Your largest bones, like those in your arms and legs, have a centre core of bone marrow. This living tissue produces 500 billion blood cells daily: red blood cells, which carry oxygen around your body, and white blood cells, which fight off invading viruses, bacteria and allergens. You have about 5.7 litres of blood circulating in your body and, if you lose small quantities, your body will quickly manufacture more. This is why you can donate limited amounts of blood safely.