An electronic device such as a portable computer may be provided with a lower housing and an upper housing. The electronic device may include hinge structures which allow the upper housing to rotate about a rotational axis relative to the lower housing. The electronic device may include a ventilation port structure with intake openings that allow air to be drawn into the electronic device. The ventilation structure may also include exhaust openings that are used to expel air from the lower housing. When the electronic device is in an open position, it may be desirable for the ventilation structure to form more exhaust openings than when the electronic device is in a closed position. The ventilation structure may form lower exhaust openings when the electronic device is in the closed position and form upper and lower exhaust openings when the electronic device is in the open position.