Abdominal & Pelvic Anatomy

Peritoneal relations and the retroperitoneum

SAD PUCKER. The retroperitoneal structures are the Suprarenal glands, Aorta and IVC, Duodenum (2nd–4th parts), Pancreas (except the tail), Ureters, Colon (ascending and descending), Kidneys, Esophagus (abdominal part only), and Rectum (partly). The ascending and descending colon, the 2nd–4th parts of the duodenum, and the pancreas began inside the peritoneum and fused to the posterior wall, so they are secondarily retroperitoneal; kidneys, adrenals, ureters, aorta, and IVC are primarily retroperitoneal.

Retroperitoneal injury (duodenal or pancreatic trauma, a leaking aortic aneurysm) can bleed extensively without peritoneal signs; flank ecchymosis (Grey Turner sign) may be the only external clue.

Hepatoduodenal ligament. The free edge of the lesser omentum carries the portal triad: portal vein (posterior), proper hepatic artery (medial), and common bile duct (lateral). It forms the anterior border of the omental (Winslow) foramen, the passage from the greater to the lesser sac, whose posterior border is the IVC. Clamping it (Pringle maneuver) stops hepatic inflow; bleeding that continues despite the clamp is coming from the IVC or hepatic veins.

Other ligaments. The falciform ligament carries the ligamentum teres (obliterated umbilical vein); the hepatogastric ligament carries the gastric vessels; the gastrosplenic ligament carries the short gastric vessels; and the splenorenal ligament carries the splenic vessels and the tail of the pancreas, which is why the tail is the one intraperitoneal part of the pancreas.

Abdominal wall, groin, and hernias

Wall layers. From outside in: skin, Camper (fatty) then Scarpa (membranous) fascia, external oblique, internal oblique, transversus abdominis, transversalis fascia, extraperitoneal fat, peritoneum. McBurney point, one-third of the way from the ASIS to the umbilicus, marks the appendix and is the site of maximal tenderness in appendicitis.

Inguinal canal. The deep (internal) ring is an opening in the transversalis fascia just lateral to the inferior epigastric vessels; the superficial (external) ring is a gap in the external oblique aponeurosis. The spermatic cord takes a covering from each layer it traverses: internal spermatic fascia (transversalis fascia), cremasteric muscle and fascia (internal oblique), and external spermatic fascia (external oblique).

Hesselbach (inguinal) triangle. Bounded by the inferior epigastric vessels (superolateral), the lateral border of the rectus abdominis (medial), and the inguinal ligament (inferior). Its floor is the weak transversalis fascia through which direct hernias protrude.

Femoral triangle. Bounded by the inguinal ligament (superior), sartorius (lateral), and adductor longus (medial). Its contents from lateral to medial spell NAVEL: femoral Nerve, Artery, Vein, Empty space (the femoral canal), Lymphatics. The femoral sheath encloses the artery, vein, and canal but not the nerve, which lies outside it. The femoral canal is bordered by the lacunar ligament medially and the femoral vein laterally, and the femoral vein for central access lies just medial to the palpable arterial pulse.

Abdominal aorta and gut blood supply

Three unpaired branches. The celiac trunk, SMA, and IMA are the unpaired midline branches, and each supplies one embryologic gut segment. Autonomic supply and the site of poorly localized visceral pain follow the same division.

Watershed zones. The splenic flexure lies at the border of SMA and IMA territory and is the classic site of ischemic colitis in low-flow states; the rectosigmoid junction is a second watershed.

Compression by the SMA. The SMA leaves the aorta at an acute angle. After rapid weight loss the third part of the duodenum can be trapped between the two (SMA syndrome: postprandial pain and vomiting), and the left renal vein can be pinched the same way (nutcracker syndrome: hematuria, left varicocele).

Portal system and portosystemic anastomoses

Portal vein. Formed behind the pancreatic neck by the splenic vein and the SMV; the IMV usually drains into the splenic vein. When portal pressure rises, most often from cirrhosis, blood detours through sites where portal and systemic veins meet, and those channels dilate.

TIPS. A transjugular intrahepatic portosystemic shunt joins the portal vein to a hepatic vein and decompresses the portal system. Because portal blood then bypasses the liver, hepatic encephalopathy is the main trade-off.

The pectinate line is the portal–systemic divide of the anal canal: above it, drainage is via the superior rectal vein to the IMV and pain fibers are visceral (internal hemorrhoids are painless); below it, drainage is via the inferior rectal vein to the internal iliac system and innervation is somatic (external hemorrhoids are painful).

Pelvis: ureter and pelvic floor

Water under the bridge. The ureters run retroperitoneally, cross the iliac vessels at the pelvic brim, and pass under the uterine artery in women and the ductus deferens in men: the ureter is the water, the vessel or duct is the bridge above it. The ureter can be ligated during hysterectomy where the uterine artery is clamped, and it also lies close to the ovarian vessels in the suspensory ligament.

Stone impaction sites. Stones lodge at the three physiologic narrowings, proximal to distal:

Pelvic diaphragm. Levator ani (pubococcygeus, puborectalis, iliococcygeus) plus coccygeus. It supports the pelvic viscera and maintains continence. The puborectalis sling pulls the anorectal junction forward to preserve the anorectal angle and relaxes to straighten it during defecation. Weakening, typically from childbirth, causes pelvic organ prolapse and stress urinary incontinence.