Erythrocytes, more commonly known as red blood cells, are the most abundant cell type in human blood. These microscopic cells are the body’s primary delivery service, responsible for transporting oxygen from the lungs to every tissue and organ. This continuous distribution of oxygen is fundamental for cellular respiration, the process that generates the energy required for life’s functions.
Anatomy of an Erythrocyte
An erythrocyte has a distinct biconcave disc shape, resembling a flattened donut with a depressed center. This shape provides a large surface area-to-volume ratio compared to a sphere, which facilitates more efficient diffusion of gases across the cell membrane. This morphology is maintained by a flexible cytoskeleton, allowing the cell to deform and squeeze through the narrowest blood vessels, known as capillaries.
A defining characteristic of a mature erythrocyte is the absence of a nucleus and most organelles, such as mitochondria. This maximizes the internal space, dedicating nearly the entire volume to its primary cargo: hemoglobin. The lack of mitochondria also means the erythrocyte does not consume the oxygen it transports, ensuring maximum delivery to the body’s tissues.
Hemoglobin is the iron-containing protein that fills each red blood cell and is responsible for its signature color. A single erythrocyte can contain approximately 270 to 300 million hemoglobin molecules. Each hemoglobin molecule is composed of four protein subunits, called globins, and four iron-containing pigment molecules called heme. The iron atom within each heme group reversibly binds to an oxygen molecule, allowing each hemoglobin molecule to carry up to four oxygen molecules.
Oxygen and Carbon Dioxide Transport
In the oxygen-rich environment of the lung’s alveoli, oxygen diffuses into red blood cells and binds to the iron atoms of hemoglobin, forming oxyhemoglobin. This binding process is cooperative, meaning that the binding of one oxygen molecule increases hemoglobin’s affinity for the next, facilitating rapid oxygen uptake. This newly oxygenated blood, which is bright red, is then pumped by the heart to the rest of the body.
As blood circulates through the capillaries of body tissues, the environmental conditions change. Tissues actively performing cellular respiration have lower oxygen concentrations and higher carbon dioxide levels. These conditions, along with a slightly lower pH, decrease hemoglobin’s affinity for oxygen, causing it to release the oxygen molecules. The released oxygen then diffuses out of the capillaries and into the surrounding cells.
Erythrocytes also transport carbon dioxide from the tissues back to the lungs. While some CO2 dissolves in plasma or binds to hemoglobin, the majority is transported as bicarbonate ions. Inside the red blood cell, the enzyme carbonic anhydrase converts CO2 into carbonic acid, which becomes bicarbonate. These ions are then moved into the plasma through a process known as the chloride shift.
When blood returns to the lungs, the process reverses. High oxygen levels promote CO2 release from hemoglobin. Carbonic anhydrase converts bicarbonate back into carbon dioxide, which diffuses into the lungs to be exhaled.
The Life Cycle of a Red Blood Cell
The journey of an erythrocyte begins in the red bone marrow through a process called erythropoiesis, which takes approximately seven days. Production is regulated by the hormone erythropoietin (EPO), which is primarily produced by the kidneys in response to low oxygen levels. EPO stimulates the bone marrow to increase the rate of red blood cell production.
During maturation, the developing cell, known as an erythroblast, synthesizes vast amounts of hemoglobin. In its final stages, the cell ejects its nucleus and other organelles, becoming a reticulocyte. These reticulocytes, which still contain some residual ribosomal RNA, are released from the bone marrow into the bloodstream and mature into erythrocytes within one to two days.
A mature red blood cell has a lifespan of approximately 100 to 120 days. As the cell membrane becomes damaged and less flexible over time, it is removed from circulation. Old or damaged erythrocytes are primarily removed by macrophages, a type of white blood cell, in the spleen, liver, and bone marrow.
Macrophages engulf the old cells and break down the hemoglobin. The globin protein portions are catabolized into amino acids, which can be reused by the body to synthesize new proteins. The iron is extracted from the heme groups and transported by a protein called transferrin back to the bone marrow to be incorporated into new hemoglobin. The remaining part of the heme molecule is converted into bilirubin, which is processed by the liver and eventually excreted.
Common Erythrocyte Disorders
Disruptions in the number or function of erythrocytes can lead to various medical conditions, generally categorized by either a deficiency or an overproduction of red blood cells.
Anemia
The most common type of erythrocyte disorder is anemia, a condition characterized by a lower-than-normal number of healthy red blood cells or a reduced quantity of hemoglobin. This deficiency impairs the blood’s ability to carry adequate oxygen, leading to symptoms like fatigue, weakness, and shortness of breath. Iron-deficiency anemia occurs when the body lacks sufficient iron to produce adequate hemoglobin. Another example is sickle cell disease, a genetic disorder where an abnormality in hemoglobin causes red blood cells to become rigid and sickle-shaped, which can block blood flow.
Polycythemia
In contrast to anemia, polycythemia is a condition characterized by an excessive number of red blood cells. This overproduction increases the viscosity, or thickness, of the blood. Thicker blood flows more slowly and has an increased tendency to form clots, which can obstruct blood vessels and lead to serious complications such as stroke or heart attack. Polycythemia can be a primary condition caused by a bone marrow disorder or a secondary response to conditions that cause chronic low oxygen levels.