<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>
where are red blood cells made in adults
in the bone marrow