The small intestine is a long tubular organ that is approximately 460-1000cm in adults. It is divided into three sections, Duodenum, Jejunum, and Ileum. The surface area is greater than one might think due to the folds, villi, and microvilli. The surface area is approximately 30 square meters. Most of human digestion and absorption takes place within these three sections of small intestine.
The diagram of normal anatomy absorption in the small intestine is pictured to the right. It is color coded based on the area of the digestive tract the absorption takes place. You may also view here: master-normal-anatomy-with-text
Length: 22 ft. (6.7 m)
Width: 1 inch
pH: neutral or slightly alkaline (5-7)
- Neutralization in stomach, where enzymes act to breakdown food
- Digestion through greater breakdown with help of bile and pancreatic juices
- Absorption through assimilation of digested food, vitamins, and salts. Nutrients are taken into the bloodstream via specialized epithelial cells to the liver through the hepatic portal vein.
Length: 1 ft. (0.30 m)
- Main site of breakdown
- C shaped turn with 4 parts: superior, descending, inferior, ascending
- Mixes food (now in form of chyme) with bile and other digestive juices
- Passes chyme through duodenojejunal flexure which contains suspensory muscle to widen the duodenal angle and increase movement.
Length: 8.2 ft. (2.5 m)
- Coiled, vascular tube that contains a thick intestinal wall
- The wall contains epithelial projections called intestinal villi
- Smaller projections in the villi, called microvilli work to:
- project specialized transport cells called enterocytes
- increase surface area
- allow more absorption
Length: 11.5 ft. (3.5 m)
- Less vascularized and thinner intestinal wall
- Absorbs nutrients that preceding sections of the gut did not
- particularly works with vitamin B12 and bile salt absorption
- Connects to the colon through the ileocecal valve for further breakdown.
Click for:RNY Absorption Anatomy (1.1 MB)
- Campbell NA, Reece JB, Mitchell LG. Biology. 5th ed. Benjamin-Cummings Pub Co. 1999-2002; 802-805.
- Ovesen L, Bendtsen F, Tage-Jensen U, Pedersen NT, Gram BR, Rube SJ. Intraluminal pH in the stomach, duodenum, and proximal jejunum in normal subjects and patients with exocrine pancreatic insufficiency. Gastroenterology. 1986; 90(4): 958-62.
- Stevens C. E., and Hume, I. D. 1995.Comparative Physiology of the Vertebrate Digestive System. 2nd ed. New York: Cambridge University Press.
- Schmidler C. Anatomy and Function of the Digestive System. Healthpages.org. 2016.
Duodenal switch (DS) operation results in the highest success rate of all weight loss surgical procedures. Patients, over time, will experience some weight gain many years after DS procedure. There are a number of suspected mechanisms that may be responsible for the weight gain:
1-Ageing may slow the metabolism and the activity down.
2-Over time patients may not be as adherent to healthy dietary and lifestyle changes as they may have been immediately after surgery.
3- Hypertrophy of the alimentary and common channels over time increases in surface area f allowing greater caloric absorption leading to increased weight gain.
It has been demonstrated in bowel resection studies, as well as rat studies, that the nutrient stimulated regions of small intestine increase villus height and total weight, crypt depth and proliferation as well as wall thickness, as an adaptation to compensate for the loss of absorptive capacity in the resected bowel. This observation may be applied to DS procedure as seen in histological slides from a patient who had to have an operation done requiring bowel resection. The segment of the bowel resected included the junction of the biliopancreatic, common and alimentary limbs.