A robotic surgical system, comprising: a surgical portal; a surgical robot including a robot arm (2, 3) and a controller, the controller configured to establish a software-based remote center of motion, RCM, of a surgical instrument (200, 300) attached to the robot arm based on a location of the surgical portal in a patient through which the surgical instrument is inserted; and a guide tube having a trailing end supported by the robot arm of the surgical robot, a leading end inserted in the surgical portal and wherein alignment is maintained between the robotic arm and the surgical portal during a surgical instrument exchange and including when the surgical instrument is not in the surgical portal via the software based RCM, the guide tube including an elongated tubular body through which an elongated shaft of the surgical instrument is inserted or removed during the surgical instrument exchange, wherein the guide tube is slidably movable relative to the surgical portal in response to movement of the robotic arm, and wherein the guide tube and the robotic arm define a first longitudinal axis that extends between the leading and trailing ends of the guide tube, and wherein the surgical portal defines a second longitudinal axis that extends between the leading and trailing ends thereof, the guide tube configured to maintain coaxial alignment of the first longitudinal axis and the second longitudinal axis during the surgical instrument exchange via the software based RCM wherein the robot arm and the surgical instrument are robotically movable axially relative to the first and second longitudinal axes by sliding the guide tube relative to the surgical portal. a surgical portal; a surgical robot including a robot arm (2, 3) and a controller, the controller configured to establish a software-based remote center of motion, RCM, of a surgical instrument (200, 300) attached to the robot arm based on a location of the surgical portal in a patient through which the surgical instrument is inserted; and a guide tube having a trailing end supported by the robot arm of the surgical robot, a leading end inserted in the surgical portal and wherein alignment is maintained between the robotic arm and the surgical portal during a surgical instrument exchange and including when the surgical instrument is not in the surgical portal via the software based RCM, the guide tube including an elongated tubular body through which an elongated shaft of the surgical instrument is inserted or removed during the surgical instrument exchange, wherein the guide tube is slidably movable relative to the surgical portal in response to movement of the robotic arm, and wherein the guide tube and the robotic arm define a first longitudinal axis that extends between the leading and trailing ends of the guide tube, and wherein the surgical portal defines a second longitudinal axis that extends between the leading and trailing ends thereof, the guide tube configured to maintain coaxial alignment of the first longitudinal axis and the second longitudinal axis during the surgical instrument exchange via the software based RCM wherein the robot arm and the surgical instrument are robotically movable axially relative to the first and second longitudinal axes by sliding the guide tube relative to the surgical portal.