The National Blood Service has expressed concern about the acute shortage of blood in the national blood bank.The bank usually operates with about 400 units of blood in stock but now has as low as 100 units which can only last for two days.Although the blood bank is taking measures to replenish the stock, its managers are calling on all Ghanaian to walk into blood centers and donate blood.Head of Accra Area blood center of the National Blood Transfusion Service, Dr Paul Mensah lamented that citizens donate blood only when their relatives need blood – a situation he said accounts for the current shortage.He said until Ghanaians voluntarily donate blood to the national blood bank, “we will not be able to sustain the demand for blood.”