GI Blood Supply

Perforated Ulcers
Ulcers on the lesser curvature of the stomach can cause bleeding from the left gastric artery.
Posterior duodenal ulcers can cause bleeding from the gastroduodenal artery.

Short Gastric Arteries
The short gastric arteries are branches off the splenic artery. They supply the fundus. Unlike other arteries of the stomach, the fundus does not have dual blood supply. Thus it is more vulnerable to ischemia.

Pringle's Maneuver 
During an abdominal surgery, you can clamp down on the hepatoduodenal ligament to see if the bleeding stops. If it does, the bleed was from the proper hepatic artery or the portal vein. That's because the hepatoduodenal ligament contains the proper hepatic artery, portal vein, and cystic duct. If the bleeding continues, the bleed is from somewhere else (like the IVC).

SMA Syndrome
The superior mesenteric artery courses over the duodenum. Usually mesenteric fat provides padding, but if there isn't enough fat, the artery can be compresses. This leads to bowel obstruction.

Dual Blood Supply
stomach - celiac trunk and superior mesenteric artery (superior and inferior pancreaticoduodenal arteries)
transverse and descending colon - superior and inferior mesenteric (marginal artery of Drummond)
rectum - inferior mesenteric artery and iliac artery (superior rectal and middle rectal)

Mesenteric Ischemia
-small intestine
-embolism and arterial thrombosis
-venous thrombosis
-under perfusion from shock (affects watershed areas of colon which include splenic flexure and rectosigmoid junction)
-intestinal angina (fear of eating)
-increased WBC, lactate, acidosis
-occult blood in stool

Ischemic Colitis 
-colon