Hematopoiesis is the process used to create new blood cells to replace those that are aging, and it is continually in progress in the human body. It involves developing immature cells, known as precursor cells, into working blood cells that help the body to fight infection, prevent excess bleeding and carry oxygen to tissues. Several types of blood cells exist, and the body follows specific pathways to create each type. A stem cell acts as the initial foundation to create mature cells through hematopoiesis.

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; by adulthood, hematopoiesis occurs in several main sites. These include the skull, the sternum, the vertebrae in the back, the pelvis and the bones of the upper thigh.

Stem cells are also referred to as pluripotent cells. 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, it is typically one of five kinds of blood cells: erythrocytes, monocytes, lymphocytes, granulocytes or platelets. Granulocytes are further divided into three kinds of blood cells, called neutrophils, basophils and eosinophils. Each type of blood cell has a specific function that protects the body or helps it to work more smoothly.

Erythrocytes 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. Low levels of each type of blood cell can have a damaging effect on the body, so hematopoiesis is necessary to keep blood cells at appropriate levels.

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 of cells maintains a constant form to be available to differentiate into other blood cells. This occurs because part of the process of hematopoiesis is for the body to continuously create new stems as well as precursor cells. In this way, the body sets up its own defense mechanism to react in times of illness.