PITTSBURGH — The American Red Cross supplies about 40% of the nation’s blood, and while volunteers in the Pittsburgh area are looking for all types, there’s one they need the most. On Monday, the organization issued a call to action for local blood donors as the summer months continue and the Fourth of July holiday weekend comes to an end. “Holidays are typically a hard time for us to collect blood. That’s because people ar traveling, doing fun things,” said Nicole Roschella of the American Red Cross. Right now, donations are down in the region because people are busy this time of year, so the Red Cross is encouraging anyone of any blood type to make an appointment, especially those who are Type O, because it will likely run out first. “Type O positive is what’s given to patients more than any other blood type. Type O negative is the universal blood type, so that’s what doctors will reach for in an emergency,” Roschella said.