A red blood cell is a tiny, disc-shaped cell whose primary job is carrying oxygen from your lungs to every tissue in your body and ferrying carbon dioxide back to the lungs for disposal. Your blood contains roughly 70% red blood cells by volume, making them by far the most abundant cell type in circulation. An adult male typically has 4.7 to 6.1 million of them in a single microliter of blood (about the size of a pinhead), while an adult female carries 4.2 to 5.4 million per microliter.
Shape and Structure
Red blood cells look like a flattened doughnut: round with an indentation on each side, but not hollow through the middle. This concave shape gives them a large surface area relative to their volume, which makes gas exchange more efficient. It also lets them flex and fold to squeeze through capillaries narrower than the cell itself.
What makes red blood cells unusual is what they’re missing. Unlike most human cells, mature red blood cells have no nucleus, no mitochondria, and very few internal structures. Shedding the nucleus frees up interior space for more hemoglobin, the iron-rich protein that actually binds oxygen. It also makes the cell more flexible. The tradeoff is that without a nucleus, a red blood cell can’t repair itself or divide, which is why each one has a limited lifespan of about 120 days.
How They Carry Oxygen
Hemoglobin is the workhorse molecule inside every red blood cell. Each hemoglobin molecule contains four iron atoms, and each iron atom can bind one molecule of oxygen. When blood passes through the lungs, oxygen attaches to those iron sites. When the cell reaches oxygen-hungry tissue, the oxygen releases. Hemoglobin then picks up carbon dioxide for the return trip to the lungs.
Healthy hemoglobin levels range from 13.2 to 16.6 grams per deciliter for men and 11.6 to 15 grams per deciliter for women. When hemoglobin drops below these ranges, you have anemia, which can cause fatigue, shortness of breath, and pale skin because your tissues aren’t getting enough oxygen.
How They’re Powered Without Mitochondria
Most cells burn fuel using mitochondria, but red blood cells don’t have any. Instead, they rely entirely on breaking down glucose through a simpler, less efficient process that doesn’t require oxygen. This design makes sense: if red blood cells consumed the oxygen they carry, less would reach the rest of your body. They essentially act as delivery trucks that never dip into their own cargo.
This glucose-based energy production also generates a molecule called 2,3-DPG, which fine-tunes how readily hemoglobin releases oxygen. When your tissues need more oxygen (during exercise, for example), 2,3-DPG levels rise, encouraging hemoglobin to let go of its oxygen more easily.
Where They Come From
Red blood cells are made inside your bone marrow, the spongy tissue at the center of large bones. The process starts with a versatile stem cell that can become several types of blood cell. If it heads down the red blood cell path, it passes through several stages over about a week: first accumulating hemoglobin, then shedding its nucleus, and finally entering the bloodstream as a fully mature cell ready to transport oxygen.
The whole process is controlled by a hormone called erythropoietin, or EPO, produced mainly by the kidneys. When specialized kidney cells detect that blood oxygen is low, they ramp up EPO production, which tells the bone marrow to make more red blood cells. Once oxygen levels recover, EPO production drops and the marrow slows down. This feedback loop keeps your red blood cell count remarkably stable under normal conditions.
How Old Cells Are Recycled
After about 120 days, red blood cells become stiff and damaged. The body needs to clear them out and salvage their iron. For decades, textbooks named the spleen as the primary site for this cleanup. More recent research from the Massachusetts General Hospital Center for Systems Biology paints a different picture: the liver is actually the major site of red blood cell disposal and iron recycling.
Here’s how it works. Specialized immune cells called monocytes detect damaged red blood cells circulating in the blood, engulf them, and travel to the liver. There, they mature into macrophages that break the cells apart and extract the iron. That iron gets sent back to the bone marrow to build new hemoglobin. When this recycling system is impaired, free iron and hemoglobin can build to toxic levels, potentially damaging the liver and kidneys.
Blood Types and Surface Markers
The outer membrane of every red blood cell is studded with molecules called antigens, and the specific combination of antigens you carry determines your blood type. The most familiar system is ABO: type A cells carry A antigens, type B carry B antigens, type AB carry both, and type O carry neither. A second important marker is the Rh factor. If your cells carry it, you’re Rh-positive; if not, Rh-negative.
These distinctions matter most during blood transfusions. If you receive blood with antigens your immune system doesn’t recognize, it mounts an attack against the donated cells. The reaction can be especially severe if an Rh-negative person receives Rh-positive blood. Beyond ABO and Rh, scientists have identified more than 300 other antigen variants on red blood cells. Some patients with rare blood types or those who need repeated transfusions require matching across several of these additional markers to avoid complications.
When Counts Are Too Low or Too High
A red blood cell count that falls below the normal range is called anemia. It has many possible causes: iron deficiency (the most common worldwide), vitamin B12 or folate deficiency, chronic kidney disease that reduces EPO production, heavy blood loss, or conditions that destroy red blood cells faster than the body can replace them. Symptoms typically include fatigue, weakness, dizziness, and shortness of breath, all stemming from reduced oxygen delivery to tissues.
The opposite problem, too many red blood cells, is called erythrocytosis (also known as polycythemia). It comes in two broad forms. Absolute erythrocytosis means the bone marrow genuinely produces more red blood cells than normal. This can happen because of a genetic defect in the marrow itself (primary erythrocytosis) or because something is driving EPO levels too high, like chronic lung disease or living at high altitude (secondary erythrocytosis). Relative erythrocytosis, on the other hand, is a concentration issue: dehydration, vomiting, diarrhea, or diuretic medications reduce the liquid portion of your blood, making it look like you have more red blood cells than you actually do. Rehydrating usually resolves it.
Too many red blood cells thicken the blood and increase the risk of clots, while too few starve tissues of oxygen. In both cases, the root cause matters more than the number itself, which is why an abnormal count on a blood test typically prompts further investigation rather than immediate treatment.