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Hematopoiesis is a biological process the body uses to create new blood cells to replace those that are aging or that have died off. It is continually in progress in all healthy humans and most animals. The process is somewhat complex from a scientific standpoint, but generally speaking, it involves developing and converting immature cells, known as precursor cells, into working blood cells. These new cells help the body to fight infection, prevent excess bleeding, and carry oxygen to tissues. There are several types of blood cells, each with its own specific purpose and function; the body follows specific pathways to create each type. There are nuanced differences depending on the end goal, but the process follows the same basic framework no matter what. In most cases the process begins with a stem cell as the foundation, and growth happens from there.
Blood courses through almost all parts of the body, and cells die off and age fairly regularly — even in people and animals considered young by most other measures. This cyclical creating and recreating is a normal part of blood health, and is one of the ways that cells stay active and able to carry nutrients and other particles efficiently and effectively.
The body creates new blood cells in various locations depending on a person’s age. During fetal development, hematopoiesis occurs in the liver, spleen and bone marrow. After birth and through development, the bone marrow takes over as the main site of blood cell formation, and by adulthood, the process is mostly streamlined to several main sites. These include the skull, the sternum, the vertebrae in the spinal column, the pelvis, and the bones of the upper thigh.
In nearly all cases blood cell creation begins with stem cells. Stem cells are also referred to as pluripotent cells, and they’re often considered one of the “building blocks” of life because of how essential they are in determining future growth and development. Each pluripotent cell functions to form either new stem cells or precursor cells that will eventually form specific types of blood cells.
When a pluripotent stem cell forms a precursor cell, which is the beginning of the blood cell creation process, it typically takes one of five forms. Specifically, blood cells are usually classed as erythrocytes, monocytes, lymphocytes, granulocytes, or platelets. Granulocytes are further divided into three kinds of blood cells, namely neutrophils, basophils and eosinophils. Each type of blood cell has a specific function that protects the body and helps it to work more smoothly.
Erythrocytes, for instance, are red blood cells responsible for carrying oxygen to the body’s tissues from the heart and lungs. Monocytes and lymphocytes are important for protecting against invading viruses and bacteria. Granulocytes are also important to the immune system, moving to a site of infection to consume foreign toxins and cells. Platelets are necessary for the blood to clot and collect at bleeding sites to clump together.
Healthy people usually have processes in place to make sure that they have adequate numbers of each sort of cell, and also to make sure that the cells are balanced proportionally with each other. When infection sets in, for instance, or in the case of injury or trauma, certain cells need to be elevated — but there also need to be checks in place to bring things back to normal once the threat has passed. Unusually low or high levels of certain types of blood cell can have a damaging effect on the body. One of the main goals of hematopoiesis from this perspective is to keep everything stabilized, and to replenish certain types of cells as needed.
The bone marrow also contains a collection of stem cells that can work to create more blood cells when needed, particularly if the body is in a crisis. This reserve acts sort of like a storehouse of more or less “neutral” cells that can be called up and activated when needed. Cells kept in the marrow can typically become almost any type of blood cell, depending on what’s needed and where. Part of the process of hematopoiesis is for the body to continuously create new stem as well as precursor cells. In this way, the body sets up its own defense mechanism to react in times of illness.
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