Red blood cells, or erythrocytes, are the body’s transporters of oxygen. Each cell is packed with hemoglobin, an iron-rich protein that binds to oxygen in the lungs and releases it to tissues. Produced in the bone marrow, these cells have a limited lifespan and require constant replacement, with the body making roughly 2 million new ones every second.
The Lifespan and Aging of a Red Blood Cell
A red blood cell circulates for approximately 100 to 120 days. Unlike most other cells, mature red blood cells lack a nucleus and other organelles. This structure maximizes the space for hemoglobin, but it also means the cell cannot synthesize new proteins or repair itself from constant wear and tear.
Over its lifespan, the red blood cell’s membrane gradually stiffens, losing the flexibility needed to pass through narrow capillaries. This loss of deformability is a primary sign of aging. The biochemical machinery within the cell also begins to fail as essential enzymes become depleted, reducing the cell’s ability to maintain its shape and function.
The Body’s Quality Control System
The body has a system for identifying and removing aged red blood cells from circulation, a process known as eryptosis. This quality control is primarily carried out in the spleen, liver, and bone marrow. The spleen acts as a filter, containing narrow passages called splenic cords that healthy, flexible red blood cells can navigate.
As red blood cells age and their membranes become more rigid, they lose the ability to squeeze through these tight spaces and become trapped in the spleen. Specialized immune cells called macrophages then recognize these entrapped cells. The macrophages engulf and digest the old red blood cells in a process called phagocytosis, clearing them from the bloodstream.
Breaking Down and Recycling Components
Once a macrophage has engulfed an old red blood cell, a recycling program begins. The macrophage breaks down the hemoglobin molecule into its two main components: globin and heme. The globin protein is disassembled into its constituent amino acids, which are released back into the body to be used in the synthesis of new proteins.
The iron atom is extracted from the heme molecule. This recovered iron binds to a carrier protein called transferrin, which transports it through the bloodstream to the bone marrow. There, it is incorporated into new hemoglobin molecules for the next generation of red blood cells.
The remainder of the heme molecule, now stripped of its iron, is converted into a green pigment called biliverdin. This is then reduced to form bilirubin, a yellow pigment. Bilirubin is released into the plasma, where it binds to albumin and is transported to the liver. The liver processes the bilirubin and excretes it as a component of bile, which gives stool its characteristic brown color.