Digestive system IV

Department of Anatomy &Histology Dr.Raja Ali
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Digestive System
Small intestine:fig.(1&2)
? The small intestine is the longest component of the digestive tract, measuring over 6 m, and is divided into three anatomic portions:
• Duodenum (_25 cm long) is the first, shortest, and widest part of the small intestine. It begins at the pylorus of the stomach and ends at the duodenojejunal junction .
• Jejunum (_2.5 m long) begins at the duodenojejunal junction and constitutes the upper two fifths of the small intestine. It gradually changes its morphologic characteristics to become the ileum.
• Ileum (_3.5 m long) is a continuation of the jejunum and constitutes the lower three fifths of the small intestine. It ends at the ileocecal junction, the union of the distal ileum and .
? The small intestine is the principal site for the digestion of food and absorption of the products of digestion.
? Plicae circulares, villi, and microvilli increase the absorptive surface area of the small intestine.
• Plicae circulares (circular folds), also known as the valves of Kerckring, are permanent transverse folds that contain a core of submucosa. Each circular fold is circularly arranged and extends about one half to two thirds of the way around the circumference of the lumen . The folds begin to appear about 5 to 6 cm beyond the pylorus. They are most numerous in the distal part of the duodenum and the beginning of the jejunum and become reduced in size and frequency in the middle of the ileum.
• Villi are unique, fingerlike and leaflike projections of the mucosa that extend from the theoretical mucosal surface for 0.5 to 1.5 mm into the lumen. They completely cover the surface of the small intestine, giving it a velvety appearance when viewed with the unaided eye.
• Microvilli of the enterocytes provide the major amplification
of the luminal surface. Each cell possesses several thousand closely packed microvilli. They are visible in the light microscope and give the apical region of the cell a striated appearance, the so-called striated border. Enterocytes and their microvilli are described below.
? The villi and intestinal glands, along with the lamina propria, associated GALT, and muscularis mucosae, constitute the essential features of the small intestinal mucosa.
? Villi, as noted, are projections of the mucosa. They consist of a core of loose connective tissue covered by a simple columnar epithelium. The core of the villus is an extension of the lamina propria, which contains numerous fibroblasts, smooth muscle cells, lymphocytes, plasma cells, eosinophils, macrophages, and a network of fenestrated blood capillaries located just beneath the epithelial basal lamina.