3 Nutrition involves the following steps: IngestionDigestionAbsorptionDistributionDigestion: the breakdown of ingested food materials into molecules that can be delivered to and utilized by the individual cells.Can be both mechanical and chemical.human anatomy tutorials, labeled with explanations (all systems) link
4 The “Gut” (Gastrointestinal Tract): Fig.49-2 pg. 983 Made of 4 layers (see pictures on page 985)Mucosa: (innermost) -epithelial, connective, some smooth muscleSubmucosa: connective, nerve fibers, blood & lymph vesselsMuscularis Externa: muscle tissue (circular & longitudinal)Serosa: outer covering of connective tissue
5 Peristalsis: coordinated contractions of circular muscles along digestive tract which move food alongSphincters: thickened, heavy bands of muscle along tract that act as valves by contracting (closing) and relaxing (opening) to control of materials
6 The Oral Cavity: Initial Processing: Mechanical DigestionTongue - swallowing (microscopic picture)Teeth: “mastication” (chewing)incisors- cutting, clippingcanines- tearing, stabbingpremolars- "bicuspids" 2 cusps (points)molars- 4 or 5 cusps (for grinding)Other Animals lack teeth, have alternatives:rasping tongue: ex- snails, lamprey, frogsgrinding organ (gizzard): ex- birds, earthwormsChemical Digestion (enzymatic hydrolysis): saliva, mucous3 pairs of salivary glands and buccal glands in jaw: lubricates, softens foodEnzyme- salivary amylase for hydrolysis of starch into smaller sugar molecules; slightly alkaline (sodium bicarbonate)
7 The Pharynx & Esophagus: Swallowing Swallowing: passage of food to the esophagus ----> stomachbegins as voluntary action (striated muscles in upper part of esophagus) lower portion involuntary (smooth muscle)= "peristalsis" Esophagus opens into StomachAbdominal cavity lined with peritonium / organs are suspended by mesentaries(animation)
8 The Stomach: Storage & Liquefaction …a collapsible, muscular, j-shaped bag with many folds and ridgesmucosal lining folds into gastric pits (microscopic picture)lined with mucous-secreting epithelial cellsgastric glands secrete HCl and pepsinogen (precurser to pepsin)pH between 1.5 & 2.5 (mucous barrier between acids + stomach lining )- ulcers?
9 Stomach acids, enzymes HCl: kills bacteria loosens tissues for digestion converts pepsinogen to pepsinPepsin: breaks down proteins into smaller peptidesSemi-liquid mass "Chyme" moves to lower end of stomach => Pyloric Sphincter (takes about 4 hrs.)Rate of leaving stomach = H2O,carbs, proteins, fat, protein/fat comboThe three phases of gastric secretion animation
10 The Small Intestine: Digestion and absorption lined with VILLI ( microscopic fingerlike projections) and tiny microvilli on surface of epithelial cellsabout 21 feet long (adult), total surface area about 300 m2Digestion:Duodenum- 1st 25 cm (10 inches)most digestion takes place here;receives several digestive juices/enzymes from ducts leading from ACCESSORY GLANDS (liver pancreas, gall bladder)
11 Accessory Glands neutralize stomach acids with alkaline secretions produces several enzymes to chemically digest nutrients
12 The PancreasEnzymes made here enter the duodenum via the pancreatic ductAmylase: continues starch digestionStarchesdisaccharidesmonosaccharidesTrypsin: breaks down proteins amino acidsLipase: hydrolyzes fats
13 The Liver: an accessory gland Bile-produced by liverstored in gall bladdercontains salts (emulsify fats)sodium bicarbonate (neutralize pH to optimum 7 or 8)
14 Absorption of Nutrients: food is absorbed through epithelial cells of intestinal mucosa into the bloodstream, for distribution to all cellsActive Transport: Monosaccharides, Dipeptides, Amino AcidsDiffusion: fatty acidsLipids resynthesize into triglycerides, phospholipids (blood & lymph vessels), Cholesterol
15 The Large Intestine: Water Absorption & Elimination The Large Intestine (colon)continues absorption of water, sodium, and other minerals (which begins in small intestine)E. coli bacteria: makes Vit.K, amino acid synthesisAppendix: vestigial organ (gastric caecum?)Waste: "fecal matter"water, bacteria, dead cells, indigestible matter (cellulose)lubricated with mucustemporary storage in rectum
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