Ebook Gastrointestinal physiology (8th edition): Part 2
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Ebook Gastrointestinal physiology (8th edition): Part 2
GASTRIC SECRETIONOBJECTIVES■Identify the secretory products of the stomach, their cells of origin, and their functions.■Understand the mechanisms maki Ebook Gastrointestinal physiology (8th edition): Part 2ing it possible for the stomach to secrete 150 mN hydrochloric acid.■Describe the electrolyte composition of gastric secretion and how it varies with the rale of secretion.■Identify the major stimulants of the parietal cell and explain their interactions.■Discuss the phases involved in the stimulati Ebook Gastrointestinal physiology (8th edition): Part 2on of gastric acid secretion and the processes acting in each.■Identify factors that both stimulate and inhibit rhe release of the hormone gastrin.■ExEbook Gastrointestinal physiology (8th edition): Part 2
plain the processes that result in the inhibition of gastric acid secretion following rhe ingestion of a meal and its emptying from the stomach.■DescrGASTRIC SECRETIONOBJECTIVES■Identify the secretory products of the stomach, their cells of origin, and their functions.■Understand the mechanisms maki Ebook Gastrointestinal physiology (8th edition): Part 2us, and water have physiologic functions. They are secreted by the various cells present within the gastric mucosa. The only indis pensable ingredient in gastric juice is intrinsic factor, required for the absorption of vitamin B|Z by the ileal mucosa. Acid is necessaiy for the conversion of inactiv Ebook Gastrointestinal physiology (8th edition): Part 2e pepsinogen to the enzyme pepsin. Acid and pepsin begin the digestion of protein, but in their absence pancreatic enzymes hydrolyze all ingested protEbook Gastrointestinal physiology (8th edition): Part 2
ein, so no nitrogen is wasted in the stools. Acid also kills a large number of bacteria that enter the stomach, thereby reducing the number of organisGASTRIC SECRETIONOBJECTIVES■Identify the secretory products of the stomach, their cells of origin, and their functions.■Understand the mechanisms maki Ebook Gastrointestinal physiology (8th edition): Part 2ll of the stomach and protects it from damage. Mucus acts primarily as a lubricant, protecting the mucosa from physical injury. Together with bicarbonate (HCOf), mucus neutral izes acid and maintains the surface of the mucosa at a pH near neutrality. This is part of the gastric mucosal barrier that Ebook Gastrointestinal physiology (8th edition): Part 2protects the stomach from acid and pep sin digestion. Water acts as the medium for the action ol acid anil enzymes and solubilizes many ol the constitEbook Gastrointestinal physiology (8th edition): Part 2
uents of a meal.Gastric juice and many of its functions originally were described by a young army surgeon, William Beaumont, stationed al a fort on MaGASTRIC SECRETIONOBJECTIVES■Identify the secretory products of the stomach, their cells of origin, and their functions.■Understand the mechanisms maki Ebook Gastrointestinal physiology (8th edition): Part 2close range with a shotgun. St. Martin unexpectedly survived but was left with a permanent opening into his stomach from the outside (gastric fistula), rhe accident occurred in 1822, and during the ensuing 3 years Beaumont nursed St. Martin back to health. Beaumont retained St. Mar tin “for the purp Ebook Gastrointestinal physiology (8th edition): Part 2ose of making physiological expert ments,” which were begun in 1825. Beaumont’s observations and conclusions, many of which remain unchanged today, inEbook Gastrointestinal physiology (8th edition): Part 2
clude the description of the juice itself and its digestive and bacteriostatic functions, the identification of the acid as hydrochloric, the realizatGASTRIC SECRETIONOBJECTIVES■Identify the secretory products of the stomach, their cells of origin, and their functions.■Understand the mechanisms maki Ebook Gastrointestinal physiology (8th edition): Part 2 thorough study of the ability of gastric juices to digest various foodstuffs.648 ■ G.FUNCTIONAL ANATOMYFunctionally, the gastric mucosa is divided into the oxyntic gland area and the pyloric gland area (Fig. 8-1). The oxyntic gland mucosa secretes acid and is located in the proximal 80% of the stom Ebook Gastrointestinal physiology (8th edition): Part 2ach. It includes the body and the Hindus. The distal 20% of the gastric mucosa, referred to as the pyloric gland tmicơsa, synthesizes and releases theEbook Gastrointestinal physiology (8th edition): Part 2
hormone gastrin. This area of the stomach often is designated the antrum.The gastric mucosa is composed of pits and glands (Fig. 8-2). The pits and sGASTRIC SECRETIONOBJECTIVES■Identify the secretory products of the stomach, their cells of origin, and their functions.■Understand the mechanisms maki Ebook Gastrointestinal physiology (8th edition): Part 2 toward the outside or serosa. The oxyntic glands contain the acid-producing parietal cells and the peptic or chief cells, which secrete the enzyme precursor pepsinogen. Pyloric glands contain the gastrin-producing G cells and mucous cells, which also produce pepsinogen. Mucous neck cells are presen Ebook Gastrointestinal physiology (8th edition): Part 2t where the glands open into the pits, liach gland contains a Siem cell in this region. These cells divide; one daughter cell remains anchored as theEbook Gastrointestinal physiology (8th edition): Part 2
stem cell, and lhe Ollier divides several limes. The resulting new cells migrate both lo lhe surface, where they dil'ferenliale into mucous cells, andGASTRIC SECRETIONOBJECTIVES■Identify the secretory products of the stomach, their cells of origin, and their functions.■Understand the mechanisms maki Ebook Gastrointestinal physiology (8th edition): Part 2lls. Peptic cells are capable of mitosis, but evidence indicates that they also can arise from stem cells during the repair of damage to the mucosa. Cells of the surface and pits are replaced much more rapidly than are those of the glands.Tire parietal cells secrete hydrochloric acid (HCI) and, in h Ebook Gastrointestinal physiology (8th edition): Part 2umans, intrinsic factor. In some species thePyloric gland mucosaFIGURE 8-1Areas of the stomach.chief cells also secrete intrinsic factor. The normal hEbook Gastrointestinal physiology (8th edition): Part 2
uman stomach contains approximately 1 billion parietal cells, which produce acid at a concentration of 150 to 160 mF.q/L rhe number of parietal cells GASTRIC SECRETIONOBJECTIVES■Identify the secretory products of the stomach, their cells of origin, and their functions.■Understand the mechanisms maki Ebook Gastrointestinal physiology (8th edition): Part 2use the pH of the final juice at high rates of secretion may be less than 1 and that of the blood is 7.4, the parietal cells must expend a large amount of energy’ to concentrate H*. The energy for the production of this more than a million fold concentration gradient comes from adenosine triphosphat Ebook Gastrointestinal physiology (8th edition): Part 2e (ATP), which is produced byr the numerous mitochondria located within the cell (Fig. 8-3).-*------Gastric lumenFIGURE 8-2 ■ Oxyntic gland and surfacEbook Gastrointestinal physiology (8th edition): Part 2
e pit. Note the positions of the various cell types.668 GASTRIC SECRETIONFIGURE 8-3 Parietal cell. A, Electron photomicrograph. B, Schematic. (A, CourGASTRIC SECRETIONOBJECTIVES■Identify the secretory products of the stomach, their cells of origin, and their functions.■Understand the mechanisms maki Ebook Gastrointestinal physiology (8th edition): Part 2ntracellular canaliculus that is continuous with the lumen of the oxyntic gland. The tubulovesides contain the enzymes carbonic anhydrase (CA) and H+, potassium (K+)-ATPase (H+,K*-ATPase), necessary for the production and secretion of acid, on their apical membranes. Titus, in the resting parietal c Ebook Gastrointestinal physiology (8th edition): Part 2ell, any basal secretion is directed into the lumen of the tubulovesicles and not into the cytoplasm of the cell. Stimulation of acid secretion causesEbook Gastrointestinal physiology (8th edition): Part 2
the migration of the tubulovesicles and their incorporation into the membrane of the canaliculus as microvilli. As a result, the surface area of the GASTRIC SECRETIONOBJECTIVES■Identify the secretory products of the stomach, their cells of origin, and their functions.■Understand the mechanisms maki Ebook Gastrointestinal physiology (8th edition): Part 2cantly during acid secretion. Acid secretion liegins within 10 minutes of administering a stimulant. This lag time probably is expended in the morphologic conversion and enzyme activations described previously, following the removal ol stimulation, the tubulovesicles reform and the canaliculus regai Ebook Gastrointestinal physiology (8th edition): Part 2ns its resting configuration.rhe surface epithelial mucous cells arc recognized primarily by the large number ol mucous granules al their apical surfaEbook Gastrointestinal physiology (8th edition): Part 2
ces. During secretion, the membranes of the granules fitse with the cell membrane and expel mucusPeptic cells contain a highly developed endoplasmic rGASTRIC SECRETIONOBJECTIVES■Identify the secretory products of the stomach, their cells of origin, and their functions.■Understand the mechanisms maki Ebook Gastrointestinal physiology (8th edition): Part 2 zymogen granules migrate to the apical surface, where, during secretion, they empty their contents into the lumen by exocytosis. This entire procedure of enzyme synthesis, packaging, and secretion is discussed in greater detail in Chapter 9.Endocrine cells of the gut also contain numerous granules. Ebook Gastrointestinal physiology (8th edition): Part 2 Unlike in the peptic and mucous cells, however, these hormone-containing granules are located at the base of the cell. The hormones are secreted intoEbook Gastrointestinal physiology (8th edition): Part 2
the intercellular space, from which they diffuse into the capillaries. The endocrine cells have numerous microvilli extending from their apical surfaGASTRIC SECRETIONOBJECTIVES■Identify the secretory products of the stomach, their cells of origin, and their functions.■Understand the mechanisms maki Ebook Gastrointestinal physiology (8th edition): Part 2priate stimuli.SECRETION OF ACIDThe transport processes involved in the secretion of HCI are shown in Figure 8-4. The exact biochemical steps for the production of H+ are not known, but the reaction can be summarized as follows: Ebook Gastrointestinal physiology (8th edition): Part 2Gọi ngay
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