Red blood cells do not have mitochondria. In fact, the cytoplasm of a mature, mammalian red blood cell has no organelles at all, not even a nucleus.
Red blood cells, or erythrocytes, transport oxygen to other cells and transport carbon dioxide to the lungs. The cytoplasm of a red blood cell is full of a protein called hemoglobin, an iron-containing molecule than bonds with oxygen in the alveoli of the lungs. Blood carries the oxygen-charged erythrocytes throughout the body until they reach the capillaries. Capillaries are the smallest blood vessels in the body and are so small that the red blood cells have to line up single-file to pass through the capillaries. This tight squeeze allows close contact with the cells surrounding the capillaries, which makes the exchange of oxygen and carbon dioxide molecules possible.