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The Digestive System
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
Components of the Digestive System
Alimentary Canal
• Around 9 meters long• Muscular tube that
passes through the ventral cavity
• Lumen
Alimentary Wall Structure
• Mucosa• Submucosa• Muscular layer
– Circular fibers
– Longitudinal fibers
– Oblique fibers
• Serosa
Movements of the Alimentary Canal
• Mixing movements• Propelling movements
Mouth Structure
• Surrounded by lips, cheeks, tongue, and palate
• Oral cavity• Vestibule• Cheeks• Lips
Tongue
• Frenulum• Papillae• Hyoid bone• Lingual tonsils
Palate
• Hard palate• Soft palate• Uvula• Palatine tonsils• Pharyngeal tonsils
(adenoids)
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
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
General Tooth Structure• Crown • Root• Neck• Enamel• Dentin• Pulp• Root canals• Cementum• Periodontal ligament
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
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
Major Salivary Glands• Parotid glands
– Largest– Mostly serous
secretions
• Submandibular glands– Mostly serous
secretions
• Sublingual glands– Smallest– Mostly mucous
secretions
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
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.
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.
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.
Esophagus
• Straight, collapsible tube
• Approximately 25 cm long
• Passageway from pharynx to stomach
• Cardiac sphincter• Mucous glands for
lubrication
Movement through Esophagus - Peristalsis
Stomach
• J-shaped, pouchlike organ
• Hangs under the diaphragm
• 1 liter capacity• Rugae
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
Stomach Regions
• Cardiac• Fundic• Body • Pyloric
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
Mixing and Emptying Actions of the Stomach
Pancreas
• Secretes pancreatic juice from acinar cells
• Mixed gland• Pancreatic duct• Hepatopancreatic
sphincter
Pancreatic Secretions
• Pancreatic juice contains several enzymes:– Pancreatic amylase– Pancreatic lipase– Nucleases– Trypsin, chymotrypsin,
and carboxypeptidase– Bicarbonate ions
• Release regulated by secretin
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
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
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
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
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
Gallbladder and Liver Problems
• Jaundice• Hepatitis• Gallstones
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
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
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
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
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
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
Regions of Large Intestine• Cecum
– Vermiform appendix
• Colon– Ascending colon– Transverse colon– Descending colon– Sigmoid colon
• Rectum• Anal canal
Anal Canal Structure• Anal columns• Anus
– Internal anal sphincter
– External anal sphincter
• Hemorrhoids
Large Intestine Anatomy
• Lack villi• Teniae coli• Many goblet cells
– Protect intestinal wall
– Bind particles of fecal matter
– Help control pH
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)
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
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