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Overview of Anemia


Evan M. Braunstein

, MD, PhD, Johns Hopkins University School of Medicine

Last full review/revision Apr 2021| Content last modified Apr 2021
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Anemia is a condition in which the number of red blood cells 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 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 three major mechanisms that produce anemia:


Anemia caused by excessive bleeding

Anemia may be caused by excessive bleeding (see Anemia due to 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 Abnormal Uterine Bleeding (AUB) Abnormal uterine bleeding is bleeding from the vagina that occurs frequently or irregularly or lasts longer or is heavier than normal menstrual periods. The most common type of abnormal bleeding... read more Abnormal Uterine Bleeding (AUB) . Chronic bleeding typically leads to low levels of iron, which leads to worsening anemia (see Iron Deficiency Anemia).

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 Formation of Blood Cells Red blood cells, most white blood cells, and platelets are produced in the bone marrow, the soft fatty tissue inside bone cavities. Two types of white blood cells, T and B cells ( lymphocytes)... read more ). 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.

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, hemolytic anemia results. Hemolytic anemia Autoimmune Hemolytic Anemia Autoimmune hemolytic anemia is a group of disorders characterized by a malfunction of the immune system that produces autoantibodies, which attack red blood cells as if they were substances... read more 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 more 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 Vitamin B12 Deficiency Vitamin B12 deficiency can occur in vegans who do not take supplements or as a result of an absorption disorder. Anemia develops, causing paleness, weakness, fatigue, and, if severe, shortness... read more .

Anemia in older adults

Symptoms of anemia are basically the same regardless of age. Also, even when anemia is mild, older people 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 people with mild anemia have no symptoms at all, particularly when anemia develops gradually, as it often does.

In older people, 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 people. 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.

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