A venous reservoir in an extracorporeal blood circuit, comprising a rigid outer enclosure for a filtering mass in the form of a hollow cylinder, provided with an upper lid which comprises couplings for the inflow of blood with the portion of space delimited by the filtering mass and with a connector at the bottom, the reservoir further comprising a diaphragm which is arranged below the upper lid and is adapted to receive the blood that enters through the couplings provided in the peripheral region of the lid, at the axis of a central hole of the diaphragm there is a column with longitudinal vanes which protrude upwardly until they skim the rim of the hole and are adapted to follow the blood in its flow by gravity toward the collection region at the base of the enclosure.