Nutrients are digested in the human body extracellularly, outside of cells. Digestive lumen - space within a tube, enzymes are secreted from cells lining the lumen of the organs of the digestive such as a blood vessel or the tract. The digestive enzymes, or exocrine secretions, hydrolyze large molecules in esophagus food into small, soluble molecules that can be transported across the lining of the gastrointestinal tract into blood and hence to body cells as nutrients.
Nutrients are ground into finer particles by the teeth, partially digested by amylase from the salivary glands, and carried by peristalsis along the esophagus and into the stomach. Chief cells, parietal cells, and mucus-secreting cells line the lumen of the stomach. Chief cells synthesize and secrete pepsin and pepsinogen, the precursor to pepsin. Pepsin is a proteolytic enzyme that degrades polypeptide chains by cleaving peptide bonds. Parietal cells produce hydrochloric acid and intrinsic factor. Hydrogen ions are secreted by active transport across the parietal cell membrane via a transmembrane protein called H+-ATPase. Through this process, the pH of the stomach may reach levels below pH of 1. Hydrogen ion secretion is stimulated by the hormone gastrin, which itself is produced by the cells of the pyloric region of the stomach and the duodenum. As previously mentioned, intrinsic factor is a protein that binds and helps transport vitamin B12 through the lining of the intestine to blood. Very little absorption of nutrients into the circulatory system occurs in the stomach. The contents of the stomach are mixed and liquefied and passed into the duodenum, the first section of the small intestine.
Both the gallbladder and the pancreas drain into the duodenum. The gallbladder drains bile, which contains bile acids and bile pigments produced by the liver, into the duodenum. The release of bile from the gallbladder is stimulated by the hormone cholecystokinin. Bile acids contain lipids that have been conjugated to hydrophilic groups to make them more soluble in the water-based fluids of the intestinal system. Bile pigments consist of the breakdown products of hemoglobin, which has metabolized by the liver. The color of the bile pigments contributes to the brown color of feces.
The pancreas contains both exocrine and endocrine cells. Pancreatic fluid contains sodium bicarbonate, which neutralizes the acidic gastric fluid, increasing the pH to approximately 8. Pancreatic proteolytic enzymes attack proteins that are carried in the gastric fluid. Arginine- and lysine-containing peptides are cleaved by trypsin; tyrosine-, phenylalanine-, and tryptophan-containing peptides are cleaved by chymotrypsin; elastase cleaves peptide bonds associated with alanine and serine; and carboxypeptidase removes amino acids at the C-terminal of peptides. Pancreatic nucleases hydrolyze nucleic acids, which are also contained in the gastric fluid, into smaller, more absorbable units. The secretion of pancreatic fluid into the duodenum is regulated by gastrointestinal hormones. Secretin controls the release of sodium bicarbonate, and cholecystokinin controls the release of the cata-bolic enzymes. The pancreatic hormones, produced in the islets of Langerhans, also contribute to nutrient degradation. Amylase breaks down starch polymers into monosaccharides, and lipase hydrolyzes triglycerides into fatty acids and mono-glycerides.
Final digestion of nutrients occurs within the villi, which line the lumen of the small intestine. Peptidases and disaccharidases break their substrates into amino acids and monosaccharide subunits that are small enough to be absorbed through the lining of the intestine and into the lymphatic and circulatory systems. Nutrients travel to the liver, adipose tissue, and other organs, where they are further processed.
Nutrients that are not absorbed through the small intestine mucosa pass into the large intestine. This undigested material consists of mainly of water and cellulose. Resident bacteria of the large intestine complete the degradation process. The chief function of the large intestine is the reabsorption of water from the undigested material. Failure to reabsorb water causes watery diarrhea, while absorption of too much water may produce constipation.
peristalsis - wavelike movement that occurs involuntarily in hollow tubes of the body chief cells - secretory cells that line the gastric glands and secrete pepsin or its precursor, pepsinogen pyloric - pertaining to the distal portion of the stomach or to the opening between the stomach and duodenum duodenum - the first part of the small intestine that is adjacent to the pyloric region of the stomach cholecystokinin - hormone secreted by the upper small intestine that stimulates contraction of the gallbladder and pancreatic secretion chymotrypsin - digestive enzyme produced by the pancreas that, with trypsin, hydrolyzes proteins to peptones or amino acids elastase - an enzyme that dissolves elastin nuclease - an enzyme that participates in the hydrolysis of nucleic acids
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