Describe lower GIT, rectum, anal canal and anal sphincters with O&G relevance.

Lower GIT, Anal Canal and Sphincter Anatomy

Lower GIT anatomy is O&G anatomy because the rectum and anal sphincter complex lie immediately behind the cervix, vagina, perineal body and pelvic floor. The obstetrician must recognise a third- or fourth-degree tear before repair. The gynaecologist must understand rectovaginal septum disease, posterior compartment prolapse, rectovaginal fistula, deep endometriosis and pelvic sidewall dissection. The urogynaecologist must connect levator injury, perineal body deficiency and anal sphincter damage to later faecal incontinence.

The most economical way to learn this region is to start with one developmental fact and let it explain almost everything else. The anal canal is built from two embryonic territories that meet partway down its length. The upper canal grows from the hindgut (endoderm), continuous with the rectum. The lower canal grows from the proctodeum, an ectodermal pit in the perineal surface. They fuse at what becomes the pectinate (dentate) line. Because the two halves come from different germ layers fed by different vessels and nerves, the pectinate line stays a sharp watershed for the rest of life: epithelium, arterial supply, venous drainage, lymphatic drainage and innervation all change as you cross it. If you can hold that one idea, the rest of the chapter is mostly its consequences. We will therefore build upward in deliberate layers: first the embryology that sets the watershed, then the rectum that stores stool, then the canal and its sphincters that close it, then the perineal body and pelvic floor that anchor everything, then the nerves and vessels that supply it, and finally the obstetric and gynaecological problems that follow when any layer fails.

The working model is continence by layered closure:

• rectum stores stool and senses filling;
• puborectalis maintains the anorectal angle;
• internal anal sphincter provides resting tone;
• external anal sphincter provides voluntary squeeze;
• anal cushions and mucosa help fine discrimination of gas, liquid and solid stool;
• perineal body and levator complex support the posterior vaginal wall and anal canal;
• pudendal and autonomic nerves coordinate reflex and voluntary control.

Where the Anal Canal Comes from

The cloaca is the common chamber at the caudal end of the early embryo into which the hindgut and the developing urinary tract both open. During the fourth to sixth weeks a wedge of mesoderm, the urorectal septum, descends and divides the cloaca into an anterior urogenital sinus (bladder, urethra and lower vagina) and a posterior anorectal canal. The point where the septum reaches the surface becomes the perineal body, which is why this single fibromuscular node is the meeting place of so many structures later. The anal membrane that closes the posterior compartment breaks down to open the canal to the exterior.

This embryology gives the region three facts that pay off clinically:

• The upper anal canal is hindgut endoderm and shares the rectum's visceral nerve supply, columnar epithelium and portal venous drainage; the lower anal canal is proctodeal ectoderm and shares the perineal skin's somatic supply, squamous epithelium and systemic venous drainage. Their boundary is the pectinate line.
• The perineal body is a developmental hinge between anterior and posterior perineum. Anything that thins it — childbirth, deep dissection, infiltrating endometriosis — destabilises both the vagina in front and the anal canal behind.
• When the urorectal septum forms abnormally, the result is an anorectal malformation or a congenital fistula between the anorectal canal and the urogenital tract. The same plane that the septum once occupied is the plane in which an acquired rectovaginal fistula later tracks. The adult problem follows the embryonic seam.

With the watershed established, we can climb the functional chain from storage upward.

Rectum: Reservoir Behind the Genital Tract

The rectum begins at about S3 as the sigmoid colon loses its mesentery and follows the concavity of the sacrum to the anorectal junction. It is about 12-15 cm long and forms a distensible ampulla. It is built like the rest of the large bowel — an inner circular and an outer longitudinal layer of smooth muscle, lined by columnar mucosa — but with two differences worth knowing: the longitudinal layer is a complete coat rather than three taeniae, and there are no appendices epiploicae. The mid-rectum carries three transverse folds, the valves of Houston, which a finger or sigmoidoscope passes; these are mucosal shelves, not sphincters, and have no role in continence.

Its anterior relations in women are the posterior cervix and upper vagina superiorly, then the posterior vaginal wall and rectovaginal septum lower down. Posteriorly lie the sacrum, coccyx, median sacral vessels and lower sacral nerves; laterally the rectum is supported by levator ani. The anterior fascial layer separating rectum from vagina (the rectovaginal or Denonvilliers fascia) is the surgical plane entered in rectovaginal fistula repair and posterior compartment surgery. The pouch of Douglas lies between posterior uterus/cervix and anterior rectum superiorly and usually contains small bowel or sigmoid colon.

Peritoneal coverage changes with level. The upper rectum has peritoneum anterolaterally; the middle rectum is covered anteriorly; the lower rectum is extraperitoneal. This explains why a high posterior vaginal injury may enter the peritoneal cavity, while lower rectovaginal disease may stay in the rectovaginal septum and perineal body. A fourth-degree tear passes through anal sphincter complex and anorectal mucosa; if the tear extends high enough — above the level of the pelvic floor — peritoneal contamination becomes a real risk and the situation is no longer a routine perineal repair. The decision point on rectal examination is whether the anorectal mucosal defect reaches above puborectalis; if it does, senior obstetric and coloproctology help should be sought because the woman may need defunctioning rather than primary closure alone.

The rectum has a compliant wall. Rectal filling activates stretch receptors and the rectoanal inhibitory reflex: the internal sphincter relaxes briefly so the sensitive epithelium just above the pectinate line can sample whether the rectal contents are gas, liquid or solid. Continence then depends on rectal compliance, stool consistency, sphincter closure, sensory discrimination and voluntary control. A rectum scarred by radiation or inflammatory bowel disease loses compliance, so urgency and leakage can occur even with intact sphincters.