Blood Formation Step 1
Blood contains a clear fluid called plasma and three types of blood cells -- red blood cells, white blood cells, and platelets. These cells are also called erythrocytes, leukocytes, and thrombocytes, respectively.
The process of blood formation is called hematopoiesis. It begins with stem cells, immature cells that are produced in the bone marrow and have the ability to differentiate, or mature, into other types of cells. The bone marrow contains a replenishing supply of stem cells; some remain as stem cells, while others go on to mature into the various types of blood cells.
In the first step of blood formation, stem cells differentiate into one of two slightly more mature types of stem cells: lymphocytic progenitor cells and myeloid progenitor cells.
Blood Formation Step 2
In the next step, the lymphocytic stem cells and myeloid stem cells undergo further specialization.
A lymphocytic stem cell can mature into other types of white blood cells.
- T lymphocyte
- B lymphocyte
- Natural killer cell
A myeloid stem cell can mature into one of several types of cells.
- Erythrocyte, or red blood cell
- Platelet
- Granulocyte - Three types of granulocytes are possible: neutrophils, basophils, and eosinophils, each with a specific job in the immune system.
- Monocyte, another type of white blood cell
Most stem cells remain in the bone marrow during the process of maturation. Others, particularly those that are differentiating into T lymphocytes and the monocytes, travel out of the bone marrow and mature in other places, such as the thymus, spleen, lymph nodes, and tonsils.
Cells that mature in the bone marrow leave the bones once they are mature. They enter the bloodstream or the lymphatic system, where they help the body to function. Some stem cells also leave the marrow and circulate in the bloodstream.
Any of these cells can start to malfunction and become leukemic, but the malfunctioning usually starts when the cells are still in the bone marrow.
Red Blood Cells
Red blood cells give blood its color. They contain the protein hemoglobin, which picks up oxygen as blood circulates through the lungs and carries it to other parts of the body. Oxygen is used in the tissues of the body to break down nutrients. The waste product from this process, carbon dioxide, is carried in the blood back to the lungs, where it is exhaled.
The normal hemoglobin range is 13 to17 grams/dL for men and 11.5 to 16 grams/dL for women.
A condition called anemia occurs when there are not enough red blood cells in the body. Too little oxygen is being carried to tissues, resulting in symptoms such as fatigue, shortness of breath, weakness, dizziness, irritability, pale skin, and headache.
Red blood cells are also called erythrocytes.
Platelets
Platelets, are tiny particles that circulate in the blood. They help to stop bleeding by forming clots at the site of a tear in a blood vessel. After the clot forms the vessel wall can heal.
When too few platelets are circulating in the blood the result is a condition called thrombocytopenia. It can lead to excessive bleeding.
White Blood Cells: Lymphocytes and Monocytes
White blood cells fight off organisms, such as bacteria, that can produce disease in the body. They can leave the bloodstream and move into surrounding tissues in order to act on these foreign substances. White blood cells are also called leukocytes, neutrophils, or granulocytes.
There are several different kinds of white blood cells, each with a specific job. Lymphocytes and monocytes belong to a group called agranulocytes or mononuclear cells.
Lymphocytes, which develop from lymphocytic stem cells, are key players in the immune system. Lymphocytes are considered to be a form of lymphoid tissue, and part of the lymph system.
The lymph system also includes the lymph vessels, a large system of channels somewhat like blood vessels; lymph fluid, a clear fluid that contains immune system cells and wastes from the body; and the lymph nodes, pea-sized nodules found along the lymph vessels where immune system cells sometimes gather. Lymph fluid circulates through the lymph vessels and filters through the lymph nodes, where lymphocytes can act on foreign substances. The lymph fluid then drains into the bloodstream.
Lymphocytes are usually found in the lymph nodes, the lymph vessels, and the spleen. Some also enter the bloodstream as needed. There are three types of lymphocytes.
- T-cell lymphocytes act to destroy foreign substances, cells that have been invaded by viruses, and possibly, cancer cells. They can release substances that help other immune system cells to do their jobs and can also help to regulate the immune system response.
- B-cell lymphocytes produce antibodies that help destroy invading foreign substances. Some antibodies then remain circulating in the blood to provide long-term protection from foreign substances.
- Natural killer cells may attack tumor cells and cells that have been invaded by viruses.
Monocytes, which develop from myeloid stem cells, leave the bloodstream, turn into macrophages, and act on invading substances by phagocytosis -- they engulf and destroy the foreigner.
White Blood Cells: Granulocytes
In the granulocyte series are three types of white blood cells.
- Neutrophils are the most common of all white blood cells; they defend the body from infection by bacteria. They leave the blood vessels and travel to the site of the bacterial invasion. There they engulf the invaders in a process called phagocytosis. They are also known as polymorphonuclear neutrophils, polymorphs, or polys.
- Eosinophils also get rid of foreign substances by way of phagocytosis, and are also important in responding to allergic reactions.
- Basophils respond during allergic reactions.
