Functions of the Gastrointestinal Tract

The gastrointestinal (GI) tract plays a crucial role in digestion and absorption of nutrients through two primary functions: mixing and propulsion.

Mixing

Characterized by a "backward-and-forward" motion that lacks directional movement.
Facilitates blending of ingested materials with digestive secretions, enhancing food breakdown.

Propulsion

Involves directional movement of materials through the GI tract.
Occurs through peristalsis, a sequential contraction of the muscularis layer creating a wave-like motion that moves contents along the digestive pathway.

Structure of the Gastrointestinal Tract Wall

Adventitia and Serosa

The outermost layer of the GI tract wall can be classified as either adventitia or serosa, depending on the organ's location.
Adventitia: Composed of areolar connective tissue, found outside the peritoneal cavity.
Serosa: Similar in composition to adventitia, covered by the visceral peritoneum and located within the peritoneal cavity.

Regulation of the Digestive System

Enteric Nervous System (ENS)

Composed of sensory and motor neurons located within the submucosal and myenteric plexuses, innervating the smooth muscle and glands of the GI tract.
Coordinates reflexes for mixing and propulsion of contents.

Autonomic Nervous System (ANS)

Influences GI tract activity through two branches:
Parasympathetic innervation: Promotes increased activity of the GI tract.
Sympathetic innervation: Opposes GI tract activity, often inhibiting digestive processes.

Nerve Reflexes

Baroreceptors: Detect stretch in the GI tract wall, providing feedback on fullness.
Chemoreceptors: Monitor chemical contents within the lumen, aiding in digestive regulation.
Reflexes can be categorized into:

Short reflex: A local reflex involving only the ENS, coordinating small segments of the GI tract.
Long reflex: Involves sensory input to the central nervous system (CNS) and autonomic motor output, coordinating broader GI motility and secretions.

Hormonal Control

Several hormones regulate digestion, including:

Gastrin: Stimulates gastric acid secretion.
Secretin: Regulates water homeostasis and influences digestive enzyme secretion.
Cholecystokinin: Stimulates digestion of fats and proteins.
Motilin: Regulates gastrointestinal motility.

Serous Membranes of the Abdominal Cavity

Peritoneum

The peritoneum is a serous membrane associated with the abdominopelvic cavity, consisting of two layers:

Parietal peritoneum: Lines the inside surface of the abdominal wall.
Visceral peritoneum: Covers the surface of internal organs.

The space between these layers is the peritoneal cavity, containing lubricating serous fluid for free movement of abdominal organs.

Mesentery

A double layer of peritoneum that supports, suspends, and stabilizes intraperitoneal GI tract organs, containing blood vessels, lymph vessels, and nerves.
Associated with specific organs, including:

Greater omentum: Extends from the inferolateral stomach and covers most abdominal organs.
Lesser omentum: Connects the stomach and proximal duodenum to the liver.
Falciform ligament: Attaches the liver to the anterior abdominal wall.
Mesentery proper: Suspends most of the small intestine from the posterior abdominal wall.
Mesocolon: Attaches parts of the large intestine to the posterior abdominal wall.

Clinical View: Peritonitis

Peritonitis is the inflammation of the peritoneum, often resulting from perforation of the GI tract, allowing contents to contact the peritoneum.
Causes can be internal (e.g., ulcers, ruptured appendix) or external (e.g., gunshot wounds, surgical complications).

Overview of the Upper Gastrointestinal Tract Organs

Upper GI Tract Organs and Accessory Structures

Oral cavity and salivary glands: The site where mechanical digestion begins, with saliva containing salivary amylase mixing with ingested materials to form a bolus.
Pharynx: Moves the bolus to the pharynx during swallowing, with mucus secreted to facilitate this process.
Esophagus: Transports the bolus from the pharynx to the stomach, lubricated by mucus secretions.
Stomach: Mixes the bolus with gastric secretions through smooth muscle contractions, forming chyme.
Duodenum: Plays a critical role in the initial stages of digestion.

Carbohydrate Digestion

Carbohydrate breakdown occurs primarily in the small intestine:

Monosaccharide Absorption: Monosaccharides like glucose, fructose, and galactose are absorbed across the small intestinal epithelial lining into the bloodstream.
Transport to the Liver: Venous blood carrying monosaccharides is transported to the liver.
Conversion to Glucose: Fructose and galactose are converted into glucose in the liver.
Utilization of Glucose: Glucose can be used for energy, converted to glycogen for storage, or transformed into fat.
Cellulose: Indigestible by humans, contributes to dietary fiber, adding bulk to the lumen content.

Celiac Disease

Celiac disease, or gluten-sensitive enteropathy, is an autoimmune disorder affecting approximately 1% of the U.S. population. Key points include:

Gluten: A protein found in wheat, rye, and barley.
Immune Response: Gluten stimulates an immune response that damages the villi of the small intestine, impairing nutrient absorption.
Symptoms: Common symptoms include abdominal pain, diarrhea, and nutrient deficiencies.

Protein Digestion

Protein digestion involves breaking down proteins into amino acids through enzymatic processes:

Protein Structure

Proteins are polymers made of amino acid subunits linked by peptide bonds.
Digestion releases individual amino acids, broken down by specific enzymes targeting peptide bonds.
Enzymes are released in inactive forms to prevent them from digesting proteins within the cells that produce them.

Protein Breakdown in the Stomach

Begins in the stomach lumen with the enzyme pepsin, formed from its inactive precursor, pepsinogen.
The stomach's low pH activates pepsinogen to pepsin, which denatures proteins, facilitating their breakdown.

Protein Breakdown in the Small Intestine

Three enzymes from the pancreas continue protein digestion: trypsinogen, chymotrypsinogen, and procarboxypeptidase, all released in inactive forms.
Enteropeptidase: Activates trypsinogen to trypsin in the small intestine.
Activation Cascade: Trypsin activates more trypsinogen, chymotrypsinogen, and procarboxypeptidase, leading to the breakdown of proteins into smaller peptides.

Final Steps of Protein Digestion

Dipeptidase: Breaks the final bond between two amino acids.
Aminopeptidase: Generates free amino acids, absorbed across the small intestinal epithelial lining.
Free amino acids serve as building blocks for new proteins or can be converted into glucose.

Lipid Digestion

Lipid digestion involves breaking down lipids, which are not water-soluble:

Lipid Structure

Triglycerides: Composed of glycerol and three fatty acids; enzymes are required to break the bonds between them.
Cholesterol: Does not require breakdown for absorption.

Lipid Breakdown in the Stomach

Lingual Lipase: Present in saliva, activated upon reaching the stomach.
Gastric Lipase: Produced by chief cells, digests about 30% of triglycerides into diglycerides and fatty acids without bile salts.

Lipid Breakdown in the Small Intestine

Pancreatic Lipase: Released into the duodenum, digests triglycerides into monoglycerides and two fatty acids.
Emulsification: Required to separate large lipid droplets into smaller droplets, enhancing pancreatic lipase action.

Emulsification Process

Occurs through bile salts, which are amphipathic molecules that form micelles, allowing greater access for pancreatic lipase to triglycerides.
Bile salts are recovered by active transport in the last portion of the ileum.

Lipid Absorption

Lipids are transported to the epithelial lining by micelles and enter the cells.
Bile salts are reused and remain in the lumen.
Within epithelial cells, triglycerides are re-formed and combined with cholesterol and other lipids to form chylomicrons, which are released by exocytosis into the lacteals and subsequently the bloodstream.

Nucleic Acid Digestion

Nucleic acids, polymers of nucleotides, consist of DNA and RNA. Each nucleotide comprises a sugar, phosphate group, and nitrogenous base.

Nucleic Acid Breakdown in the Small Intestine

Deoxyribonuclease and Ribonuclease: Nucleases released by the pancreas that break the phosphodiester bonds between nucleotides of DNA and RNA.
Brush Border Enzymes: Include phosphatase and nucleosidase, which break bonds holding phosphate and between sugar and nitrogenous bases, respectively.
The components (phosphate, sugar, nitrogenous bases) are absorbed across the epithelium of the small intestine.

Water, Electrolyte, and Vitamin Absorption

The small intestine is the primary site for the absorption of water, electrolytes, and vitamins:

Water Absorption

The small intestine absorbs nearly all ingested water, averaging about 8 out of 9 liters, while the large intestine absorbs some, with the remainder passed in feces.
Water is absorbed across the epithelial lining and into blood vessels via osmosis.

Electrolyte Absorption

The small intestine absorbs almost all electrolytes that enter, with most absorption being unregulated and dependent on dietary intake.
Iron Absorption: Controlled by the hormone hepcidin, which inhibits iron absorption. When iron levels are low, hepcidin levels decrease, allowing for greater iron absorption.

Vitamin Absorption

Fat-Soluble Vitamins: Vitamins A, D, E, and K are absorbed in the small intestine along with lipids within micelles.
Water-Soluble Vitamins: Vitamins B and C are absorbed through diffusion and active transport.
Vitamin B12: Absorbed by receptor-mediated endocytosis, requiring intrinsic factor produced by parietal cells in the stomach.

Accessory Digestive Organs and Ducts

Duodenum and Hepatopancreatic Sphincter

The duodenum is the first section of the small intestine where important digestive processes occur. The hepatopancreatic sphincter regulates the movement of bile and pancreatic juice into the duodenum.

Liver: Structure and Function

The liver is the largest internal organ, located in the right upper quadrant of the abdomen, and produces bile, vital for fat digestion.

Gross Anatomy of the Liver

Covered by a connective tissue capsule and visceral peritoneum, divided into four lobes.
The right and left lobes are separated by the falciform ligament.
The round ligament is a remnant of the fetal umbilical vein.
The quadrate and caudate lobes are situated within the right lobe.

Additional Anatomical Features

The porta hepatis is the entry and exit point for blood vessels, lymph vessels, bile ducts, and nerves.

Histology of the Liver

The liver's histological structure is characterized by a connective tissue capsule forming septa that partition the liver into hepatic lobules. Each lobule contains:

Hepatocytes: Main functional cells responsible for metabolic processes.
Portal triads: Located at the periphery of each lobule, consisting of a hepatic bile ductule, branches of the hepatic portal vein, and a hepatic artery.
Central vein: Drains blood from the lobule into hepatic veins.

Blood Supply to the Liver

The hepatic artery delivers oxygenated blood to the liver.
The hepatic portal vein transports deoxygenated, nutrient-rich blood from the GI tract, contributing approximately 75% of the blood supply.
As blood flows through the hepatic lobules, nutrients and oxygen are absorbed into hepatocytes.
Stellate cells (Kupffer cells) act as macrophages, clearing harmful substances from the blood.

Bile Production

Bile is secreted by the liver and consists of:

Water
Bicarbonate ions
Bile salts and pigments
Cholesterol
Lecithin
Mucin

Bile salts and lecithin are important for the mechanical digestion of lipids through emulsification.

Clinical View: Cirrhosis of the Liver

Cirrhosis is a condition where hepatocytes are replaced by fibrous scar tissue, often resulting from chronic injury to hepatocytes due to:

Chronic alcoholism
Liver diseases
Drugs or toxins
Viral infections, particularly hepatitis B or C

Symptoms include fatigue, weight loss, nausea, and abdominal pain, with advanced cases presenting jaundice, edema, and ascites.

Gallbladder

The gallbladder is a sac-like organ attached to the liver, responsible for storing and releasing bile. Its structure includes:

Three tunics: inner mucosa, middle muscularis, and external serosa.
Connected to the common bile duct via the cystic duct, which contains a sphincter valve controlling bile flow.

Clinical View: Gallstones (Cholelithiasis)

Gallstones are solid particles that form in the gallbladder, often resulting from cholesterol or calcium and bile salts. Risk factors include:

Female sex
Obesity
Age
Caucasian ethnicity

Symptoms may include severe pain in the right hypochondriac region, nausea, and vomiting, often requiring surgical removal (cholecystectomy).

Oral Cavity and Salivary Glands

Submandibular Glands

Located beneath the jaw, contributing approximately 3% to 5% of total saliva production.

Histology of Salivary Glands

Salivary glands contain two main types of secretory cells:

Mucous Cells: Secrete mucin, which hydrates to form mucus.
Serous Cells: Produce a watery fluid rich in electrolytes and salivary amylase.

Composition and Functions of Saliva

Daily secretion ranges from 1.0 to 1.5 liters, primarily during mealtime.
Saliva is composed of 99.5% water along with various solutes, including:

Salivary amylase
Mucin
Lysozyme

Functions include moistening food, initiating starch breakdown, dissolving food molecules for taste, cleansing oral cavity structures, and providing antibacterial properties.

Regulation of Salivary Secretions

Regulated by salivary nuclei in the brainstem, with a basal level of salivation due to parasympathetic stimulation.
Higher brain centers send signals in response to food thoughts, increasing salivation.

Mechanical Digestion: Mastication

Mastication mechanically reduces food bulk to facilitate swallowing and increases surface area for digestive enzymes.
Controlled by nuclei in the medulla and pons, known as the mastication center.

Teeth Structure

Collectively referred to as dentition, with structure including:

Crown: The exposed part of the tooth.
Neck: The constricted area between the crown and root.
Roots: Anchor the tooth to the jaw.

Tooth Composition

Dentin: The primary mass of the tooth, harder than bone.
Enamel: The outer layer on the crown, composed of calcium phosphate crystals.
Pulp Cavity: The center filled with connective tissue and pulp.
Root Canal: Continuous with the pulp cavity, containing blood vessels and nerves.
Cementum: Covers the root.

Deciduous and Permanent Teeth

Humans have 20 deciduous teeth that erupt between 6 and 30 months.
There are 32 permanent teeth that replace deciduous teeth.
Third molars (wisdom teeth) typically emerge in late teens or 20s.

Types of Teeth

Incisors: Anterior teeth for slicing food.
Canines: Pointed tips for puncturing and tearing food.
Premolars: Flat crowns for crushing and grinding.
Molars: Large crowns designed for grinding and crushing.

Gingivae (Gums)

Composed of dense irregular connective tissue, covered by nonkeratinized stratified squamous epithelium.

Gross Anatomy of the Pharynx

The pharynx is a funnel-shaped muscular passageway for air and food, formed by three pairs of skeletal muscles.
Lined with nonkeratinized stratified squamous epithelium for protection.

Small Intestine

Gross Anatomy

Circular Folds: Internal folds of the mucosal and submucosal tunics that increase surface area for nutrient absorption.

Histology

Villi: Fingerlike projections of the mucosa that increase surface area for absorption.
Lacteal: Lymphatic capillary within each villus responsible for absorbing lipids.
Microvilli: Extensions of the plasma membrane of epithelial cells that form a brush border with embedded enzymes.

Small Intestine Secretions

Intestinal Glands: Secrete intestinal juice and extend to the base of the mucosa.
Goblet Cells: Produce mucin, increasing in number from the duodenum to the ileum.
Unicellular Gland Cells: Synthesize enteropeptidase.
Enteroendocrine Cells: Release hormones such as cholecystokinin (CCK) and secretin.
Submucosal Gland: Produces alkaline mucus secretion to protect the duodenum from chyme.

Motility of the Small Intestine

Smooth Muscle Function: Mixes chyme with gland secretions through segmentation and propels contents through peristalsis.

Regulation of Small Intestine Motility

Intestinal Phase: Segmentation is prevalent early, while peristalsis is prevalent late, initiated by motilin.

Chyme Movement into Large Intestine

Gastroileal Reflex: Ileum contracts, and the ileocecal sphincter relaxes, allowing contents to move from the ileum to the cecum.

Accessory Digestive Organs and Ducts

Biliary Apparatus: Network of ducts draining the liver.
Accessory Pancreatic Duct: Allows a small amount of pancreatic juice to enter the duodenum.
Hepatopancreatic Ampulla: Where the common bile duct and main pancreatic duct pierce the wall of the duodenum.

Liver

Gross Anatomy: Largest internal organ, located in the right upper quadrant of the abdomen.

Gallstones (Cholelithiasis)

Overview: Gallstones can form in the gallbladder or biliary apparatus, often asymptomatic until lodged in the cystic duct.

Pancreas

Functions: Produces insulin and glucagon (endocrine) and pancreatic juice (exocrine) to assist with digestion.

Histology of the Pancreas

Structure: Contains acinar cells that produce and release digestive enzymes organized into lobules.

Pancreatic Secretions

Composition: Pancreatic juice formed from secretions of acinar and duct cells, including digestive enzymes.

Pancreatic Cancer

Overview: Cancer typically arises from exocrine cells of the pancreas, often difficult to detect early.

Regulation of Accessory Structures

Cholecystokinin (CCK): Stimulates gallbladder contraction and bile release, and pancreas to release pancreatic juice.
Secretin: Causes the release of alkaline solution from the liver and pancreas, neutralizing acidic chyme.

Large Intestine

Overview: Responsible for absorbing water and electrolytes, compacting chyme into feces, and storing feces until elimination.

Gross Anatomy of the Large Intestine

Cecum: First portion of the large intestine, receiving chyme from the ileum.
Vermiform Appendix: Thin sac projecting from the cecum, lined by lymphatic nodules.
Colon: Divided into ascending, transverse, descending, and sigmoid colon.
Rectum: Connects to the sigmoid colon, expanding to store feces.
Anal Canal: Last few centimeters of the large intestine, containing internal and external anal sphincters.

Specialized Structures of the Large Intestine

Teniae Coli: Longitudinal muscle bands that contribute to haustra formation.
Omental Appendices: Lobules of fat hanging off the external surface of haustra.

Appendicitis

Overview: Inflammation of the appendix, often due to fecal matter obstruction, leading to bacterial growth.

Histology of the Large Intestine

Mucosa: Lined by simple columnar epithelium with numerous goblet cells.
Muscularis: Consists of two layers of smooth muscle, with the outer longitudinal layer forming teniae coli.

Bacterial Action in the Large Intestine

Indigenous Microbiota: Normal bacterial flora that break down carbohydrates, proteins, and lipids in chyme, generating B vitamins and vitamin K.

Motility and Regulation in the Large Intestine

Peristalsis: Characterized as weak and sluggish, with haustral churning and mass movements occurring 2-3 times a day.
Gastrocolic Reflex: Initiated by stomach distension, causing mass movement in the large intestine.
Defecation Reflex: Filling of the rectum triggers the urge to defecate.

Clinical Views on Large Intestine Disorders

Colorectal Cancer

Second most common cancer type in the U.S., characterized by malignant growths in the large intestine or rectum.
Risk Factors: Include a low-fiber diet, family history, age, and a history of ulcerative colitis.
Symptoms: Rectal bleeding, abdominal pain, and weight loss.
Screening Methods: Include fecal occult blood tests, sigmoidoscopy, and colonoscopy.

Fecal Transplant

A low-cost, low-risk procedure used to treat Clostridium difficile colitis.

Diverticulosis and Diverticulitis

Diverticulosis: Characterized by small bulges in the intestinal lining.
Diverticulitis: Involves inflammation of the diverticula, which can be life-threatening if they rupture.

Constipation and Diarrhea

Constipation: Temporary impairment in the ability to defecate.
Diarrhea: Results from a disruption in the normal mechanisms that absorb intestinal water.

Overview of Gastrointestinal Disorders and Anatomy

Gastrointestinal Disorders

Nausea and Vomiting: Characteristic symptoms include nausea and vomiting, which can lead to erosion and perforation of the gastric lining.

Clinical View: Vomiting

Definition: Rapid expulsion of gastric contents through the oral cavity.
Control: Governed by the vomiting center in the medulla oblongata.
Triggers: Can respond to various stimuli including head injury, motion sickness, and food irritation.

Lower Gastrointestinal Tract

Function: Responsible for the continuation of digestion and absorption, as well as the elimination of indigestible materials.
Small Intestine: Divided into three regions: duodenum, jejunum, and ileum.
Accessory Digestive Organs: Include the liver, pancreas, and gallbladder.
Large Intestine: Primarily responsible for absorbing water, electrolytes, and producing feces.

Inflammatory Bowel Disease and Irritable Bowel Syndrome

Inflammatory Bowel Disease (IBD): Autoimmune disorders characterized by inflammation in the intestines.
Irritable Bowel Syndrome (IBS): A common condition affecting colon function, presenting with cramps and bloating.

Small Intestine Anatomy

General Structure: The small intestine is a long tube where ingested nutrients reside for at least 12 hours.
Segments: Include duodenum, jejunum, and ileum.

Functions and Structure of the Gastrointestinal Tract

Functions and Structure of the Gastrointestinal Tract