Digestive System - Lecture Notes

Digestive System Components: Gastrointestinal Tract (GI)

• Continuous tube for processing food

• Lined with a mucous membrane

• Includes: Oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, and anus

• Within the lumen, food is broken down into smaller components to be absorbed

Digestive System Components: Accessory Digestive Organs

• Assist in digestion

• Some produce secretions that empty into the GI tract

• Include: teeth, tongue, salivary glands, liver, gallbladder, and pancreas

Functions of the Digestive System

• Ingestion

• Motility

• Secretion

• Digestion:

  • Mechanical and Chemical digestion

• Absorption

• Elimination

Function: Ingestion

• Introduction of solid and liquid nutrients into the oral cavity

• First Step in process of digesting and absorbing nutrients

Function: Motility

• Voluntary and involuntary muscular contractions

• Mixing and moving materials through the GI tract

Function: Secretion

• Process of producing and releasing fluid products facilitating digestion

• Example: digestive enzymes, acid, and bile

Function: Digestion

• Breakdown of ingested food into smaller structures

• Mechanical Digestion:

  • Material physically broken down by chewing and mixing

• Chemical Digestion:

  • Involves specific enzymes to break chemical bonds

  • Change large complex molecules into smaller molecules

Function: Absorption

• Transport of digested molecules, electrolytes, vitamins, water

• Move from G tract into blood or lymph

Function: Elimination

• Expulsion of indigestible components that are not absorbed

GI Tract Wall: Innermost to Outermost

• Hollow tube of 4 concentric layers, tunics

  • Mucosa

  • Submucosa

  • Muscularis

  • Adventitia (or serosa)

GI Tract Wall: Mucosa

• Epithelium in contact with food; secretes mucus & absorbs nutrients

• Lamina propria (connective tissue) with blood and lymph capillaries

• Muscularis mucosae facilitates secretion and absorption

GI Tract Wall: Submucosa

• Connective tissue, blood vessels, lymphatic tissue (MALT), and nerves

• Submuscosal nerve plexus are nerves ganglia innervting smooth muscle and glands

• Peyer patches are larger aggregates of lymphatic nodules in distal small intestine

GI Tract Wall: Muscularis

• Inner circular for peristalsis & segmentation

  • Contraction constricts tube lumen

• Outer longitudinal layers for peristalsis & segmentation

  • Contraction shortens tube

• Myenteric nerve plexus controls contractions

• Sphincters regulate passage of food

GI Tract Wall: Muscularis - Motility

• Functions to mix and propel contents within GI tract

• Mixing backward-and-forward motion that lacks directional movement

  • Blends ingested materials with secretions

• Propulsion: Occurs by Peristalsis

  • Sequential contraction of muscularis, GI tract wall moves like a wave

GI Tract Wall: Adventitia (Serosa)

• Adventitia: Found outside peritoneal cavity

• Serosa: Found within peritoneal cavity (covered by visceral peritoneum)

Enteric Nervous System (ENS)

• Sensory and motor neurons within submucosal plexus and myenteric plexus

• Innervates smooth muscle and glands of GI tract

• Coordinates mixing and propul

Autonomic Nervous System (ANS)

• Parasympathetic innervation promotes GI tract activity (Increases digestion)

• Sympathetic innervation opposes GI tract activity (Decreases digestion)

Hormonal Regulation

• Gastrin: Stimulates gastric activity

• Secretion & Cholecystokinin (CCK): Inhibit gastric activity and stimulate accessory organs

Receptors

• Initiates reflexes

• Baroreceptors detect stretch in GI tract wall

• Chemoreceptors monitor chemical contents in lumen

Serous Membranes: Peritoneum

• Serous membrane associated with abdominopelvic cavity

• Parietal peritoneum: Lines inside surface of abdominal wall

• Visceral peritoneum: Serous membrane reflecting over and covering the internal organs surface

• Peritoneal cavity: Lubricating serous fluid secreted between both peritoneum.

  • Allows abdominal organs to move freely

Serous Membranes: Mesentery

• Supports, suspends, stabilizes intraperitoneal GI tract organs

• Falciform ligament: Flat, thin crescent-shaped fold that attaches the liver to internal surface of anterior abdominal wall

• Mesentery proper: Suspends most of small intestine from posterir abdominal wall

• Mesocolon: Fold of peritoneum; attaches part of large intestine to posterior abdominal wall

Serous Membranes: Omenta

• Greater omentum

  • Extends inferiorly from inferolateral stomach

  • Covers most abdominal organs

  • Accumulates large amounts of adipose tissue (fatty apron)

• Lesser omentum

  • Connects superomedial surface of stomach and proximal end of duodenum to the liver

Clinical View: Peritonitis

• Inflammation of peritoneum

• Abdominal pain

• Most common cause: GI tract perforation

  • Allows contents to contact peritoneum

  • May result within (ulcer, ruptured appendix) or without (gunshot wound, surgery)

Upper GI tract organs and accessory

• Oral cavity and salivary glands

• Pharynx & Esophagus

• Stomach

• Gastric Motility

Oral Cavity and Salivary Glands

• Functions: Mechanical digestion (chewing) and enzymatic breakdown

• Saliva Components: Water mucus, salivary amylase (starch digestion), and lingual lipase

• Teeth: Incisors (cutting), canines (tearing), premolars/molars (grinding)

Salivary Glands

• Intrinsic Salivary Glands: Within oral cavity

  • Contain lingual lipase, enzyme that begins digestion

• Extrinsic Salivary Glands: Outside of oral cavity

  • Produce most saliva

  • Parotid, submandibular, and sublingual glands

Parotid salivary glands

• Largest salivary glands

• 25%-30% of saliva

• Conducted through parotid duct to oral cavity

• Extends from gland across masseter, opening near second upper molar

• Infection of the parotid gland causes mumps

Submandibular and Sublingual Salivary Gland

• Submandibular:

  • Produces 60% to 70% of saliva

  • Submandibular duct opens from each gland to floor of cavity

• Sublingual

  • Extends tiny ducts opening into inferior surface of cavity

  • Contribute only 3% to 5% of saliva

Oral Cavity - Anterior View

Oral Cavity and Pharynx - Sagittal Section

Pharynx & Esophagus

• Swallowing (Deglutition) Phases:

  • Voluntary Phase: Tongue pushes bolus to oropharynx

  • Pharyngeal Phase: Involuntary; soft palate & epiglottis close passageways

  • Esophageal Phase: Peristalsis moves food to stomach

• Esophageal Sphincters: Prevent air entry & acid reflux

Phases of Swallowing

Stomach

• Regions: Cardia, fundus, body pylorus (controls chyme entry to small intestine)

• Gastric Secretions:

  • Mucous Cells: Produce mucus for protection

  • Parietal Cells: Secrete intrinsic factor (B12 absorption) & HCL

  • Chief Cells: Secrete pepsinogen (activated to pepsin for protein digestion)

  • G Cells: Release gastrin, stimulating stomach motility & secretion)

Gross Anatomy of the Stomach

Stomach Mucosa

Stomach: Gastric Motility

• Mixing: Forms chyme

• Emptying: Peristalsis moves chyme into duodenum

Clinical View: Reflux Esophagitis and Gastroesophageal Reflux Disease

• Inflammation of esophagus due to acidic chyme refluxing into esophagus

• Pain posterior to sternum, heartburn

• Seen frequently

  • Overweight individuals, smokers, after large meals

  • Hiatal hernias: portion of stomach protruding through disaphragm

• Treatment

  • Lifestyle changes, head elevation, limiting meal size

Clinical View: Reflux Esophagitis and Gastroesophageal Reflux Disease (continue)

• Can result from chronic reflux esophagitis

• Erodes esophageal tissue

• Scar tissue buildup, leading to narrowed lumen

• May change from stratified squamous to columnar secretory epithelium, Barrett esophagus

  • Increases risk of cancerous growth

• May be treated with multiple medications

  • Includes proton pump inhibitors, histamine blockers

Motility in the Stomach: Gastric Mixing and Gastric Emptying

Stomach: Cephalic Phase

• Cephalic reflex, initiated by the thought of food

  • Nerve signals from higher regions of the brain are sent to the hypothalamus, which relays nerve signals to medulla oblongata

  • Causes increase in motility and secretory activity

  • Stomach “growls”

Stomach: Gastric Phase

• Process following bolus reaching stomach

• Regulated via gastric reflex and through release of gastrin hormone

• Gastric reflex is initiated as baroreceptors detect stomach stretch

• Chemoreceptors detect protein and increased pH

• Nerve signals relayed to medulla oblongata

• Results in increased stomach motility and secretory activity

• Presence of food (especially protein) in stomach causes release of hormone gastrin

Stomach: Intestinal phase

• Process following chyme reaching small intestine

• Involves intestinal reflex

  • Intestinal reflex opposing cephalic and gastric reflexes

  • Chyme in duodenum signals the medulla oblongata

  • Causes decrease in motility and secretory activity of stomach

• Cholecystokinin (CCK) and secretin also decrease stomach motility and secretory activity

  • Slows emptying stomach

Regulation of Digestive Processes in the Stomach

Clinical View: Gastric Bypass

• Surgical treatment for obesity

• Small part of stomach sectioned off and attached to lower part of small intestine

• Less nutrients absorbed

• Reduced appetite

• Altered response to hormones like insulin

• Cann induce remission of type 2 diabetes within a few days of surgery

Clinical View: Peptic Ulcers

• Peptic ulcer

  • Solitary erosion of portion of stomach or duodenum

  • Gastric ulcers in stomach, duodenal ulcers in duodenum

  • Symptoms of gnawing, burning pain in epigastric region

  • May erode and cause perforation, medical emergency

  • Helicobacter pylori causes erosion of gastric lining

  • Treatment similar to those of gastric reflux

Clinical View: Vomiting

• Rapid expulsion of gastric contents through oral cavity

• Controlled by vomiting center in the mdeulla oblongata

• Responds to head injury, motion sickness, infection, toxicity, food irritation

• Closure of nasal passages and the glottis

• Skeletal muscle contraction increasing intragastric pressure

• Gastric contents forced into and through the esophagus

• Danger of aspiration

Lower GI Tract Organs and Associated Accessory Digestive Organs

Clinical View: Inflammatory Bowel Disease (IBD)

Autoimmune disorders, both with a region of inflamed intestine

Clinical View: Inflammatory Bowel Syndrome (IBS)

• More common, affecting 1 in 5 Americans

• Abnormal function of colon

• More common in women than men

• Cramps, bloating, constipation, diarrhea

• Treatment with diet, medication, stress reduction

Clinical View: Crohn Disease and Ulcerative Colitis

• Chron Disease:

  • Young adults

  • Intermittent and relapsing episodes of cramping and diarrhea

• Ulcerative Colitis:

  • Similar to Crohn’s but only involves large intestine

Small Intestine: Regions

• Duodenum: Receives chyme and secretions from liver & pancreas

• Jejunum: Primary site of digestion and absorption

• Ileum: Final absorption; connects to large intestine

Small Intestine: Surface Area Enhancements

• Circular Folds: Slow movement for more absorption

• Villi: Finger-like projections containing capillaries & lacteals

• Microvilli (Brush Border): Contain enzymes for final digestion

Small Intestine: Accessory Digestive Organs

• Liver: Produces bile (fat emulsification), detoxifies blood, stores nutrients

• Gallbladder: Stores & concentrates bile, releases it via the bile duct

• Pancreas: Produces digestive enzymes & bicarbonate to neutralize stomach acid

Gross Anatomy of the Liver: Posterior inferior View

Large Intestine Regions

Cecum, colon (ascending, transverse, descending, sigmoid), rectum, anal canal

Large Intestine: Functions

• Water & electrolyte absorption

• Compacts feces

• Houses gut microbiota for vitamin production & fermentation

• Defection Reflex: Triggered by rectal distension; controlled by internal (involuntary) and external (voluntary) and sphincter

Anal Canal

• Lined by stratified squamous epithelium

• Passes through opening in levator ani muscle and terminates at anus

• Longitudinal ridges, anal columns

• Depression between, anal sinuses

  • Release mucus when pressure exerted

• Internal anal sphincter

  • Involuntary smooth muscle at base of anal canal

• External anal sphincter

  • Voluntary skeletal muscle

  • Sphincters normally closed off but relax during defacation

Small Intestine Tunics

Section of Small Intestine

Bile

• Secreted by liver, no digestive enzymes

• Contains: Water, bicarbonate ions, bile salts and pigments, cholesterol, lecithin, mucin

• Bile salts and lecithin help emulsify lipids

Hepatic Lobules: Structure and Function

• Hepatic lobules are hexagonal structural and functional units.

• They are composed of hepatocytes, which are liver cells that filter and process nutrient-rich blood.

Hepatic Lobules: Components

• Each hepatic lobule contains a central vein.

• In each corner of the lobule, there is a portal triad, which consists of:

  • Hepatic portal vein

  • Hepatic artery

  • Bile duct

Liver Lobule

Hepatic Portal System

Gallbladder

• Stores, concentrates, and releases bile produced in liver

• 3 tunics: Inner mucosa, middle muscularis, external serosa

  • Mucosa with folds that allow distension of wall

• Connected to the common bile duct by cystic duct

• Sphincter valve

  • Controls flow of bile into and out of gallbladder

Clinical View: Gallstones

• In Gallbladder or biliary apparatus

• Risk factors: Female sex, obesity age, caucasian

• Condensation of cholesterol or calcium and bile salts

• Majority asymptomatic until lodged in neck of cystic duct: cholecystitis

• Severe pain in right hypochondriac region

• Symptoms worse after fatty meal

  • May require surgical removal, cholecystectomy

Pancreas

• Endocrine function: Produces and secretes insulin and glucagon

• Exocrine function: Produces pancreatic juice to assist with digestive activities

Pancreatic Juice

• Formed from secretions of acinar cells and pancreatic duct cells

• Alkaline fluid

• Mostly water: HCO₃^-, digestive enzymes

  • Pancreatic amylase to digest starch

  • Pancreatic lipase to digest triglycerides

  • Inactive proteases that digest proteins when activated

  • Nucleases for digestion of nucleic acids

Cholecystokinin (CCK)

• Hormone released from small intestine in response to fatty chyme

• Stimulates gallbladder to strongly contract and release bile

• Stimulates pancreas to release pancreatic juice

• Relaxes smooth muscle within hepatopancreatic ampulla

  • Allows entry of bile and pancreatic juice into small intestine

• Inhibits stomach motility and release of gastric secretions

Defacation

• Gastrocolic Reflex

  • Initiated by stomach and causes a mass movement

• Defecation Reflex

  • Filling of rectum initiates urge to defecate

  • Receptors signal to spinal cord

  • Increases parasympathetic output to sigmoid colon and rectum

  • Decreases output to internal (involuntary) anal sphincter

• Voluntary defecation

  • Involves Valsalva Maneuver and relaxation of external (voluntary) anal sphincter

Clinical View: Constipation

• Temporary impaired ability to defecate

• Compacted feces difficult to eliminate

• From low fiber, dehydration, lack of exercise, poor bowel habits, anesthesia

Clinical View: Diarrhea

• Disrupted in normal mechanism to absorb intestinal water

• Example: from osmotically active solutes that move large volumes of water into colon

Carbohydrate Digestion

• Starts in mouth: Salivary amylase

• Continues in small intestine: Pancreatic amylase & brush border enzymes

Protein Digestion

• Starts in stomach: Pepsin breaks proteins into peptides

• Continues in small intestine: Trypsin, chymotrypsin, and peptidases

Lipid Digestion

• Starts in stomach: lingual and gastric lipase

• Major digestion in small intestine: Bile emulsifies fats pancreatic lipase digests triglycerides

Digestion of Startch

• Brush border enzymes complete starch breakdown

• Includes dextrinase and glucoamylase

  • Break bond between glucose subunits of oligosaccharides

• Maltase

  • Breaks bonds between the two glucose molecules

Digestion of Other Disaccharides

• Digestion of other disaccharides requires one enzyme each

• Named for substrate it digests

  • Example: lactase digests lactose into glucose and galactose

  • Reduced amount or lack of lactase = lactose intolerance

Clinical View: Celiac Disease

• Gluten-Sensitive Enteropathy

• Autoimmune disorders affect 1% of the U.S. population

• Gluten = protein in wheat, rye, barley

  • Stimulates immune response, damages villi of small intestine, interferes with absorption

• Symptoms:

  • Abdominal pain, diarrhea, nutrient deficiencies

Nucleic Acid Digestion

• Small intestine: Pancreatic nucleases break down DNA & RNA into nucleotides

• Brush border enzymes:

  • Phosphatase: breaks bonds holding phosphate

  • Nucleosidase: breaks bond between sugar and nitrogenous base

• Components absorbed across epithelium of small intestine

  • Phosphate, sugar, nitrogenous base