Do we have the same blood that we have always had since birth? Does it constantly recycle? If it dies, where does it go?
In: Biology
Red blood cells only live for about 120 days. Red blood cells are *super* specialized to carry hemoglobin, the protein that holds and transports oxygen. They are *so* specialized that they can’t reproduce themselves. They are made in your bone marrow, and after maturing they lose their nuclei entirely, so they don’t carry DNA and can’t divide and create new red blood cells. They also lose their mitochondria, so they can’t metabolize sugar (and therefore don’t actually use any of the oxygen they are carrying). Their metabolism is super limited – basically, they’re just squishy boxes of hemoglobin with almost none of the other functions or abilities of typical cells.
Old red blood cells are filtered out in your spleen and liver. They are consumed by *macrophages*, which are a kind of white blood cell that essentially “eats” unwanted cells like old or damaged red blood cells, along with deactivated viruses and dead or crippled bacteria. Enzymes inside the macrophages tear the cells apart along with anything in them and spit out any useful amino acids that will be absorbed into other cells as needed to build new proteins. Anything that isn’t useful or is dangerous (like virus bits) is expelled in your poop.
Your bone marrow is constantly producing fresh blood cells and your spleen and liver are constantly filtering old and damaged cells out of your blood.
Incidentally, this is one of the reasons why blood cancers like leukemia are so challenging to treat. Most of the cells in your blood, including most white blood cells, are made in your bone marrow. When that tissue becomes cancerous, the only way to physically remove it is to remove (and replace) the bone marrow. Not that *any* cancer is pleasant, but most tumors can at least be accessed fairly easily through surgery and don’t involve drilling into your bones – an *intensely* painful operation.
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