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Overview of Blood and Blood Components

What is blood?

Blood is the life-maintaining fluid that circulates through the entire body.

What is the function of blood?

Blood carries the following to the body tissues:

  • Nourishment

  • Electrolytes

  • Hormones

  • Vitamins

  • Antibodies

  • Heat

  • Oxygen

  • Immune cells (cells that fight infection)

Blood carries the following away from the body tissues:

  • Waste matter

  • Carbon dioxide

What are the components of blood?

The components of human blood are:




      • Lymphocytes

      • Monocytes

      • Eosinophils

      • Basophils

      • Neutrophils

    • Platelets (thrombocytes). These help in blood clotting.

Where are blood cells made?


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 function of cells. The production and development of new cells in the bone marrow is a process called hematopoiesis.

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

What are the functions of blood cells?



  • Neutrophils

  • Eosinophils

  • Lymphocytes

  • Monocytes

  • Basophils 

White blood cells:


  • Protect you from foreign bodies that enter the blood stream, such as allergens.



What is a complete blood cell count (CBC)?

A CBC count is a measurement of size, number, and maturity of the different blood cells in the blood sample. A CBC can be used to find problems 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 mean there is an infection or disease process. Often with an infection, the number of white blood cells will be elevated. Many forms of cancer can affect the production of blood cells. For instance, an increase in the immature white blood cells in a CBC can be associated with leukemia. Blood diseases, such as anemia and sickle cell disease, will cause an abnormally low hemoglobin.

Common blood tests



CBC, which includes:

  • White blood cell count (WBC)

  • Red blood cell count (RBC)

  • Platelet count

  • Hematocrit red blood cell volume (Hct)


  • Differential blood count

To aid in diagnosing anemia and other blood disorders and certain cancers of the blood; to monitor blood loss and infection; or to monitor response to cancer therapy, such as chemotherapy and radiation.

Platelet count

To diagnose and monitor bleeding and clotting disorders.

Prothrombin time (PT) and partial thromboplastin time (PTT)


Your  healthcare provider will explain the purpose and results of any blood tests with you.

Online Medical Reviewer: Donna Freeborn PhD CNM FNP
Online Medical Reviewer: Lu Cunningham
Online Medical Reviewer: Richard LoCicero MD
Date Last Reviewed: 11/1/2018
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