The human body continuously requires billions of new blood cells to replace those that have reached the end of their lifespan. This production process, known as hematopoiesis, is essential for life, ensuring the blood can transport oxygen, fight infection, and prevent blood loss. The specific location where these cellular components of blood—collectively known as formed elements—are created changes throughout a person’s development, but it settles in a highly specialized tissue in adulthood.
What Exactly Are Formed Elements?
The formed elements are the cellular components suspended within the plasma, the liquid portion of the blood. They make up approximately 45% of total blood volume and are broadly categorized into three types: erythrocytes, leukocytes, and thrombocytes.
Erythrocytes, or red blood cells, are the most numerous and are primarily responsible for transporting oxygen. Leukocytes, or white blood cells, are the body’s immune defense system, neutralizing foreign invaders and abnormal cells. Thrombocytes, commonly called platelets, are fragments of bone marrow cells that are instrumental in forming clots to stop bleeding.
The Central Factory: Bone Marrow in Adults
In a healthy adult, the primary site of formed element production is the red bone marrow. This soft, spongy tissue is found within the interior of certain bones. The bone marrow’s cellular environment provides the necessary signals and structure to support the immense volume of cell creation.
Red marrow is distinct from yellow marrow, which is mainly composed of fat cells and stores energy. As a person matures, the red marrow recedes from the long bones of the limbs, becoming concentrated in the flat and irregular bones of the torso. The most active production sites in adults are the axial skeleton, specifically the vertebrae, sternum, ribs, and the pelvic bones.
Production also continues in the proximal ends of large long bones, such as the femur and humerus. This continuous production replaces billions of short-lived cells daily, including an estimated 200 billion red cells and 400 billion platelets. If the body experiences extreme demand, such as severe blood loss, the liver and spleen retain the capacity to resume blood cell production in a process called extramedullary hematopoiesis.
How Formed Elements Are Made: The Process of Hematopoiesis
All mature blood cells trace their origin back to hematopoietic stem cells (HSCs) residing within the red bone marrow. These cells are multipotent, meaning they have the unique ability to self-renew and differentiate into any type of blood cell. The process unfolds as HSCs divide into progenitor cells, which are progressively committed to becoming a specific formed element.
These progenitor cells follow two main pathways: the myeloid lineage, which yields red blood cells, platelets, and most types of white blood cells, and the lymphoid lineage, which produces lymphocytes. Differentiation from an unspecialized stem cell to a mature cell is tightly regulated by chemical signals called growth factors.
One well-known regulatory factor is the hormone erythropoietin (EPO), which is produced primarily by the kidneys in response to low oxygen levels. EPO stimulates the bone marrow to increase the production of red blood cells. Similarly, thrombopoietin (TPO), produced by the liver and kidneys, regulates the formation of platelets.
Production Sites in Fetal Development
Before birth, the sites of formed element production shift locations as the fetus develops. This initial production supports the rapidly growing embryo before the skeletal system has fully formed its marrow cavities. The first site of hematopoiesis is the yolk sac, an extraembryonic membrane that begins production during the first weeks of gestation.
As the embryo grows, the liver takes over as the primary blood-forming organ, beginning around the second trimester and continuing until shortly before birth. The spleen also plays a role in this second phase of fetal production. Eventually, during late gestation, the function gradually transitions to the bone marrow, where it remains the main site for the rest of an individual’s life.