Anatomy of Digestive System

Digestive System Anatomy

• The digestive system consists of the digestive tract (alimentary canal) and accessory organs.
• The digestive tract begins at the oral cavity (mouth), passes through the pharynx (throat), esophagus, stomach, small intestine, and large intestine, and ends at the anus.

Major Organs of the Digestive Tract

• Oral Cavity (Mouth):
  • Mechanical processing of food with teeth and tongue
  • Salivary secretion and mixing with food
• Pharynx:
  • Muscular propulsion of food into the esophagus
• Esophagus:
  • Transport of food to the stomach (25-30 cm)
• Stomach:
  • Chemical breakdown of food by acid and enzymes
  • Mechanical processing through muscular contractions
• Small Intestine:
  • Enzymatic digestion and absorption of nutrients
• Large Intestine:
  • Dehydration and compaction of indigestible materials for elimination

Accessory Organs of the Digestive System

• Teeth:
  • Mechanical processing of food by chewing
• Tongue:
  • Assists mechanical processing with teeth
  • Sensory analysis of food
• Salivary Glands:
  • Secretion of lubricating fluid with enzymes that break down carbohydrates
• Liver:
  • Secretion of bile for lipid digestion
  • Storage of nutrients
  • Multiple vital functions
• Gallbladder:
  • Storage and concentration of bile
• Pancreas:
  • Exocrine cells secreting digestive enzymes
  • Endocrine cells secreting hormones

Digestive System Functions

• Ingestion: intake of food
• Digestion: breakdown of complex nutrients into simple molecules
• Absorption: uptake of nutrients into the bloodstream and lymphatic system
• Defecation: elimination of undigested material

Stages of Digestion

• Mechanical Digestion: physical breakdown of food into smaller particles
• Chemical Digestion: series of hydrolysis reactions breaking down nutrient macromolecules into monomers

Oral Cavity

• Bound by teeth, tongue, hard palate, and soft palate
• Structures work together to break down food into small masses for swallowing

Tongue

• Dorsal surface covered by keratinized stratified squamous epithelium with papillae
• Papillae contain taste buds
• Filiform papillae do not have taste buds

Esophagus

• Straight muscular tube from pharynx to stomach
• Non-keratinized stratified squamous epithelium in the mucosa
• Esophageal glands in the submucosa release mucus during swallowing

Peritoneum and Peritoneal Cavity

• Parietal peritoneum lines the deep surface of the abdominal walls and the inferior surface of the diaphragm
• Visceral peritoneum directly covers the abdominal organs
• Peritoneal cavity is a narrow, potential space between the opposing layers of the peritoneum

Stomach

• Expandable muscular sac with a capacity of 50 ml to 4,000 ml
• Regions of the stomach: cardiac region, fundus, body, and pyloric region
• Rugae increase surface area for secretions and allow for expansion

Small Intestine

• Duodenum: receives stomach contents, pancreatic juice, and bile
• Jejunum: upper 2.5 meters
• Ileum: lower 4.5 meters, ends at the ileocecal junction with the large intestine

Intestinal Motility

• Peristalsis: wave-like contractions that gradually move contents towards the colon
• Segmentation: random, ring-like constrictions that mix and churn contents for absorption and digestion

Liver

• Organ with four lobes located in the right upper quadrant of the abdomen
• Produces bile and plasma proteins, stores minerals, vitamins, and glycogen, and detoxifies the blood
• Can regenerate after disease or surgical removal of part

Gallbladder

• Stores and concentrates bile
• Receives bile from the liver through the cystic duct

Ducts of Gallbladder, Liver, and Pancreas

• Bile flows from the liver into the right and left hepatic ducts
• Common hepatic duct and cystic duct merge to form the common bile duct
• Pancreatic duct merges with the common bile duct at the hepatopancreatic ampulla
• Bile and pancreatic fluid enter the duodenum through the major duodenal papilla### Digestive System
• Bile:
  • Backs up into the gallbladder between meals when the hepatopancreatic sphincter is closed
  • Concentrated 20 times by a highly folded mucosa
  • Contains minerals, fats, bile pigments, and bile acids
• Bile pigments:
  • From the breakdown of hemoglobin from old RBCs destroyed by macrophages in the liver
  • Mainly bilirubin, which is green-yellow in color
  • Excreted as waste
• Bile acids (bile salts):
  • Emulsify fats and aid in their digestion
  • 80% recycled from the ileum through enterohepatic circulation
• Pancreas:
  • Retroperitoneal gland posterior to the stomach
  • Contains endocrine and exocrine tissue
  • Secrete insulin and glucagon into the blood
  • Secrete 1500 ml exocrine fluid into the duodenum per day, containing water, enzymes, and bicarbonate

Gross Anatomy of Large Intestine

• Also called colon
• 2 meters long
• Begins at the cecum in the lower right quadrant and leads to the ascending, transverse, descending, and sigmoid portions
• Sigmoid colon is S-shaped portion leading down to the rectum in the pelvis
• Rectum is straight portion ending at the anus
• Lining of the colon is smooth with only a few folds and no villi

Colon Absorption and Motility

• Transit time is 12 to 24 hours
• Reabsorbs water and electrolytes
• Feces consist of water and solids (bacteria, mucus, undigested fiber, fat, sloughed epithelial cells)
• Contractions occur about every 30 minutes
• Distension of the colon stimulates contraction
• Mass movements usually occur 1 to 3 times a day

Appendix

• Thin, closed tube attached to the cecum
• Submucosa contains numerous lymphoid follicles with abundant lymphocytes that play a role in immunity
• Receives small amounts of processed food and can respond to dangerous antigens
• Infection and inflammation of the mucosa can lead to pain, swelling, and possible rupture, which can cause peritonitis
• Contains a diverse population of intestinal bacteria, which can help repopulate the colon with a normal flora faster than individuals without an appendix

Anatomy of the Rectum and Anus

• Anal canal is about 3 cm in length
• Anal columns are longitudinal ridges separated by mucus-secreting anal sinuses
• Hemorrhoids are distended veins in the lamina propria