Red blood cells are produced in the bone marrow

In humans, about 25 percent of the cells in the body are RBCs. They are produced in the bone marrow at a rate of about 2 million per second, and they circulate around the body for 100–120 days before they are destroyed. Each circuit around the body takes about a minute when at rest. RBCs are among the smallest cells of the body. They have to pass through the tiniest blood vessels, the capillaries. To be so small yet carry a lot of hemoglobin (at least 25% of their volume), RBCs lose their nuclei and most cell organelles prior to being released into the circulation. Because they have no mitochondria, they depend on *glycolysis for energy.

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*connect the concepts In Key Concept 9.1, glycolysis is described as a sequence of reactions that by the sixth step has converted one molecule of glucose to two molecules of 1,3-bisphosphoglycerate and by the tenth step to two molecules of pyruvate. However, some of the 1,3-bisphosphoglycerate can be enzymatically converted to 2,3-bisphosphoglycerate which plays an important role in regulating the oxygen-binding properties of hemoglobin, as you will learn below.