Significant blood loss as a result of trauma or internal bleeding due to illnesses such as gastric ulcer can lead to life-threatening anaemia necessitating blood transfusion.
Anaemia can be extremely dramatic when blood loss is sudden and acute, as in severe injury or ruptured ectopic pregnancy. Haemoglobin levels plummet, causing acute shock and the risk of circulatory failure when they fall below 6 g/dl. Loss of over two litres of blood over a short period is an acute threat to life.

Many cases of anaemia are due to various deficiency states. Malnutrition or the daily loss of four to five millilitres of blood, for example, can deprive the body of nutrients such as the iron, vitamin B12 or folic acid needed for red blood cell production.