Small Intestine – Tutorial
Please read Unit 10 – Introduction to Digestive System Tissues and study the image slider of the four major layers of the digestive tract wall prior to completing the activities in this chapter.
Introduction to the Small Intestine
The small intestine (small bowel) is a highly coiled, tube-like organ averaging approximately 1.5 inches (4 cm) in diameter and approximately 20 feet (6 m) in length that occupies most of the central abdominal cavity inferior to the liver and stomach. The small intestine begins at the pyloric sphincter where chyme exits the stomach and ends at the ileocecal valve (sphincter) where unabsorbed contents of the small intestine enter the large intestine. The small intestine serves as the primary site of enzymatic digestion and nutrient absorption in the human digestive tract, and it is divided into three segments known as the duodenum, jejunum, and ileum.
The duodenum is the first segment of the small intestine, beginning just after the pyloric sphincter and extending approximately 10 inches (25 cm) to the point where it turns inferiorly to become the jejunum. When the pyloric sphincter opens, small amounts of chyme exit the stomach and enter the lumen of the duodenum. A short distance (approximately 4-5 in. or 7-10 cm) from the pyloric sphincter, pancreatic secretions from the pancreatic duct and bile from the common bile duct enter the duodenum at a common opening known as the duodenal papilla. The pancreatic juice and bile mix with chyme in the lumen of the small intestine and serve as important substances for buffering chyme, enzymatic digestion of food, and absorption of nutrients.
The second portion of the small intestine is the jejunum which is continuous with the end of the duodenum and measures approximately 8 feet (2.5 m) in length. The jejunum is the portion of the small intestine where most of the enzymatic digestion and nutrient absorption occur.
The final and longest portion of the small intestine is the ileum which is continuous with the end of the jejunum and measures approximately 11 feet (3.5 m) in length. Any nutrients not absorbed along the jejunum are typically absorbed from the lumen of the ileum before chyme reaches the large intestine. Any unabsorbed material remaining in the lumen pass into the cecum of the large intestine through the ileocecal valve (sphincter).
The wall of the small intestine exhibits many structural adaptations that give it huge amounts of internal surface area to facilitate efficient secretion of digestive enzymes into the lumen and absorption of digested nutrients out of the lumen. Large (macroscopic) transverse internal folds known as plicae (circular folds) provide the first order of surface area enhancement along most of the length of the small intestine. The plicae are covered with microscopic fingerlike projections of the mucosa known as villi that serve to greatly increase the overall internal surface area. Individual intestinal villi are covered with simple columnar epithelium and the apical surface of individual columnar cells is composed of tiny folds of the cell membrane known as microvilli (brush border). Collectively, the plicae, villi, and microvilli on columnar cells greatly amplify the surface area of the intestinal lining in direct contact with contents of the lumen. These adaptations are critically important for efficient digestion and absorption of nutrients.
The lamina propria of the small intestinal mucosa extends up into each individual villus. The lamina propria within each villus contains a capillary network that serves to transport the majority of absorbed nutrients away from the small intestine toward the liver via the hepatic portal system. Also found within the lamina propria of each villus is a small lymphatic vessel known as a lacteal. Lacteals absorb packages of digested fats known as chylomicrons and transport them to larger lymphatic vessels that eventually deliver the chylomicrons to the bloodstream at the left subclavian vein.
Tutorial: Use the image slider below to learn more about the structure and characteristics of the small intestine.
Microscopy: Use the image slider below to learn how to use a microscope to study the small intestine on a microscope slide.
Tutorial: Use the hotspot image below to learn more about the structure of the wall of the small intestine on a low power image.
Tutorial: Use the hotspot image below to learn more about the structure and function of the small intestine.
