Digestive System Notes PDF

Digestive System - Notes 2025-01-24 7:57 AM Digestive System: Part 1 Digestive System Anatomy: Consists of 1. Gastrointestinal (GI) Tract Muscular tube also called digestive tract or alime...

Digestive System - Notes 2025-01-24 7:57 AM Digestive System: Part 1 Digestive System Anatomy: Consists of 1. Gastrointestinal (GI) Tract Muscular tube also called digestive tract or alimentary canal Food passes along length from mouth to anus 2. Accessory Organs Produce or store secretions that flow into GI tract through ducts Functions 1. Ingestion Occurs when solid food and liquid enter the oral cavity 2. Mechanical digestion and propulsion Involves crushing and shredding of food in the oral cavity and mixing and churning in the stomach 3. Chemical digestion Chemical and enzymatic breakdown of food into small organic molecules that can be absorbed by the digestive organs 4. Secretion The release of water, acids, enzymes, buffers, and salts by the digestive tract epithelium and by accessory digestive organs 5. Absorption Movement of nutrients across the digestive epithelium and into the bloodstream The king of the digestive system – w/out this there would be no point of eating 6. Defecation Indigestible food is compacted into material waste called feces, which are eliminated by defecation Mechanical Digestion Includes all movements that help digestion: Mastication (chewing) Swallowing Mixing Peristalsis Muscular contractions in the wall of the GI tract physically break down food by churning it and propelling it along the tract and also help dissolve foods by mixing them with fluids into the GI tract Chemical Digestion Enzymes secreted by accessory organs and cells that line the tract break down the food chemically Mainly accomplished by using water to break down chemical bonds (hydrolysis) Fats are broken down into fatty acids and glycerol Carbohydrates are broken down into monosaccharides Proteins are broken down into amino acids Major Organs & Functions 1. Oral cavity (mouth) Mechanical processing (with the teeth and tongue), moistening, mixing with salivary secretions 2. Pharynx Muscular propulsion of food into the esophagus 3. Esophagus Transport of materials to the stomach 4. Stomach Chemical breakdown and mechanical processing 5. Small intestine Enzymatic digestion and absorption 6. Large intestine Dehydration and compaction of indigestible materials Accessory Digestive Organs 1. Salivary glands Produce saliva containing mucins and enzymes 2. Gallbladder Stores and concentrates bile secreted by the liver 3. Pancreas Exocrine cells secrete buffers and digestive enzymes Endocrine cells secrete several hormones 4. Liver Almost 200 known function Layers of the GI Tract: Four major layers of the digestive tract Same basic layers from esophagus to anal canal The Layers: 1. Mucosa 2. Submucosa 3. Muscular Layer 4. Serosa or adventitia 1. Mucosa Inner lining/Superficial – 3 components Epithelium Epithelium in the mouth, pharynx, esophagus, and anal canal is nonkeratinized, stratified squamous epithelium For PROTECTION Epithelium in stomach and intestines is simple columnar epithelium For SECRETION and ABSORPTION Lamina propria Areolar connective tissue with blood and lymphatic vessels and mucosa – associated lymphatic tissue (MALT) MALT contains immune cells that protect against disease Muscularis mucosae Thin layer of smooth muscles making folds to increase surface area specifically in the stomach and the small intestine 2. Submucosa Layer of dense irregular connective tissue Contains blood vessels and lymphatic vessels Also contains exocrine glands in some regions Secrete buffers and enzymes into the digestive tract Has a network of neurons called submucosal (Meissner's) plexus 3. Muscular layer Smooth muscle in two layers Inner circular layer Outer longitudinal layer Involved in mechanical processing and movement along tract – Peristalsis Another network of neurons exists here call the myenteric (Auerbach's) plexus; between muscle layers 4. Serosa Outermost covering of organs along the digestive tract in the abdominal cavity Also called the visceral peritoneum along the digestive tract in the abdominal cavity No serosa in oral cavity, pharynx, esophagus and rectum Covered instead by adventitia (sheath formed from a dense network of collagen fibers) Firmly attaches tract to adjacent structures Peritoneum: Largest serous membrane of the body Divided into: 1. Parietal Peritoneum – lines the inner surface of the peritoneal cavity 2. Visceral peritoneum (aka. Serosa) – covers organs enclosed by the peritoneal cavity Space between is peritoneal cavity and contains lubricating serous fluid Serous Membrane Continuously secretes peritoneal fluid into peritoneal cavity ~7 liters/day secreted and reabsorbed Volume at any one time is about 50 mL Thin layer separates parietal and visceral surfaces Allows sliding movements without friction or irritation Rate of fluid moving into the cavity is accelerated by liver disease, kidney disease, and heart failure Accumulation of peritoneal fluid can create abdominal swelling (ascites) Retroperitoneal Organs Some organs are considered retroperitoneal which means the peritoneum only covers their anterior surface Kidneys Ascending and descending colon Duodenum Pancreas Ureters 3 major Peritoneal folds in the peritoneum: 1. Mesentery 2. Omenta 3. Falciform ligaments Peritoneal Folds: 1. Mesentery Double sheets of peritoneal membrane Areolar tissue lie between mesothelial layers - provides access route for blood vessels, nerves, and lymphatics Stabilizes attached organs Prevents entanglement of intestines Includes the Mesocolon Mesentery of the colon 2. Omenta There are two omenta in the body, they are continuations of the mesentery: 1. Greater omentum 2. Lesser omentum Greater Omentum Attach to stomach and transverse colon Forms a large pouch extending inferiorly b/w anterior body wall and anterior surface of the small intestine Contains adipose tissue that provides padding and protection Also contains blood vessels and lymphatic tissue Lesser Omentum Suspends the stomach and duodenum from the liver Provides an access route for blood vessels and other structures entering/leaving the liver Hepatic portal vein Hepatic artery Common bile duct 3. Falciform Ligament Connects the liver to the anterior body wall The liver is the only digestive organ that is attached to the ANTERIOR abdominal wall Remnant part of the umbilical vein Nerve Plexuses: Involved in local control of digestive activities Parasympathetic stimulation increases digestive muscle tone and activity Sympathetic stimulation decreases muscle tone and activity 2 major nerve plexuses: 1. Submucosal neural plexus (Meissner's) 2. Myenteric plexus (Auerbach's) 1. Submucosal neural plexus Meissner's Located in the submucosal layer Innervates the mucosa and submucosa Contains sensory neurons, autonomic nerve fibers 2. Myenteric plexus Auerbach's Network of sensory neurons and autonomic nerve fibers Located in the muscularis externa b/w the circular and longitudinal layer Works with the submucosal plexus to coordinate local control of digestive activity Enteric Nervous System (ENS) The submucosal and myenteric plexuses make up the ENS Intrinsic set of nerves – the "brain of the gut" Neurons extending from esophagus to anus Quasi autonomous – meaning it can act on its own with some input from the autonomic nervous system Autonomic Nervous System Extrinsic set of nerves Vagus nerve (CN X) supplies parasympathetic fibers to most of the GI tract Except the last ½ of the large intestine which is supplied from the sacral spinal cord Parasympathetic stimulation will increase secretion and motility by increasing activity of ENS neurons Sympathetic nerve supply arises from thoracic and lumbar region of the spinal cord Stimulation decreases secretions and activity by inhibiting ENS The Oral Cavity: Space that contains the tongue, teeth, and gums Lined by oral mucosa (stratified squamous epithelium) Thin mucosa inferior to the tongue allows for rapid absorption of lipid-soluble drugs (nitroglycerin) Little to no nutrients are absorbed here Digestion of carbohydrates and lipids begins here Oral Cavity Boundaries Superior Boundary Hard palate Formed by the palatine processes of the maxillary bones horizontal plates of the palatine bones Thick mucosa w/ ridges covering the hard palate Provides traction for compression of food by the tongue Soft palate Muscular region posterior to the hard palate Anterior/Lateral Boundary Cheeks Form the lateral walls of the oral cavity Supported by pads of fat and the buccinator muscles Anteriorly, cheek mucosa is continuous with the labia Labia Aka lips Form anterior boundary Inferior Boundary Tongue – Body Body of the tongue Anterior, mobile portion Posterior Boundary Uvula Dangling process extending from the soft palate Helps prevent food from entering pharynx prematurely Swings upward during swallowing to prevent food from entering the nasopharynx Palatine tonsils One located on either side of the oropharynx Root of tongue Fixed portion projecting into the oropharynx Marked by a V-shaped line of vallate papillae Lingual tonsils Located in the root of the tongue Other Features Oral vestibule The space b/w the cheeks (or lips) and teeth Frenulum – upper lip Attaches gums to upper lip Frenulum – lower lip Attaches gums to lower lip Gingivae (gums) Ridges or oral mucosa surround the base of each tooth Firmly attached to the periostea of the underlying bone Tongue: Accessory digestive organ Skeletal muscle covered by mucous membrane Extrinsic muscles – move the tongue side to side and in and out to maneuver food for chewing and forcing food to the back of the mouth for swallowing Intrinsic muscles – originate in and insert into connective tissue within the tongue Dorsum and lateral surfaces are covered w/ papillae which contain taste buds and some receptors for touch Surface flushed by secretions of small glands Secretions contain water, mucins, and lingual lipase (an enzyme that starts the digestion of lipids) Attached to the floor of the mouth by the frenulum of the tongue – lingual frenulum Salivary Glands: Salivary glands release saliva into the oral cavity Just enough is secreted to keep mouth and pharynx moist and clean When food enters mouth, secretion increases to lubricate, dissolve and begins the chemical digestion of food 3 pairs for major salivary glands secrete most of the saliva: 1. Parotid glands 2. Submandibular glands 3. Sublingual glands 1. Parotid Glands Lie inferior to the zygomatic arch, deep to the skin, covering the mandible Secretes saliva via parotid duct Produce serous secretion containing large amounts of salivary amylase ~25% of total saliva 2. Sublingual Glands Located beneath the tongue Sublingual ducts open into the floor of the mouth Produce a mucous secretion that acts as a buffer and lubricant ~5% of total saliva 3. Submandibular Glands Located medial and inferior to the body of the mandible and the submandibular ducts enter the oral cavity lateral to the lingual frenulum Secrete a mixture of buffers, mucins, salivary amylase Amylase is an enzyme that breaks down starches Cells transport IgA antibodies into the saliva Provides protection against pathogens ~70% of total saliva Saliva: Mostly water – 99.5% 0.5% solutes – ions (sodium, potassium, chloride, bicarbonate and phosphate), dissolved gases, urea, uric acid, mucous, immunoglobulin A, lysozyme, and salivary amylase Functions Constantly flushes oral surfaces Buffers keep pH of mouth near 7.0 and prevent buildup of acids produced by bacteria Contains antibodies (IgA) and lysozyme to help control oral bacteria populations Mixes with food to form a bolus to be easily swallowed Small glands called Ebner's glands secrete lingual lipase Salivation Controlled by autonomic nervous system Parasympathetic stimulation promotes secretion of moderate amount of saliva Sympathetic stimulation decreases salivation The taste and smell of food are potent stimulators of salivary gland secretions - "cephalic phase" of digestion Teeth Teeth or Dentes Accessory digestive organ Dentin forms the majority of the tooth and is harder than bone Covered by enamel, which is even harder 2 Dentitions 1. Deciduous or primary teeth Baby teeth 2. Permanent or secondary teeth Components of a Tooth Dentin The bulk to each tooth is composed of dentin Mineralized matrix similar to bone but contains no cells Pulp cavity The interior chamber of the tooth Enamel Covers the dentin of the crown Hardest biologically manufactured substance Composed of calcium phosphate Requires calcium, phosphate, and vit D for formation and resistance to decay Root canal Narrow tunnel within the root of the tooth Passageway for blood vessels and nerves to the pulp cavity Opening into the root canal is the apical foramen Regions of a Tooth Crown Portion projecting into the oral cavity from the surface of the gums Neck The boundary between the crown and root Root Portion below the gum line Sits in a bony tooth socket called an alveolus Digestion in the Mouth: The mouth is the 1st place where digestion begins, both mechanical and chemical Mechanical Digestion In the mouth Chewing or mastication Food is manipulated by tongue, ground by teeth, and mixed w/ saliva Forms a soft, flexible mass called a bolus Chemical Digestion Salivary Amylase Secreted by salivary glands and acts on starches Only monosaccharides can be absorbed into the bloodstream Continues to act until inactivated by stomach acid Works well in the mouth - doesn’t work in the stomach Lingual Lipase Secreted by lingual glands of tongue acts on triglycerides Active in the mouth, but becomes even more activated in acidic environment of stomach Acts to breakdown dietary triglycerides into fatty acids and diglycerides Works well in the stomach - doesn’t work well in the mouth Pharynx: Membrane-lined cavity posterior to the nose and mouth (throat) Skeletal muscle wall of the pharynx play a key role in swallowing (along w/ esophagus) Continuous w/ the esophagus Common passageway for solid food, liquids, and air Three regions: 1. Nasopharynx (respiration only) 2. Oropharynx 3. Laryngopharynx The Esophagus: Function Actively moves food and liquids to the stomach No enzymes produced here, no absorption Structure Hollow, muscular tube Narrowest point at the beginning (posterior to cricoid cartilage) Descends posterior to the trachea Enters the abdominopelvic cavity through the esophageal hiatus (opening in the diaphragm) Histology Mucosa Nonkeratinized stratified squamous epithelium – for protection Mucosa and submucosa form large folds extending the length of the esophagus Allows for expansion with passage of bolus Submucosa Contains areolar CT, blood vessels, and mucous glands Muscularis externa Superior third – composed of skeletal muscle Middle third – a mix of skeletal and smooth muscle Inferior third – composed of smooth muscle only No serosa Adventitia of connective tissue anchors esophagus to posterior body wall Control of Movement Upper Esophageal Sphincter Band of smooth muscle that functions as a sphincter Prevents air from entering the esophagus Lower Esophageal Sphincter Aka cardiac sphincter At the inferior end of the esophagus Normally contracted – prevents backflow of stomach contents Swallow of Deglutition Initiated voluntarily but proceeds automatically Facilitated by secretions of saliva and mucus Involves mouth, pharynx, and esophagus Three phases of swallowing: 1. Buccal Phase 2. Pharyngeal Phase 3. Esophageal Phase 1. Buccal Phase Or voluntary phase Begins with compression of bolus against hard palate Tongue forces bolus into the oropharynx Also elevates soft palate (sealing off nasopharynx) Entry into the oropharynx triggers reflex response 2. Pharyngeal Phase Begins with stimulation of tactile receptors in uvula and palatine arches Motor commands from the swallowing center (in medulla oblongata) coordinate muscle contraction in pharyngeal muscles Larynx is elevated; epiglottis is folded; uvula and soft palate are elevated Bolus is moved through the pharynx into the esophagus 3. Esophageal Phase Begins as bolus is forced through the entrance to the esophagus Bolus is pushed toward the stomach by peristalsis Approach of bolus triggers the opening of the lower esophageal sphincter Bolus enters the stomach Typical travel time is 9 seconds Liquids may travel faster A dry (poorly lubricated) bolus may require secondary peristaltic waves Peristalsis: Smooth muscle contractions produce motility of the digestive tract Wave of muscle contraction Food enters the digestive tract as a bolus Moist, compact mass of material Bolus is propelled along the tract by contractions of the muscularis externa (peristalsis) GI Pathology: Periodontal Disease Most common cause for loss of teeth Occurs when dental plaque forms between gums and teeth Resulting bacterial activity may cause: Gingivitis (inflammation of the gums) Tooth decay Eventual breakdown of periodontal ligaments and surrounding bone Link to cardiovascular disease? Inflammation and atherosclerosis