Apparatus for percutaneous access to a body of a patient, comprising: a first steerable tube, having a proximal end and a distal end, and shaped to define: a first lumen between the proximal end and the distal end, and a first coupling at a longitudinal site of the first steerable tube; a second steerable tube, shaped to define a second lumen and a second coupling, the second coupling being intracorporeally couplable to the first coupling; and an extracorporeal locking system, comprising: a housing shaped to define a groove; and a protrusion, configured to be coupled to the housing by being disposed in the groove, the apparatus having: an unlocked state in which at least the second coupling is disposed within the first lumen, and the second steerable tube is rotatable within the first lumen, and a locked state in which the second coupling is coupled to the first coupling, the coupling of the second coupling to the first coupling inhibiting rotation of the second steerable tube within the first lumen, and the apparatus being configured such that: the second coupling is advanceable through the first lumen until at least the longitudinal site, the apparatus remains in the unlocked state when the second coupling is disposed within the first lumen, proximal to the longitudinal site, when the second coupling becomes disposed at the longitudinal site in a given rotational orientation of the second steerable tube within the first lumen, the apparatus moves into the locked state by the second coupling automatically coupling to the first coupling, and the coupling of the protrusion to the housing further inhibits rotation of the second steerable tube within the first lumen. a first steerable tube, having a proximal end and a distal end, and shaped to define: a first lumen between the proximal end and the distal end, and a first coupling at a longitudinal site of the first steerable tube; a first lumen between the proximal end and the distal end, and a first coupling at a longitudinal site of the first steerable tube; a second steerable tube, shaped to define a second lumen and a second coupling, the second coupling being intracorporeally couplable to the first coupling; and an extracorporeal locking system, comprising: a housing shaped to define a groove; and a protrusion, configured to be coupled to the housing by being disposed in the groove, a housing shaped to define a groove; and a protrusion, configured to be coupled to the housing by being disposed in the groove, the apparatus having: an unlocked state in which at least the second coupling is disposed within the first lumen, and the second steerable tube is rotatable within the first lumen, and a locked state in which the second coupling is coupled to the first coupling, the coupling of the second coupling to the first coupling inhibiting rotation of the second steerable tube within the first lumen, and an unlocked state in which at least the second coupling is disposed within the first lumen, and the second steerable tube is rotatable within the first lumen, and a locked state in which the second coupling is coupled to the first coupling, the coupling of the second coupling to the first coupling inhibiting rotation of the second steerable tube within the first lumen, and the apparatus being configured such that: the second coupling is advanceable through the first lumen until at least the longitudinal site, the apparatus remains in the unlocked state when the second coupling is disposed within the first lumen, proximal to the longitudinal site, when the second coupling becomes disposed at the longitudinal site in a given rotational orientation of the second steerable tube within the first lumen, the apparatus moves into the locked state by the second coupling automatically coupling to the first coupling, and the coupling of the protrusion to the housing further inhibits rotation of the second steerable tube within the first lumen. the second coupling is advanceable through the first lumen until at least the longitudinal site, the apparatus remains in the unlocked state when the second coupling is disposed within the first lumen, proximal to the longitudinal site, when the second coupling becomes disposed at the longitudinal site in a given rotational orientation of the second steerable tube within the first lumen, the apparatus moves into the locked state by the second coupling automatically coupling to the first coupling, and the coupling of the protrusion to the housing further inhibits rotation of the second steerable tube within the first lumen.