An extracorporeal circuit for extracorporeally circulating blood, comprising: a blood reservoir (3, 3A, 3B) possessing a storage chamber for temporarily storing liquid, the reservoir comprising a tube having an upper end, the reservoir having a lower end opening at a bottom region of the storage chamber; an arterial line connectable to a patient during extracorporeal circulation to receive blood from the patient; a venous line connectable to a patient during extracorporeal circulation to return blood to the patient; a pump connected to the lower end opening of the blood reservoir; the upper end of the tube of the blood reservoir (3, 3A, 3B) being adapted to be connected to a source of priming solution; an oxygenator connected to the arterial line and the pump to effect gas exchange on the blood from the patient; characterized by a flexible collection container possessing an interior for collecting the priming solution; a partition in the interior of the collection container dividing the interior of the collection container into a first space positioned above a second space; a branch line in fluid communication with the venous line and branching from the venous line, the first space being in fluid communication with the branch line; the interior of the collection container comprising a communication part for establishing communication between the first space and the second space; and wherein the priming solution is introduced into the first space by way of the branch line, and is collected in the second space via the communication part; the communication part comprising a tube in the flexible collection container, and including at least one side hole formed in a wall of the tube and opening toward the first space; thereby being configured to collect liquid into the second space, and changing an internal pressure in the second space with the internal pressure of the first space not substantially changing due to the changing of the internal pressure in the second space. a blood reservoir (3, 3A, 3B) possessing a storage chamber for temporarily storing liquid, the reservoir comprising a tube having an upper end, the reservoir having a lower end opening at a bottom region of the storage chamber; an arterial line connectable to a patient during extracorporeal circulation to receive blood from the patient; a venous line connectable to a patient during extracorporeal circulation to return blood to the patient; a pump connected to the lower end opening of the blood reservoir; the upper end of the tube of the blood reservoir (3, 3A, 3B) being adapted to be connected to a source of priming solution; an oxygenator connected to the arterial line and the pump to effect gas exchange on the blood from the patient; characterized by a flexible collection container possessing an interior for collecting the priming solution; a partition in the interior of the collection container dividing the interior of the collection container into a first space positioned above a second space; a branch line in fluid communication with the venous line and branching from the venous line, the first space being in fluid communication with the branch line; the interior of the collection container comprising a communication part for establishing communication between the first space and the second space; and wherein the priming solution is introduced into the first space by way of the branch line, and is collected in the second space via the communication part; the communication part comprising a tube in the flexible collection container, and including at least one side hole formed in a wall of the tube and opening toward the first space; thereby being configured to collect liquid into the second space, and changing an internal pressure in the second space with the internal pressure of the first space not substantially changing due to the changing of the internal pressure in the second space.