part of contractile vacuole central bladder central vacuole A specialized vacuole of eukaryotic cells, especially Protozoa, that fills with water from the cytoplasm and then discharges this externally by the opening of contractile vacuole pores. One of its functions is osmoregulatory. cellular_component Wikipedia:Contractile_vacuole GO:0000331 vacuole cytoplasmic vesicle contractile vacuole complex