A cassette useful for measuring one or more parameters of blood in, for example, a cardiopulmonary bypass circuit has a convex coupling for releasable connection with a mating, concave coupling of a measuring device. The coupling of the cassette includes flexible leg portions that can be moved toward each other under the influence of finger pressure to detach the cassette from the measuring device. Optionally, the cassette includes a body carrying one or more sensors, and also includes a casing that can be detached from the body when desired. A shunt circuit arrangement enables the cassette to be connected to the bypass circuit either before or during a surgical procedure.