Asked by: Ross Kennedy, New Malden
According to information from the Red Cross and the Mayo Clinic, it takes between 600 and 650 kilocalories of energy for your body to replace the contents of a pint of blood. This sounds like a lot until you realise that the scientist's 'kilocalorie' is actually the same unit of energy that nutritionists confusingly call a 'calorie', and that the energy involved is about that in a 100g bar of chocolate.
The organisations do not give the source of their figures, but most of the energy is likely to be used to manufacture the 2.5 trillion red blood cells that are needed to replace those that have been taken. This manufacturing process, which is called erythropoiesis, takes place in the marrow of the large bones at the rate of about two million cells per second.