A hemostatic unit for the determination of arteriovenous bloodstreams comprises a flat cuff (15) having a length sufficient to be wound by about one and a half turns around the region to be measured and being chargeable with compressed air, a pump (11, 21) for feeding said compressed air into said cuff (15), a pressure regulating valve (13, 23) for regulating the pressure of said compressed air, an air supply/discharge circuit for connecting an outlet (11a, 21a) of said pump (11, 21) to an air inlet (15b) of said cuff (15), and an air suction circuit branching out of said air supply/discharge circuit and connected to an inlet of said pump (11, 21). The unit further includes an electromagnetic valve (12, 22, 22') provided in said air/supply circuit and said air suction circuit with its given valve port being in open communication with the atmosphere.