Ever since doctors have had the ability to transfuse blood, we've needed a basically unlimited supply of the stuff. There's nothing worse for a person than running out of blood, since it kind of starves your organs of the nutrients and oxygen that they need to survive, and there's a distinct risk of this in severe injuries and intensive surgery. Even a single donor can save several lives. Discovering blood-types and how they deal with combinations was a major breakthough here. In the absense of properly available blood, a saline solution can still keep the circulatory system's volume up, making the remaining available blood more useful to the body.
But the need for blood is tempered by the risk of blood-bourne disease. Severe diseases like Hepatitus and AIDS can be transmitted through blood transfusions. The ideal donor would be an O- virgin, 12 years old or younger, who subsisted on a pure vegan diet of sunshine, water, and unicorn farts, and was proven to be free of all known diseases. Since the pool would be extremely limited if doctors actually held out for this, compromises have been made to remove the most likely carriers of disease, and to categorize blood to make the most use of the available supply. Blood donors can give up to a pint at a time, which the donor will slowly re-grow over a period of about 56 days. Blood donors typically get a snack of punch and cookies to replenish lost fluid and blood sugars, minimizing any negative effect that losing blood could have on them. Though many milions of people worldiwde heroically donate, people still die from lack of access to blood, because the need is far bigger than the supply.
Discovery news reports today that a biotech firm is now cloning blood from umbilical cords, producing massive amounts of disease-free blood from even one. Even better, the process is extremely fast. The blood is produced in far less time than the natural version, so in case of a massive emergency, a large supply of blood could be quickly made available.
A better process still would be completely artificial blood, as that could be made without limit. Blood is made of things that aren't very expensive: Carbon, Oxygen, Hydrogen, and a small amount of Iron. Though the chemistry is complex, once a process could be set up, machines could crank out gallon after gallon after gallon of the stuff and rush it to hospitals worldwide. The volume would be so great that pipe-based transport would be a worthwhile investment. Unlimited transfusable blood would have all sorts of insane implications. For one, poisoning could be cured by blood flushing, in which your blood would be replaced until no trace of the poison remained. For another, hemodialysis would become a very different, and probably much cheaper, process. Bacterial or viral diseases could also probably be flushed away. The waste blood could be used to grow flowers or something.