the digestive system. digestion digestion = the mechanical and chemical breakdown of foods into...
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The Digestive System
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Digestion• Digestion = the
mechanical and chemical breakdown of foods into nutrients that cell membranes can absorb
• 2 Components of the digestive system:– Alimentary canal – mouth,
pharynx, esophagus, stomach, small intestine, large intestine, anal canal
– Accessory organs – secrete products into the canal; salivary glands, liver, pancreas, gallbladder
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Components of the Digestive System
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Alimentary Canal
• Around 9 meters long• Muscular tube that
passes through the ventral cavity
• Lumen
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Alimentary Wall Structure
• Mucosa• Submucosa• Muscular layer
– Circular fibers
– Longitudinal fibers
– Oblique fibers
• Serosa
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Movements of the Alimentary Canal
• Mixing movements• Propelling movements
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Mouth Structure
• Surrounded by lips, cheeks, tongue, and palate
• Oral cavity• Vestibule• Cheeks• Lips
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Tongue
• Frenulum• Papillae• Hyoid bone• Lingual tonsils
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Palate
• Hard palate• Soft palate• Uvula• Palatine tonsils• Pharyngeal tonsils
(adenoids)
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Primary and Secondary Teeth
• Primary teeth– Deciduous teeth
– Erupt between 6 months and 2-4 years
– 20 teeth
• Secondary teeth– Appear around 6 years
– 32 teeth
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Tooth Types
• Incisors – Chisel-shaped– Bite off large pieces of
food
• Cuspids– Cone-shaped– Grasp and tear food
• Bicuspids and Molars– Flattened surfaces– Grind food
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General Tooth Structure• Crown • Root• Neck• Enamel• Dentin• Pulp• Root canals• Cementum• Periodontal ligament
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Salivary Glands
• Secrete saliva:– Moistens food particles– Helps bind food
particles– Begins chemical
digestion of carbohydrates
– Dissolves food for tasting
– Helps cleanse mouth and teeth
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Salivary Cells
• Serous cells– Produce watery fluid
that contains amylase
– Amylase splits starch and glycogen into disaccharides
• Mucous cells– Secrete mucus to bind
food particles and lubricate during swallowing
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Major Salivary Glands• Parotid glands
– Largest– Mostly serous
secretions
• Submandibular glands– Mostly serous
secretions
• Sublingual glands– Smallest– Mostly mucous
secretions
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Regions of the Pharynx• Nasopharynx
– Open to nasal cavity– Passage for air during
breathing
• Oropharynx – Behind soft palate– Passage for air and
food
• Laryngopharynx– Passage for food to the
esophagus
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Swallowing Reflex
• Food is chewed and mixed with saliva to form a mass called a bolus.
• Bolus is forced into the pharynx.
• Swallowing reflex is stimulated by sensory receptors around the pharyngeal opening.
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Swallowing Reflex
• Soft palate rises to prevent food from entering the nasal cavity.
• Hyoid bone and larynx are elevated, and the epiglottis of the larynx closes off the top of the trachea.
• Breathing is briefly inhibited.
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Swallowing Reflex
• Tongue presses against the soft palate, sealing the oral cavity off from the pharynx.
• Longitudinal muscles in the pharyngeal wall contract, moving the pharynx up toward the bolus.
• Muscles in the lower pharynx relax, and the esophagus opens.
• Peristalsis moves the bolus through the esophagus.
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Esophagus
• Straight, collapsible tube
• Approximately 25 cm long
• Passageway from pharynx to stomach
• Cardiac sphincter• Mucous glands for
lubrication
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Movement through Esophagus - Peristalsis
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Stomach
• J-shaped, pouchlike organ
• Hangs under the diaphragm
• 1 liter capacity• Rugae
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Stomach Functions
• Receives food from the esophagus
• Mixes food with gastric juices
• Initiates protein digestion
• Performs limited absorption of water, salts, alcohol, and lipid-soluble drugs
• Moves food into the small intestine
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Stomach Regions
• Cardiac• Fundic• Body • Pyloric
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Gastric Secretions
• Gastric pits
• Gastric glands– Goblet cells
– Chief cells – pepsinogen
– Parietal cells – HCl and intrinsic factor
• Gastric juice
• Regulated by ACh, gastrin, and cholecystokinin
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Mixing and Emptying Actions of the Stomach
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Pancreas
• Secretes pancreatic juice from acinar cells
• Mixed gland• Pancreatic duct• Hepatopancreatic
sphincter
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Pancreatic Secretions
• Pancreatic juice contains several enzymes:– Pancreatic amylase– Pancreatic lipase– Nucleases– Trypsin, chymotrypsin,
and carboxypeptidase– Bicarbonate ions
• Release regulated by secretin
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Liver• Located in upper right
quadrant below the diaphragm
• Color from rich supply of blood vessels
• Divided into left and right lobes by fibrous capsule
• Each lobe separated into hepatic lobules functional units of liver
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Hepatic Lobule Structure
• Consists of many hepatic cells radiating out from a central vein
• Hepatic sinusoids• Portal vein• Central veins• Kupffer cells• Bile canals• Common hepatic duct
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Liver Functions• Cells respond to insulin and glucagon to maintain normal
glucose levels• Carbohydrate, lipid, and protein metabolism
– Glucose Glycogen– Noncarbs Glucose– Makes cholesterol and fats– Amino acids Urea– Makes plasma proteins
• Storage of glycogen, iron, vitamins A, D, and B12• Blood filtering• Detoxification• Secretion of bile
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Bile• Yellowish-green
liquid that contains:– Bile salts – for
emulsification and absorption of fatty acids, cholesterol, vitamins A, D, E, and K
– Bile pigments – bilirubin and biliverdin
– Cholesterol– Electrolytes
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Gallbladder• Pear-shaped sac on the
inferior liver surface• Connects to the cystic duct
which feeds into the common hepatic duct
• Stores bile between meals• Reabsorbs water to
concentrate bile• Releases bile into the small
intestine• Common bile duct• Stimulated by cholecystokinin
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Gallbladder and Liver Problems
• Jaundice• Hepatitis• Gallstones
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Small Intestine• Extends from pyloric
sphincter to the large intestine
• Receives secretions from the pancreas and liver
• Completes digestion of nutrients in chyme and absorbs products of digestion
• Mixing movements and peristalsis – chyme moves through in 3-10 hours
• Transports digestive residue to the large intestine
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Regions of the Small Intestine
• Duodenum– 25 cm long
– Most fixed portion of the small intestine
• Jejunum• Ileum
– Jejunum and ileum are not distinctly separate
– Both are mobile
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Mesentery• Double-layered fold of
peritoneal membrane• Suspends the jejunum
and ileum from the posterior abdominal wall
• Supports the blood vessels, nerves, and lymphatic vessels that supply the intestinal wall
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Greater Omentum
• Filmy, double-layered fold of the peritoneal membrane
• Drapes like an apron from the stomach over the transverse colon and the folds of the small intestine
• May adhere to infected areas of the alimentary canal to wall it off
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Intestinal Villi• Tiny projections on the
inner wall off the small intestine
• Densest in the duodenum• Increase surface area for
absorption• Lacteals – absorb fatty
acids and glycerol• Goblet cells• Intestinal glands• Microvilli
– Secrete peptidases, sucrase, maltase, lactase, intestinal lipase
• Capillaries absorb simple sugars, amino acids, electrolytes, and water
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Large Intestine
• Ileocecal valve• 1.5 meters long• Extends up right side,
crosses obliquely to the left side, and descends into the pelvis
• Opens to the outside of the body as the anus
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Regions of Large Intestine• Cecum
– Vermiform appendix
• Colon– Ascending colon– Transverse colon– Descending colon– Sigmoid colon
• Rectum• Anal canal
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Anal Canal Structure• Anal columns• Anus
– Internal anal sphincter
– External anal sphincter
• Hemorrhoids
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Large Intestine Anatomy
• Lack villi• Teniae coli• Many goblet cells
– Protect intestinal wall
– Bind particles of fecal matter
– Help control pH
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Large Intestine Functions• Proximal end functions primarily in water
and electrolyte absorption
• Distal end functions primarily to store feces
• Little to no digestive function
• More sluggish movements – peristaltic waves 2-3 times per day (mass movements)
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Defecation Reflex
• Can be initiated by person (deep breath and abdominal contraction)
• Forces feces into rectum• Reflex involves relaxation of the internal
anal sphincter and peristaltic waves through the descending colon
• Can be prevented by contraction of the external anal sphincter
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Feces
• Made of materials not digested or absorbed – Water– Electrolytes– Mucus– Bacteria
• 75% water• Color from bile pigments
altered by bacterial action• Odor from compounds
produced by bacteria