A blood pump, in particular a pneumatic ventricular assist device ( 1 ) has a cap ( 2 ), which has an air chamber ( 4 ) and a blood chamber ( 3 ), separated by a membrane mounted around the perimeter of the cap ( 2 ). The blood chamber ( 3 ) of the cap ( 2 ) has an inlet channel ( 5 ) and an outlet channel ( 6 ) for blood, which channels have circular seats ( 7, 8 ) for mounting valves regulating the blood flow through the device ( 1 ). To the inlet channel ( 5 ) and the outlet channel ( 6 ) are connected with their first ends the connectors ( 9 ) used for joining the pump to cannulas connecting the pump with the circulatory system of a patient. The diameter ( D 1 ) of the seat ( 7 ) for the valve in the inlet channel ( 5 ) is smaller than the diameter ( D2 ) of the seat ( 8 ) for the valve in the outlet channel ( 6 ), whereas the inside channel of each connector ( 9 ) along the full working length thereof is uniformly convergent towards the second end of said connector ( 9 ).
