Digestive System Overview

Digestive System Overview

• The digestive system breaks down food into smaller molecules usable by body cells.
• Two main groups of organs: digestive canal (GI tract) and accessory digestive organs.

Digestive Canal (GI Tract)

• Includes: mouth, pharynx, esophagus, stomach, small intestine, and large intestine.

Accessory Digestive Organs

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

Learning Objectives

• Identify digestive system organs.
• Describe basic digestive processes.
• Describe digestive canal layers' structure and function.
• Describe the nerve supply of the digestive canal.
• Describe digestive system components.

Digestive System Functions

• Ingestion: taking food into the mouth.
• Secretion: releasing water, acid, buffers, and enzymes into the digestive canal.
• Mixing and propulsion: churning and movement of food through the canal.
• Digestion: mechanical and chemical breakdown of food.
• Absorption: passage of digested products into blood or lymph.
• Defecation: elimination of undigested food material.

Layers of the Digestive Canal

• Mucosa: innermost layer, moist epithelial membrane lining the alimentary canal.
  • Functions: secretes mucus, digestive enzymes, hormones, absorbs end products, protects against disease.
• Submucosa: below mucosa, containing blood and lymphatic vessels, and submucosal neural plexus (Meissner plexus).
• Muscular layer: responsible for movement.
  • Component differs based on location. (e.g., voluntary muscle in mouth, pharynx, superior/middle esophagus; smooth muscle in other areas with inner circular and outer longitudinal muscle layers and a myenteric neural plexus).
• Serosa/Adventitia: outermost layer. serosa (peritoneum) is the largest serous membrane, with a layer of simple squamous epithelium (mesothelium) underlying areolar connective tissue. Esophagus replaces with adventitia
• These layers are arranged from deep to superficial.

Peritoneum

• Divided into parietal and visceral layers.
• Parietal layer lines the abdominal wall, visceral layer covers organs.
• Peritoneal fluid is between layers to reduce friction.
• Peritonitis: inflammation of the peritoneum, often due to infection, injury, or surgery.

Blood Supply (Splanchnic Circulation)

• Branches off the aorta that supply digestive organs.
• Arterial supply includes the celiac trunk (liver, stomach, spleen), mesenteric arteries (small and large intestines).
• Digestive organs receive 25% of total cardiac output.
• Hepatic portal circulation collects nutrient-rich venous blood draining digestive organs to deliver to the liver.

Nerve Supply (Enteric Nervous System)

• Intrinsic nervous system (~100 million neurons) in the digestive canal.
• Controls digestive secretion and enfolding with submucosal plexus, located in the submucosa.
• Controls digestive canal motility using myenteric plexus, located between smooth muscle layers.
• Autonomic nervous system influences functions: parasympathetic increases secretion and motility, sympathetic decreases both.

The Mouth

• Formed by cheeks, lips, hard and soft palates, oral cavity, teeth, salivary glands, and tongue.
• Major functions of mouth include: chewing (mastication), forming a bolus, initiating swallowing.

Salivary Glands

• Accessory digestive organs releasing saliva into the oral cavity.
• Include parotid, submandibular, and sublingual glands.
• Minor glands are scattered throughout.
• Saliva contains water, electrolytes, and enzymes (amylase).
• Secretion is regulated by parasympathetic and sympathetic stimuli.
• Mumps is an inflammation or swelling of salivary glands.

Tongue

• Accessory digestive organ.
• Covered by a mucous membrane, consists of skeletal muscle.
• Functions include repositioning food during chewing, mixing with saliva, initiating swallowing, forming consonants for speech.

Pharynx

• Funnel-shaped tube extending from internal nares to esophagus posteriorly and larynx anteriorly.
• Composed of skeletal muscle and a mucous membrane.
• Divided into nasopharynx, oropharynx, and laryngopharynx.

Esophagus

• Muscular tube connecting pharynx to stomach.
• Bolus (food mass) passes through, guarded by upper and lower esophageal sphincters (UES and LES).
  • UES and LES relax when relaxed.
• Submucosa secretes mucus to lubricate bolus passage.

Stomach

• J-shaped enlargement.
• Converts bolus into chyme.
• Five regions: cardia, fundus, corpus, antrum, pylorus.
• Pylorus is separated by the pyloric valve from the duodenum.
• Gastric folds (rugae) present when stomach is empty.
• Functions: mixes food, serves as reservoir, secretes gastric juice (HCl, pepsin, intrinsic factor, gastric lipase), secretes gastrin into blood.

Stomach Mucosa

• Layer of simple columnar cells (surface mucous cells).
• Gastric glands extend into the lamina propria.
• Secretion from glands flow to gastric pits and then to the stomach lumen.
• Different cell types (surface mucous cells, mucous neck cells, parietal cells, chief cells, G cells) produce various secretions.

Gastric Mucosal Barrier

• Three factors create a mucosal barrier that prevents stomach self-digestion: a thick coat of bicarbonate-rich mucus, tight junctions in epithelial cells, rapid replacement of damaged epithelial cells by undifferentiated stem cells.
• Anything that breaches the mucosal barrier can lead to inflammation and peptic ulcers.

Small Intestine

• Divided into three sections: duodenum, jejunum, ileum.
• Unique structural modifications (circular folds, intestinal villi, microvilli).
  • Circular folds are in the mucosa and submucosa.
  • Intestinal villi and microvilli (brush borders) increase surface area.
• Duodenum: receives chyme from stomach, neutralizes acidity, mixes chyme with pancreatic and liver secretions.
• Jejunum: Primarily digestion and absorption.
• Ileum: Final segment of the small intestine, ends at ileocecal valve.
• Functions: completes digestion, absorbs nutrients (90%), and regulates motility (segmentation and migrating motor complexes (MMC) between meals).

Small Intestine Cell Types

• Absorptive cells, goblet cells, enteroendocrine cells, and Paneth cells.
  • All assist in digestion and absorption of material.

Large Intestine

• Extends from ileocecal valve to anus, contains cecum, appendix, colon, rectum, and anal canal.
  • Colon is divided into ascending, transverse, descending, and sigmoid sections.
• Functions:
  • Water absorption and reabsorption.
  • Bacteria synthesizes vitamins.
  • Feces formation and storage.
  • Defecation.
• Haustra and teniae coli are unique components located in the muscular and serosa layers that create a protective function and aid in motility.

Anal Canal

• Last portion of the large intestine, opens to the exterior.
• Guarded by internal (involuntary) and external (voluntary) anal sphincters.

Defecation Reflex

• Stretch receptors in rectum initiate spinal reflex via parasympathetic reflexes.
• This leads to relaxation of internal anal sphincter and contraction of sigmoid colon and rectum.
• Voluntary control of external sphincter allows for timing of defecation.

Pancreas

• C-shaped, retroperitoneal accessory digestive organ.
• Pancreatic juice neutralizes acidic chyme and contains enzymes (proteases, lipases, amylase, nucleases) that digest all types of food.
• Secretion is regulated by local hormones and parasympathetic nervous system.
• Empties contents into duodenum.
  • Pancreas also contains pancreatic acini and islets producing hormones such as glucagon, insulin, and somatostatin.

Liver and Gallbladder

• Liver produces bile (emulsifies fats)
• Gallbladder stores bile.
• Functions of both: carbohydrate, lipid, protein metabolism, drug processing, bilirubin excretion, bile salt synthesis, storage, phagocytosis and Vitamin D activation.

Liver Histology

• Hepatocytes form complex 3-D shapes called hepatic laminae.
• Surrounded by hepatic sinusoids (deliver blood to central vein then to hepatic veins).
• Bile secreted by hepatocytes, collected by bile canaliculi, and ultimately formed into bile ducts.
• Bile duct + hepatic artery + hepatic portal vein = portal triad.

Blood Supply of the Liver

• Oxygenated blood from hepatic artery.
• Nutrient-rich blood from hepatic portal vein
• Blood flows through hepatic sinusoids into central veins.
• Outflows through hepatic veins to inferior vena cava.

Hepatopancreatic Ampulla (Sphincter of Oddi)

• Bile ducts and pancreatic ducts merge to form hepatopancreatic ampulla at the duodenum.
• The hepatopancreatic sphincter (sphincter of Oddi) controls entry of bile and pancreatic juice into the duodenum.

This quiz covers the essential components and functions of the digestive system, including both the digestive canal and accessory organs. You'll learn about the structure, function, and processes involved in digestion, helping you understand how the body utilizes food.