A system for collecting small amounts of body fluids, comprising a drive unit with a holder which moves from a first position to a second position when the drive unit is activated, and a disposable piercing unit with a retaining zone which is removably disposed in the holder, enabling the proximal end of a longitudinal capillary structure to be connected to at least one capillary channel for conveying body fluid and the distal end of the capillary structure is suitable for piercing skin. When the holder is arranged in the first position, the distal end of the capillary structure is located outside the skin. In the second position, said end penetrates the skin to a specific depth which is the depth of penetration. The invention is characterized in that the at least one capillary channel is outwardly open, at least in one region encompassing at least one part of the longitudinal extension of the capillary structure.