The internal urethral orifice is the opening from the bladder into the urethra, at the lowest point of the bladder. It forms the bottom corner of the trigone.
| Location | Neck of the bladder, at the apex of the trigone |
|---|---|
| Parts |
|
| Nerve Supply | Inferior hypogastric plexus (sympathetic keeps it closed; parasympathetic opens it) |
| Blood Supply | Inferior vesical artery (males); Vaginal artery (females) |
| Venous drainage | |
| Lymphatic drainage |
|
Lets urine leave the bladder; in males the surrounding smooth muscle closes during ejaculation to stop semen entering the bladder.
An enlarged prostate middle lobe can bulge into and block the orifice. If it stays open during ejaculation, semen passes backwards into the bladder (retrograde ejaculation), a side effect of some prostate operations and drugs.
Semen entering the bladder because the bladder neck does not close.
Blockage at the bladder neck, often from prostate enlargement.
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