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Overview of Blood

What is blood?

Blood is the life-maintaining fluid that circulates through the body's:

Anatomy of a bone, showing blood cells
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What is the function of blood?

Blood carries the following to the body tissues:

Blood carries the following away from the body/tissues:

What are the components of blood?

Human blood consists of about 22 percent solids and 78 percent water. The components of human blood are:

Where are blood cells made?

Blood cells are made in the bone marrow. The bone marrow is the spongy material in the center of the bones that produces all of the types of blood cells.

There are other organs and systems in our bodies that help regulate blood cells. The lymph nodes, spleen, and liver help regulate the production, destruction, and differentiation (developing a specific function) of cells. The production and development of new cells is a process called hematopoiesis.

Blood cells formed in the bone marrow start out as a stem cell. A "stem cell" (or hematopoietic cell) is the initial phase of all blood cells. As the stem cell matures, several distinct cells evolve such as the red blood cells, white blood cells, and platelets. Immature blood cells are also called blasts. Some blasts stay in the marrow to mature and others travel to other parts of the body to develop into mature, functioning blood cells.

What are the functions of blood cells?

The primary function of red blood cells, or erythrocytes, is to carry oxygen and carbon dioxide. Hemoglobin (Hgb) is an important red blood cell that carries oxygen from the lungs to all parts of our body.

The primary function of white blood cells, or leukocytes, is to fight infection. There are several types of white blood cells and each has its own role in fighting bacterial, viral, fungi, and parasitic infections. Types of white blood cells that are most important for helping protect the body from infection and foreign cells include the following:

White blood cells:

The primary function of platelets, or thrombocytes, is blood clotting. Platelets are much smaller in size than the other blood cells. They group together to form clumps, or a plug, in the hole of a vessel to stop bleeding.

What is a complete blood cell count (CBC)?

A complete blood cell count is a measurement of size, number, and maturity of the different blood cells in a specific volume of blood. A complete blood cell count can be used to determine many abnormalities with either the production or destruction of blood cells. Variations from the normal number, size, or maturity of the blood cells can be used to indicate an infection or disease process. Often with an infection, the number of white blood cells will be elevated.

Common hematology tests:

Some common hematology tests include the following:

Test Uses
Complete blood count (CBC), which includes:
  • white blood cell count (WBC)
  • red blood cell count (RBC)
  • platelet count
  • hematocrit red blood cell volume (HCT)
  • hemoglobin concentration (HB) - the oxygen-carrying pigment in red blood cells
  • differential blood count
To aid in diagnosing anemia and other disorders of the blood; to monitor blood loss and infection.
Platelet count To diagnose and/or monitor bleeding and clotting disorders.
Prothrombin time (PT) To evaluate bleeding and clotting disorders and to monitor anticoagulation (anti-clotting) therapies.

Your child's physician will explain the purpose and results of any blood tests with you.

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