Abstract: A swallowable capsule for obtaining fluid samples from the gastrointestinal (GI) tract of a subject. The capsule comprises an outer shell having an inlet port that allows a fluid sample to enter into the interior space of the capsule. A piston is arranged in the interior space and can be moved via a pressure generating device to exert pressure on one side of the piston to propel the piston from an open position, in which fluid can enter the capsule from outside to a closed position, in which entrance of fluid from outside the capsule is prevented. The piston has at least one seal portion that closes the shell opening in the closed position. A control unit is configured to control the pressure generating device so as to move the piston in a forward direction from the open position to the closed position.