<P> In humans, mature red blood cells are flexible and oval biconcave disks . They lack a cell nucleus and most organelles, in order to accommodate maximum space for hemoglobin; they can be viewed as sacks of hemoglobin, with a plasma membrane as the sack . Approximately 2.4 million new erythrocytes are produced per second in human adults . The cells develop in the bone marrow and circulate for about 100--120 days in the body before their components are recycled by macrophages . Each circulation takes about 60 seconds (one minute). Approximately a quarter of the cells in the human body are red blood cells . Nearly half of the blood's volume (40% to 45%) is red blood cells . </P> <P> Packed red blood cells (pRBC) are red blood cells that have been donated, processed, and stored in a blood bank for blood transfusion . </P> <P> Almost all vertebrates, including all mammals and humans, have red blood cells . Red blood cells are cells present in blood in order to transport oxygen . The only known vertebrates without red blood cells are the crocodile icefish (family Channichthyidae); they live in very oxygen - rich cold water and transport oxygen freely dissolved in their blood . While they no longer use hemoglobin, remnants of hemoglobin genes can be found in their genome . </P> <P> Vertebrate red blood cells consist mainly of hemoglobin, a complex metalloprotein containing heme groups whose iron atoms temporarily bind to oxygen molecules (O) in the lungs or gills and release them throughout the body . Oxygen can easily diffuse through the red blood cell's cell membrane . Hemoglobin in the red blood cells also carries some of the waste product carbon dioxide back from the tissues; most waste carbon dioxide, however, is transported back to the pulmonary capillaries of the lungs as bicarbonate (HCO) dissolved in the blood plasma . Myoglobin, a compound related to hemoglobin, acts to store oxygen in muscle cells . </P>

Where are red blood cells found in the body