulum or ruptured esophageal varices).

Anemia due to inadequate red blood cell production

Anemia may also result when the body does not produce enough red blood cells (see also Formation of Blood Cells). Many nutrients are needed for red blood cell production. The most critical are iron, vitamin B12, and folate (folic acid), but the body also needs trace amounts of copper, as well as a proper balance of hormones, especially erythropoietin (a hormone that stimulates red blood cell production). Without these nutrients and hormones, production of red blood cells is slow and inadequate, or the red blood cells may be deformed and unable to carry oxygen adequately.

Chronic inflammation, infection, or cancer also may suppress red blood cell production. In other circumstances, the bone marrow space may be invaded and replaced (for example, by leukemia, lymphoma, or metastatic cancer), resulting in decreased production of red blood cells.

Anemia due to excessive red blood cell destruction

Anemia may also result when too many red blood cells are destroyed. Normally, red blood cells live about 120 days. Scavenger cells in the bone marrow, spleen, and liver detect and destroy red blood cells that are near or beyond their usual life span. If red blood cells are destroyed prematurely (hemolysis), the bone marrow tries to compensate by producing new cells faster. When destruction of red blood cells exceeds their production, anemia results. Hemolytic anemia is relatively uncommon compared with the anemia caused by excessive bleeding and decreased red blood cell production. Hemolytic anemia may result from disorders of the red blood cells themselves, but often it results from other disorders that cause red blood cells to be destroyed.

Symptoms of Anemia

Symptoms vary depending on the severity of the anemia and how rapidly it develops. Some people with mild anemia, particularly when it develops slowly, have no symptoms at all. Other people may experience symptoms only during physical exertion. More severe anemia may cause symptoms even when people are resting. Symptoms are more severe when mild or severe anemia develops rapidly, such as when bleeding occurs when a blood vessel ruptures.

Mild anemia often causes fatigue, weakness, and paleness. In addition to these symptoms, more severe anemia may cause faintness, dizziness, increased thirst, sweating, a weak and rapid pulse, and rapid breathing. Severe anemia may cause painful lower leg cramps during exercise, shortness of breath, and chest pain, especially if people already have impaired blood circulation in the legs or certain types of lung or heart disease.

Some symptoms may also give clues to the cause of the anemia. For example, black tarry stools, blood in the urine or stool, or coughing up blood suggests that anemia is caused by bleeding. Dark urine or jaundice (a yellowish tinge to the skin or the whites of the eyes) suggests that red blood cell destruction may be the cause of anemia. A burning or prickling feeling in the hands or feet may indicate vitamin B12 deficiency.

Anemia in older adults

Many disorders that cause anemia, such as cancer, including blood cancers such as myelodysplastic syndrome and multiple myeloma, tend to be more common among older adults. Thus, many older adults develop anemia. Anemia of chronic disease (caused by chronic inflammation, infection, or cancer) and iron deficiency anemia caused by abnormal bleeding are the most common causes of anemia among older adults. Anemia is not a normal consequence of aging, and a cause should always be sought when anemia is identified.

Symptoms of anemia are basically the same regardless of age. Even when anemia is mild, older adults are more likely to become confused, depressed, agitated, or listless than younger people. They may also become unsteady and have difficulty walking. These problems can interfere with being able to live independently. However, some older adults with mild anemia have no symptoms at all, particularly when anemia develops gradually, as it often does.

In older adults, anemia caused by vitamin B12 deficiency may be mistaken for dementia because this type of anemia may affect mental function.

Having anemia may shorten the life expectancy of older adults. Thus, identifying the cause and correcting it are particularly important.

Diagnosis of Anemia

  • Blood tests

Sometimes anemia is detected before people notice symptoms when routine blood tests are done.

Low levels of hemoglobin or a low hematocrit (the percentage of red blood cells in the total blood volume) found in a blood sample confirm the anemia. Other tests, such as examining a blood sample under a microscope and, less often, examining a sample taken from the bone marrow, help determine the cause of the anemia.

Treatment of Anemia

  • Treatment for the cause of anemia

  • Blood transfusion if needed

Treatment of anemia depends on identifying the cause.

For people with excessive bleeding, stopping the bleeding is most urgent. For example, if a wound has significant bleeding, pressure can stop the bleeding, but surgery may be needed. Bleeding in the gastrointestinal tract, such as from an ulcer, can often be stopped by endoscopy or other measures. Chronic uterine bleeding may require, for example, oral contraceptives or removal of uterine fibroids.

Inadequate production of blood cells is often caused by an inadequate amount of a vitamin or nutrient required to make blood, such as iron or B12.

People whose red blood cells are destroyed prematurely (hemolysis) may require treatment with medications that suppress the immune system.

Ultimately, if the anemia is severe or causes symptoms, blood transfusions may be needed and can be life-saving.

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