English to Portuguese Science Biology (-tech,-chem,micro-) Physiology

O contexto é: Initially (when flows are first “turned on”) there is a large gradient of component X from fluid A to fluid B at all points in the system; but even after flow has continued for some time there will still be a small gradient from A to B throughout the length of the system. At the left-hand side, B has gained a large concentration of component X but still cannot be as concentrated as the incoming fluid A, and will continue to pick up more of X. Almost complete transfer of X from A to B occurs, at all but the highest of flow rates.

Não consigo entender o que o autor quer dizer com o final do trecho "but the highest of flow rates". Alguma luz?