Red blood cells (RBCs), also known as erythrocytes, are the most abundant cell type circulating in the blood, accounting for approximately 40 to 45% of the total blood volume. Produced in the bone marrow, their sole function is transporting respiratory gases throughout the body. They collect oxygen from the lungs and deliver it to peripheral tissues, while simultaneously picking up carbon dioxide waste for exhalation. The unique structure of the erythrocyte allows it to perform this task efficiently.
The Defining Feature Biconcave Disc Shape
The most distinguishing feature of a mature red blood cell is its biconcave disc shape. This means the cell is shaped like a flattened, round disc that is indented in the center on both sides. A typical human RBC measures about 6 to 8 micrometers in diameter, but only about 2 micrometers thick at its edges. This contour is a structural adaptation that enhances the cell’s performance.
This flattened shape maximizes the cell’s surface area-to-volume ratio, improving the efficiency of gas exchange. A larger surface area allows oxygen and carbon dioxide to diffuse more rapidly across the cell membrane, speeding up loading and unloading. The unique shape also provides the cell with flexibility, necessary for its journey through the circulatory system. Red blood cells must often squeeze through capillaries that are narrower than the cell itself. The biconcave structure allows the cell to deform, fold, and twist without rupturing as it navigates these tight passages.
Internal Structure and Contents
The specialized function of the red blood cell dictates a highly streamlined internal structure. Mature erythrocytes are anucleated, meaning they lack a nucleus, which is expelled during maturation in the bone marrow. The cell also discards nearly all other organelles, including mitochondria, endoplasmic reticulum, and ribosomes. This absence of internal machinery maximizes the internal volume available for the cell’s primary payload.
The internal space is dominated by hemoglobin (Hb). Hemoglobin is an iron-containing protein responsible for binding to oxygen in the lungs and releasing it in the body’s tissues. Each red blood cell is densely packed, containing approximately 270 million hemoglobin molecules. This concentration accounts for about a third of the cell’s total volume and gives the blood its characteristic red color.
Since the cell lacks mitochondria, it cannot perform aerobic respiration that consumes oxygen. Instead, the red blood cell relies exclusively on anaerobic glycolysis to generate the energy it needs for survival. This ensures that the transported oxygen is delivered to the surrounding tissues. Hemoglobin also plays a role in transporting carbon dioxide waste, though most is carried in the blood plasma as bicarbonate.
The Elastic Shell Membrane and Cytoskeleton
The integrity and shape of the red blood cell are maintained by a specialized cell membrane supported by an underlying protein framework. The outer boundary is a typical lipid bilayer, but its mechanical properties are reinforced to withstand the shear stress of blood flow. Directly beneath this lipid layer lies the membrane skeleton, a dense network of structural proteins that acts like an internal scaffolding. This flexible meshwork is composed primarily of the protein spectrin.
Spectrin molecules link together to form a hexagonal mesh that lines the inner surface of the cell membrane. This scaffolding is securely tethered to the lipid bilayer through various adapter proteins, such as ankyrin and Protein 4.1. These proteins connect the spectrin network to integral membrane proteins, creating a stable, interconnected structure. This elastic framework provides the cell with its deformability. The spectrin mesh allows the cell to fold, stretch, and contort itself to pass through microvessels far smaller than its resting size, enabling it to spring back to its original biconcave shape once the pressure is released.