Abdominal Viscera & Urinary System

🩸 Arterial supply to abdominal viscera

Abdominal aorta enters at T12 (aortic hiatus), descends left of midline, bifurcates at L4. Branches:

Visceral (unpaired)

Celiac — foregut
Superior mesenteric — midgut
Inferior mesenteric — hindgut

Visceral (paired)

Middle suprarenal
Renal
Gonadal

Parietal

Inferior phrenic (paired)
Lumbar (paired)
Common iliac (paired)
Median sacral (unpaired)

⚠️ Aneurysm & atherosclerosis: Most common site for abdominal aneurysm is between renal arteries and bifurcation. Atherosclerotic plaque common at bifurcation.

Celiac trunk (foregut)

Origin: T12–L1, anterior aorta. Divides into 3 retroperitoneal branches:

Left gastric → lesser curvature, esophageal branch.
Splenic (longest, tortuous) → spleen, pancreatic body/tail, left gastroepiploic, short gastric.
Common hepatic → proper hepatic (→ right/left hepatic + cystic a.) and gastroduodenal (→ right gastroepiploic + superior pancreaticoduodenal).

Penetrating ulcer risk: posterior wall stomach → splenic artery; lesser curvature → left gastric; duodenal bulb → gastroduodenal artery.

Superior mesenteric artery (midgut)

Origin: L1, deep to pancreas neck. Branches:

Inferior pancreaticoduodenal (anastomoses with superior from gastroduodenal).
Intestinal arteries (12–15) → jejunum/ileum via arcades and vasa recta.
Ileocolic → terminal ileum, cecum.
Right colic → ascending colon.
Middle colic → proximal ⅔ transverse colon.

Clinical: left renal vein compression by SMA aneurysm → left renal/adrenal hypertension, left varicocele in males.

Inferior mesenteric artery (hindgut)

Origin: L3, aorta just above bifurcation. Branches:

Left colic → distal transverse & descending colon.
Sigmoid arteries → sigmoid colon.
Superior rectal → upper rectum/anal canal.

Marginal artery (of Drummond) connects SMA and IMA branches along large intestine → collateral pathway.

Ischemia: splenic flexure (watershed) is the most common site of bowel ischemia.

Paired visceral arteries

Middle suprarenal → suprarenal glands.
Renal (L2) → right longer, passes posterior to IVC.
Gonadal → testicular/ovarian, descend retroperitoneally.

🔄 Venous drainage

Inferior vena cava (IVC)

Formed by common iliac veins at L5, ascends right of aorta, caval hiatus at T8. Receives from lower limbs, pelvis, paired viscera, and body wall. Does NOT receive GI venous drainage (except lower rectum).

Right gonadal & right suprarenal → direct into IVC.
Left gonadal & left suprarenal → left renal vein → IVC.
Left renal vein crosses anterior to aorta (below SMA origin).

Hepatic portal system

Portal vein = union of SMV (midgut) + splenic vein (foregut) behind pancreas neck. IMV (hindgut) usually drains into splenic vein.

Portal blood flows to liver sinusoids → hepatic veins → IVC → right atrium.

Portal hypertension: retrograde flow through portacaval anastomoses → esophageal varices, hemorrhoids, caput medusae.

Site	Portal tributary	Caval tributary	Clinical sign
Esophagus	Left gastric (esophageal veins)	Azygos system	Esophageal varices
Rectum	Superior rectal (IMA)	Inferior rectal (internal iliac)	Internal hemorrhoids
Umbilicus	Paraumbilical veins	Superficial abdominal wall veins	Caput medusae

🧬 Embryology of kidney & ureter

Three overlapping systems: pronephros (week 4, regresses), mesonephros (week 5, temporary), metanephros (permanent kidney).

Ureteric bud (from mesonephric duct) → renal pelvis, calyces, collecting tubules, ureter.
Metanephric blastema → nephrons (proximal tubule, loop of Henle, distal tubule).

Kidneys ascend from pelvis; ureters elongate; vascular supply from aorta.

Congenital: renal agenesis (bilateral fatal, Potter sequence), pelvic kidney, horseshoe kidney (fused lower poles, hooks under IMA), double ureter, urachal remnants.

🧪 Kidney & ureter anatomy

Kidneys: T12–L3 (right lower due to liver). Related to diaphragm, psoas major, quadratus lumborum. Right: 12th rib; Left: 11th & 12th ribs.

Ureters: fibromuscular, retroperitoneal, cross external iliac at pelvic brim. Constrictions (renal calculi lodgment):

Pelviureteric junction
Pelvic inlet (crossing iliac vessels)
Intramural (bladder wall)

Relations: ureter passes posterior to ductus deferens (male) / uterine artery (female) — “water under the bridge”.

🚻 Bladder, urethra & innervation

Bladder: covered superiorly by peritoneum. Trigone (smooth mucosa) bounded by ureteric orifices and urethral opening.

Detrusor (smooth muscle) — parasympathetic (S2–4) contracts during micturition.
Internal urethral sphincter (sympathetic T11–L2) — prevents leakage during filling.
External urethral sphincter (skeletal, pudendal nerve) — voluntary control.

Spastic bladder Upper motor neuron lesion (above sacral) → loss of inhibition, urge incontinence.

Atonic bladder Sacral cord/root lesion → loss of detrusor contraction, overflow dribbling.

Male urethra: 20 cm, divided into prostatic, membranous, spongy (penile). Female urethra: 4 cm.

Blood supply: vesical branches from internal iliac & umbilical arteries. Venous plexus → internal iliac veins.

🔬 Renal histology & filtration

Nephron: ~1–1.3 million per kidney. Bowman’s capsule + glomerulus = renal corpuscle.

Podocytes (visceral layer) have foot processes, slit diaphragms (20–50 nm) — size-selective filter.
Filtration barrier: fenestrated endothelium (50–100 nm) + shared basal lamina (blocks >70 kDa) + slit diaphragms.

Kidney functions: fluid/electrolyte balance, waste excretion, acid–base, glucose reabsorption, blood pressure (renin), erythropoiesis (EPO), vasodilation (prostaglandins).

Blood flow: renal artery → interlobar → arcuate → interlobular → afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries → venules. Unique arteriole–capillary–arteriole–capillary sequence.

💡 Clinical pearls & high-yield facts

Abdominal aneurysm: most common infrarenal (between renal arteries and bifurcation).
Splenic flexure – watershed area, most susceptible to ischemia.
Left renal vein compression (nutcracker phenomenon) by SMA → left varicocele, left adrenal/renal hypertension.
Portal hypertension → portacaval anastomoses: esophageal varices, hemorrhoids, caput medusae.
Ureteric constrictions: PUJ, pelvic brim, intramural – common sites for stone impaction.
Spastic vs atonic bladder – upper vs lower motor neuron lesion.
Horseshoe kidney – fused lower poles, trapped under IMA, normal function but increased stones.
Renal agenesis: unilateral common, bilateral fatal (oligohydramnios, Potter sequence).

Aorta → Celiac / SMA / IMA → Foregut / Midgut / Hindgut → Portal system → liver