The Digestive System

Two Divisions Alimentary Canal (or gastrointestinal (GI) tract) Where food passes from mouth to anus Organs: mouth, pharynx, esophagus, stomach, small intestine, large intestine Accessory Digestive Organs Contribute to digestion and absorption Teeth, tongue, salivary glands, liver, gallbladder, pancreas

Layers of the GI Tract Same four layers from esophagus to anus

Layers of the GI Tract Mucosa – innermost lining of tract Secretion – mucus, enzymes, hormones Protection – disease Absorption – nutrients, ions, water, vitamins Plica – folds that increase surface area

Layers of the GI Tract Submucosa – dense connective tissue Binding – mucosa to muscularis Contains blood vessels, lymph vessels, lymph nodes, nerves

Layers of the GI Tract Muscularis externa – smooth muscle Contains circular and longitudinal fibers Serosa - outermost layer AKA visceral peritoneum

Functions of the Digestive System Ingestion – “eating” Propulsion – moving food through GI Deglutition (swallowing) – voluntary Peristalsis – wave-like contractions that move food – involuntary Mass movements – large contractions in large intestine

Functions of the Digestive System Mechanical digestion – food breakdown chewing & mixing by tongue & teeth (mastication) churning in the stomach Segmentation – rhythmic contractions of small intestine that mixes food

Functions of the Digestive System Chemical digestion – food breaks down into chemical building blocks Uses enzymes Mouth through small intestine Hydrolysis reactions Carbohydrates monosaccharides Protein amino acids DNA nucleotides

Functions of the Digestive System Absorption - movement of digested end products from lumen of GI into blood or lymph Defecation – elimination of wastes

The Mouth, Pharynx, & Esophagus Salivary glands produce salivary amylase to start break down of complex carbohydrates. Mastication – why? Bolus formed in palate Deglutition Epiglottis protects trachea as food passes through pharynx Peristalsis moves food through esophagus to stomach

The Stomach Cardioesophageal sphincter controls entrance Rugae expand to accommodate food Enteroendocrine cells produce the hormone gastrin to stimulate release of gastric juice Parietal cells release hydrochloric acid to activate enzymes and kill bacteria Mucous neck cells secrete mucus to protect the stomach wall from acid

The Stomach Chief cells produce pepsinogen which is activated by acid to form pepsin (digests protein). Processed food (chyme) enters the small intestine through the pyloric sphincter

The Small Intestine Three Parts: Duodenum – 5% of length, but most activity is here, C-shaped Jejunum – almost 40%, coiled Ileum – almost 60%, coiled Suspended from abdominal wall by mesenteries

Duodenum Trypsin produced by pancreas furthers protein digestion Pancreatic amylase continues carbohydrate breakdown Brush border enzymes of small intestine finish carb and protein digestion Bile (produced by liver & stored in gall bladder) emulsifies fats for absorption Pancreatic enzymes & bile production is controlled by hormones secretin and CCK, produced by small intestine when chyme is present

Jejunum & Ileum Villi & microvilli (brush border) increase surface area of small intestine Fat is absorbed by diffusion Other molecules are absorbed through active transport

Large Intestine Remaining water is absorbed Residue is further digested by resident bacteria, who produce vitamins (K & some B) and gas (more with carb- rich food) Goblet cells produce mucus to ease passage of solid waste

Rectum & Anus Mass movements push feces past internal anal sphincter (involuntary) Relaxation of external anal sphincter is voluntary and results in defecation

Leptin & Ghrelin