Blood collecting devices for collecting blood from a patient using a single needle insertion are disclosed. Each blood collecting device typically includes a housing which is removably attached to a test tube or blood reservoir. A blood flow tube is disposed in fluid communication with the housing for distributing blood from a patient into the test tube or blood reservoir. The blood is distributed through a membrane cavity having a liquid-impervious and air-permeable membrane. At least one air opening is provided in the housing at the membrane cavity. Accordingly, air in the test tube or blood reservoir is displaced by the incoming blood through the membrane cavity and membrane and out the air opening or openings. Vacuum pressure in the test tube or blood reservoir is dispelled by the flow of air through the air opening or openings, membrane cavity and membrane and into the test tube or blood reservoir, respectively.