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Digestive System
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Function
The function of the digestive system is to change complex nutrient molecules that can be absorbed into the blood or lymph to be transported to cells
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Divisions of digestion
1.The alimentary tube extends from the mouth to the anus. It consist of the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.
2. The accessory organs of the digestion are the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
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Types of digestion
Food is broken down in two complementary processes
1. Mechanical digestion is the physical breaking up of food into smaller pieces
2. Chemical digestion uses enzymes to break up food particles, and turn them into something the body can utilize
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Oral Cavity
Food enters the oral cavity by way of the mouth
The boundaries are the hard and soft palates, the cheeks, and the floor of the mouth
Within the oral cavity are the teeth and tongue
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Teeth
The function of the teeth is to chewIndividuals develop two sets of teeth
1) deciduous and 2) permanentDeciduous teeth arrive around 6 months
of age, and are a set of 20 teethPermanent teeth begin to replace
deciduous teeth at the age of six, and consist of 32 teeth
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Teeth
Enamel is the hard outer covering of the tooth, and is more resistant to decay than other parts of the tooth
Dentin is within the enamel, and is very similar to bone; dentin also forms the root of tooth
The innermost portion of a tooth is the pulp cavity, which contains the blood vessels and nerve endings
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TongueThe tongue is made of skeletal muscle On the upper surface are small
projections called papillae, which contain taste buds
Sensory nerves for taste are the cranial nerves
Elevation of the tongue is the first step in swallowing, and is a voluntary action
The mass of food, called a bolus, is pushed backward toward the pharynx
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Salivary glands
Saliva is the digestive secretion in the oral cavity produced by three pairs of glands
1. Parotid glands are just below the ears2. Submandibular are posterior to the
mandible3. Sublingual are on the floor of the mouthEach gland has at least one duct to take saliva
to the oral cavity
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Cont.
Secretion of saliva is continuous, but varies
The presence, sight, or smell of food increases the amount saliva
Stressful situations decrease the amount of saliva
Salivary amylase is the digestive enzyme that breaks down starch molecules
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Pharynx No digestion takes place in the
pharynx, its only related function is swallowing
The reflex center for swallowing is the medulla
The medulla coordinates:1. Constriction of the pharynx2. Closure of the epiglottis3. Peristalsis of the esophagus
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Esophagus
The esophagus is a muscular tube that takes food from the pharynx to the stomach; no digestion takes place
Peristalsis of the esophagus propels food The lower esophageal sphincter relaxes to
permit food to enter the stomach, then contracts to prevent backup
If contraction is not complete gastric juices may splash up, I.e. heartburn
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Layers of alimentary tube
4 layers1. Mucosa2. Submucosa3. External muscle layer4. serosa
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Mucosa
The mucosa, or lining, is made of epithelial tissue and smooth muscle
The epithelium secretes mucus, which lubricates the passage of food, and secretes digestive enzymes
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Submucosa
The submucosa is made of connective tissue
Meissner’s plexus is the nerve network that regulate secretions
Also known as the brain of the gut
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External muscle layer
The external layer has two layers, inner and outer
Contractions of the external layer help break up food and mix it with digestive juices
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Serosa
The outermost layer, is fibrous connective tissue
The serosa is the mesentary lining the abdominal cavity
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Stomach The stomach is a sac that extends from the
esophgus to the small intestine Both mechanical and chemical digestion
take place in the stomach Parts of the stomach:1. Cardiac is the opening of the esophagus2. Fundus is the level above the opening3. Body is the large central portion4. Pylorus is where most digestion takes place
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Cont.
The folds within the stomach called rugae allow the stomach to stretch without tearing
Parietal cells secrete hydrochloric acid, which begins the digestion of proteins
Mucus cells secrete mucus, which coats the stomach, and prevents the erosion of gastric juices
The pH of the stomach is around 1, which helps to kill microorganisms
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Small intestine Extends from the stomach to the large
intestine The small intestine has 3 parts1. Duodenum2. Jejunum3. Ileum -Digestion is completed in the small
intestine -Peyer’s patches are lymph nodes that
destroy pathogens in the sm. intestine
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Liver
Consist of two large lobes, right and left The capillaries of the liver are called
sinusoids The only digestive function of the liver is the
production of bile Bile carries bilirubin and excess cholesterol to
the intestines for elimination Bile salts emulsify fats in the small intestine;
this a mechanical process The hormone secretin, stimulates the
production of bile
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Gallbladder
Is located on the underside of the liverBile from the liver is stored in the
gallbladder until neededCholecystokinin is the hormone that
stimulates the contraction of the smooth muscle in the gallbladder, which forces out bile
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Gallstones
Are caused by cholesterol crystals, that if large enough cannot pass out of the gallbladder
The backing up of bile can cause juandice
Lithotripsy is the generation of shock waves that break down gallstones into smaller pieces
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Pancreas
Pacreatic enzymes1. Amylase- digest starch 2. Lipase- converts fats to fatty acids3. Trypsin- digest polypeptides to amino acidsThe pancreas also produces bicarbonate juice,
which neutralizes the gastric acid that come from the stomach
Secretin stimulates the production of bicarbonate juice in the pancreas
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Disorders of the stomach
Pyloric stenosis means that the opening of the pyloric sphincter is narrowed, blocking the emptying of the stomach
A gastric ulcer is an erosion of the mucosa of the stomach; it is the result of over secretion of HCl
The bacteria Helicobacter pylori causes most gastric ulcers
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Large Intestine
Extends from the small intestines to the anus
The first portion is called the cecum; the ileocecal valve prevents fecal matter from back flowing into the small intestine
The appendix is attached to the cecumInfection of the appendix caused by the
fecal matter is called appendicitis
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Cont.
The last portion of the large intestine is called the colon; no digestion occurs here
The only secretion of the colon is mucus that coats for the passage of feces
The functions of the colon are the absorption of minerals, water, and vitamins
About 80% of the water that enters the colon is absorbed
The flora of the colon prevents the growth of pathogens
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Disorders of the Intestines
Duodenal ulcers are erosions of the intestinal wall, caused by gastric juices
Lactose intolerance is the inability to digest lacose; when lactose is not digested it undergoes fermentation within the intestine
Salmonella is caused by a bacteria found in meats and eggs, they cause the intestines to secrete large amounts of fluid
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Cont.
Paralytic ileus is the ceasing of contraction of the smooth muscle layer of the intestine, causing intestinal blockage
Infant botulism is caused by the ingestion of bacterial spores; an infant’s stomach does not produce enough HCl to destroy spores
Diets high in fiber have been thought to reduce the frequency of certain diseases
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Absorption
Most absorption takes place in the small intestine
Absorption needs a large surface area, projections called villi help increase surface area
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Elimination of feces
Feces consist of cellulose and other undigestible material, and water
A spinal cord reflex called the defecation reflex controls the elimination of feces
The relaxation of the internal anal sphincter permits defecation to occur
The external anal sphincter makes it possible to delay defecation
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Problems of the Liver
Hepatitis is the inflammation of the liver caused by viruses
Hepatitis A is an intestinal virus that is spread by the digestion of fecal matter
Hepatitis B is contracted by exposure to body fluids of an infected person (blood, semen)
Hepatitis C virus is spread through body fluids, blood, and mucus membranes
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Other Functions of Liver1. Carbohydrate metabolism- excess glucose
is stored as glycogen2. Amino acid metabolism- amino acids are
changed to carbohydrates 3. Lipid metabolism- synthesis of cholesterol4. Formation of bilirubin- old RBC’s are
phagocytized and bilirubin is formed5. Phagocytosis- phagocytized old RBC’s and
bacteria are absorbed by the colon6. Detoxification- liver enzymes change
poisons to less harmful substances