Two Types of Systems Incomplete digestive system –One-way, saclike digestive cavity Complete digestive system –Tube with opening at each end

5 Tasks of Digestive Systems 1. Mechanical processing - break up, mix, and move food material 2. Secretion - of enzymes into tube where digestion occurs 3. Digestion - break down of food particles into smaller molecules

5 Tasks of Digestive Systems 4. Absorption of nutrients and fluids 5. Elimination of wastes and residues

Human Digestive System Complete system with many specialized accessory glands and organs About 10 meters long Lined with mucus-secreting epithelium Movement is one way, from mouth to anus

Major Components Mouth (oral cavity) Pharynx (throat) Esophagus Gut –Stomach –Small intestine –Large intestine –Rectum –Anus

Accessory Organs Salivary glands –Secrete saliva Liver –Secretes bile Gallbladder –Stores and concentrates bile Pancreas –Secretes digestive enzymes

Saliva Produced by salivary glands at back of mouth and under tongue Saliva includes –Salivary amylase (enzyme) –Bicarbonate (buffer) –Mucins (bind food into bolus) –Water

Structure of Stomach J-shaped organ lies below the diaphragm Sphincters at both ends Outer serosa covers smooth muscle layers Inner layer of glandular epithelium faces lumen mucosa sphincters muscle serosa

Stomach Secretions Secreted into lumen (gastric fluid) –Hydrochloric acid (HCl) –Mucus (protective) –Protein-digesting enzymes Stomach cells also secrete the hormone gastrin into the bloodstream

Small Intestine Longest segment of digestive tract Receives chyme from stomach Receives secretions from liver, gallbladder, and pancreas Digests food into absorbable subunits

Walls of Small Intestine Projections into the intestinal lumen increase the surface area available for absorption one villus

mucosa (inner lining) Fig. 24-6a, p.407

microvilli at the free surface of an absorptive cell (brush border cell) cytoplasm mucus secretion hormone secretion lysozyme secretion nutrient absorption Fig. 24-6b2, p.407

Nutrient Absorption Passage of molecules into internal environment Occurs mainly in small intestine Various methods of absorption –Osmosis, transport proteins, diffusion

Large Intestine (Colon) Concentrates and stores feces Actively transports sodium ions out of lumen; water follows Resident bacteria produce vitamins Large and small intestines reclaim 90% of water that enters alimentary

Dietary Recommendations Whole foods help prevent chronic disease

Vitamins and Minerals Vitamins –Essential organic substances Minerals –Essential inorganic substances