Small Intestine
Objective 5
Indicate the location of the following regions: duodenum, jejunum, ileum. Describe the histology and function of the unique features of the small intestine: plicae circularis, villi, and microvilli. Identify the organs and specific cellular and molecular structures involved in the absorption of each of the major nutrient types: monosaccharides; peptides and amino acids; fatty acids and monoglycerides. Explain the classification of fat-soluble and water-soluble vitamins. Discuss the absorption of vitamin B12 and its role in pernicious anemia.
The small intestine has three regions. From stomach to large intestine, they are:
- duodenum (Latin: “12 finger breadths”) — about 0.25 m in length
- jejunum (Latin: “empty”) — about 1 m long
- ileum (Latin: “flank”) — about 2 m long
The lumen is wrinkly, with plicae circularis, the Latin name for “circular folds.”
The histology of the small intestine is exactly what we learned earlier, because the small intestine is used as the “model” for the histology of the GI tract.
In the mucosa, the epithelial layer consists of a simple columnar epithelium called enterocytes or absorptive cells. The shape of these cells allows them to absorb water and nutrients on their apical (luminal) surfaces, process the nutrients as necessary, and then drop absorbed substances into the capillary bed on the basal (abluminal) surface.
For this reason, the lamina propria of the mucosa has a rich capillary bed, with numerous arterioles and venules. Also, a small lymph vessel called a lacteal carries lipid from the absorptive epithelium. These lymphatics eventually gather into the thoracic duct which, in turn, releases into the bloodstream at the left jugular/subclavian junction.
The muscularis layer reverts to the “normal” two sublayers of smooth (autonomic) muscle fibers: a circular and a longitudinal layer. Between them lie the clusters of neurons that control autonomic movements of the intestines: the myenteric plexi. As we’ll see later, these neurons generate the peristaltic waves that keep things moving in the GI tract.
The luminal surface of the small intestine appears velvety. This is because of the presence of villi (sing.: Latin villus, “shaggy”). These finger-like projections are about 1 mm long by 0.1 mm diameter. Their diameter is just at the limit of human vision, so that’s why it looks like velvet.
Most of the villi’s epithelial cells are specialized for absorption — the enterocyte. These cells are coated with microvilli, a smaller version of the villi. Microvilli are about 1/1000 the size of villi (micro–, 1/1000, get it?) and are thus about 1 μm long.
The epithelium has its share of mucus-secreting goblet cells.
Enzyme-secreting cells of the intestine are called Paneth cells. They secrete many enzymes. They are also capable of phagocytosis.
Enteroendocrine cells of the intestine secrete secretin from S cells, cholecystokinin (CCK) from CCK cells, and glucose-dependent insulinotropic peptide