Overview of Blood and Blood Components

Anatomy of a bone, showing blood cells

What is blood?

Blood is the life-maintaining fluid that circulates through:

  • Heart

  • Arteries

  • Veins

  • Capillaries

  • Brain

  • Rest of the 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:

  • Plasma. The liquid component of the blood in which the following blood cells are suspended:

    • Red blood cells (erythrocytes). These carry oxygen from the lungs to the rest of the body

    • White blood cells (leukocytes). These help fight infections and aid in the immune process. Types of white blood cells include:

      • Lymphocytes

      • Monocytes

      • Eosinophils

      • Basophils

      • Neutrophils

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

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 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 in the bone marrow is a process called hematopoiesis.

Blood cells formed in the bone marrow start out as a stem cell. A stem cell (or hematopoietic stem 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 from the lungs to the body tissues and carbon dioxide as a waste product, away from the tissues and back to the lungs. Hemoglobin (Hgb) is an important protein in the red blood cells 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, fungal, 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:

  • Neutrophils

  • Eosinophils

  • Lymphocytes

  • Monocytes

  • Basophils 

White blood cells:

  • Help heal wounds not only by fighting infection but also by ingesting matter, such as dead cells, tissue debris, and old red blood cells.

  • Are our protection from foreign bodies that enter the blood stream, such as allergens.

  • Are involved in the protection against mutated cells, such as cancer.

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 CBC count is a measurement of size, number, and maturity of the different blood cells in a specific volume of blood. A CBC 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. Many forms of cancer can affect the bone marrow 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 hematological tests



CBC, which includes:

  • White blood cell count (WBC)

  • Red blood cell count (RBC)

  • Platelet count

  • Hematocrit red blood cell volume (Hct)

  • Hemoglobin (Hgb) concentration. The oxygen-carrying pigment in red blood cells

  • Differential blood count

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

Platelet count

To diagnose and/or monitor bleeding and clotting disorders.

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

To evaluate bleeding and clotting disorders and to monitor anticoagulation (anticlotting) therapies.

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