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Overview of Blood Disorders


David J. Kuter

, MD, DPhil, Harvard Medical School

Last full review/revision Jun 2021| Content last modified Jun 2021
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Disorders that affect the blood are called blood disorders or hematologic disorders. There are many blood disorders, and they can affect the quantity as well as the function of the cells in the blood (blood cells) or proteins in the blood clotting system or immune system.

Some blood disorders cause the number of cells in the blood to decrease:

Other blood disorders cause the numbers of blood cells to increase:

  • An increased number of red blood cells is called erythrocytosis.

  • An increased number of white blood cells is called leukocytosis.

  • An increased number of platelets is called thrombocytosis or thrombocythemia.

Other blood disorders affect proteins within the blood cells or blood plasma (the liquid portion of the blood):

  • Hemoglobin, the oxygen-carrying protein inside red blood cells

  • Immune system proteins, such as antibodies (also called immunoglobulins)

  • Blood clotting factors

Blood flows to every cell in the body and is important to the health and function of all of the body's organs.

Blood cells are produced in the bone marrow, and many blood proteins are made in the liver or the blood cells themselves. Blood cells and blood proteins provide the following functions:

  • Red blood cells contain hemoglobin, which carries oxygen to every part of the body.

  • White blood cells and antibodies fight infections and cancers.

  • Platelets and blood clotting factors make bleeding stop or prevent bleeding from occurring.

Blood disorders cause symptoms resulting from disruption of these functions, and symptoms can arise from any tissues and organs that are adversely affected.

Doctors will ask about a person's symptoms and do a physical examination, but often, the presence of a blood disorder is first discovered by a blood test such as the complete blood count (CBC) done when the doctor is doing a complete evaluation of a person or when investigating why a person is not feeling well. The doctor usually must do further blood tests to diagnose a person's blood disorder, and sometimes a bone marrow biopsy is necessary.

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Vitamin Deficiency Anemia
Deficiencies of vitamin B12 and folate (folic acid) cause a condition called megaloblastic anemia. In patients with megaloblastic anemia the bone marrow produces red blood cells that are large and abnormal (megaloblasts). Which of the following is NOT a cause of Vitamin B12 or folate deficiency? 
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