A rack-mountable computer system enables an airflow that cools components in an upstream portion of the computer system interior to be cooled through mixing with a bypass airflow downstream of the components in the upstream portion. The mixed airflow can cool components in a downstream portion of the interior. The bypass airflow is directed by a bypass plenum that is unencompassed by the separate plenum that directs the airflow to cool the upstream portion components. The bypass plenum can be at least partially established by an external surface the computer system and one or more external structures, including an external surface of an adjacently mounted computer system. Relative flow rates through the separate plenums can be adjusted, via flow control elements, to separately control heat removal from components upstream and downstream of the air mixing, based at least in part upon air temperatures in the separate interior portions.