The Physiology of the Small and Large Intestines
The digestive system processes the food you eat. Food travels via the esophagus into the stomach and then into the small and large intestines. The small intestine starts at the pylorus of the stomach and ends at the cecum of the large intestine. The main function of the small intestine is continued digestion and absorption of nutrients.
The duodenum is the first segment of the small intestine and is also the shortest (about 25 centimeters, or roughly 10 inches). It starts at the pylorus and ends at the duodenojejunal junction. It has four parts:
Superior part: Horizontal to and in front of the 1st lumbar vertebra.
Descending part: Runs inferiorly along the right borders of the 2nd and 3rd lumbar vertebrae. It’s close to the head of the pancreas.
Inferior part: Crosses to the left, in front of the inferior vena cava, close to the level of the 3rd lumbar vertebra.
Ascending part: Starting near the 3rd lumbar vertebra, this part runs upward along the left side of the aorta. It joins the jejunum at the duodenojejunal junction. It’s supported by the suspensory muscle of the duodenum, also called the ligament of Treitz.
Parasympathetic nerve supply to the duodenum comes from the vagus nerve, and sympathetic nerves in this area include the greater and lesser splanchnic nerves. Blood is supplied to the duodenum by the superior pancreaticoduodenal artery, a branch of the gastroduodenal artery, and the inferior pancreaticoduodenal artery, a branch of the superior mesenteric artery. Blood is drained by the corresponding veins into the hepatic portal system. Lymphatic vessels follow the arteries to the pancreaticoduodenal lymph nodes, pyloric lymph nodes, and the superior mesenteric lymph nodes. Lymph flows from those nodes to the celiac lymph nodes.
The jejunum and the ileum
The jejunum is the middle portion of the small intestine. It starts at the duodenojejunal junction and changes into the ileum, which is the third portion. The jejunum takes up about two-fifths of the length of the small intestine, but no clear line demarcates where it turns into the ileum. The ileum ends at the ileocecal junction. The ileum and jejunum are attached to the posterior abdominal wall by the mesentery.
Sympathetic and parasympathetic nerves are brought by the superior mesenteric plexus. Blood is brought to the jejunum and ileum by branches from the superior mesenteric artery. Blood is drained by the superior mesenteric vein. Lymph nodes that drain this area include the juxtaintestinal lymph nodes, mesenteric lymph nodes, and central nodes. Lacteals are specialized lymphatic vessels found in the small intestine that absorb fat from the foods you eat.
The large intestine
Most of the large intestine is located in the abdomen; the sigmoid colon and rectum are in the pelvic cavity. The abdominal portion of the large intestine includes the cecum and the ascending, transverse, and descending colon. The main function of the large intestine is to absorb water from fecal material before it’s eliminated from the body. The colon is also home to friendly bacteria that synthesize vitamin K and keep bad microbes in check.
The cecum is a pouch of intestine that hangs below the ileocecal junction in the right lower quadrant of the abdomen. Folds of mucosal tissue form the ileocecal valve that covers the ileal orifice. The appendix extends from the posteromedial part of the cecum.
Sympathetic and parasympathetic nerves come from the superior mesenteric plexus. Blood supply to the cecum comes via the ileocolic artery, a branch of the superior mesenteric artery. The appendicular artery branches from the ileocolic artery. Lymphatic vessels pass to the ileocolic lymph nodes and the superior mesenteric lymph nodes.
The ascending colon
The ascending colon travels from the cecum upward on the right side of the abdominal cavity to the right colic flexure near the right side of the liver. This part of the colon is retroperitoneal.
Nervous supply is brought to the ascending colon by the superior mesenteric plexus. The ileocolic and right colic arteries supply blood. Blood is drained away by the ileocolic and right colic veins. Lymph is drained by the epicolic and paracolic lymph nodes, and then it travels to the ileocolic and right colic lymph nodes.
The transverse colon
The transverse colon crosses from the right side of the abdomen to the left, ending at the left colic flexure. The sympathetic nerves that serve the transverse colon come from the superior and inferior mesenteric plexuses; the parasympathetic nerves arise from the vagus nerves and the pelvic splanchnic nerves.
Blood is brought to the transverse colon primarily by the middle colic artery. The distal portion of the transverse colon is served by the left colic artery, a branch of the inferior mesenteric artery. Venous blood is removed by the superior mesenteric and inferior mesenteric veins. Lymph is drained into the colic lymph nodes and into the colic nodes.
The descending colon
The descending colon travels behind the peritoneum and downward from the left colic flexure to the left iliac fossa where it continues as the sigmoid colon. Sympathetic nerve supply comes from the lumbar splanchnic nerves, the inferior mesenteric plexus, and the periarterial plexuses that surround the inferior mesenteric artery. Parasympathetic nerve supply comes from the pelvic splanchnic nerve.
Blood is brought to the descending colon by the left colic and sigmoid arteries, branches of the inferior mesenteric artery. Blood is drained away by the inferior mesenteric vein. Lymph is drained into the epicolic and paracolic lymph nodes, which drain into the intermediate colic lymph nodes. From here the lymph drains into the inferior mesenteric lymph nodes.