Red blood cells are the most abundant cells in human blood and one of the body’s most important transport systems. Their main job is to carry oxygen from the lungs to tissues throughout the body and help transport carbon dioxide back to the lungs for removal.
Also called erythrocytes, red blood cells are highly specialized for this work. Their unusual shape, flexible membrane, lack of a nucleus when mature, and high concentration of a protein called hemoglobin all help them move gases efficiently through the circulatory system.
What are red blood cells?
Red blood cells are cells produced in the bone marrow, the soft tissue inside many bones. Once mature, they circulate through blood vessels, where they repeatedly pass between the lungs and the body’s tissues.
Unlike most other cells, mature human red blood cells do not contain a nucleus. They also lack mitochondria and several other internal structures. This leaves more room for hemoglobin and means the cells do not consume the oxygen they are carrying for their own energy needs.
A typical red blood cell has a biconcave shape: it is thinner in the center than around the edges, with a shallow depression on both sides. This shape gives the cell a large surface area relative to its volume and helps it exchange gases efficiently. It also makes the cell highly flexible, allowing it to bend as it travels through tiny blood vessels called capillaries.
Red blood cells are relatively small—only a few micrometers across—but they are produced in enormous numbers because the body continually removes and replaces them.
How do red blood cells carry oxygen?
The key to oxygen transport is hemoglobin, an iron-containing protein inside red blood cells.
When you inhale, oxygen enters the lungs and reaches tiny air sacs called alveoli. Oxygen then moves across the thin walls of the alveoli and nearby capillaries into the blood. There, oxygen binds to hemoglobin inside red blood cells.
Each hemoglobin molecule can bind several oxygen molecules. In the lungs, where oxygen levels are high, hemoglobin readily picks up oxygen. Red blood cells then carry that oxygen through the bloodstream.
When they reach tissues that need oxygen, conditions such as lower oxygen concentration and higher carbon dioxide concentration favor the release of oxygen from hemoglobin. Oxygen can then move from the blood into cells, where it is used in cellular metabolism to help produce energy.
This process is why red blood cells are essential to organs with high energy demands, including the brain, heart, and muscles.
What does hemoglobin do besides carry oxygen?
Hemoglobin is best known for transporting oxygen, but it also participates in carbon dioxide transport and helps regulate blood chemistry.
Cells produce carbon dioxide as a waste product of metabolism. Some carbon dioxide binds to hemoglobin, while much of it is transported in the blood in another form, primarily as bicarbonate. Red blood cells contain the enzyme carbonic anhydrase, which helps convert carbon dioxide and water into carbonic acid and, ultimately, bicarbonate and hydrogen ions.
When blood reaches the lungs, these reactions can be reversed. Carbon dioxide is formed and then moves into the alveoli, where it is exhaled.
Hemoglobin also helps buffer acids in the blood, contributing to the body’s control of pH, a measure of how acidic or alkaline a solution is.
Why are red blood cells red?
Their color comes from hemoglobin.
Hemoglobin contains heme, a chemical structure that includes iron. The way heme interacts with light gives oxygen-rich blood its characteristic bright red appearance. Blood with less oxygen is typically darker red, although it is not actually blue when it is inside veins.
The difference between oxygen-rich and oxygen-poor blood is a difference in shade, not a change from red to blue.
Where are red blood cells made?
Most red blood cells are made in the red bone marrow through a process called erythropoiesis.
The process begins with blood-forming stem cells. Through several stages of development, these cells become increasingly specialized until they form mature red blood cells.
The hormone erythropoietin (EPO) plays a major role in regulating production. When the kidneys detect that tissues are not receiving enough oxygen, they increase the release of erythropoietin. The hormone signals the bone marrow to increase red blood cell production.
This feedback system helps the body adjust its supply of red blood cells to its oxygen needs. For example, living at high altitude can stimulate increased red blood cell production because the air contains less available oxygen than at sea level.
How long do red blood cells live?
A healthy red blood cell circulates for roughly 120 days.
Because mature red blood cells cannot divide, repair themselves in the same way as many other cells, or produce new proteins, they eventually become less able to function effectively. Older or damaged cells are removed from circulation, mainly by specialized cells in the spleen, liver, and bone marrow.
The body recycles many of their components. Iron from hemoglobin can be recovered and reused to make new hemoglobin. Other components of hemoglobin are broken down and processed by the liver and other tissues.
The continuous cycle of production and removal keeps the red blood cell population relatively stable.
What happens when you have too few red blood cells?
A reduced number of red blood cells or an insufficient amount of hemoglobin can lead to anemia. Because hemoglobin carries oxygen, anemia can reduce the amount of oxygen delivered to tissues.
Anemia is not a single disease. It has many possible causes, including blood loss, iron deficiency, deficiencies of certain vitamins, problems with red blood cell production, chronic diseases, and conditions that cause red blood cells to be destroyed too quickly.
Common symptoms can include fatigue, weakness, shortness of breath, dizziness, headache, or a rapid heartbeat. The symptoms and their severity depend on the cause, how quickly the anemia develops, and how severe it is.
Iron deficiency is a particularly important cause because the body needs iron to make hemoglobin. But not every type of anemia is caused by too little iron, so taking iron supplements is not an appropriate treatment for every case.
What happens when you have too many red blood cells?
The opposite problem can also occur. An unusually high concentration of red blood cells is called erythrocytosis; the term polycythemia is also used in certain contexts.
Having more red blood cells can increase the proportion of blood occupied by cells, known as the hematocrit. If the concentration becomes substantially elevated, blood can become more viscous, or resistant to flow.
An increased red blood cell count can occur for different reasons. Sometimes the body is responding to reduced oxygen availability, such as at high altitude or in certain medical conditions. In other cases, the bone marrow itself produces excessive numbers of red blood cells.
The significance of a high red blood cell count therefore depends on why it occurred and on the person’s overall health.
What can a blood test tell you about red blood cells?
A complete blood count (CBC) is one of the standard blood tests used to evaluate red blood cells.
A CBC can measure or calculate several features, including:
- Red blood cell count: the number of red blood cells in a given volume of blood.
- Hemoglobin: the amount of hemoglobin in the blood.
- Hematocrit: the percentage of blood volume occupied by red blood cells.
- Mean corpuscular volume (MCV): the average size of red blood cells.
- Mean corpuscular hemoglobin (MCH): the average amount of hemoglobin in each red blood cell.
- Red cell distribution width (RDW): a measure of variation in red blood cell size.
These measurements can provide clues about problems such as anemia and help clinicians determine what additional testing may be useful.
For example, anemia associated with iron deficiency often produces smaller-than-normal red blood cells, while some other conditions can produce larger cells. These patterns are clues rather than diagnoses by themselves.
Why does the shape of a red blood cell matter?
The red blood cell’s shape is closely tied to its function.
Its biconcave structure provides a relatively large surface area through which oxygen and carbon dioxide can move. Its thin central region also reduces the distance gases need to travel within the cell.
Just as important, the cell membrane and internal structure allow a red blood cell to deform without breaking. This flexibility is essential because capillaries can be narrower than the diameter of a resting red blood cell. The cell temporarily changes shape as it squeezes through these vessels and then returns toward its usual form.
Diseases that alter the shape or flexibility of red blood cells can therefore interfere with circulation and oxygen delivery.
Red blood cells are more than simple oxygen carriers
Red blood cells are often described simply as the cells that carry oxygen, but their role is more sophisticated. They coordinate oxygen delivery with the chemical conditions in different tissues, participate in carbon dioxide transport, help buffer blood pH, and continuously adapt to the physical demands of moving through the smallest blood vessels.
Their streamlined structure reflects these jobs. A mature red blood cell sacrifices many features that ordinary cells need so that it can be densely packed with hemoglobin, remain flexible, and spend its roughly four-month lifespan transporting gases through the body’s circulation.
