Comprehensive Notes on the Human Digestive System

Subdivisions and General Structure of the Digestive System

• Definition and Length: The digestive system is a $30\text{-foot-long}$ tube that extends from the mouth to the anus. It is comprised of the Alimentary Canal (Digestive tract or GI tract) and various accessory organs.

• Digestive Tract (GI Tract): Includes the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine (colon).

• Accessory Organs: Includes the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

• Relationship to the Peritoneum:     * External Environment: The digestive tract is technically open to the environment; therefore, undigested food is never technically "in" the body.     * Peritoneal Cavity: Most digestive organs reside within the peritoneal cavity.     * Retroperitoneal Organs: Includes the duodenum, pancreas, and parts of the large intestine.     * Peritoneum Membranes: A moist serous membrane consisting of the Parietal peritoneum (outer membrane) and the Visceral peritoneum (inner membrane).     * Dorsal Mesentery: Suspends the GI tract and forms the serosa (visceral peritoneum) of the stomach and intestines.     * Ventral Mesentery: Forms the lesser and greater omenta, which consists of a lacy layer of connective tissue containing lymph nodes, lymphatic vessels, and blood vessels.

• Omenta and Mesocolon:     * Lesser Omentum: Attaches the stomach to the liver.     * Greater Omentum: Covers the small intestines like an apron.     * Mesentery of Small Intestines: Holds many blood vessels.     * Mesocolon: Anchors the colon to the posterior body wall.

Digestive Functions and Processes

• Primary Functions:     * Ingestion: Selective intake of food.     * Digestion: Breakdown of food into smaller molecules.     * Absorption: Uptake of nutrients into the blood or lymph.     * Defecation: Elimination of undigested material.

• The Three Processes:     * Motility: Muscular contractions that break up food, mix it with enzymes, and move it along the tract.     * Secretion: Release of digestive enzymes and hormones.     * Membrane Transport: The physical absorption of nutrients.

• Stages of Digestion:     * Mechanical Digestion: The physical breakdown of food into smaller particles. Examples include the cutting and grinding of teeth and the churning action of the stomach and intestines.     * Chemical Digestion: Hydrolysis reactions involving enzymes in the saliva, stomach, pancreas, and intestines. This stage breaks macromolecules into their monomers:         * Polysaccharides into monosaccharides.         * Proteins into amino acids.         * Fats into glycerol and fatty acids.         * Nucleic acids into nucleotides.

The Mouth (Oral or Buccal Cavity)

• Anatomical Features:     * Oral Orifice: The anterior opening.     * Fauces: The posterior opening to the throat.     * Cheeks and Lips: Essential for speech, suckling (in infants), and keeping food between the teeth during chewing.     * Tongue: A sensitive, muscular manipulator of food. Its dorsal surface contains papillae and taste buds. The root contains tonsils, and lingual glands secrete saliva.     * Palate: The hard and soft palate allow for breathing and chewing to occur simultaneously.

• Specific Structures of the Oral Cavity:     * Vestibule, Upper/Lower lips.     * Superior and Inferior labial frenulum.     * Palatine raphe, Palatoglossal arch, Palatopharyngeal arch.     * Palatine tonsil.     * Uvula.     * Lingual frenulum.     * Salivary duct orifices (Sublingual and Submandibular).

• Functions of the Mouth:     * Ingestion, taste, and mastication (chewing).     * Beginning of chemical digestion of starch.     * Deglutition (swallowing), speech, and respiration.

Dentition (Teeth)

• Deciduous (Baby) Teeth: Total of $20$ teeth typically erupted by age $2$.     * Central incisor: $6\text{--}9$ months.     * Lateral incisor: $7\text{--}11$ months.     * Canine: $16\text{--}20$ months.     * $1\text{st}$ molar: $12\text{--}16$ months.     * $2\text{nd}$ molar: $20\text{--}26$ months.

• Permanent Teeth: Total of $32$ teeth erupted between ages $6$ and $25$.     * Central incisor: $6\text{--}8$ years.     * Lateral incisor: $7\text{--}9$ years.     * Canine: $9\text{--}12$ years.     * $1\text{st}$ premolar: $10\text{--}12$ years.     * $2\text{nd}$ premolar: $10\text{--}12$ years.     * $1\text{st}$ molar: $6\text{--}7$ years.     * $2\text{nd}$ molar: $11\text{--}13$ years.     * $3\text{rd}$ molar (wisdom tooth): $17\text{--}25$ years.

• Tooth Structure:     * Periodontal Ligament: Modified periosteum that anchors the tooth into the bone.     * Cementum and Dentin: Living tissues of the tooth.     * Enamel: An acellular secretion formed during development; it covers the crown and neck.     * Root Canal: Leads into the pulp cavity, containing nerves and blood vessels.     * Gingiva: The gums.

Saliva and Salivary Glands

• Composition and Characteristics: Production of $1$ to $1.5\,L$ per day. It is a hypotonic solution of $99.5\%$ water and various solutes.

• Solutes in Saliva:     * Amylase: Begins starch digestion.     * Lingual Lipase: Digests fat after reaching the stomach; must be activated by $HCl$ in the stomach.     * Mucus: Lubricates and binds food for swallowing.     * Lysozyme: Enzyme that kills bacteria.     * IgA: Antibodies that inhibit bacterial growth.     * Electrolytes: $Na^{+}$, $K^{+}$, $Cl^{-}$, phosphate, and bicarbonate.     * pH: Ranges between $6.8$ and $7.0$.

• Functions of Saliva: Moistens the mouth, partially digests starch, inhibits bacterial growth, dissolves molecules to stimulate taste buds, and binds food into a bolus for swallowing.

• Intrinsic Salivary Glands: Indefinite number dispersed throughout oral tissue (Buccal/cheeks, Labial/lips, Lingual/tongue). They secrete continuously at a constant rate.

• Extrinsic Salivary Glands: Three pairs connected to the oral cavity by ducts: Parotid, Submandibular, and Sublingual.     * Mucous Glands: Secrete mucous.     * Serous Glands: Secrete amylase and electrolytes.

The Pharynx and Esophagus

• Pharynx (Throat): Composed of skeletal muscle (longitudinal and circular).     * Contains superior, middle, and inferior constrictors to force food downward.     * The inferior constrictor remains constricted when food is not being swallowed to exclude air from the esophagus.

• Tissue Layers of the GI Tract:     1. Mucosa: Consists of epithelium (stratified squamous in esophagus), lamina propria (loose connective tissue), and muscularis mucosae (thin smooth muscle layer).     2. Submucosa: Contains esophageal glands and blood vessels.     3. Muscularis Externa: Inner circular layer and outer longitudinal layer.     4. Adventitia or Serosa: Areolar tissue or mesothelium.     5. Enteric Nervous System: Myenteric plexus and submucosal plexus (with parasympathetic ganglions).

• Esophagus Characteristics: A straight muscular tube $25\text{--}30\,cm$ long, located dorsal to the trachea.     * Upper part is skeletal muscle; bottom is smooth muscle.     * Passes through the esophageal hiatus in the diaphragm.     * Lower esophageal sphincter closes the orifice to prevent reflux.

Deglutition (Swallowing)

• Coordination: Series of muscular contractions coordinated by the swallowing center in the medulla and pons.

• Buccal Phase: Voluntary phase where the tongue collects food and pushes it into the oropharynx.

• Pharyngeal-Esophageal Phase: Involuntary phase.     * Soft palate rises to block the nasopharynx.     * Infrahyoid muscles lift the larynx; the epiglottis folds back.     * Pharyngeal constrictors push the bolus down the esophagus via peristaltic waves.

• Transit Times: Liquids reach the stomach in $2$ seconds; a food bolus may take $8$ seconds.

The Stomach

• Anatomy: A J-shaped muscular sac in the upper left abdominal cavity, inferior to the diaphragm.     * Features include the lesser and greater curvatures and rugae (wrinkles).     * Volume: $50\,ml$ when empty; $1$ to $1.5\,L$ after a typical meal; maximum capacity of $4\,L$.

• Functions: Mechanical breakdown of food, liquefication of food into chyme, and the beginning of chemical digestion of protein and fat.     * Absorption: Very limited; absorbs aspirin and some lipid-soluble drugs.     * Filtering: All blood drained from the stomach is filtered through the liver before returning to the heart.

• Regions of the Stomach:     * Cardiac region: Just inside the cardiac orifice.     * Fundus: Domed portion superior to the esophageal opening.     * Body (Gastric region): Main portion.     * Pyloric region: Narrow inferior end containing the antrum, funnel, and pyloric canal.     * Pylorus: Opening to the duodenum controlled by the pyloric sphincter (thick ring of smooth muscle).

• Stomach Wall Layers: Mucosa has simple glandular epithelium; lamina propria is filled with tubular glands called gastric pits. Muscularis externa has three layers: outer longitudinal, middle circular, and inner oblique.

Gastric Glands and Secretions

• Cells of the Gastric Glands:     * Mucous cells: Produce mucus.     * Regenerative cells: Divide rapidly to produce new cells.     * Parietal cells: Secrete $HCl$ (Hydrochloric acid) and intrinsic factor.     * Chief cells: Secrete chymosin (rennin) and lipase in infants; secrete pepsinogen throughout life.     * Enteroendocrine (G) cells: Secrete hormones and paracrines.

• Gastric Juice: Production of $2$ to $3\,L$ per day, mostly water, $HCl$, and pepsin.

• Hydrochloric Acid ($HCl$):     * pH as low as $0.8$.     * Activates pepsin and lingual lipase.     * Breaks up connective tissues and plant cell walls.     * Converts ferric ions ($Fe^{+3}$) to ferrous ions ($Fe^{+2}$) for absorption.

• Pepsin: Digestion of protein.     * Secreted as pepsinogen (a zymogen).     * Activation: $Pepsinogen + HCl \rightarrow Pepsin$. It is an autocatalytic process where active pepsin can activate more pepsinogen.

• Other Factors:     * Intrinsic Factor: Secreted by parietal cells; essential for $B_{12}$ absorption by the small intestine and $RBC$ production. Deficiency leads to pernicious anemia.     * Gastric Lipase: Digests butterfat in infants.     * Chymosin (Rennin): Curdles milk in infants.

• Chemical Messengers:     * Gastrin: Stimulates $HCl$ and enzyme secretion; stimulates intestinal motility; relaxes ileocecal valve.     * Serotonin: Stimulates gastric motility.     * Histamine: Stimulates $HCl$ secretion.     * Somatostatin: Inhibits gastric secretions and motility; inhibits pancreatic and gallbladder secretions.

Gastric Motility and Regulation

• Motility Process: Swallowing center sends impulses to the stomach to relax (receptive-relaxation response). Peristaltic contractions, controlled by pacemaker cells in the longitudinal muscle layer, occur every $20$ seconds.

• Excretion: Approximately $3\,ml$ of chyme enters the duodenum at a time.     * Meal emptying: Typical meal ($4$ hours), liquid (sooner), high fat ($6$ hours).

• Vomiting: Induced by stretching, psychological stimuli, or chemical irritants. Controlled by the emetic center in the medulla.

• Stomach Protection: Alkaline mucous coat, rapid epithelial cell replacement (every $3\text{--}6$ days), and tight junctions.

• Peptic Ulcers: Caused by erosion of the stomach wall by $HCl$ and pepsin. Major cause is the bacterium Helicobacter pylori; treatment involves antibiotics.

• Three Phases of Regulation:     1. Cephalic Phase: Brain control via the vagus nerve; triggered by sight, smell, or thought of food.     2. Gastric Phase: Stomach control via stretching and presence of food; stimulated by acetylcholine, histamine, and gastrin.     3. Intestinal Phase: Duodenum regulates gastric activity. Initially stimulates, then inhibits gastric secretion/motility via the Enterogastric reflex and hormones: Secretin, Cholecystokinin (CCK), and gastric inhibitory peptide (GIP).

The Liver and Gallbladder

• Liver Anatomy: $3\,lb$ organ with four lobes: right, left, quadrate, and caudate.     * Falciform ligament: Separates left and right lobes.     * Round ligament: Remnant of umbilical vein.     * Hepatic Lobules: Tiny cylinders ($2\,mm$ by $1\,mm$) with a central vein and hepatocytes separated by sinusoids.     * Hepatic Triad: Hepatic portal vein, hepatic artery, and bile duct.

• Liver Functions: Filters blood (glucose, amino acids, iron, fat-soluble vitamins A, D, E, K, hormones, toxins); secretes albumin, lipoproteins, clotting factors, and angiotensinogen; manages glycogen storage and production.

• The Gallbladder: $10\,cm$ long sac for storing and concentrating bile (by factor of $20$ through water/electrolyte absorption).

• Bile Pathway: Bile canaliculi $\rightarrow$ bile ductules $\rightarrow$ right/left hepatic ducts $\rightarrow$ common hepatic duct $\rightarrow$ (joins cystic duct) $\rightarrow$ common bile duct $\rightarrow$ (joins pancreatic duct) $\rightarrow$ hepatopancreatic ampulla $\rightarrow$ duodenum at the major duodenal papilla (regulated by hepatopancreatic sphincter).

• Bile Composition: Yellow-green fluid containing minerals, bile acids (salts), cholesterol, phospholipids, and pigments (Bilirubin).     * Bilirubin is metabolized to urobilinogen (brown color of feces).     * Bile acids are synthesized from cholesterol and emulsify fats.

• Gallstones (Biliary Calculi): Composed of cholesterol, calcium carbonate, and bilirubin. Can block bile flow, causing jaundice and poor fat digestion.

• Diet and Colon Cancer: High fat/low fiber diets are associated with colon cancer (e.g., US, Germany). Fiber binds bile acids, preventing their modification into cancer promoters.

The Pancreas

• Anatomy: Retroperitoneal gland with head, body, and tail. Acts as both endocrine (insulin/glucagon) and exocrine gland.

• Pancreatic Juice: $1.5\,L$ secreted per day containing water, enzymes, zymogens, electrolytes, and sodium bicarbonate ($NaHCO_3$).

• Zymogens/Proteases:     * Trypsinogen $\rightarrow$ Trypsin (activated by intestinal epithelium).     * Chymotrypsinogen $\rightarrow$ Chymotrypsin (activated by trypsin).     * Procarboxypeptidase $\rightarrow$ Carboxypeptidase (activated by trypsin).

• Other Enzymes: Pancreatic amylase (starch), Pancreatic lipase (fats), Ribonuclease (RNA), and Deoxyribonuclease (DNA).

• Hormonal Control:     * CCK: Released due to acid/fat; causes gallbladder contraction and pancreatic enzyme secretion.     * Secretin: Released due to acidic chyme; stimulates bicarbonate secretion.     * Gastrin: Weakly stimulates gallbladder and pancreatic enzymes.

The Small Intestine

• Dimensions: $6\text{--}7\,m$ in cadavers; $2\,m$ in living persons due to muscle tone.

• Regions:     1. Duodenum: (~$10\,in$) Curves around the pancreas; receives bile and pancreatic juice; neutralizes acid.     2. Jejunum: ($8\,ft$) Primary site of nutrient absorption.     3. Ileum: ($12\,ft$) Ends at the ileocecal junction.

• Surface Area Enhancements:     * Circular folds: Slow chyme and create spiral flow.     * Villi: $1\,mm$ tall projections containing blood vessels and lacteals.     * Microvilli: $1\,micron$ tall brush border containing digestive enzymes.

• Histology: Intestinal crypts (pores between villi) containing absorptive, goblet, and rapidly dividing cells.     * Brunner’s glands: Secrete bicarbonate mucus.     * Peyer patches: Lymphocyte populations.

• Intestinal Motility:     * Segmentation: Random ringlike constrictions ($12\text{ times/min}$ in duodenum) to mix, not move material.     * Peristalsis: Migrating motor complex moves contents toward the colon.     * Gastroileal reflex: Food in stomach triggers relaxation of the ileocecal valve.

Digestion and Absorption Details

• Carbohydrates:     * $50\%$ of starch is digested before reaching the small intestine.     * Pancreatic amylase completes the first step in $10$ minutes.     * Brush border enzymes (Maltase, dextrinase, glucoamylase) act on oligosaccharides.     * Absorption: Sodium-glucose transport proteins (SGLT) for glucose and galactose. Fructose is absorbed via facilitated diffusion.

• Proteins:     * Stomach pepsin (optimal pH $1.5\text{--}3.5$) is inactivated in the duodenum (pH $8$).     * Pancreatic enzymes (Trypsin, Chymotrypsin) hydrolyze polypeptides.     * Carboxypeptidase, Aminopeptidase, and Dipeptidase (brush border) finish the task.     * Infants absorb proteins by pinocytosis (maternal IgA).

• Fats:     * Fat globules are emulsified by lecithin and bile acids.     * Pancreatic lipase hydrolyzes triglycerides into free fatty acids and monoglycerides.     * Micelles pick up lipids and pass to absorptive cells.     * Inside cells, triglycerides are resynthesized and packaged into protein-coated chylomicrons.

• Nucleic Acids, Vitamins, and Minerals:     * Nucleases/Brush border enzymes break DNA/RNA into phosphate ions, sugars, and bases.     * Vitamins A, D, E, K are absorbed with lipids; B and C by diffusion; $B_{12}$ with intrinsic factor.     * Iron and calcium are absorbed as needed; $Na^{+}$ is cotransported with sugars; $Cl^{-}$ is exchanged for bicarbonate.

• Water Balance: GI tract receives $9\,L$ water/day ($0.7\,L$ food, $1.6\,L$ drink, $6.7\,L$ secretions). $8\,L$ absorbed by small intestine; $0.8\,L$ by large intestine. Diarrhea occurs if absorption is insufficient.

The Large Intestine and Defecation

• Anatomy: $5\,ft$ long, $2.5\,in$ diameter. Features haustra (pouches).     * Regions: Cecum (with appendix), ascending, transverse, descending, and sigmoid colon, rectum (with $3$ valves), and anal canal.     * Appendix: Rich in lymphocytes; collection of pathogens.

• Bacterial Flora: Ferment cellulose, synthesize vitamins B and K.

• Gas (Flatus): Total of $500\,ml$ produced daily (mostly swallowed air; contains methane, hydrogen sulfide, indole, and skatole).

• Absorption and Motility: Transit time of $12\text{--}24\,hours$. Haustral contractions every $30\,min$; Mass movements $1\text{--}3\,times\text{ daily}$ triggered by gastrocolic/duodenocolic reflexes.

• Anal Canal: $3\,cm$ length; contains anal columns and sinuses. Hemorrhoids are distended veins.

• Defecation Reflexes:     1. Intrinsic reflex: via myenteric plexus.     2. Parasympathetic reflex: via spinal cord (splanchnic nerves).     * Requires voluntary relaxation of the external anal sphincter and abdominal contractions.

• Colon Cancer: Third leading cause of cancer deaths in the US. Begins as a polyp. Early colonoscopy recommended (age $50$).