Digestive system

DIGESTIVE VOCABULARY

• Alimentary Canal: Also known as the digestive tract.

• Digestion: The process of breaking down food into usable molecules.

• Absorption: The process of taking usable molecules, vitamins, and minerals into the blood.

• Peristalsis: Involuntary contractions that move food along the gastrointestinal (GI) tract; characterized as powerful.

MAIN FUNCTIONS OF THE DIGESTIVE SYSTEM

• Ingestion: Physically taking food into the digestive tract.

• Propulsion: Moving food through the alimentary canal through peristalsis.

• Mechanical Digestion: Physical breakdown of food, such as chewing, tongue mixing, and stomach churning (segmentation).

• Chemical Digestion: Enzymatic breakdown of food, primarily occurring in the small intestine (catabolic process).

• Absorption: Uptake of nutrients (vitamins, minerals, molecules) from the lumen of the GI tract into the blood.

• Defecation: The elimination of waste (pooping).

PERISTALSIS VS SEGMENTATION

• Peristalsis:

  • Purpose: To move food.

  • Mechanism: Alternating contraction and relaxation for forward movement.

• Segmentation:

  • Purpose: To mix food.

  • Mechanism: Contraction and relaxation of different parts of the GI tract, allowing for backwards and forwards movement.

PERITONEUM AND GI COVERINGS

PERITONEUM

• Not to be confused with the perineum.

• Most extensive abdominal membrane:

  • Visceral: On organs.

  • Parietal: On body wall.

  • Peritoneal Cavity: Fluid-filled space between visceral and parietal peritoneum; its main job is to make organs slippery, preventing friction and injury.

MESENTERY

• Double-layered peritoneum (back-to-back serous membranes).

• Functions:

  • Routes for blood vessels, lymph, nerves.

  • Holds organs in place.

  • Serves as fat storage (e.g., greater omentum, visceral fat).

RELATION TO THE PERITONEUM

• Organs are described based on their position relative to the peritoneum:

  • Retroperitoneal: Behind the peritoneum (e.g., kidneys, parts of small and large intestines).

  • Intraperitoneal: Within the peritoneum (e.g., stomach).

HISTOLOGY AND GI TRACT

LAYERS OF THE GI TRACT

• Layers from deep to superficial (innermost to outermost):

  1. Mucosa

  2. Submucosa

  3. Muscularis Externa

  4. Serosa

MUCOSA

• Innermost layer; moist epithelial tissue.

• Functions:

  • Secretes mucus, digestive enzymes, and hormones.

  • Absorbs digestive end products.

  • Protects against disease (macrophages).

• 3 Sub-layers:

  • Lining Epithelium: simple columnar (except mouth, anus, esophagus, which are stratified squamous).

  • Lamina Propria: Loose areolar CT with lymphoid function (MALT); contains tonsils and appendix.

  • Muscularis Mucosae: Smooth muscle enhancing absorption and secretion.

SUBMUCOSA

• Layer external to the mucosa.

• Contains areolar CT with rich blood, lymph, and nerve supply.

• Elastic fibers allow stretching and recoiling of GI organs.

MUSCULARIS EXTERNA

• Main functions are segmentation and peristalsis.

• Composition: Smooth muscle.

  • Inner layer: Circular.

  • Outer layer: Longitudinal.

  • Forms sphincters to prevent backflow.

SEROSA

• Outermost layer, also known as the visceral peritoneum.

• Composition includes:

  • Areolar connective tissue.

  • Simple squamous epithelium covering.

GI BLOOD AND NERVE SUPPLY

GI BLOOD SUPPLY

• Splanchnic Circulation: Refers to the blood supply to abdominal organs via branches of the abdominal aorta.

• Contributes to the hepatic portal system (liver blood supply).

• Celiac Trunk: Arterial supply to most GI organs.

• The gut receives ¼ of cardiac output, increasing after meals.

GI NERVE SUPPLY

• Enteric Nervous System: The gut's own system often referred to as its own brain.

• Consists of:

  • Submucosal Plexus: In the submucosal layer.

  • Myenteric Plexus: Between circular and longitudinal muscles in the muscularis externa.

REGULATING DIGESTION

BASIC IDEAS

• Digestion is triggered by mechanical and chemical stimuli via receptors in the GI organs.

• Smooth muscles and glands act to mix and propel food.

• Enteric neurons and hormones coordinate digestive activities, with both short-and long-term control mechanisms.

ANATOMY OF THE GI SYSTEM

ORAL CAVITY & ESOPHAGUS

MOUTH

• Composed of various structures, including the palatoglossal arch, tongue, and hard/soft palates.

TONGUE

• Taste: Anterior 2/3 (CN VII), Posterior 1/3 (CN IX).

• Movement: CN XII.

• Types of papillae vary in structure and function (Fungiform, Circumvallate, Foliate).

SALIVARY GLANDS

• Functions:

  • Clean mouth.

  • Dissolve food for taste.

  • Initiate chemical digestion with enzymes such as amylase and lingual lipase.

• Major/Extrinsic Glands: Outside oral cavity (e.g., Parotid, Submandibular, Sublingual).

• Minor/Intrinsic Glands: Found within the oral cavity.

COMPOSITION OF SALIVA

• Mostly water (pH = 6.75-7, slightly acidic).

• Contains solutes including sodium, potassium, chloride, amylase, lingual lipase, and immunoglobulins.

TEETH

• Types of teeth: Baby teeth (deciduous), incisors, canines, premolars, molars.

• Structure includes crown, enamel, dentin, root, pulp, and gums.

ESOPHAGUS

• Divided into three sections: 1/3 voluntary skeletal muscle, 1/3 partially voluntary, and 1/3 involuntary smooth muscle.

• Contains the esophageal hiatus and esophageal/cardiac sphincter.

STOMACH

STOMACH CHARACTERISTICS

• Contains rugae to help mash food.

• Pyloric sphincter regulates food entry into the small intestine.

• Highly innervated and contains lymphatic tissue for immune response.

• Acidity in the stomach helps kill pathogens.

SPECIAL CELLS IN THE STOMACH

• Gastric Glands: Produce gastric juice containing HCl and other enzymes.

• Mucous Neck Cells: Secrete acidic mucus.

• Parietal Cells: Secrete HCl and intrinsic factor.

• Chief Cells: Produce pepsinogen and lipases.

• Enteroendocrine Cells: Secrete various hormones (histamine, serotonin).

LIVER, GALLBLADDER & PANCREAS

LIVER

• Main functions include filtering blood and synthesizing key molecules.

• Highly vascularized, storing fat-soluble vitamins and secreting bile (900ml).

GALLBLADDER

• Serves as a bile storage pouch, can form stones, and is often removed if inflamed.

DIGESTIVE FUNCTION OF PANCREAS

• Produces powerful digestive enzymes (protease, lipase, amylase, nuclease) released in a controlled manner to prevent activation within the pancreas.

SMALL INTESTINES

• Length: 7-13 feet. Major site for chemical digestion and nutrient absorption.

• Sections:

  1. Duodenum: Attached to stomach, site for pancreatic and biliary secretion.

  2. Jejunum: Limited activity.

  3. Ileum: Connects to the large intestine.

SMALL INTESTINES MICRO ANATOMY

• Microvilli: Increase absorption and contain digestive enzymes, forming the brush border.

SMALL INTESTINE CELLS

• Enterocytes: Common cells for nutrient and enzyme absorption.

• Goblet Cells: Secrete mucus.

• Enteroendocrine Cells & Paneth Cells: Contribute to digestion and immune function.

LARGE INTESTINE

• Main job: Reabsorb water and form feces.

• No digestion occurs, and is divided into five major sections.

LARGE INTESTINE LANDMARKS

• Important anatomical features include right/left colic flexures, ascending and descending colon, haustra, and taenia coli.

LARGE INTESTINE AND MICROBIOME

• The microbiome synthesizes vitamin K and supports immune functions; antibiotics can disrupt this balance.

ANUS AND RECTUM

• Rectum: Storage zone for feces, triggers urge to void.

• Anus: Exit portal for fecal matter with both voluntary and involuntary control.

CHEMICAL BREAKDOWN AND ABSORPTION

CARBOHYDRATES

• Enzymatic actions include salivary amylase, pancreatic amylase, and brush border enzymes, with absorption occurring in the small intestine via sodium-ion cotransport.

PROTEINS

• Enzymes involved: Pepsin in stomach, pancreatic enzymes in small intestine; absorption primarily involves cotransport with sodium.

LIPIDS

• Emulsification occurs in the small intestine with the help of bile salts. Fatty acids and monoglycerides enter intestines via diffusion and are transported as chylomicrons.

NUCLEIC ACIDS

• Pancreatic ribonuclease and deoxyribonuclease help in digestion; units are absorbed via active transport.

GI TRACT DISORDERS

GI DISORDERS

• Mouth Disorders: Periodontal disease, canker sores, cold sores, bruxism.

• Esophageal Disorders: Barrett's Esophagus, achalasia, heartburn.

• Stomach Disorders: Gastritis, H. Pylori infections.

• Small Intestine Disorders: Celiac disease, IBS, Crohn's disease.

• Liver and Gallbladder Disorders: Hepatitis A, B, C (foodborne or via bodily fluids), cholecystitis.

• Large Intestine Disorders: IBS, Crohn's disease, colon cancer.