may be configured to open in response to a differential pressure threshold being reached or exceeded. FIG. 3B is a schematic diagram illustrating an example slit valve 210. Slit valve 210 may include two or more sides in inner catheter 212. Slit valve 210 may be configured to deform in response to a differential pressure threshold being exceeded. FIG. 3C is a schematic diagram illustrating an example flap valve 220. Flap valve 220 may include one or more flaps 224 extending from inner catheter 222. Flap valve 220 may be configured to open in response to a differential pressure threshold being exceeded and may be aided by a direction of fluid flow past the flaps 224. [0084] The alignment elements discussed herein may have a variety of configurations. FIGS. 4A-4C are schematic longitudinal cross-sectional diagrams illustrating example aspiration catheter systems 300, 320, 340 having various example alignment elements for positioning an inner catheter at one or more predetermined positions within an outer catheter. Each aspiration catheter system 300, 320, 340 may be functionally similar to, for example, aspiration catheter system 10 of FIGS. 1A-1C. [0085] FIG. 4A is a schematic longitudinal cross-sectional diagram illustrating example aspiration catheter system 300 that includes visible markers as an alignment element. Aspiration catheter system 300 includes an outer catheter 306, an inner catheter 308 within outer catheter 306, a plurality of sidewall openings 310 at a distal portion 302 of aspiration catheter system 300, and an alignment element at a proximal portion 304 of aspiration catheter system 300, the alignment element including an inner catheter visible marker 312 on inner catheter 308. When inner catheter visible marker 312 is aligned with an outer catheter proximal opening 314 of outer catheter 306, inner catheter 308 may be at the predetermined position Pi relative to outer catheter 306. In this way, a clinician operating aspiration catheter system 300 may adjust aspiration catheter system 300, such as by operating a proximal portion of inner catheter 308, using an easy to view visual indication. While visible marker 312 is described at proximal portion 304, in some examples, visible marker 312 may be located at other locations, such that a clinician may more easily observe a relationship between visual marker 312 and outer catheter proximal opening 214. In other examples, outer catheter 306 may include a visual marker, such that visual marker 312 may be aligned with the outer catheter visual marker. [0086] FIG. 4B is a schematic longitudinal cross-sectional diagram illustrating example aspiration catheter system 320 that includes an alignment element including a plurality of visible markers. Aspiration catheter system 320 includes an outer catheter 326, an inner catheter 328 within outer catheter 326, a plurality of sidewall openings 330