Medical connectors and methods of use

A retrograde-flow-resistant needleless medical connector comprising:

• a housing comprising a body member and a base member permanently coupled together, and a female luer-receiving proximal region and a male luer distal end, and an internal fluid volume configured to receive medicinal fluid during use;
• the proximal region of the housing comprising a proximal closure system with a first stage at which a proximal end of the housing is closed and a second stage at which the proximal end of the housing is open, the proximal closure system being configured to move between the first and second stages such that a proximal surface of the proximal closure system is configured to be displaced in a distal direction when a medical implement is inserted into the proximal end of the housing;
• a volume adjuster positioned within the housing that is capable of operating independently of the proximal closure system, the volume adjuster being configured to change the internal fluid volume in the proximal region of the connector automatically in response to changes in fluid pressure within the medical connector;
• and
• an internal closure system positioned in a distal direction from the volume adjuster, the internal closure system being capable of operating independently of the proximal closure system and spaced from the distal male end of the housing, the internal closure system comprising a first stage at which the internal closure system is closed and a second stage at which the internal closure system is open, the internal closure system being capable of being closed when a male luer of the medical implement is seated within the female luer-receiving proximal end, and the proximal closure system is open, to resist retrograde flow of fluid through the distal end of the housing when the proximal end is open.