An average adult has around 4.5–5.5 litres of blood, depending on height, weight and sex. Children have less because their bodies are smaller, and the total volume gradually increases as they grow. The body can lose a small amount of blood without serious problems, but large or rapid blood loss (for example after a major accident or complicated surgery) can be life‑threatening.

To keep up with daily needs, the body is constantly making new blood cells in the bone marrow (the soft tissue inside certain bones). Red blood cells live for about 120 days, platelets for about 7–10 days, and some white cells for only a few hours or days. Old cells are removed by the spleen and liver, and new cells are released from the marrow to replace them.