Most cells in your body have a nucleus - the central compartment that holds the cell’s DNA and acts as its control room. Red blood cells are different. They start out with a nucleus, but on their way to being mature, they push it out and become essentially hollow.
The reason is space. Red blood cells have one job: carry oxygen from your lungs to the rest of your body. They do that with a protein called hemoglobin, which can bind to oxygen and release it where needed. Each red blood cell can carry around a billion oxygen molecules. By dumping the nucleus, the cell makes more room for hemoglobin - and gets the shape of a flexible disc that can squeeze through tight blood vessels.
You have about 25 trillion red blood cells coursing through your body at any moment. They live for about 120 days before being recycled, and your bone marrow builds about 2.5 million new ones every second to keep up. Take a deep breath and millions more just started carrying air through your blood.
