Devices and a system for detection of blood within in-vivo fluids are provided. A device comprises a housing that includes a gap. The gap has at least one opening through which in-vivo fluids may enter and/or exit the gap. The device further comprises an illumination source for illuminating the in-vivo fluids in the gap, a light detector for detecting light which passes through the in-vivo fluids in the gap, and flexible fins disposed on the housing in the vicinity of the gap's opening for covering the opening when the fins are folded and for pumping fluids into and out of the opening by repeated closure and opening of the opening by the fins, due to repeated peristaltic waves. This pumping effect may lead to continuous flow of fluids into and out of the opening and thus into and out of the gap of the device.