Red blood cells, also known as erythrocytes, are an abundant and fundamental component of blood within the bodies of vertebrates. These microscopic cells perform a constant journey throughout the circulatory system, delivering substances to every tissue. In a healthy adult, there are 5 million to 5.5 million red blood cells per cubic millimeter of blood. This vast number highlights their continuous contribution to sustaining life.
The Anatomy of a Red Blood Cell
A mature red blood cell has a biconcave disc shape, resembling a doughnut with a central indentation. This specific morphology increases the cell’s surface area relative to its volume, which aids efficient gas exchange. This flexible, disc-like structure also allows the cells to bend and squeeze through the narrowest capillaries, ensuring oxygen delivery to distant tissues.
Mammalian red blood cells lack a nucleus and other internal organelles. This absence of internal structures maximizes the available space within the cell. This increased internal volume is dedicated to housing hemoglobin, the iron-containing protein that gives blood its characteristic red color.
Oxygen and Carbon Dioxide Transport
The primary function of red blood cells is the transport of respiratory gases. Hemoglobin, the protein packed within these cells, binds oxygen. In the lungs, where oxygen levels are high, hemoglobin picks up oxygen molecules, forming oxyhemoglobin.
As red blood cells travel through the bloodstream to the body’s tissues, where oxygen levels are lower, the hemoglobin releases its bound oxygen. This release ensures that cells receive the oxygen necessary for their metabolic processes. The entire circulation cycle for a human red blood cell takes about 60 seconds.
Beyond oxygen delivery, red blood cells also play a role in transporting carbon dioxide, a waste product, back to the lungs for exhalation. While a small amount of carbon dioxide dissolves directly in the blood plasma, and some binds to the globin portion of hemoglobin to form carbaminohemoglobin, the majority is transported in a different form. Inside red blood cells, the enzyme carbonic anhydrase converts carbon dioxide and water into carbonic acid, which then dissociates into bicarbonate ions.
These bicarbonate ions then diffuse into the blood plasma and are transported to the lungs. In the lungs, the process reverses, and bicarbonate ions are converted back into carbon dioxide. This carbon dioxide then diffuses into the lung’s air sacs and is expelled from the body through breathing.
The Lifecycle of a Red Blood Cell
The journey of a red blood cell begins in the red bone marrow through a process called erythropoiesis. This continuous production generates 2.4 million new red blood cells every second in human adults. The hormone erythropoietin (EPO) regulates this production, increasing it when oxygen levels in the body are low.
Once mature, red blood cells circulate in the bloodstream for a lifespan of 100 to 120 days. During this period, they traverse extensive distances, navigating through various blood vessels, including very narrow capillaries. Over time, their cell membranes become less flexible and more fragile due to the absence of a nucleus to repair damage.
Old or damaged red blood cells are recognized and removed from circulation by macrophages. This removal primarily occurs in the spleen, often referred to as the “graveyard of red blood cells.” These macrophages engulf and break down the senescent cells.
During the breakdown of red blood cells, their components are recycled. Hemoglobin is separated into its heme and globin parts. The globin is broken down into amino acids, which the body can reuse to synthesize new proteins, including new hemoglobin. The iron from the heme component is also conserved and recycled, binding to transport proteins to be returned to the bone marrow for the production of new red blood cells.
When Red Blood Cells Malfunction
Problems with red blood cells can impact health, often categorized by whether there are too few, too many, or abnormally shaped cells. These conditions can lead to various symptoms because they disrupt the body’s ability to transport oxygen efficiently.
Too Few (Anemia)
Anemia is a condition where the blood has a reduced number of red blood cells or insufficient hemoglobin. This leads to less oxygen being delivered to the body’s tissues, resulting in symptoms such as fatigue, weakness, shortness of breath, and pale skin. One common cause is iron deficiency anemia, which occurs when the body lacks enough iron to produce adequate hemoglobin. Another type, pernicious anemia, results from the body’s inability to properly absorb vitamin B12, a vitamin necessary for red blood cell formation.
Abnormal Shape (e.g., Sickle Cell Disease)
Certain genetic conditions can cause red blood cells to develop an abnormal shape, impairing their function. Sickle cell disease is an example, an inherited disorder where a genetic mutation causes hemoglobin to form stiff rods inside the red blood cells. This results in the red blood cells becoming rigid and crescent-shaped, resembling a sickle. These misshapen cells are less flexible, cannot easily pass through small blood vessels, and tend to break down prematurely. This leads to chronic anemia and can cause painful blockages in capillaries, damaging organs and tissues.
Too Many (Polycythemia)
In contrast, polycythemia is a condition characterized by too many red blood cells in the bloodstream. This increase in cell count makes the blood thicker, increasing its viscosity. Thicker blood flows less smoothly through vessels and elevates the risk of blood clot formation, which can lead to cardiovascular complications such as strokes or heart attacks. Polycythemia can arise from various factors, including certain cancers, chronic lung diseases that cause low oxygen levels, or dehydration.
To diagnose these conditions, a healthcare provider uses a complete blood count (CBC). This common blood test measures various components of the blood, including the red blood cell count, hemoglobin levels, and hematocrit, which is the percentage of blood volume made up of red blood cells. This test helps identify abnormalities in red blood cell numbers or characteristics, guiding medical interventions.