Human red blood cells, making up 40-45% of blood volume, are essential for survival. These cells primarily deliver oxygen throughout the body. An adult human has approximately 20-30 trillion red blood cells, about one-quarter of all human cells, highlighting their importance in maintaining health and energy.
Structure and Primary Function
Red blood cells, or erythrocytes, have a unique biconcave disc shape, like a flattened donut. This shape provides a large surface area for efficient gas exchange and grants them flexibility. This flexibility allows them to deform and navigate through the body’s narrowest blood vessels, including capillaries.
Mature red blood cells lack a nucleus and most other organelles, such as mitochondria and ribosomes. This absence creates more internal space for hemoglobin, the protein responsible for oxygen transport. Without a nucleus, these cells cannot divide or synthesize new proteins, which limits their lifespan but maximizes their gas carrying function.
Hemoglobin is an iron-containing protein that gives red blood cells their red color. Each hemoglobin molecule has four subunits, each with a heme group containing an iron atom that binds one oxygen molecule. This allows a single hemoglobin molecule to transport up to four oxygen molecules.
Red blood cells bind oxygen in the lungs and transport it to tissues. Simultaneously, they help transport carbon dioxide, a waste product, from tissues back to the lungs for exhalation. Carbon dioxide binds to the globin portion of hemoglobin, separate from the oxygen-binding site.
The Red Blood Cell Life Cycle
Red blood cells are continuously produced in the bone marrow through a process called erythropoiesis. In adults, production primarily occurs in bones like the vertebrae, ribs, breastbone, and pelvis. Immature red blood cells, called erythroblasts, develop in the bone marrow, accumulating hemoglobin while losing their nucleus and other organelles.
Once matured, red blood cells are released into the bloodstream, a process taking about seven days. The average lifespan of a red blood cell in circulation is about 120 days. During this time, they deliver oxygen and collect carbon dioxide.
As red blood cells age or become damaged, their flexibility decreases, and they are removed from circulation. This removal primarily occurs in the liver and spleen by specialized immune cells called macrophages. Macrophages engulf and break down these cells. Components are recycled; for instance, iron from hemoglobin is recovered and reused for new red blood cell production in the bone marrow. The remaining heme molecule degrades into bilirubin, which is then eliminated from the body.
Common Conditions Affecting Red Blood Cells
Anemia is a common condition characterized by an insufficient number of healthy red blood cells or reduced hemoglobin, impairing the blood’s oxygen-carrying ability. This leads to decreased oxygen supply to tissues and organs, causing symptoms like fatigue, weakness, and pale skin. Anemia can stem from blood loss, decreased red blood cell production, or increased destruction. Iron deficiency can also contribute to anemia, as iron is a component of hemoglobin.
Polycythemia involves an excess of red blood cells. This overproduction thickens the blood, making it difficult to flow smoothly through vessels. Sluggish circulation heightens the risk of blood clots, potentially leading to serious complications like heart attacks or strokes. Symptoms can include dizziness, headaches, and itching.
Sickle cell disease is an inherited disorder affecting hemoglobin within red blood cells. A genetic mutation causes hemoglobin to form stiff rods, distorting red blood cells into a rigid, crescent or “sickle” shape. These abnormally shaped cells are less flexible and can get stuck in small blood vessels, blocking blood flow and preventing oxygen from reaching tissues. This blockage leads to episodes of severe pain, known as pain crises, and can cause organ damage.