Most red blood cells die in the liver. For decades, textbooks named the spleen as the primary site, but research from Massachusetts General Hospital’s Center for Systems Biology showed that the liver is actually where the majority of red blood cell elimination and iron recycling takes place. The spleen still plays an important role, but it handles a smaller share of the workload than previously believed.
Why the Liver, Not the Spleen
Red blood cells live an average of about 120 days, with a normal range of 70 to 140 days. When they age or become damaged, immune cells called monocytes grab them while they’re still circulating in the bloodstream. These monocytes then travel to both the liver and the spleen, but within several hours, almost all of the captured red blood cells end up inside a specialized population of immune cells found only in the liver.
The liver draws these red-blood-cell-laden monocytes to itself by releasing signaling proteins called chemokines. Once there, the monocytes mature into temporary macrophages (a type of immune cell that engulfs and digests cellular debris) capable of breaking down the red blood cells and recycling their iron. This system essentially works as a buffer: the bone marrow produces the monocytes, they capture dying red blood cells in the blood, and the liver collects them for processing.
The liver’s resident immune cells, known as Kupffer cells, also continuously filter dead and dying red blood cells from the blood as it passes through. Because the liver receives a massive volume of blood flow, it’s well positioned to serve as the body’s primary recycling center.
What the Spleen Still Does
The spleen acts more like a quality-control filter. Its internal structure contains narrow slits that are significantly smaller than a red blood cell. Healthy red blood cells are flexible enough to squeeze through these gaps quickly. Older or damaged cells, which are stiffer and less deformable, get stuck. The longer a struggling cell remains trapped, the more time the spleen’s resident macrophages have to identify and consume it.
This mechanical filtering is remarkably precise. Ion channels in the red blood cell membrane help regulate the cell’s volume and shape during the squeeze. When those channels don’t work properly, or when a red blood cell has lost too much flexibility with age, the cell spends more time in contact with macrophages and is more likely to be destroyed. The spleen is especially important for catching cells with structural defects, like those seen in sickle cell disease or hereditary spherocytosis.
How Your Body Flags Old Red Blood Cells
Your immune system doesn’t destroy red blood cells at random. Aging cells undergo specific surface changes that flip them from “leave me alone” to “eat me” status. One key player is a protein called CD47, which normally sits on the red blood cell surface and acts as a “don’t eat me” signal. As the cell ages, CD47 changes shape. This altered form attracts a binding protein, and the combination tells macrophages that the cell is ready for removal.
Two other signals reinforce the message. First, a major structural protein in the red blood cell membrane (called Band 3) changes shape with age, triggering the body to produce antibodies against it. Second, a fat molecule called phosphatidylserine, normally hidden on the inner surface of the cell membrane, flips to the outside. Macrophages recognize all three of these changes as markers of a cell past its prime.
Where the Parts Go After Destruction
Nearly all red blood cell death happens outside the bloodstream, inside the macrophages of the liver and spleen. This process is called extravascular hemolysis, and it accounts for the vast majority of normal red blood cell turnover. A small number of red blood cells do rupture directly in the bloodstream (intravascular hemolysis), but this is minimal under healthy conditions.
Once a macrophage digests a red blood cell, it breaks down hemoglobin into its component parts. The protein chains are recycled into amino acids. The iron atom is extracted and either stored inside a protein called ferritin or shipped back to the bone marrow to build new red blood cells. About 90% of the iron needed for new red blood cell production comes from this recycling process, which is why the body loses very little iron under normal circumstances.
The non-iron portion of heme is converted first into biliverdin (a green pigment) and then into bilirubin (a yellow pigment). A healthy adult produces roughly 250 to 350 milligrams of bilirubin per day, and about 80% of that comes directly from red blood cell breakdown. The remaining 20% comes from the turnover of other iron-containing proteins throughout the body. Bilirubin is processed by the liver, attached to a sugar molecule, and excreted in bile. It’s what gives stool its brown color and, in excess, causes the yellowing of jaundice.
The breakdown process also releases a small amount of carbon monoxide, which is exhaled through the lungs. This is actually useful clinically: measuring exhaled carbon monoxide is one way to estimate how quickly the body is destroying red blood cells.
What Happens Without a Spleen
People who have had their spleen removed (splenectomy) still clear old red blood cells effectively because the liver handles the larger share of the work. The liver’s Kupffer cells and the temporary macrophages derived from monocytes continue to filter and recycle aging cells. However, the loss of the spleen’s mechanical filtering means that some abnormal or stiff red blood cells that would normally be caught in the splenic slits can remain in circulation longer. This is why blood smears from people without a spleen often show unusual red blood cell shapes that would otherwise have been removed.