A needleless medical connector configured to resist retrograde fluid flow, the medical connector comprising: a housing comprising a proximal body member and a base member, the proximal body member comprising a threaded female end and the base member comprising a shroud and a male tip protrusion; a proximal closure system comprising a seal member positioned within the housing; a plug member slidably received inside the seal member; and an internal closure system comprising a backflow resist valve that is dome-shaped: wherein the dome-shaped backflow resist valve is configured to open to permit fluid to flow from a proximal side of the dome-shaped backflow resist valve to a distal side of the dome-shaped backflow resist valve under a first fluid pressure differential between a proximal region of the medical connector and a distal region of the medical connector, and the dome-shaped backflow resist valve is configured to open to permit fluid flow from the distal side of the dome-shaped backflow resist valve to the proximal side of the dome-shaped backflow resist valve under a second fluid pressure differential between the distal region of the medical connector and the proximal region of the medical connector that is higher than the first fluid pressure differential; and wherein the proximal closure system and the internal closure system are configured to open and close independently from each other. a housing comprising a proximal body member and a base member, the proximal body member comprising a threaded female end and the base member comprising a shroud and a male tip protrusion; a proximal closure system comprising a seal member positioned within the housing; a plug member slidably received inside the seal member; and an internal closure system comprising a backflow resist valve that is dome-shaped: wherein the dome-shaped backflow resist valve is configured to open to permit fluid to flow from a proximal side of the dome-shaped backflow resist valve to a distal side of the dome-shaped backflow resist valve under a first fluid pressure differential between a proximal region of the medical connector and a distal region of the medical connector, and the dome-shaped backflow resist valve is configured to open to permit fluid flow from the distal side of the dome-shaped backflow resist valve to the proximal side of the dome-shaped backflow resist valve under a second fluid pressure differential between the distal region of the medical connector and the proximal region of the medical connector that is higher than the first fluid pressure differential; and wherein the proximal closure system and the internal closure system are configured to open and close independently from each other. wherein the dome-shaped backflow resist valve is configured to open to permit fluid to flow from a proximal side of the dome-shaped backflow resist valve to a distal side of the dome-shaped backflow resist valve under a first fluid pressure differential between a proximal region of the medical connector and a distal region of the medical connector, and the dome-shaped backflow resist valve is configured to open to permit fluid flow from the distal side of the dome-shaped backflow resist valve to the proximal side of the dome-shaped backflow resist valve under a second fluid pressure differential between the distal region of the medical connector and the proximal region of the medical connector that is higher than the first fluid pressure differential; and wherein the proximal closure system and the internal closure system are configured to open and close independently from each other.