Biology 157-The Digestive System

Gastroenterology- medical specialty that deals with the structures, functions, diagnosis, and treatment of diseases of the stomach and intestines
Proctology- medical specialty that deals with the diagnosis and treatment of disorders of the lower portion of the large intestine, rectum, and anus
Diseases/Conditions: Diverticulosis- diverticula (outpouchings of the large intestine) form, usually in the sigmoid colon. May become infected, inflamed, painful, and bleed
The organs of the digestive tract can be divided into 2 groups: 1. digestive tract- continuous muscular tube that extends from the mouth to anus 2. accessory digestive organs- organs that assist in digestion
The digestive system performs 6 basic processes
1. 1. Ingestion- process of taking food and liquid into mouth 2. Mechanical digestion- crushing and churning that makes materials easier to propel along digestive tract 3. Chemical Digestion- chemical breakdown of ingested food into small molecules that can be absorbed 4. Secretion- cells lining the digestive tract and accessory digestive organs release water, acid, enzymes, buffers, salts, and hormones into the lumen of the tract 5. Absorption- passage of products of digestion across epithelial cells that line the digestive tract, and then into the interstitial fluid of the digestive tract 6. Defecation/Egestion- elimination of feces through anus
The digestive tract has a four-layered arrangement of tissues
1. 1. Mucosa- mucous membrane that forms the inner lining of the digestive tract. Composted of two layers: digestive epithelium- includes cells that protect the walls, secrete substances, and absorb small molecules. And lamina propria- connective tissue containing small blood and lymphatic capillaries which are the routes by which nutrients that are absorbed by the epithelium travel to the body cells 2. Submucosa- connective tissue that binds the mucosa to the muscularis, it contains blood and lymphatic vessels 3. Muscularis mucosae- main muscular layer of digestive tract 4. Visceral peritoneum- superficial layer of the digestive tract
The peritoneum is the largest serous membrane of the body. It is divided into 2 parts: visceral peritoneum- covers the outer surface of the digestive tube and the surface of the abdominal organs. The parietal peritoneum- lines the wall of the abdominopelvic cavity
Peritonitis- an acute inflammation of the peritoneum that can be caused by accidental or surgical wounds followed by microbial contamination, or by rupture of the appendix
Oral Cavity has 3 important structures: tongue, salivary glands, and teeth
1. Tongue- composed of skeletal muscle covered with mucous membrane. Assists in mechanical processing of food by compression. Secretes watery fluid and enzyme lingual lipase
2. Salivary Glands- release saliva in the mouth which includes water, ions, antibodies, and enzymes. Functions of saliva include: lubricating the mouth, providing a medium for dissolving foods, buffers acidic foods, antibodies and the enzyme lysozyme help control oral bacteria, and salivary calculi : development of stones in the salivary glands
3. Teeth- hardest structures in the body and highly calcified
Chemical digestion is based on 2 enzymes: salivary amylase- initiates the breakdown of polysaccharides into disaccharides. Lingual lipase- initiates the breakdown of triglycerides into fatty acids and monoglycerides
Deglutition (swallowing) occurs in 3 stages: buccal stage, pharyngeal stage, and esopharyngeal stage
1. Buccal stage- the tongue compresses the bolus against the palate and then forces the bolus into the oropharynx
2. Pharyngeal stage- involuntary passage of bolus through the pharynx into the esophagus
3. Esopharyngeal stage- involuntary passage of bolus through the esophagus into the stomach
The stomach is a J-shaped enlargement of the digestive tract that connects the esophagus to the small intestine. A hiatus hernia may occur when the stomach protrudes through the opening in the diagpharm
Mechanical digestion-several minutes after food enters the stomach, peristaltic movement called mixing waves pass over the stomach every 15-25 seconds. This creates a soupy liquid called chyme
Chemical digestion- cells lining the stomach secrete about 1.5 L of gastric juice in a day. They cells are found in shallow depressions called gastric pits they include:
1. 1. Mucous cells- secrete mucus into the lumen of the gastric pit 2. Parietal cells- secrete hydrochloric acid 3. Chief cells- secrete pepsinogen an inactive proenzyme which is converted into the enzyme pepsin by the acid in the lumen of the stomach 4. G cells- secrete gastrin
Events in gastric digestion occur in 3 phases: 1. Cephalic phase- prepares stomach for the arrival of food 2. Gastric phase- begins with the arrival of food in the stomach 3. Intestinal phase- controls the rate of chime entry into the duodenum to give time for the duodenum to deal with the arriving acids
Caffeine and alcohol stimulate gastric secretions and motility
Stomach absorbs very little including ions, water, aspirin, some antibodies, and alcohol
Vomiting (emesis)- forcible expulsion of contents of the upper digestive tract through the mouth
The acinar cells of the pancreas produce about 1L of pancreatic juice per day
1. Pancreatic juice includes: 1.Sodium bicarbonate 2. Enzymes including pancreatic amylase, pancreatic lipase, nucleases (digest nucleic acids into nucleotides), and trypsinogen- and inactive proenzyme which is converted into the enzyme trypsin in small intestine. Trypsin continues the digestion of proteins into short peptides
Gallstones- form when bile contains insufficient bile salts or excessive cholesterol. Bile becomes saturated with cholesterol which precipitates out of solution to form crystals and gallstones. They may partially or completely obstruct the flow of bile from the gallbladder into the duodenum
The walls of the small intestine have 3 structures that increase surface area: 1. Plicae circulares- ridges that extend out from the small intestine 2. Intestinal villi- fingerlike projections of the mucosa 3. Microvilli- hairlike projections on the free side of the plasma membrane of absorptive cells
Two types of movement occur in small intestine: 1. Segmentation- localized, mixing contractions 2. Peristalsis- contractions that move chime along
Brush border enzymes finally digestive disaccharides into monosaccharaides. These enzymes include: maltase- breaks down maltose into two glucoses. Sucrase- breaks down into glucose and fructose. Lactase- breaks down lactose into glucose and galactose
1. Lactose intolerance- mucosal cells of the small intestine fail to produce lactase
Pancreatic lipase completes the breakdown of triglycerides into fatty acids and monoglycerides. As fatty acids and monoglycerides are released,, the interact with bile salts in chime to form small lipid/bile salt complexes called micelles
Absorption- all mechanical and chemical phases of digestion are directed towards changing foods into forms that can pass through the epithelial cells lining the mucosa and into the underlying blood and lymphatic vessels
Normally all carbohydrates are absorbed except cellulose and fibers. Normally 95-98% of proteins are absorbed. Normally 95% of lipids are absorbed
All nutrients are absorbed into the blood go directly to the liver through the hepatic portal system. Blood capillaries of the small intestine make their way via the hepatic portal veins to the livers hepatic portal system. The liver acts as a filter of absorbed material before it enters the general circulation through the hepatic portal system via the hepatic veins
1. Functions of liver: maintaining metabolism of blood levels of carbohydrates, proteins, and lipids. Storage of iron, glycogen, lipids, vitamins, processing of drugs and hormones, synthesis of bile and plasma proteins, excretion of bilirubin, kupffer cells- phagocytosis of pathogens
Hepatitis- disease of the liver caused by viruses. Hepatocytes- are destroyed due to drugs and alcohol, viral infections, inadequate blood supply, blocked hepatic ducts
Large Intestine- terminal portion of digestive tract which extends from the ileocecal valve to the anus
Haustral churning mass peristalsis- drives the contents of the second half of the colon into the rectum by strong waves
Bacteria also breaks down remaining proteins and amino acids into simpler compounds, including ammonia, hydrogen sulphide, indole, and skatole
When chime has remained in the large intestine for about 3-10 hours it becomes solid because of water absorption. It is now called feces which consists of: water, bacteria, unabsorbed digestive material, ingestible parts of food, sloughed off epithelial cells, products of bacterial decomposition
Defecation Reflex- when the stretch receptors detect distension of the rectum as fecal material is moved by peristalsis into the rectum from the sigmoid colon, the defecation reflex is initiated which involves 2 reflexes: the involuntary internal anal sphincter to relax, feces now moves into anal canal. The second reflex causes the contraction of the external anal sphincter under voluntary control
1. Hemorrhoids- venous pressures in the lamina propria and submucosa in the anal canal become too high causing veins to become distended
2. Diarrhea- excessive amounts of fecal material and fluid contents is eliminated from the body due to enhanced peristaltic activity and decreased absorption
3. Constipation- infrequent or difficult defecation due to decreased motility

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