Red blood cells, also known as erythrocytes, are tiny, disc-shaped cellular components making up a significant portion of blood volume. They are responsible for the distinctive red color of blood. Their primary role is transporting gases throughout the body. These specialized cells undergo a cycle of creation, breakdown, and recycling, ensuring oxygen delivery to tissues and carbon dioxide removal.
Red Blood Cell Formation
The production of red blood cells, a process called erythropoiesis, begins in the bone marrow, the spongy tissue found within certain bones. This process starts with hematopoietic stem cells (HSCs), which are foundational cells capable of developing into all blood cell types. These HSCs differentiate into common myeloid progenitor cells, which then specifically commit to becoming red blood cells.
The development proceeds through several identifiable stages, including proerythroblasts, erythroblasts (also called normoblasts), and reticulocytes, before maturing into erythrocytes. During this maturation, the developing cell accumulates hemoglobin, the protein responsible for oxygen transport, and it expels its nucleus and other organelles. The kidneys play a regulatory role by producing a hormone called erythropoietin (EPO). When oxygen levels in the body are low, the kidneys release more EPO, which then stimulates the bone marrow to accelerate red blood cell production, ensuring the body maintains an adequate supply.
Purpose and Duration
Mature red blood cells primarily function to transport oxygen from the lungs to the body’s tissues and carry carbon dioxide back to the lungs for exhalation. This task is performed by hemoglobin, an iron-containing protein packed within each red blood cell. The presence of iron in hemoglobin gives red blood cells their characteristic color and allows them to bind effectively with oxygen.
The unique biconcave disc shape of red blood cells increases their surface area, enhancing the efficiency of gas exchange. This shape also provides flexibility, enabling these cells to deform and squeeze through the body’s narrowest capillaries. Because red blood cells shed their nucleus and other organelles during maturation, they are unable to repair themselves. This lack of repair mechanisms limits their lifespan to approximately 120 days.
End of Life and Recycling
As red blood cells approach the end of their lifespan, their membranes become more fragile and less flexible. These aged or damaged cells are recognized and removed from circulation, primarily by specialized immune cells called macrophages. This removal process largely occurs in the spleen, liver, and bone marrow, which are parts of the reticuloendothelial system.
Once engulfed by macrophages, the red blood cells are broken down, and their hemoglobin is separated into two main components: globin and heme. The globin portion, a protein, is broken down into its constituent amino acids, which the body recycles for synthesizing new proteins. The heme group undergoes a more complex breakdown process.
The iron from the heme group is conserved and recycled; it can be stored by the body, often bound to proteins like ferritin, or transported back to the bone marrow by transferrin to be reused in the production of new red blood cells. The remaining part of the heme is converted into biliverdin, a green pigment, which is then reduced to bilirubin, a yellow pigment. Bilirubin is transported to the liver, where it is made water-soluble and excreted as a component of bile into the intestines.