Complex sentence: The effectiveness of retia is primarily determined by how readily the heat/gas/ions can be exchanged; thus for a given length they are most effective with respect to gases or heat, then small ions, and decreasingly so with respect to other substances.
Split into simpler sentences: The effectiveness of retia is primarily determined by how readily the heat, ions, or gases can be exchanged. And, for a given length they are most effective with respect to gases or heat, then small ions, and decreasingly so with respect to other substances.