CHAPTER 25 Section 25.1—General Anatomy and Digestive Process Digestive system —organ system that processes food, extracts nutrients from it, and eliminates the residue o 5 stages (1) Ingestion —selective intake of food (2) Digestion —mechanical and chemical breakdown of food into a form usable by the body (3) Absorption —uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood (4) Compaction —absorbing water and consolidating the indigestible residue into feces (5) Defection —elimination of feces o Digestion has two facets Mechanical digestion —physical breakdown of food into smaller particles Cutting and grinding action of teeth Churning contractions of the stomach Exposes more food surface to the action of digestive enzymes Chemical digestion —series of hydrolysis reactions that break dietary macromolecules into their monomers (polysaccharides into monosaccharides; proteins into amino acids; fats into monoglycerides and fatty acids; nucleic acids into nucleotides) Carried out by digestive enzymes produced by salivary glands, stomach, pancreas, and small intestine Some nutrients are already present in usable form in ingested food and can be absorbed without being digested—vitamins, free amino acids, minerals, cholesterol, and water General Anatomy Digestive system—two subdivisions o Digestive tract (alimentary canal) —muscular tube extending from mouth to anus Stomach and intestines—gastrointestinal (GI) tract Most of material in it has not entered any body tissues—considered external to the body until absorbed Tissue layers (inner to outer surface) Mucosa (mucous membrane)— abundant in lymphocytes and lymphatic nodules o Epithelium Simple columnar in most of tract Stratified squamous from oral cavity through the esophagus, and lower anal canal (more scraping) o Lamina propria —loose connective tissue layer o Muscularis mucosae —smooth muscle
Tenses the mucosa—creates grooves and ridges that enhance its surface area and contact w/ food Improves efficiency of digestion and nutrient absorption Submucosa —thicker layer of loose connective tissue o Contains blood vessels and lymphatics, nerve plexus, and glands (in some places) that secrete lubricating mucus into the lumen Muscularis externa —two layers of muscle; responsible for motility that propels food and residue through digestive tract o Inner circular layer Thickened to form valves (sphincters) that regulate the passage of material through tract o Outer longitudinal layer Serosa —begins in lower esophagus and ends before rectum; the rest has no serosa but has adventitia (fibrous connective tissue layer) o Areolar tissue o Mesothelium o Accessory organs —teeth, tongue, salivary glands, liver, gallbladder, and pancreas Enteric nervous system —nervous network in esophagus, stomach, and intestines o Regulates digestive tract motility, secretion, and blood flow o 100 million neurons (more than spinal cord) o
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- Fall '19
- digestive tract, muscularis externa, Pyloric Glands