A red blood cell is a tiny, disc-shaped cell that travels through your bloodstream carrying oxygen to every part of your body. These cells are so small that millions of them could fit on the head of a pin, yet your body contains about 25 trillion of them at any given moment.

Red blood cells get their color from a protein called hemoglobin, which is what makes your blood red. Hemoglobin works like a molecular delivery truck: it picks up oxygen in your lungs, carries it through your arteries to your muscles, organs, and tissues, then hauls carbon dioxide (a waste product) back to your lungs so you can breathe it out. Without red blood cells constantly making these rounds, your body's cells would run out of oxygen in minutes.

Unlike most cells in your body, red blood cells don't have a nucleus (the control center most cells have). This gives them more room to pack in hemoglobin and makes them flexible enough to squeeze through the tiniest blood vessels in your body. Your bone marrow constantly produces new red blood cells because each one only lives about four months before wearing out. When you feel dizzy or tired from anemia, it can mean you don't have enough red blood cells to deliver the oxygen your body needs.