Replacement of Red and White Blood Cells

In humans, the bone marrow contains haematopoetic stem cells which divide by mitosis and differentiate into a number of different types of blood cells.Red blood cells - erythrocytes - carry oxygen around the body.They have no nucleus and therefore can only survive for 120 days. They therefore need to be replaced constantly at the rate of about 3 billion per kg of body mass per day.White blood cells - leucocytes - form the cells of the immune system.Neutrophils are phagocytic cells that engulf bacteria.They live for about 6 hours and are produced from the haematopoetic cells of the bone marrow at the rate of about 1.5 billion per kg per day. This increases during infection.