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

Overview

Length: 22 ft. (6.7 m)

Width: 1 inch

pH: neutral or slightly alkaline (5-7)

• Steps:
  1. Neutralization in stomach, where enzymes act to breakdown food
  2. Digestion through greater breakdown with help of bile and pancreatic juices
  3. 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.

Duodenum

Length: 1 ft. (0.30 m)

pH~ 5

• 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.

Jejunum

Length: 8.2 ft. (2.5 m)

pH~ 6

• 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:

1. project specialized transport cells called enterocytes
2. increase surface area
3. allow more absorption

Ileum

Length: 11.5 ft. (3.5 m)

pH~ 7

• 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.

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Click for: Duodenal Switch Absorption & Anatomy (1.2 MB)

Click for:RNY Absorption Anatomy (1.1 MB)

References:

1. Campbell NA, Reece JB, Mitchell LG. Biology. 5th ed. Benjamin-Cummings Pub Co. 1999-2002; 802-805.
2. 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.
3. Stevens C. E., and Hume, I. D. 1995.Comparative Physiology of the Vertebrate Digestive System. 2nd ed. New York: Cambridge University Press.
4. Schmidler C. Anatomy and Function of the Digestive System. Healthpages.org. 2016.