The Digestive System

The Digestive System

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 2 Main Divisions of the Digestive System

Alimentary Canal/ Gastrointestinal Tract Continuous, muscular digestive tube winding throughout the body (extends from the mouth to the anus through the thoracic and abdominopelvic cavities) Digests and absorbs food particles Contains the following organs: Mouth, Pharynx, Esophagus, Stomach, Small and Large Intestines Length: 5-7 meters for living human; 7-9 meters for cadaver

Accessory Digestive Organs Contains the following organs: Teeth, Tongue, Gallbladder, Salivary Glands, Liver, and Pancreas Physical breakdown of food, chewing, swallowing and secretes enzyme for chemical breakdown. Digestive System Divisions Peritoneum serous

membrane of the abdominal cavity Visceral covers external surface of most digestive organs Parietal lines the body wall Peritoneal cavity- small space between visceral and parietal peritoneum.

Lubricates digestive organs Allows them to slide across one another Overview of Digestive Processes 1. INGESTION 2. SECRETION 3. MIXING & PROPULSION 4.a.MECHANICALDIGESTION 4.b. CHEMICAL DIGESTION

5. ABSORPTION 6. DEFECATION 1. INGESTION--- process of taking food and liquid into the mouth (eating). 2. SECRETION--- Each day, cells within the walls of GIT and accessory digestive organ secrete a total of 7 liters of water, acid, buffers and enzymes into the lumen of tract. 3. MIXING & PROPULSION--- alternating

contractions and relaxations of smooth muscle in the walls of GIT mix food and secretions and slowly propel them to anus(motility.) 4.a.MECHANICALDIGESTION--- teeth cut and grind food before it is swallowed; small intestine churn the food 4.b. CHEMICAL DIGESTION---- macromolecules such as proteins, carbohydrates, lipids and nucleic

acids are split into their monomers by hydrolysis aid with actions of enzymes secreted. 5. ABSORPTION--- passage of digested products from GIT into blood and lymph. 6. DEFECATION elemination of feces from GIT. Peristalsis Peristalsis moves contents steadily in one direction along

the alimentary canal. Histology of the Alimentary Canal From esophagus to the anal canal the walls of the GI tract have the same four layers (tunics) From the lumen outward they are the mucosa, submucosa, muscularis externa, and serosa MUCOSA The innermost layer epithelial layer that lines the lumen of the alimentary canal

Stomach and small intestine mucosa contain: Enzyme-secreting cells that aid in the absorption of nutrients for digestion Mucus producing cells protects the lumen from hydrolytic enzymes while aiding the movement food through the alimentary canal Has many lymphatic vessels that protection against infectious disease Anatomy of the Alimentary Canal

Diaphragm Serosa Mucosa Muscularis Submucosa Blood vessels

Other layers of the Alimentary Canal Submucosa Dense connective tissue containing elastic fibers Blood and lymphatic vessels Nerves (Submucosal Nerve Plexus) Muscularis externa Composed of involuntary smooth muscle innervated by the (Myenteric Nerve Plexus) responsible for segmentation and peristalsis

Serosa the protective visceral peritoneum Outer most layer Enteric Nervous Control Is often referred to the second brain because it can regulate digestive tract mobility, secretion and blood flow independently of CNS. It is thought that there are more neurons in the ENS

than the spinal cord. Composed of two nerve networks submucosal plexus controls glandular secretion and muscular contraction of the mucosa myenteric plexus controls peristalsis contractions of muscularis external

Enteric Nervous Control Myenteric Nerve Plexus Submucosal Nerve Plexus Histology of the GI Tract MOUTH Features and Functions: 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 Anatomy of the Mouth TONGUE

Features and Functions: Helps grind food into a bolus/chyme 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)

Anatomy of the Tongue SALIVARY GLANDS Main function: Produces and secretes saliva 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

Functions of saliva cleanse teeth, inhibit bacteria, Dissolves food chemicals so they can be tasted Moistens food, compacting it into a bolus

Begins the chemical breakdown e.g.Salivary amylase: starch Composed of 99.5% water and solutes

salivary amylase, begins starch digestion lingual lipase, digests fat activated by stomach acid mucus, aids in swallowing lysozyme, enzyme kills bacteria immunoglobulin A, inhibits bacterial growth electrolytes = Na+, K+, Cl-, phosphate and bicarbonate pH of 6.8 to 7.0 Anatomy of the Salivary Glands

TEETH Features and Functions: 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 Pharynx Deglutition = swallowing Oropharynx and Laryngopharynx are common passageways for food and air Pharynx contains stratified squamous epithelium (frictionresistant) ESOPHAGUS

Features and Functions : 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

Anatomy of the Esophagus Digestive Processes in the Mouth, Pharynx, and Esophagus Mouth processes: Ingestion Mastication: chewing chemical digestion (e.g. salivary amylase)

Initiation of Propulsion Pharyngeal processes: Deglutition = swallowing Voluntary Buccal phase Involuntary Pharyngeal-Esophageal Phase Esophageal processes:

Peristalsis (rhythmic contractions, involuntary) STOMACH Features and Functions : 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) Gross Anatomy of the Stomach 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

Microscopic Anatomy of the Stomach 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

Digestive Processes in the Stomach SMALL INTESTINE Features and Functions : Receives chyme from stomach; performs majority of digestion and absorption of

nutrients Runs from pyloric sphincter to the ileocecal valve SMALL INTESTINE Has three subdivisions: duodenum, jejunum, and ileum Duodenum (10 in.) Receives stomach contents through the pyloric sphincter. Pancreatic juice and bile aid in chemical digestion of chyme.

Neutralizes stomach acids, emulsifies fats, pepsin inactivated by pH increase, pancreatic enzymes hydrolyze their specific substrate. Mostly digestion with minimal absorption occurs here. Jejunum - next 8 ft. (in upper abdomen) has large tall circular folds; walls are thick, muscular nutrient digestion and absorption occurs here Ileum - last 12 ft. (in lower abdomen) has peyers patches clusters of lymphatic nodules

ends at ileocecal junction with large intestine Primarily nutrient absorption occurs here Microscopic Anatomy of Small Intestine Lined simple columnar epithelium which functions in absorption and secretion. Villi fingerlike extensions of the mucosa containing blood and lymph vessels for the absorption of nutrients Microvilli

tiny folds in the individual simple columnar epithelial cells that further increase digestive surface area. Brush-border enzymes hydrolytic enzymes responsible for chemical digestion of organic molecules and activation of pancreatic enzymes. Pancreatic enzymes and bile enter the duodenum through for the sphincter of Oddi Crypts of Lieberkuhn: secrete intestinal juice and special

lysozymes that protect against bacteria Peyers Patches: aggregated lymphoid tissues containing lymphocytes Microscopic Anatomy of the Small Intestine 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, proteinrich 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 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 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 Gallbladder: found underneath left lobe, stores bile Bile emulsifies fats, separating them into smaller parts Gross Anatomy of the Liver PANCREAS Features and Functions :

Pancreatic Juice secreted by acinar cells Islets of Langerhans release insulin and glucagon (important in glucose metabolism) Insulin uses a second messenger which allows more glucose to enter the cell Lowers blood glucose levels by increasing membrane permeability to both glucose and amino acids. Pancreatic Juice contains:

Sodium Bicarbonate (buffers HCl in stomach) Proteases (break down polypeptides) Pancreatic amylase (digests oligosaccarides and disaccharides into monosaccharides) Pancreatic lipases (break down lipids into fatty acids and glycerol) Pancreatic nucleases (break down nucleic acids) Anatomy of the Pancreas LARGE INTESTINE

Features and Functions: Reabsorption of remaining water and electrolytes Production and absorption of Vitamins B and K Elimination of feces Diameter is only 7 cm but is larger than that of the small intestine Gross Anatomy of the Large Intestine Teniae Coli: bands of smooth muscle that create pocket-like sacs (haustra)

Cecum: sac-like connection between the small and large intestines Appendix: small structure containing lymphoid tissue; small immune function Ascending, Descending, Transverse, and Sigmoid Colon Splenic and hepatic flexure Rectum: storage area Anus: regulates defecation with two sphincter muscles; internal and external

Anatomy of the Colon Microscopic Anatomy of the Large Intestine Simple columnar epithelium for absorption except in the anal canal where there is stratified squamous No villi, no digestivesecreting cells

Goblet cells produce mucus for lubrication of feces Bacterial flora synthesize vitamin B and most of the vitamin K needed for blood clotting

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