A medical connector for use in a fluid pathway, the connector comprising: a housing (22, 24) having a proximal end with a proximal opening and a distal end with a distal opening; a first internal closure system positioned substantially within the housing, the first internal closure system being configured to selectively seal the proximal opening; and characterized by a second internal closure system positioned within the housing, the second internal closure system being configured to selectively seal within the housing in a distal direction from the first internal closure system, wherein: the second internal closure system opens or remains open so as to permit fluid flow in a first direction when the pressure in a proximal region within the connector is greater than the pressure in a distal region within the connector by a first threshold value, the first direction being from the proximal end portion to the distal end portion; the second internal closure system opens or remains open so as to permit fluid flow in a second direction when the pressure in the distal region within the connector is greater than the pressure in the proximal region within the connector by a second threshold value, the second direction being from the distal end portion to the proximal end portion, and the second threshold value being greater than the first threshold value; and a volume adjuster comprising a side wall formed from a flexible material, the side wall being configured to buckle or flex so as to reduce the internal volume of the volume adjuster from a first internal volume to a second internal volume when a pressure change occurs at the proximal end, thereby diminishing the pressure change within the proximal region of the connector. a housing (22, 24) having a proximal end with a proximal opening and a distal end with a distal opening; a first internal closure system positioned substantially within the housing, the first internal closure system being configured to selectively seal the proximal opening; and characterized by a second internal closure system positioned within the housing, the second internal closure system being configured to selectively seal within the housing in a distal direction from the first internal closure system, wherein: the second internal closure system opens or remains open so as to permit fluid flow in a first direction when the pressure in a proximal region within the connector is greater than the pressure in a distal region within the connector by a first threshold value, the first direction being from the proximal end portion to the distal end portion; the second internal closure system opens or remains open so as to permit fluid flow in a second direction when the pressure in the distal region within the connector is greater than the pressure in the proximal region within the connector by a second threshold value, the second direction being from the distal end portion to the proximal end portion, and the second threshold value being greater than the first threshold value; and the second internal closure system opens or remains open so as to permit fluid flow in a first direction when the pressure in a proximal region within the connector is greater than the pressure in a distal region within the connector by a first threshold value, the first direction being from the proximal end portion to the distal end portion; the second internal closure system opens or remains open so as to permit fluid flow in a second direction when the pressure in the distal region within the connector is greater than the pressure in the proximal region within the connector by a second threshold value, the second direction being from the distal end portion to the proximal end portion, and the second threshold value being greater than the first threshold value; and a volume adjuster comprising a side wall formed from a flexible material, the side wall being configured to buckle or flex so as to reduce the internal volume of the volume adjuster from a first internal volume to a second internal volume when a pressure change occurs at the proximal end, thereby diminishing the pressure change within the proximal region of the connector.