A hemodialysis circuit component kit, comprising: a cassette including: a body forming a loop adaptor recess, a portion of the body forming the loop adaptor recess having an aperture, the body having a first portion and a second portion adjacent to the first portion, the first portion having a plurality of recesses (212, 218, 222, 230, 235, 241, 243) configured to retain tubes, wherein a first aperture in the first portion intersects at least one of the recesses, the second portion having a first slot along a top edge of the second portion and a second slot along a side edge of the second portion, the first and second slots being arranged to allow the body to capture a dialyzer; a loop adaptor in the loop adaptor recess of the body, wherein the loop adaptor includes a locking pin disposed in the aperture in the portion of the body forming the loop adaptor recess; a plurality of tubes, wherein each tube is in a recess of the plurality of recesses; a capsule in fluid communication with a tube of the plurality of tubes; and an air release chamber in fluid communication with a tube of the plurality of tubes. a cassette including: a body forming a loop adaptor recess, a portion of the body forming the loop adaptor recess having an aperture, the body having a first portion and a second portion adjacent to the first portion, the first portion having a plurality of recesses (212, 218, 222, 230, 235, 241, 243) configured to retain tubes, wherein a first aperture in the first portion intersects at least one of the recesses, the second portion having a first slot along a top edge of the second portion and a second slot along a side edge of the second portion, the first and second slots being arranged to allow the body to capture a dialyzer; a loop adaptor in the loop adaptor recess of the body, wherein the loop adaptor includes a locking pin disposed in the aperture in the portion of the body forming the loop adaptor recess; a plurality of tubes, wherein each tube is in a recess of the plurality of recesses; a capsule in fluid communication with a tube of the plurality of tubes; and an air release chamber in fluid communication with a tube of the plurality of tubes. a body forming a loop adaptor recess, a portion of the body forming the loop adaptor recess having an aperture, the body having a first portion and a second portion adjacent to the first portion, the first portion having a plurality of recesses (212, 218, 222, 230, 235, 241, 243) configured to retain tubes, wherein a first aperture in the first portion intersects at least one of the recesses, the second portion having a first slot along a top edge of the second portion and a second slot along a side edge of the second portion, the first and second slots being arranged to allow the body to capture a dialyzer; a loop adaptor in the loop adaptor recess of the body, wherein the loop adaptor includes a locking pin disposed in the aperture in the portion of the body forming the loop adaptor recess; a plurality of tubes, wherein each tube is in a recess of the plurality of recesses; a capsule in fluid communication with a tube of the plurality of tubes; and an air release chamber in fluid communication with a tube of the plurality of tubes.