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The Digestive System. Functions of the Digestive System. Ingest food Break down food into nutrient molecules Absorb molecules into the bloodstream Rid the body of indigestible remains. Main Divisions of the Digestive System. Alimentary Canal
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Functions of the Digestive System • Ingest food • Break down food into nutrient molecules • Absorb molecules into the bloodstream • Rid the body of indigestible remains
Main Divisions of the Digestive System • Alimentary Canal • Continuous, muscular digestive tube winding throughout the body • Digests and absorbs food particles • Contains the following organs: • Mouth, Pharynx, Esophagus, Stomach, Small and Large Intestines • Accessory Digestive Organs • Contains the following organs: • Teeth, Tongue, Gallbladder, Salivary Glands, Liver, and Pancreas
Digestive Processes • Ingestion • Propulsion • Mechanical digestion • Chemical digestion • Absorption • Defecation
Circulation of the Alimentary Canal/GI Tract • Splanchnic Circulation • Includes all arteries branching off the abdominal aorta and the hepatic portal circulation • Arterial supply: • Celiac Trunk (hepatic, splenic, and left gastric) • Mesenteric Arteries: sup. and inf. • Receives ¼ of the blood volume (CO); increases after a meal
Histology of the Alimentary Canal/GI Tract • From esophagus to anus, the walls of the alimentary canal have the same 4 layers: • Mucosa (secretes hormones and mucus, absorbs end products of digestion, and protects against infection • Submucosa (contains lymphoid follicles and elastic tissue) • Muscularis externa (segmentation and peristalsis, contains inner circular layer and outer longitudinal layer, area where valves are found) • Serosa (same as visceral peritoneum, made of areolar connective tissue)
Peritoneum • Peritoneum is the serous membrane lining the abdominopelvic cavity • Visceral peritoneum covers the external surfaces of most digestive organs and is continuous with the parietal peritoneum that lines the body wall • Between the two peritoneums is the peritoneal cavity • Mesentery is a double layer peritoneum; provides routes for BV, lymphatics, nerves • Alimentary canal organs are classified as • Retroperitoneal - no mesentery and organs lies posterior to the peritoneum (SADPUCKER) • Intraperitoneal - mesentery and organs lies within the peritoneal cavity
Features and Functions of the Mouth • Buccal/oral cavity • Contains stratified squamous epithelium • Vestibule: area bounded by lips and cheeks externally and teeth and gums internally • Lips: posses no sweat or oil glands • Palate: forms roof of the mouth, soft and hard palate, uvula
Features and Functions of the Tongue • Helps grind food into a bolus which contains partially digested food and saliva • Helps form words and is a sensory organ for taste • Three surface features: • Filiform papillae (roughness and grip) • Fungiform papillae (contains taste buds) • Circumvallate papillae (contains taste buds)
Features and Functions of the Salivary Glands • Main functions: • Produces and secretes saliva • Cleanses the mouth • Dissolves food chemicals so they can be tasted • Moistens food, compacting it into a bolus • Begins the chemical breakdown of food • Salivary amylase: starch
Types of Salivary Glands • Submandibular Glands • Found underneath the mandible • Sublingual Glands • Found underneath the tongue • Parotid Glands • Found anterior to the ear between masseter and skin • Saliva travels to oral orifice via ducts from all three glands
Composition of Saliva: 97-99.5% water • pH 6.75-7.0 • Sodium, potassium, chloride, phosphate, and bicarbonate • Mucin • Salivary amylase
Features and Functions of the Teeth • Break food into smaller parts, increasing surface area for digestion • Types of Teeth • Deciduous Teeth (“baby” teeth) • Permanent Teeth • Incisors- cutting and shredding • Canines- piercing and tearing • Molars- grinding • Premolars- grinding and crushing
Tooth Structure • Crown: exposed portion of tooth covered by enamel which covers dentin • Root: internal portion that is beneath the gums (gingiva) and is anchored by periodontal ligaments
Deglutition and the Pharynx • Deglutition = swallowing • Oropharynx and Laryngopharynx are common passageways for food and air • Pharynx contains stratified squamous epithelium (friction-resistant)
Features and Functions of the Esophagus • Muscular tube that propels food to stomach; bolus enters stomach through esophageal hiatus • Skeletal muscle (upper third for swallowing) and smooth muscle (lower third) for peristalsis • Esophageal glands – produce mucus to lubricate bolus • Esophageal sphincter – prevents backflow into oral cavity • Cardiac sphincter- prevents backflow into esophagus
Digestive Processes in the Mouth, Pharynx, and Esophagus • Mouth processes: • Ingestion • Mechanical digestion (e.g. salivary amylase) • Initiation of Propulsion • Mastication: chewing • Pharyngeal processes: • Deglutition = swallowing • Voluntary Buccal phase • Involuntary Pharyngeal-Esophageal Phase • Esophageal processes: • Peristalsis (rhythmic contractions, involuntary)
Features and Functions of the Stomach • Temporary storage area for food and allows it to mix with gastric juice to produce chyme • Regions: cardiac, fundus, body, and pyloric • Greater and Lesser Curvatures: connected to greater and lesser omentums • Rugae folds: longitudinal folds in stomach wall - mucous b/w folds • Muscle layers arranged circularly, longitudinally, AND obliquely (aids in digestion)
Microscopic Anatomy of the Stomach • Simple columnar epithelium – contains gastric pits that secrete gastric juices • Goblet cells – secrete mucus that coats stomach and prevents it from being digested itself • Parietal cells – secrete hydrochloric acid (converts pepsinogen into pepsin) and intrinsic factor (necessary for absorption of vitamin B12) • Chief cells – secrete pepsinogen which is converted to pepsin to aid in protein digestion • Enteroendocrine cells – release hormones such as: • Histamine, Serotonin, Gastrin, Endorphins, and Somatostatin
Histamine - activates parietal cells to release HCl • Serotonin - contraction of stomach muscle • Gastrin - gastric glands to increase secretion • Endorphins - natural opiates • Somatostatin - sympathetic n.s. - inhibits gastric secretion - inhibits gastric emptying
Digestive Processes in the Stomach • Gastric Secretion • Cephalic Phase • Stimulated by the thought, sight, taste, or aroma or food • Inputs from olfactory receptors and taste buds travel to parasympathetic enteric ganglia which then stimulate stomach glands • Gastric Phase • Stomach distension activates stretch receptors • Food chemicals (e.g. peptides, rising pH) activate chemoreceptors which activate G cells which secrete gastrin • Intestinal Phase • Presence of low pH and partially digested foods in duodenum stimulates intestinal gastrin secretion
Gastric Motility and Emptying • Peristaltic waves approach stomach and become stronger near pyloric region • Pyloric sphincter allows ~ 3 mL of chyme to pass to duodenum and the rest to return to stomach for further mixing
Features and Functions of the Small Intestine • Receives chyme from stomach; performs majority of digestion and absorption of nutrients • Regions: • Duodenum (upper region receiving chyme from stomach and digestive enzymes from pancreas and bile from liver and gallbladder) • Jejunum/Ileum (lower regions where absorption occurs) • Plicae circulares (permanent folds in mucosa and submucosa that slow movement of chyme)
Microscopic Anatomy of Small Intestine • Villi: fingerlike projections that increase the surface area of the SI • Microvilli: tiny projections on the plasma membranes of columnar cells that appear fuzzy (i.e. brush border cells) • Crypts of Lieberkuhn: secrete intestinal juice and special lysozymes that protect against bacteria • Peyer’s Patches: aggregated lymphoid tissues containing lymphocytes
Secretions of the Small Intestine • Secretin: released by enteroendocrine cells when acidic chyme enters SI; causes release of bicarbonate-rich pancreatic juices • Somatostatin: slows gastric motility and emptying and inhibits production of gastric secretions • Cholecystokinin (CCK): released when fatty, protein-rich chyme enters SI; causes release of enzyme-rich pancreatic juices and bile • Brush border enzymes: process long peptides, nucleic acids, and sugars into smaller ones
After Digestion: Absorption Protein Fats
Functions of the Liver • Largest internal organ • Functions: • Filters and processes nutrient-rich blood of carbohydrates, proteins, and lipids from intestine • Production and regulation of cholesterol • Production of bile which emulsifies fats • Removes drugs and hormones from circulation • Storage of vitamins and minerals
Anatomy of the Liver • Right and Left Lobes: separated by falciform ligament • Caudate and Quadrate Lobes: found on posterior side • Blood vessels: • Hepatic artery/vein and hepatic portal vein • Gallbladder: found underneath left lobe, stores bile
Microscopic Anatomy of the Liver • Liver Lobules: structural unit of liver • Hepatocytes: liver cells contained within the lobules • Hepatic portal vein & Hepatic Artery: the circulation of the liver, they bring blood into the liver where it is filtered through the liver sinusoidal capillaries • Kupffer cells: remove debris • Filtered blood drains into the central vein, then to the hepatic vein, and eventually to the inferior vena cava • Bile (produced by hepatocytes) drains into the bile duct after passing through portal triad • Bile then shipped to gallbladder for storage