The invention relates to a system unit of a computer. The system unit has a plug-in card populated with components and a fan which supplies an air flow to an air ducting body arranged in the system unit. The air flow is provided for cooling the plug-in card which is arranged in the system unit essentially parallel to the air ducting body. Cooling of the plug-in card is improved in that the air ducting body has a central conduit and a first and a second side conduit. The side conduits are provided with a plurality of openings which are arranged in a row and aligned essentially parallel to heat sinks arranged on the plug-in card. The central conduit has a constriction for the admission of air.