Red blood cells (RBCs) come from the bone marrow, which is housed within the bones, in a process called erythropoiesis.

Bone marrow is a part of the bones, so while it is correct that RBCs come from the marrow, it is only more specific to say as such, and not incorrect to say that the RBCs come from the bones.

In fact, the tibia and femur, the two longest bones in the body produce many RBCs until the age of 25, where they stop contributing, and the main source are the vertebrae, sternum, ribs, and cranial bones.

The spleen serves to filter and destroy old RBCs and the heart only serves as a mechanical pump in order to distribute them.

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