duodenum(redirected from pars descendens duodeni)
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muscular hoselike portion of the gastrointestinal tract extending from the lower end of the stomach (pylorus) to the anal opening. In humans this fairly narrow (about 1 in./2.
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, glandular organ that secretes digestive enzymes and hormones. In humans, the pancreas is a yellowish organ about 7 in. (17.8 cm) long and 1.5 in. (3.8 cm) wide. It lies beneath the stomach and is connected to the small intestine at the duodenum (see digestive system).
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the first section (individually distinct in man, mammals, and birds) of the small intestine, from the pylorus to the jejunum.
The length of the duodenum in man is approximately 25–30 cm (the width of about 12 fingers—hence the name); its volume is 150–250 ml. The wall of the duodenum consists of three layers: the inner layer of mucous membrane, the middle layer of muscular membrane, and the external layer of serous membrane. The mucous membrane forms numerous transverse folds, its surface studded with villi; it contains cells that manufacture an intestinal juice containing enzymes that break down proteins, fats, and carbohydrates; it also contains enterokinase, which converts trypsinogen, the inactive enzyme of pancreatic juice, to the active trypsin. In the wall of the upper portion of the duodenum are the so-called Brun-ner’s glands, which, in structure and in the composition of the juice they secrete, are closely related to the glands of the pyloric portion of the stomach. The effluent ducts of the pancreas and liver open into the duodenal cavity through a common aperture. The acidic, gruel-like food mass (chyme) passing from the stomach continues to be digested in the duodenum under the influence of enzymes of the alkaline pancreatic and intestinal juices. Proteins are broken down into amino acids, carbohydrates into monosaccharides, and fats into glycerol and fatty acids. Bile promotes the digestion and absorption of fats. Through the walls of the villi the products of proteolysis and glycolysis enter the blood; the products of lypolysis enter the lymph. In the walls of the duodenum an inactive substance is formed (prosecretin) which, under the action of hydrochloric acid entering from the stomach, is converted to the biologically active secretin. Upon entering the blood secretin, acting along with the sympathetic nervous system, stimulates the secretion of pancreatic juice. Ulcer is the most common disease of the duodenum.