FIG. IB, alignment element 26 may indicate an alignment position Mi that corresponds to outer catheter proximal opening 30. [0079] Aspiration catheter systems discussed herein can be configured to maintain suction and continuous flow of fluid on an engaged thrombus. As an illustration, FIGS. 2A and 2B are schematic longitudinal cross-sectional diagrams illustrating an example aspiration catheter system 110 that includes an outer catheter 112 and an inner catheter 114. Aspiration catheter system 110, outer catheter 112, inner catheter 114, an outer catheter lumen 116, an outer catheter distal opening 118, an inner catheter lumen 120, an inner catheter distal opening 122, and a plurality of sidewall openings 124 may correspond to aspiration catheter system 10, outer catheter 12, inner catheter 14, outer catheter lumen 16, outer catheter distal opening 18, inner catheter lumen 20, inner catheter distal opening 22, and plurality of sidewall openings 24 of FIGS. 1A-1C, respectively. [0080] In FIG. 2A, outer catheter 112 and inner catheter 114 are positioned near a thrombus 126 prior to engagement of aspiration catheter system 110 with thrombus 126. When inner catheter distal opening 122 is not blocked by thrombus 126 (or partially blocked by thrombus 126 such that adequate flow of fluid is maintained through inner catheter distal opening 122), an aspiration pump, such as aspiration pump 6, may deliver aspiration fluid through outer catheter lumen 116 between an inner surface of outer catheter 112 and an outer surface of inner catheter 114. The aspiration fluid may be delivered out of outer catheter distal opening 118 into a volume of space outside outer catheter distal opening 18, such as near thrombus 126. In some examples (not shown), the aspiration fluid may be delivered through sidewall openings 124 into inner catheter lumen 120, even when thrombus 126 is not engaged, such as if sidewall openings 124 do not have a differential pressure (i.e., difference between positive pressure and negative pressure) threshold (i.e., minimum differential pressure) or the differential pressure threshold has been reached. The aspiration fluid (or combination of fluids) may flow near a surface of the unengaged or partially engaged thrombus 126, such that pieces of thrombus 126 may be loosened and removed from thrombus 126. Aspiration pump 6 may draw the pieces of thrombus 126, along with fluid, into inner catheter lumen 120. As such, the fluid flow shown in FIG. 2A may characterize a first flow path of fluid from outer catheter lumen 116 through outer catheter distal opening 118 and inner catheter distal opening 122 to inner catheter lumen 120.