Breathing circuit systems and devices

A filler device includes a distal housing comprising a distal inner port and a distal outer port; a proximal housing comprising a proximal inner port and a proximal outer port, the proximal housing being sealingly affixed to the distal housing to form an inspiratory pathway between the distal inner port and the proximal inner port and to form an expiratory pathway between the distal outer port and the proximal outer port that is fluidly sealed from the inspiratory pathway, the inspiratory pathway being laterally adjacent the expiratory pathway; and a first filter in the inspiratory pathway or in the expiratory pathway to filter gases flowing through the inspiratory pathway or the expiratory pathway.