Marieb Human Anatomy & Physiology: The Digestive System

Overview of the Digestive System

• Essential Function of the Digestive System:     * A healthy digestive system is essential to life.     * It converts foods into raw materials required to build and fuel the body’s cells.     * Ingestion: The process of taking in food.     * Digestion: Breaking food down into nutrient molecules.     * Absorption: Taking those nutrient molecules into the bloodstream.     * Excretion: The elimination of indigestible residues.

• Two Main Groups of Organs:     1. Alimentary Canal (Gastrointestinal (GI) Tract or Gut):         * A continuous muscular tube running from the mouth to the anus.         * Length: Approximately $30\,ft$ in a cadaver (it is shorter in a living body due to muscle tone).         * Luminal contents are considered part of the external environment.         * Major organs: Mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus.         * Function: Digests food into small fragments and absorbs them through the mucosal lining into the blood.     2. Accessory Digestive Organs:         * Organs: Teeth, tongue, and gallbladder.         * Digestive glands: Salivary glands, liver, and pancreas.         * Function: Use ducts to secrete digestive fluids (enzymes and bile) into the GI tract.

Major Processes of Digestive System Activity

• Six Essential Activities:     1. Ingestion: The simple act of eating.     2. Propulsion: Movement of food through the alimentary canal.         * Swallowing: Initiated voluntarily.         * Peristalsis: The primary means of propulsion; involves alternating (involuntary) waves of contraction and relaxation of muscles in the organ walls. It is primarily propulsive, though some mixing may occur.     3. Mechanical Breakdown: Increases the surface area of nutrients, preparing food for chemical digestion by enzymes. Formerly called mechanical digestion.         * Includes chewing, mixing food with saliva (via the tongue), churning food in the stomach, and segmentation.         * Segmentation: Local rhythmic contractions of the small intestine that move food forward and backward to mix it with digestive fluids. It primarily mixes food and breaks it down mechanically.     4. Digestion: Catabolism via enzymes, breaking down large food molecules into absorbable chemical building blocks. Formerly called chemical digestion.     5. Absorption: The passage of nutrients from the lumen of the GI tract through the mucosal epithelium and into the blood or lymph.     6. Defecation: The elimination of indigestible substances from the body in the form of feces via the anus.

Relationship of Digestive Organs to the Peritoneum

• Peritoneum: Serous membranes of the abdominopelvic cavity.     * Visceral Peritoneum: Covers the external surface of most digestive organs.     * Parietal Peritoneum: Lines the body wall and is continuous with the visceral layer.     * Peritoneal Cavity: A slit-like space between the two membranes containing serous fluid, which lubricates mobile organs as they move against each other.

• Mesentery: A double layer of peritoneum (layers fused back-to-back) extending from the body wall to a digestive organ.     * Functions: Holds organs in place, stores fat, and provides a route for blood vessels, lymphatics, and nerves to reach the organs.     * Orientation: Most are dorsal (attach to the posterior body wall).     * Named mesenteries include the omenta.

• Organ Classifications:     * Intraperitoneal (Peritoneal) Organs: Surrounded by peritoneum and anchored to the body wall by mesentery.     * Retroperitoneal Organs: Lie posterior to the peritoneum. This includes most of the pancreas, the duodenum of the small intestine, and parts of the large intestine.

• Peritonitis:     * Definition: Inflammation of the peritoneum.     * Common Cause: A ruptured appendix.     * Other Causes: Piercing abdominal wounds, perforating ulcers, or poor sterile technique during surgery.     * Characteristics: Peritoneal coverings tend to stick together to localize the infection, but widespread infection is dangerous and lethal.     * Treatment: Removal of infectious debris and megadoses of antibiotics.

Histology of the Alimentary Canal

• The Four Basic Layers (Tunics):     1. Mucosa (Mucous Membrane):         * Innermost layer in contact with luminal contents.         * Functions: Secrete mucus, digestive enzymes, and hormones; absorb end products; protect against infectious disease.         * Sublayers:             * Epithelium: Simple columnar in most areas; stratified squamous in the mouth, pharynx, esophagus, and anus for protection.             * Lamina Propria: Areolar connective tissue containing capillaries for nourishment/absorption and lymphoid follicles (part of MALT: mucosa-associated lymphoid tissue).             * Muscularis Mucosae: A thin layer of smooth muscle producing local movements.     2. Submucosa:         * Areolar connective tissue with rich blood/lymphatic supply, lymphoid follicles, and nerve fibers (submucosal nerve plexus).         * Abundant elastic fibers allow organs to stretch and regain shape.     3. Muscularis Externa (Muscularis):         * Responsible for segmentation and peristalsis.         * Layers: Inner circular layer and outer longitudinal layer of smooth muscle.         * Sphincters: The circular layer thickens at certain points to form valves that regulate food movement.     4. Serosa:         * The visceral peritoneum (outermost layer) in intraperitoneal organs.         * Consists of areolar connective tissue covered with mesothelium (simple squamous epithelium).         * Adventitia: Replaces the serosa in organs outside the abdominopelvic cavity (e.g., the esophagus). It is a dense connective tissue that binds the organ to surrounding structures.         * Retroperitoneal organs have both an adventitia (on the side facing the body wall) and a serosa (on the side facing the peritoneal cavity).

Blood Supply and the Enteric Nervous System

• Splanchnic Circulation:     * Arteries serving digestive organs receive approximately $25\%$ of cardiac output, which increases after a meal.     * Celiac Trunk: Serves the spleen, liver, and stomach.     * Mesenteric Arteries (Superior and Inferior): Serve the intestines.     * Hepatic Portal Circulation: Delivers nutrient-rich venous blood from digestive organs to the liver.

• Enteric Nervous System (ENS):     * Commonly called the "gut brain"; it consists of more neurons than the spinal cord.     * Enteric Neurons: Communicate to regulate digestive activity via two major intrinsic nerve plexuses:         1. Submucosal Nerve Plexus: Regulates glands and smooth muscle in the mucosa.         2. Myenteric Nerve Plexus: Lies between muscle layers; controls GI tract motility.     * Reflex Arcs:         * Short Reflexes: Mediated entirely by the intrinsic plexuses in response to stimuli within the GI tract.         * Long Reflexes: Mediated by the CNS and extrinsic autonomic nerves.             * Parasympathetic inputs: Stimulate digestive processes.             * Sympathetic inputs: Inhibit digestive processes.

Basic Concepts of Regulating Digestive Activity

• Regulating Stimuli:     * Receptors (mechanoreceptors and chemoreceptors) in the GI walls respond to stretch, changes in osmolarity, pH, and the presence of substrates or end products.

• Effectors: Smooth muscle (for mixing and moving) and glands (for secreting juices or hormones).

• Control Mechanisms:     * Intrinsic Controls: Involve short reflexes of the ENS.     * Extrinsic Controls: Involve long reflexes and the CNS.     * Hormonal Controls: Hormones from the stomach and small intestine stimulate target cells to affect secretion or contraction.

Functional Anatomy of the Mouth

• Anatomy of the Mouth (Oral or Buccal Cavity):     * Boundaries: Lips (anterior), cheeks (lateral), palate (superior), tongue (inferior).     * Epithelium: Stratified squamous to protect against friction; slightly keratinized on the gums, hard palate, and part of the tongue.     * Lips (Labia): Orbicularis oris muscle covered with skin.     * Cheeks: Primarily the buccinator muscles.     * Oral Vestibule: Recess between the lips/cheeks and teeth/gums.     * Oral Cavity Proper: Area within the teeth and gums.     * Labial Frenulum: Median attachment of each lip to the gum.

• The Palate:     * Hard Palate: Formed by palatine bones and palatine processes of maxillae; has a midline ridge called the raphe to create friction.     * Soft Palate: Mobile fold of skeletal muscle. It rises to close the nasopharynx during swallowing.     * Uvula: Finger-like projection pointing downward from the soft palate.

• The Tongue:     * Composed of interlacing bundles of skeletal muscle.     * Functions: Repositioning food, mixing food with saliva to form a bolus (compact mass), initiating swallowing, speech, and taste.     * Lingual Frenulum: Attaches the tongue to the floor of the mouth.     * Ankyloglossia: A congenital condition ("fused tongue") where the lingual frenulum is extremely short; treated surgically by snipping.     * Papillae:         * Filiform: Smallest, most abundant, contain keratin (whitish), provide friction but no taste buds.         * Fungiform: Mushroom-shaped, vascular (reddish), house taste buds.         * Vallate: $8-12$ large papillae in a V-shape on the back of the tongue; house taste buds.         * Foliate: Pleat-like, on lateral posterior aspects; house taste buds.     * Terminal Sulcus: Groove behind vallate papillae; divides the body of the tongue from the root (which contains the lingual tonsil).

Salivary Glands and Saliva

• Functions of Saliva: Cleanses the mouth, dissolves food chemicals for taste, moistens food for bolus formation, and begins starch breakdown via amylase.

• Gland Types:     * Minor (Intrinsic) Glands: Located throughout the oral mucosa; keep it moist.     * Major (Extrinsic) Glands: Paired glands outside the oral cavity; include:         * Parotid Gland: Anterior to the ear; duct opens next to the second upper molar.         * Submandibular Gland: Medial to the mandible; duct opens at the base of the lingual frenulum.         * Sublingual Gland: Under the tongue; $10-20$ ducts open into the floor of the mouth.

• Cell Types:     * Serous Cells: Produce watery secretion with enzymes and ions.     * Mucous Cells: Produce mucus.     * Note: Parotid and submandibular are mostly serous; sublingual is mostly mucous.

• Saliva Composition:     * $97-99.5\%$ water (hypo-osmotic).     * pH: $6.75$ to $7$.     * Electrolytes: $Na^+$, $K^+$, $Cl^-$, $PO_4^{3-}$, $HCO_3^-$.     * Proteins: Amylase, lingual lipase, mucin, lysozyme, IgA, and defensins.     * Metabolic wastes: Urea and uric acid.

• Xerostomia: Uncomfortably dry mouth due to low saliva; caused by medications, diabetes, HIV/AIDS, or Sjögren’s syndrome.

• Control of Salivation:     * Average output: Approximately $1500\,ml/day$.     * Controlled by the parasympathetic division (Facial nerve VII and Glossopharyngeal nerve IX).     * Stimuli: Chemoreceptors (acidic/spicy) and mechanoreceptors; sight/smell/thought of food.     * Sympathetic stimulation causes thick, mucin-rich saliva or inhibits salivation (leading to dry mouth during stress).

The Teeth and Mechanical Breakdown

• Teeth Function: Mastication (chewing) to tear and grind food.

• Dentition:     * Deciduous (Milk/Baby) Teeth: $20$ teeth; erupt between $6-24\,months$.     * Permanent Teeth: $32$ teeth; erupt between $6-12\,years$ (except wisdom teeth at $17-25\,years$).

• Classification:     * Incisors: Chisel-shaped for cutting/nipping.     * Canines (Cuspids): Fang-like for tearing/piercing.     * Premolars (Bicuspids) & Molars: Broad crowns for grinding and crushing.

• Dental Formulae:     * Deciduous: $\frac{2I, 1C, 2M (upper)}{2I, 1C, 2M (lower)} \times 2 = 20\,teeth$     * Permanent: $\frac{2I, 1C, 2PM, 3M (upper)}{2I, 1C, 2PM, 3M (lower)} \times 2 = 32\,teeth$

• Tooth Structure:     * Crown: Part above the gingiva (gum).         * Enamel: Hardest substance in the body; heavily mineralized with calcium salts and hydroxyapatite. Cannot heal; requires artificial filling.     * Root: Embedded in the jaw bone.     * Dentin: Bone-like material under enamel, bulk of the tooth; maintained by odontoblasts.     * Pulp Cavity: Contains pulp (vessels and nerves for nourishment/sensation).     * Root Canal: Extension of the pulp cavity into the root.     * Cementum: Calcified tissue covering the root; attaches to the periodontal ligament, forming a gomphosis (fibrous joint).

• Tooth Diseases:     * Dental Caries (Cavities): Demineralization of enamel/dentin by bacterial acid from dental plaque.     * Gingivitis: Inflammation of gums due to calculus (tartar) disrupting the seal of the gums.     * Periodontitis: Immune cells attack bacteria but destroy the periodontal ligament and activate osteoclasts (dissolving bone). Risk factors: smoking, diabetes.

The Pharynx and Esophagus

• Pharynx:     * Passage for food, fluids, and air.     * Layers: Stratified squamous epithelium and two skeletal muscle layers (inner longitudinal and outer pharyngeal constrictors).

• Esophagus:     * Muscular tube approximately $10\,in$ long; collapsed when empty.     * Esophageal Hiatus: Opening in the diaphragm where the esophagus passes through.     * Cardial (Gastroesophageal) Sphincter: Surrounds the cardial orifice; protects the esophagus from acid reflux.     * Histology: Transitions from stratified squamous (esophagus) to simple columnar (stomach). Muscularis changes from skeletal (superior) to smooth (inferior).

• Heartburn: Result of stomach acid entering the esophagus. Often a symptom of GERD (Gastroesophageal Reflux Disease). Can be caused by a hiatal hernia.

Deglutition (Swallowing)

• Phase 1: Buccal Phase:     * Voluntary.     * The tongue presses food bolus against the hard palate, forcing it into the oropharynx.

• Phase 2: Pharyngeal-Esophageal Phase:     * Involuntary; controlled by the swallowing center in the medulla and lower pons via the vagus nerve.     * The tongue blocks the mouth; the soft palate/uvula rise to block the nasopharynx.     * The larynx rises so the epiglottis blocks the trachea.     * The upper esophageal sphincter relaxes to let food enter, then contracts to prevent reflux.     * Peristalsis moves food to the stomach; the lower esophageal sphincter opens for entry.

The Stomach

• Stomach Anatomy:     * Functions: Temporary storage, physical/chemical breakdown into paste-like chyme.     * Volume: $50\,ml$ (empty) to $4\,L$ (distended).     * Rugae: Mucosal folds that allow for expansion.     * Regions: Cardia (surrounds orifice), Fundus (dome), Body (midportion), Pyloric part (antrum, canal, and pylorus terminating at the pyloric sphincter).     * Curvatures: Greater (lateral) and Lesser (medial).     * Mesenteries: Lesser Omentum (liver to lesser curvature) and Greater Omentum (greater curvature over intestines/spleen; contains fat and lymph nodes).

• Microscopic Anatomy:     * Muscularis Externa has an inner oblique layer for extra churning/pummeling power.     * Mucosal Barrier: Protects against self-digestion using bicarbonate-rich mucus, tight junctions, and rapid cell replacement (every $3-6\,days$).     * Gastric Gland Cells:         1. Mucous Neck Cells: Secrete thin, acidic mucus.         2. Parietal Cells: Secrete Hydrochloric acid (HCl) (pH $1.5-3.5$; kills bacteria, denatures proteins, activates pepsin) and Intrinsic Factor (required for Vitamin $B_{12}$ absorption).         3. Chief Cells: Secrete pepsinogen (activated by HCl to pepsin) and gastric lipases.         4. Enteroendocrine Cells: Secrete chemical messengers like Gastrin, histamine, serotonin, and somatostatin.     * Gastritis: Inflammation that breaches the mucosal barrier; can lead to Peptic Ulcers (mostly caused by Helicobacter pylori).

Digestive Processes in the Stomach

• Protein Digestion: Initiated by pepsin. In infants, rennin breaks down milk protein (casein).

• Vitamin B12: Intrinsic factor is the only stomach function absolutely essential for life; lack of it causes pernicious anemia.

• Regulation of Gastric Secretion:     1. Cephalic Phase: Before food reaches the stomach; triggered by sight/smell/taste.     2. Gastric Phase: Lasts $3-4\,hours$; triggered by distension (stretch receptors) and gastrin release from G cells.     3. Intestinal Phase: Brief stimulation when food enters the duodenum, followed by inhibition via the Enterogastric Reflex and Enterogastrones (Secretin and Cholecystokinin/CCK).

• HCl Secretion Mechanism: Parietal cells pump $H^+$ into the lumen using $H^+/K^+\,ATPase$ pumps. As $H^+$ enters the stomach, $HCO_3^-$ is exported to the blood (Alkaline Tide) in exchange for $Cl^-$; $Cl^-$ then joins $H^+$ in the lumen.

• Motility:     * Basic Electrical Rhythm (BER) of $3\,per\,minute$ set by enteric pacemaker cells.     * Maximal volume before pressure increases significantly: approximately $1.5\,L$.     * Gastric Emptying: Usually takes $4\,hours$; slowed to $6\,hr+$ by fatty chyme.

The Liver, Gallbladder, and Pancreas

• The Liver:     * Largest gland ($\sim 3\,lbs$); four lobes: Right, Left, Caudate, Quadrate.     * Liver Lobules: Hexagonal units consisting of hepatocyte plates and a central vein.     * Portal Triad: Found at corners; contains a branch of the hepatic artery (oxygen), hepatic portal vein (nutrients), and bile duct.     * Hepatocyte Functions: Produce $\sim 900\,ml$ of bile daily, process nutrients (store glycogen), store fat-soluble vitamins, and detoxify (ammonia to urea).

• Bile: Yellow-green solution containing bile salts (cholesterol derivatives) that emulsify fats.     * Enterohepatic Circulation: $95\%$ of bile salts are recycled from the ileum back to the liver.     * Bilirubin: Pigment from heme breakdown; metabolized to stercobilin, giving feces brown color.

• The Gallbladder: Stores and concentrates bile. Gallstones (cholelithiasis) are crystallized cholesterol.

• The Pancreas:     * Exocrine Part: Acini produce pancreatic juice with proenzymes. Duct cells secrete water and $HCO_3^-$.     * Pancreatic Juice: Output is $1200-1500\,ml/day$ (pH $8$).     * Protease Activation: Occurs in the duodenum. Trypsinogen is activated to Trypsin by enteropeptidase. Trypsin then activates procarboxypeptidase and chymotrypsinogen.     * Other enzymes: Amylase, lipases, nucleases.

• Regulation of Bile/Pancreatic Juice:     * CCK: Stimulated by fat/protein; causes gallbladder contraction and pancreatic enzyme release.     * Secretin: Stimulated by acid; causes bicarbonate secretion from pancreatic duct cells.

The Small Intestine

• Subdivisions: Duodenum ($\sim 10\,in$), Jejunum ($\sim 8\,ft$), and Ileum ($\sim 12\,ft$). Total length in vivo: $2-4\,m$.

• Modifications for Absorption:     * Circular Folds: Force chyme to spiral to slow it down.     * Villi: Finger-like projections; each contains a lacteal (lymphatic capillary) for fat absorption.     * Microvilli: "Brush border" containing enzymes to finish carbohydrate/protein digestion.

• Histology:     * Intestinal Crypts: Secretion of intestinal juice ($1-2\,L/day$, alkaline).     * Paneth Cells: Secrete antimicrobial agents (defensins, lysozyme).     * Peyer’s Patches: Aggregated lymphoid nodules in the distal ileum.

• Motility:     * After a meal: Segmentation mixes chyme.     * Between meals: Migrating Motor Complex (MMC) (peristalsis) every $90-120\,min$ cleans out remnants.

The Large Intestine

• Features:     * Teniae Coli: Three bands of smooth muscle longitudinal layer.     * Haustra: Pocket-like sacs.     * Epiploic Appendages: Fat-filled peritoneal pouches.

• Anatomy: Cecum, Appendix (MALT), Colon (Ascending, Transverse, Descending, Sigmoid), Rectum, and Anal Canal.

• Anal Canal Sphincters: Internal (smooth muscle, involuntary) and External (skeletal muscle, voluntary).

• Microscopic Anatomy: Thick mucosa with deep crypts for mucus secretion (lubrication). No villi or circular folds.

• Bacterial Flora: Over $1000$ types ($\sim 2\,lb$ of body weight). Synthesize Vitamin K and B complex; ferment indigestible carbs, releasing gas ($\sim 500\,ml/day$).

• Motility:     * Haustral Contractions: Slow mixing movements.     * Mass Movements: Powerful peristaltic waves $3-4\,times$ per day (often via gastrocolic reflex).

• Defecation Reflex: Parasympathetic spinal reflex initiated by rectal distension. Involves Valsalva’s maneuver (increased intra-abdominal pressure).

Physiology of Digestion and Absorption

• Mechanism: Enzymatic hydrolysis (adding water to break bonds).

• Carbohydrates:     * Broke down to monosaccharides (glucose, fructose, galactose).     * Salivary/Pancreatic Amylase $+$ Brush border enzymes (lactase, sucrase, etc.).     * Absorbed via secondary active transport with $Na^+$; exit via facilitated diffusion.

• Proteins:     * Stomach (Pepsin) $+$ Pancreatic proteases (Trypsin, Chymotrypsin) $+$ Brush border (Aminopeptidase).     * Absorbed via carrier proteins with $Na^+$.

• Lipids:     1. Emulsification (Bile salts).     2. Digestion (Pancreatic lipase to monoglyceride $+$ 2 free fatty acids).     3. Micelle formation.     4. Diffusion into enterocyte.     5. Chylomicron formation and exocytosis into lacteals.

• Water/Electrolytes:     * $9\,L$ of water enters daily; $95\%$ absorbed in the small intestine via osmosis.     * $Na^+$ is actively pumped; $K^+$ follows water; Calcium requires active Vitamin D and PTH.

• Vitamins: Fat-soluble (A, D, E, K) follow fat; Water-soluble (B, C) absorbed via simple/active transport. $B_{12}$ requires endocytosis with intrinsic factor.

Development and Aging

• Embryology: Epithelial lining from endoderm; stomach/muscle from mesoderm. Alimentary canal is a continuous tube by week 5.

• Congenital Disorders: Cleft palate (failure of maxillae to fuse), Tracheoesophageal fistula (esophagus connects to trachea), Cystic fibrosis (thick mucus blocks pancreatic duct).

• Aging: Digestive juice production and motility decline; frequency of diverticulosis, fecal incontinence, and GI cancers increases.