Note that Encarta has excellent material on the heart, circulation, and the blood. Check it out.

Use this material to construct a more detailed table of blood cell types, their functions, and their origins.

Use the information on "origins" to construct a "family tree" for the various types of blood cells. Start with pluripotent stem cells, and show how the cells derived from them differentiate into the seven types of fully functional blood cells mentioned in the article.

Erythrocytes, or red blood cells, are round disks, concave on two sides, and approximately 7.5 thousandths of a millimeter in diameter. In humans, and most other mammals, the mature red blood cell contains no nucleus; in some vertebrates, it is oval, and nucleated. Hemoglobin, a protein in the red blood cells, gives blood its red color and transports oxygen from the lungs to the body cells, where it picks up carbon dioxide for transport back to the lungs to be expired. Hemoglobin also transports nitric oxide, which regulates blood pressure by expanding or contracting blood vessel walls.

Leukocytes, or white blood cells, are of two principal types: granular leukocytes, which have multilobed nuclei, and nongranular leukocytes, which have rounded nuclei. There are three kinds of granular leukocytes: neutrophils, which ingest and destroy bacteria; eosinophils, which increase and become active in the presence of certain infections and allergies; and basophils, which secrete heparin and histamine. Heparin keeps blood from clotting, and histamine stimulates inflammation. There are two kinds of nongranular leukocytes: lymphocytes and the less numerous monocytes, both associated with the immune system. Lymphocytes have an important role in producing antibodies and in cellular immunity. Monocytes ingest nonbacterial foreign substances, usually during chronic infection.

Thrombocytes, or platelets are small, round, nonnucleated bodies with a diameter about one-third that of red blood cells. Thrombocytes adhere to the walls of blood vessels at the site of an injury and thus plug the defect in the vascular wall. As they disintegrate, they release clotting agents that lead to the local formation of thromboplastin, which helps to form a blood clot, the first step in the healing of an injury.

[All blood cells originate from cells in the bone marrow called pluripotent stem cells. Pluripotent means they are capable of becoming many different things. Pluripotent stem cells give rise to two different types of cells, lymphoid stem cells, which migrate to the lymphatic tissue, and myeloid stem cells, which remain in the bone marrow. These cells then divide by mitosis to produce all the other kinds of blood cells

Red blood cells are formed in the bone marrow (see Bone). After an average life of 120 days, red blood cells are broken down and removed by the spleen. The different kinds of white blood cells are formed in various sites: neutrophils, basophils, and eosinophils (the granular leukocytes) are formed in the bone marrow; lymphocytes are formed in the thymus, the lymph glands (see Lymphatic System), and other lymphatic tissue; and monocytes are formed in the spleen, liver, lymph nodes, and other organs. Platelets are formed in the bone marrow [by the fragmenting of cells called megakaryocytes]. All these constituents of the blood are continuously worn out or consumed and must be continuously replaced.

"Blood," Microsoft(R) Encarta(R) 98 Encyclopedia. (c) 1993-1997 Microsoft Corporation. All rights reserved.