Let’s take a look at the anatomy of the large bowel. The large bowel starts at the caecum in the right lower quadrant and ends in the anal canal. It’s approximately 1.5 metres long and consists of the caecum, the ascending colon, the transverse colon, the descending colon, sigmoid colon, rectum, and the anal canal, where the GI tract ends. Embryologically, the large bowel is derived from the midgut and the hindgut. This is important as it allows us to understand the characteristic pattern of pain that patients with large bowel obstruction will develop, as well as the blood supply to the large bowel. The arterial supply to the large intestine arises from two anterior branches of the abdominal aorta, the superior mesenteric artery, and the inferior mesenteric artery. The superior mesenteric artery, via its branches, the ileocolic, right colic, and middle colic arteries, supplies the midgut parts of the large bowel, which consists of the caecum, the ascending colon, and the proximal 2/3rds of the transverse colon, whereas the rest of the large bowel, the hindgut part, so the distal 1/3rd of the transverse colon, the descending colon, sigmoid colon, rectum, and anal canal above the pectinate line, are supplied by the inferior mesenteric artery via its branches, the left colic artery, the sigmoid arteries, and the superior rectal artery. Rather than being two distinct arterial systems, the blood supply to the midgut and hindgut communicate via the marginal artery of Drummond. This is a continuous arterial arcade that runs parallel with the inner border of the colon and provides anastomosis between the terminal branches from the superior and inferior mesenteric arteries. Taking a closer look at the large bowel, we can see that the bowel wall is composed of four layers. The inner layer, which lines the lumen of the bowel, is the mucosa. Moving outwards, we have the submucosa, the muscularis propria, and finally, the outermost layer, which is the serosa.