digestive system

Digestive system:

Within the hypothalamus are two centers related to food intake. In the lateral nucleus is the feeding center. In the ventromedial nucleus is the satiety center

The digestive system is concerned with five main activities:

ingestion-- taking food into the body, i.e. eating
movement-- moving the food along the digestive tract
digestion-- breaking food down
absorption-- the passage of digested food into the body
defication-- passing of waste products and undigestables

There are two types of digestion. In the mouth and stomach, mechanical digestion takes place. This is mastication and the grinding of food. Chemical digestion takes place in the stomach, also, as well as the intestines. Chemical digestion requires acid (HCl, hydrochloric acid) and enzymes:

salivary amylase-- from the salivary glands, breaks down starches (polysaccharides), producing maltose (disaccharide)
pepsin-- secreted in the stomach (zymogenic cells), breaks down protein into peptides
pancreatic amylase-- from the pancreas, breaks starches down into maltose
trypsin-- from the pancreas, breaks down proteins into peptides
chymotrypsin-- from the pancreas, breaks proteins into peptides
carboxypolypeptidase-- from the pancreas, digests terminal amino acid carboxyl (acid) end of peptides
pancreatic lipase-- from the pancreas breaks down neutral fats (triglycerides) that have been emulsifies by bile salts into fatty acids and monoglycerides
maltase-- from the small intestine, breaks down maltose into glucose
sucrase-- from the small intestine, breaks down sucrose into glucose and fructose
lactase-- from the small intestine, breaks down lactose into glucose and galactose
aminopeptidase-- from the small intestine, breaks down terminal acids and amino and of peptides into amino acids
dipeptidase-- from the small intestine, breaks dipeptides into amino acids
ribonuclease-- from the pancreas and small intestine, breaks down ribonucleic acid nucleotides into pentoses and nitrogen bases
deoxyribonuclease-- from the pancreas and small intestine, breaks down deoxyribonucleic acid nucleotides into pentoses and nitrogen bases

The organs of the digestive system are separated into two main groups. The first is the gastrointestinal tract, or alimentary canal. The gastrointestinal tract is a long, continuous tube running through the body from the mouth to the anus. The second group is made up of the accessory structures or organs; the teeth, tongue, salivary glands, liver, gallbladder, and pancreas

The wall of the gastrointestinal tract, especially from the esophagus to the anal canal, has the same basic arrangement of tissues. The four coats or tunics of the tract from the inside out are the mucosa, submucosa, muscularis, and serosa or adventitia

the mucosa is a mucous membrane attached to a thin layer of visceral muscle. Two layers compose this membrane: the lining epithelium, which is in direct contact with digested materials, and an underlying lamina propria. The epithelial layer is made up of nonkeritinized cells that are stratified in the mouth and esophagus, but are simple throughout the rest of the tract. The lamina propria is a loose connective tissue containing the nerves, blood vessels and lymph vessels and nodules, as well as nonecapsulated lymphatic tissues. Below the lamina propria is the muscularis mucosae. which contains smooth muscle fibers that cause the small folds in the intestine. With one exception, the other three layers of the intestine contain no glandular tissue
the submucosa is made up of loose connective tissue that binds the mucosa to the third tunic, the muscularis. This layer is highly vascularized and contains a portion of the submucous plexus (plexus of Meissner), which is a part of the autonomic nerve supply to the muscularis mucosae. This plexus is also vital to the control of secretions by the gastrointestinal tract
the muscularis of the mouth, pharynx, and esophagus consists in part of skeletal muscle that produces voluntary contractions of those areas (read that: swallowing). Throughout the rest of the gastrointestinal tract, this layer is made up of smooth muscle found in two sheets. The inner layer runs around the circumference of the tract in circular arrangements of fibers. The outer sheet is made up of fibers running ninety degrees to the inner in longitudinal arrangements. Alternating contractions of the sheets produces a wave-like movement (peristalsis) that carries food along the tract, breaks it down mechanically and mixes it with digestive enzymes and acids. The muscularis also contains the major nerve supply to the alimentary tract; the myenteric plexus (plexus of Auerbach), which is made up of fibers from both autonomic divisions and controls most gastrointestinal motility
the outer peritoneum is the largest serous membrane of the body. Serous membranes are also associated with the heart (pericardium) and lungs (pleurae). The serous membrane consists of a layer of simple squamous epithelium (mesothelium) and an underlying supporting layer of connective tissue. The parietal peritoneum lines the walls of the abdominal cavity. The visceral peritoneum covers some of the organs and constitutes their serosa. The potential space between the parietal and visceral is called the peritoneal cavity and contains serous fluid. In certain diseases or disorders, the this cavity may become distended by the volume of serous fluid contained therein. Such an accumulation is referred to as ascites. Some organs lie on the posterior abdominal wall and are covered only on the anterior aspect by the peritoneum (called retroperitoneal organs). Examples would be the kidneys and pancreas. Unlike the pericardium and pleurae, the peritoneum contains large folds that weave in and out between the viscera, containing blood vessels, lymphatic tissues and nerves. One such extension of the peritoneum is called the mesentery. It is an outward fold of the peritoneum over the small intestine. A similar structure, the mesocolon, binds the large intestine to the posterior body wall. Some other important peritoneal folds are the falciform ligament (attaches the liver to the anterior abdominal wall and diaphragm), the greater omentum (four-layered fold in the serosa of the stomach that hangs like an apron over the anterior of the intestines, then wraps around the transverse colon to attach firmly to the parietal peritoneum), and the lesser omentum (two folds from the duodenum, liver and stomach serosae)

The mouth, also called the oral or buccal, cavity, is formed by the cheeks, hard and soft palates, and tongue. The cheeks are lined with nonkeritinized stratifies squamous epithelium and terminate at the lips. The lips (labia) are covered on the outside by skin containing no sweat or oil glands, and on the inside by a mucous membrane. The transition of one kind of tissue to the other is called the vermilion. The skin is nonkeritinized, so the blood is quite visible, giving lips their red appearance. With the cheeks, the lips help aid in speech and positioning food during mastication. At the midline, the inner surface of each lip is joined to the gingiva by a (superior or inferior) labial frenulum. The vestibule of the oral cavity is bounded externally by the cheek and lips and internally by the gums and teeth (read that: the crevice between the gingiva and the lips/cheek). The oral cavity proper extends from the vestibule to the fauces, the opening between the oral cavity and the pharynx. The hard palate makes up the anterior portion of the superior oral cavity, and is composed of the palatine processes of the maxillary bones and the palatine bones. The hard palate it corrugated like a washboard to aid in mechanical digestion. The soft palate forms the posterior roof of the mouth, an arch-shaped muscular area lined by a mucous membrane. Hanging from the free (posterior) border of the soft palate is the uvula. On either side of the uvula are muscular folds that run down the lateral side of the soft palate. Anteriorly, this is the palatoglossal arch, posterior is the palatopharyngeal arch (posterior pillar). Situated between the arches are the tonsils, housing for lymphatic tissues

The tongue and it's accessory muscles make up the floor of the oral cavity. The extrinsic muscles of the tongue originate outside the tongue and insert into it. They include the hyoglossus, chondroglossus, genioglossus, styloglossus, and palatoglossus. The extrinsic muscles are used to move the tongue in order to chew and form food into a round mass called a bolus and force it to the back of the throat for swallowing. The intrinsic muscles originate and insert in the tongue to alter the size and shape of the tongue for speech. The lingual frenulum aids in limiting the movement of the tongue posteriorly. The term 'tongue-tied' comes from the condition known as ankyloglossia in which the lingual frenulum is too short and makes speech difficult. The upper and lateral surface of the tongue are covered in papillae, projections of the lamina propria covered by epithelium. Filiform papillae are conical projections distributed in parallel rows over the anterior two thirds of the tongue. They are whitish and contain no taste buds. Fungiform papillae are mushroom-like in shape, evenly distributed among the the filiform papillae, but are more numerous towards the tip of the tongue. Most contain taste buds and appear as red dots. Circumvallate papillae, ten to twelve in number, are arrange to form a V on the posterior surface of the tongue. They all contain taste buds

Saliva is secreted by glands in or near the mouth. Small buccal glands in the mucous membrane aid in saliva production, but most comes from the salivary glands, of which there are three pairs:

parotid glands-- located under and in front of the ears, between the masseter and the skin. These are compound tuboacinar glands. The parotid (Stensen's) ducts empty saliva into the superior oral vestibule near the second molar
submandibular glands-- compound acinar glands, found in the posterior floor of the mouth, near the angle of the ramus. Saliva enters via the submandibular (Wharton's) ducts, just behind the central incisors
sublingual glands-- located under the tongue, also compound acinar glands. They are anterior to the submandibular glands. Their ducts, the lesser sublingual (Rivinus') ducts along the area under the tongue

Saliva contains the bacteriolyic enzyme lysozyme, urea, uric acid, serum albumin and globulin, mucin, and salivary amylase. But 99.5 percent of the volume is water. Each pair of glands supplies a different substance to the overall composition of saliva. The parotids contain cells that secrete a watery substance containing salivary amylase. The submandibulars secrete the same enzyme, but the glands contain more mucous cells, resulting in a thicker, more viscous secretion. The sublinguals are mostly mucous cells, and contribute very little salivary amylase. The water in saliva provides a medium for the food to be dissolved into for tasting and initial digestion

Teeth (dentes) are accessory structures located in the mouth, in the sockets of the alveolar processes of the maxillae and mandible. A hard enamel is the chewing surface. It is put down during the growth of the tooth and is not replenished by the body after eruption. The portion of the tooth about the gingiva (gums) is referred to as the crow. Underlying the enamel is the dentin, a bone-like substance that gives the tooth shape and rigidity. The constricted part of the tooth where the enamel terminates is the neck. ABove the neck, still in the crown, is the pulp cavity. It is filled with pulp, a connective tissue containing blood vessels, lymphatics and nerves. The pulp cavity runs down the inside of the tooth to the tooth's roots, called root canals. At the end of each root is an opening for the nerves, blood vessels and lymphatic tissues to exit, the apical foramen. The dentin of the root is covered with cementum, which attaches the root to the periodontal ligament. The periodontal ligament attaches the tooth to the gingiva. The permanent dentition contains 32 teeth: (anterior to posterior)

central incisors (4)
lateral incisors (4)
cuspids or canines (4)
first premolars or bicuspids (4)
second premolar or bicuspids (4)
first molars (4)
second molars (4)
third molars or wisdom teeth (4)

Swallowing, or deglutition, is made up of three stages. The first is the voluntary stage in which the bolus is moved from the mouth into oropharynx. Second is the pharyngeal stage, the involuntary passage into the esophagus from the pharynx. The last is also involuntary, the esophageal stage from the esophagus to the stomach

The esophagus is about 23 to 25 centimeters in length and begins at the end of the laryngopharynx, passes though the mediastinum anterior to the spine, through the esophageal hiatus of the diaphragm and terminates at the superior portion of the stomach. This termination is at the lower esophageal sphincter. The upper esophageal sphincter consists of the cricophageal muscles and the cricoid cartilage

The main part of digestion takes place in the stomach, a large, J-shaped organ directly inferior to the diaphragm. It is divided into four areas:

cardia-- surrounds the lower esophageal sphincter
fundus-- the rounded portion above and to the left of the cardia
body-- the main section of the body
pylorus-- the narrow section at the inferior end of the stomach

The medial, concave border of the stomach is called the lesser curvature, and the lateral convex is the greater curvature. The pylorus meets the small intestine at the pyloric sphincter. The internal lining of the stomach is folded into what are called rugae of the mucosa. Inspection of the mucosa reveals a layer of simple columnar epithelium, pitted with gastric glands that rest in lamina propria. These glands are lined with zymogenic or peptic cells, parietal cells, mucous cells, and enteroendocrine cells. The zymogenic cells produce the major digestive enzyme precursor, pepsinogen. Hydrochloric acid, which is involved in the conversion of the pepsinogen into pepsin, and intrinsic factor, involved in the absorption of vitamin B₁₂, are from the parietal or oxyntic cells. The mucous cells produce (surprise!) mucous. The secretions of these three cells are collectively called gastric juice. The enteroendocrine cells secrete gastrin, a hormone which prompts the secretion of HCl and pepsinogen, contracts the lower esophageal sphincter, mildly increases the motility of the lower gastrointestinal tract and relaxes the pyloric sphincter. The serosa covering the stomach is a part of the visceral peritoneum

After food passes out the pyloric sphincter, it enters the small intestine. The continued chemical breakdown required secretions of not only the small intestine, but those of the pancreas, liver and gallbladder, accessory structures outside the small intestine

the pancreas is a soft, oblong tuboacinar gland about 12 or 13 centimeters long and 2.5 thick. It lies posterior to the greater curvature of the stomach and is connected by a duct (usually two) to the duodenum. The pancreas is divided into a head, body and tail. The head is the expanded portion near the curve of the duodenum. Superior and to the left is the body. The tapering terminal end it the tail. The pancreas is connected to the small intestine through two ducts. The large passage of pancreatic secretions is the pancreatic duct (duct of Wirsung). Most of the time, this duct joins with the common bile duct to form a common duct at the hepatopancreatic ampulla (ampulla of Vater). The hepatopancreatic duct opens into the small intestine at the duodenal papilla, about 10 cm from the pylorus of the stomach. The smaller duct is the accessory duct (duct of Santorini), located above the main pancreatic duct. The pancreas is made up of glandular epithelial cells. About one percent on the cells are the pancreatic islets, or the islets of Langerhans. They are made up of alpha, beta, and delta cells which secrete the hormones glucagon, insulin, and somatostatin. These islets are endocrine glands. The remaining cells are referred to as acini. Their secretions are made up of water, some salts, sodium bicarbonate, and enzymes. The sodium bicarbonate makes the pancreatic juice slightly alkaline to counteract the actions of the pepsin. The enzymes secreted by the pancreas are pancreatic amylase, trypsin, chymotrysin, carboxypolypeptidase, pancreatic lipase, deoxyribonuclease and ribonuclease
the liver weighs about 1.4 kilograms. It is divided into two principle lobes, the right lobe and the left lobe, by the falciform ligament. Bile, one of the livers secretions, enters bile capillaries or canaliculi that empty into small bile ducts. These, in turn, lead to the right and left hepatic duct which joins to the common hepatic duct. The common hepatic duct then joins the cystic duct of the pancreas as described above, becoming the common bile duct. THe liver is made up of functional units called lobules. A lobule consists of cords of hepatic (liver) cells arranged in a radial pattern around a central vein. Between the cords are endothelial-lined spaces called sinusoids, through which blood passes. The sinusoids are also partially lined with phagocytic cells, termed stellate reticuloendothelial (Kupffer's) cells, that destroy worn our white and red blood cells. Bile has a pH of 7.6 to 8.6. It is made up mostly of water and bile salts, cholesterol, a phospholipid called lecithin, bile pigments, and several ions. Bile is yellow, brownish or olive-green in color. Bile plays a role in the emulsification of fats, or the breaking down of the fats into small droplets. Cholesterol is made soluable in bile by lecithin and bile salts. The principle bile pigment is bilirubin, which is broken down in the intestines and give feces their color. The liver has several functions:

manufacturing bile salts, used for emulsification
along with mast cells, manufacture the anticoagulant heparin and most other blood proteins
phagocytizing red and white blood cells
break down poisons
storage of nutrients
storage of glycogen, copper, iron, and vitamins A, B₁₂, D, E, and K
activation of vitamin D

the gallbladder is a pear-shaped sac about 7 to 10 centimeters in length. It is located in the fossa of the visceral surface of the liver. It's function is the storage and concentrate bile. When the small intestine is empty, a valve around the ampulla of Vater called the sphincter of the hepatopancreatic ampulla (sphincter of Oddi) closes and the backed up bile overflows into the cystic duct and into the gallbladder for storage

The major part of digestion and absorption takes place in the small intestine. It is about 2.5 centimeters in diameter and 6.35 meters in length. The small intestine is divided into three sections:

duodenum-- the shortest section, extending from the pyloric sphincter about 25 centimeters to joint the jejunum
jejunum-- about 2.5 meters long
ileum-- the longest section and the most inferior. About 3.6 meters long, joining the large intestine at the ileocecal sphincter

The mucosa on the small intestine contains many pits lined with glandular epithelium, called intestinal glands (crypts of Lieberkühn) which secrete intestinal juice. The submucosa contains duodenal (Brunner's) glands that secrete an alkaline mucous to protect the intestinal walls. The epithelial cells, except the goblet cells, contain microvilli, finger-like projections to increase the surface area of the cell for absorption. The mucosa itself lies in a series of villi, about .5 to 1 millimeter in length, for the same purpose. Each villus has a core of lamina propria, in which is embedded an arteriole, a venule, a capillary network, and a lacteal or lymphatic vessel. In addition to the microvilli and villi, plicae curculares or circular folds further increase the surface area available for absorption. Solitary lymphatic nodules are most numerous in the lower ileum. Groups of these nodules are referred to as aggregated lymphatic follicles or Peyer's patches. There are two types of movement in the small intestine. Segmentation is the major movement, used along with peristalsis. Segmentation does not move the chyme, but holds it in contact with the mucosa

The large intestine is about 1.5 meters long and about 6.5 centimeters in diameter. It joins with the small intestine at the ileocecal sphincter or valve. The purpose of the large intestine is to complete absorption, manufacture certain vitamins, the formation and expulsion of feces. The large intestine is attached to the posterior abdominal wall by the mesocolon of visceral peritoneum. Below the ileocecal valve hangs the cecum. The cecum is a blind pouch to which is attached the vermiform appendix, a coiled tube used to gather up bacterium. The vermiform appendix is attached to ileum of the small intestine by visceral peritoneum, called the mesoappendix. The main body of the large intestine is the colon. The colon, in turn, is divided into the ascending, transverse and descending colons. The angle of the ascending and transverse colons is the right colic (hepatic) flexure. The transverse and descending flexure is the left colic (splenic) flexure. The sigmoid colon begins at the slight angle in the descending colon, at about the level of the third sacral vertebra. The last 20 centimeters of the gastrointestinal tract is the rectum. The most inferior section of the rectum is the anal canal. The opening, the anus, is surrounded by the internal and external anal sphincters. The mucous membrane in the anus is arranged into longitudinal folds called anal columns. The longitudinal muscles of the large intestine show as the bands running the length of the intestine, referred to as taeniae coli. The colon is then gathered into pouched named haustra. Small pouches of peritoneum filed with fat are attached to the taeniae coli, called epiploic appendages.

Diseases and disorders of the digestive system include dental caries (tooth decay), pyorrhea (degeneration and inflammation of the gingiva and bone loss in the alveolar bone), peptic ulcers, appendicitis (inflammation of the vermiform appendix), tumors, diverticulitis (weakening of the muscles in the large intestine, forming sagging pouches called diverticula), cirrhosis (distortions or scarring of the liver), hepatitis, hemorrhoids, gallstones, anorexia, bulimia, and dietary fiber (Roughage) disorders

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