The prostatic urethra, the widest and most dilatable part of the urethra canal, is about 3 cm long.
It runs almost vertically through the prostate from its base to its apex, lying nearer its anterior than its posterior surface; the form of the canal is spindle-shaped, being wider in the middle than at either extremity, and narrowest below, where it joins the membranous portion.
A transverse section of the canal as it lies in the prostate is horse-shoe-shaped, with the convexity directed forward.
The keyhole sign, in ultrasound, is associated with a dilated bladder and prostatic urethra.