Doctors speak in a strange tongue when discussing blood and blood products. Typically a person has 85 mls of blood for each kg of body weight. So an average man weighing 72 kg or 165 pounds has about 6 litres of blood circulating around. If he has an accident and loses 2000 mls of blood , he has lost about a third of his total volume.
Red blood cells are another matter. They are measured in weight of haemoglobin per volume of whole blood, and to make things even more confusing, usually in grams per 100 mls (or decalitre). So a normal range of haemoglobin is 13.5-18 gms per 100ml in a man and 11.5-16.5 in a woman. Anaemia is defined as anything below this level, although dangerous levels are highly individual and, at the end of the day, anyone's guess.To complicate things further, the Americans measure haemoglobin in haematocrits that is the percentage of red cells per unit of total blood volume. This works out to 40-50 per cent in a healthy man and 35-47 per cent in a healthy woman.
The Witnesses believe (and medicine is coming to agree) that the actual red blood count is not as important as total circulating volume of fluid. If a person's volume is high it can speed up the flow of even a low red blood cell count. Once you lose volume, the heart has to work extremely hard to get those red blood cells around to vital organs. In cases of emergency, it's vital first to stop bleeding and then to replace circulating volumes of fluids as soon as possible.