General Anatomy and Digestive Processes

The digestive system consists of accessory organs such as the teeth and liver, and the [1], a passage from mouth to anus. In the abdominal cavity, much of it is suspended from the dorsal body wall by a connective tissue sheet, the [2]. Attached to the greater curvature of the stomach is a fatty serous membrane, the [3], which covers the small intestine like an apron. The intake of food is called [4]. Its breakdown consists of two stages: [5], a physical process achieved by the teeth and gastrointestinal contractions, and [6], a series of enzymatic hydrolysis reactions.

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The Mouth Through Esophagus

The oral, or [7], cavity opens at the oral orifice in front and the [8] at the rear. The anterior two-thirds, or [9], of the tongue is anchored to the floor of the mouth by a fold called the [10]. Most of its mass consists of the [11] muscles. The roof of the mouth consists of a bony [12] in front and a/an [13] at the rear. From 6 to 30 months of age, 20 [14] teeth appear in predictable order. The emergence of each tooth from the gum is called [15]. By age 25, these are replaced by up to 32 permanent teeth. From front to rear on one side of the jaw, these are 2 [16], 1 [17], 2 [18], and 2 or 3 [19]. Each tooth is embedded in a socket called the [20] and held in place by a collagenous membrane, the [21]. A tooth consists mostly of hard, yellowish [22], a thinner but very hard layer of [23] over its crown, and soft [24] filling its center.

Saliva is produced by [25] salivary glands within the oral tissues and [26] salivary glands that lie elsewhere but convey saliva to the mouth through their ducts. The latter include the [27] gland near the ear and the submandibular and [28] glands beneath the oral cavity.

The esophagus exhibits a stratification of its tissues that more or less typifies the rest of the digestive tract. Lining the lumen is a mucous membrane, or [29], that consists of an epithelium, a/an [30], and a muscularis mucosae. Because it is subject to abrasion by passing food, the esophageal epithelium is of the [31] type. Deep to the muscularis mucosae is a connective tissue layer, the [32], and then a thick muscular wall, the [33], which has an inner layer of circular smooth muscle and an outer layer of longitudinal muscle. The outer surface of the digestive tract is covered with a thin moist membrane, the [34], in most places, but most of the esophagus is covered with a fibrous layer, the [35]. Two nerve plexuses in the wall of the digestive tract, beginning in the esophagus, coordinate its activities: the [36] plexus in the submucosa and the [37] plexus between the two outer muscle layers. Together they form the [38] nervous system, which mediates the [39] reflexes of the tract as opposed to the longer reflex pathways that go by way of the vagus nerves to the brainstem and back.

The first use of such reflexes is swallowing, which involves sequential actions of the tongue; the superior, middle, and inferior [40] in the throat; and the muscular wall of the esophagus. In the esophagus, the 38 nervous system coordinates a traveling wave of constriction called [41], although a food mass, or [42], usually passes down the esophagus by gravity faster than this wave can keep up with it.

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The Stomach

In the stomach, food is reduced to a semidigested paste called [43]. The stomach has a J shape extending from its domelike [44] against the diaphragm to its [45] canal where it opens into the duodenum. Its superior opening is controlled by a constriction, present only in life, called the [46] sphincter, and its lower opening by a muscular ring, the [47] sphincter. The mucosa is pocked by innumerable gastric pits with two or three tubular glands opening into the bottom of each one. In the cardiac and pyloric regions, these glands secrete only [48], but in the rest of the stomach, where they are called [49] glands, they have a greater variety of cells and functions. The majority cell type, called [50] cells, secrete pepsinogen; the [51] cells secrete hydrochloric acid (HCl); and the [52] cells secrete hormones that regulate digestion.

The primary function of HCl is to convert the zymogen [53] into the active enzyme [54], which digests [55]. In infants, the stomach also secretes [56], which digests fat, and [57], which curdles milk. But the only secretion of the stomach that we can't live without is [58], which enables the small intestine to absorb vitamin B₁₂. The stomach is protected from its own powerful digestive action by a high rate of epithelial cell replacement, its internal coating of [59], and the [60], which prevent gastric juice from seeping between the epithelial cells.

Gastric activity begins in the [61] phase even before food reaches the stomach. The taste and smell of food, for example, can stimulate its contractions and secretions. The [62] phase begins when food enters the stomach. Food raises the pH and thereby stimulates [63] cells to release the hormone [64]. This hormone stimulates HCl and pepsinogen secretion. The [65] phase is triggered as chyme passes into the duodenum. There, it triggers the [66] reflex through the enteric nervous system and a long reflex through the vagus nerves and brainstem. Both reflexes slow down gastric activity until the duodenum processes the chyme that is already in it. The small intestine also secretes three hormones that inhibit gastric activity: CCK, [67], and [68].

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The Liver, Gallbladder, and Pancreas

The liver has many functions, but its only digestive role is to secrete [69]. It is composed mainly of a connective tissue capsule and stroma with innumerable tiny cylinders, the hepatic [70]. Each of these cylinders consists of plates of gland cells called [71] radially arranged around a blood vessel, the [72], in the middle. Blood filters through narrow spaces called hepatic [73] between these plates while the liver removes substances from the blood and adds other things to it. Macrophages called [74] clean up the blood by removing bacteria and other foreign matter.

Bile is secreted into narrow channels, the bile [75], between the liver cells. It flows into progressively larger channels until it leaves the liver by way of right and left [76]. Bile collects in a sac, the [77], on the inferior surface of the right lobe of the liver. This sac concentrates the bile and periodically releases it through the [78] duct into the duodenum. At its lower end, this duct joins one from the pancreas and a muscular [79] sphincter controls the emptying of bile and pancreatic juice into the duodenum. Bile is composed mainly of wastes, but contains steroids called [80] that aid in fat digestion. These steroids are subsequently absorbed by the small intestine, returned to the liver, and reused. This recycling pathway is called the [81] circulation.

The pancreas releases its secretion mainly by way of the [82] papilla in the small intestine, but in many people there is a/an [83] duct that provides a second outlet for the pancreatic juice. The pancreatic juice contains an acid-neutralizing agent, [84], and several enzymes and [85], which are inactive enzyme precursors. One of these, [86], is converted to [87] by an enzyme called enterokinase on the epithelial surface of the intestine. The enzyme 87 not only digests dietary protein but also activates other 85s in the pancreatic juice. Pancreatic juice also contains a starch-digesting enzyme called [88], a fat-digesting enzyme called [89], and enzymes that digest RNA and DNA. Chyme in the duodenum triggers the release of two hormones: [90], which stimulates the release of bile and pancreatic enzymes, and [91], which stimulates the release of 84.

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The Small Intestine

In order to absorb nutrients adequately, the small intestine requires a large surface area. This is provided by its great length, by ridges in the mucosa called [92], by barely visible projections called [93] that give it a fuzzy appearance, and by a fringe of [94] on the surface of each absorptive epithelial cell. The small intestine begins at the [95] valve and ends at the [96] valve. It is divided into three regions: a short [97] near the stomach, a longer [98] located mostly in the upper left quadrant of the abdomen, and longest of all, the [99], located mostly in the lower right quadrant.

Each villus has a core of areolar tissue with a plexus of blood capillaries and a lymphatic capillary called a/an [100]. A villus is covered with an epithelium composed of mucus-secreting [101] cells and columnar absorptive cells. The brush border of the latter has enzymes that function in [102] digestion, a process that requires that chyme come into physical contact with the epithelium. Between the villi, the small intestine has tubular pits called [103]. In the duodenum, the submucosa has [104] glands that provide extra protection from stomach acid. In the ileum, there are clusters of lymphocytes called [105] that protect against infection.

The small intestine shows two forms of motility: [106], which chops and mixes the chyme, and overlapping waves of peristalsis called the [107], which moves chyme toward the large intestine. Food in the stomach promotes emptying of the small intestine by means of the [108] reflex, which relaxes the ileocecal valve.

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The Large Intestine

The large intestine begins at a pouch called the [109], extends up the right side of the abdominal cavity as the [110] colon, crosses over to the left, descends that side, takes an S-shaped turn called the [111] into pelvic cavity, and ends with a straight segment called the [112] followed by the anal canal. It has crypts like the small intestine but lacks the upward-projecting [113]. In most places, the longitudinal layer of the muscularis externa is reduced to three longitudinal strips called [114]. Tonus of those muscles makes the intestinal wall bulge out in pockets called [115]. The average adult passes about 500 mL of gas, or [116], per day, consisting partly of swallowed air and partly of gases produced by the bacterial population of the colon, called the intestinal [117]. The large intestine exhibits movements called [118] that pass food residue from one pouch to another, and a few times a day it shows stronger [119] that pass residue for a considerable distance along the tract. These are triggered by [120] and [121] reflexes, stimulated by food in the stomach and duodenum, respectively. Feces are passed in response to a relatively weak [122] reflex mediated through the enteric nervous system and a stronger [123] reflex mediated through the spinal cord. These reflexes involuntarily relax the [124] sphincter, but defecation normally does not occur unless one voluntarily relaxes the [125] sphincter at the same time.

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Chemical Digestion and Absorption

Most digestible dietary carbohydrate is [126]. Its digestion begins with the action of the enzyme [127] in the mouth. This enzyme is quickly [128] by the acid in the stomach and then ceases to function, although if it is in the middle of a large food bolus it may act for an hour or two before the acid gets to it. Digestion continues in the small intestine, where pancreatic [129] digests most of the 126, the brush border enzyme [130] hydrolyzes maltose, and the enzymes [131] and [132] hydrolyze longer oligosaccharides. Two dietary disaccharides, cane sugar and milk sugar, are digested by the brush border enzymes [133] and [134], respectively, although most races of people do not produce 134 after infancy and cannot digest milk sugar. Glucose and galactose are absorbed by the [135], a transport protein in the plasma membranes of the epithelial cells; they also pass between epithelial cells by [136] drag in the osmotically absorbed water.

The chemical digestion of protein begins with the action of the enzyme [137] in the stomach, which breaks 10% to 15% of the protein into shorter peptides. In the small intestine, protein digestion continues with the action of the pancreatic enzymes [138] and [139]. Small peptides are then taken apart one amino acid at a time by three brush border enzymes: [140], which removes an amino acid from the –NH₂ end of the peptide; [141], which removes an amino acid from the –COOH end; and [142], which splits apart the last two amino acids. Like glucose, amino acids are absorbed by [143]-dependent transport proteins.

Lipids are digested by lingual [144], which is present in the saliva but not activated until it is swallowed; [145], present in the gastric juice of infants; and [146], secreted by the pancreas. For 146 to work efficiently, ingested fat must be broken up into smaller droplets with more exposed surface area. This process is called [147] and is carried out by lecithin and bile salts in the bile. Enzyme 146 removes the first and third [148] from a fat molecule and leaves the middle one. The lipids are then coated with bile salts to form small droplets called [149]. When these contact the surface of an intestinal absorptive cell, the lipids leave the 149 and diffuse into the cell. Here, the Golgi complex packages them into [150] and the cell releases them into the lamina propria of the villus. These 150s are too large to get through the wall of a blood capillary, so instead of being absorbed into the bloodstream they are taken up by the [151] of the villi. This produces a white, fatty lymph called [152], which is eventually emptied into the blood at the subclavian veins. Absorbed fats take [153] vitamins along with them. Vitamin C and the B vitamins are absorbed by simple diffusion, except that vitamin B₁₂ binds to [154] and is then absorbed by receptor-mediated [155].

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