Kin 110 Chapter 3 Digestions and Absorption 1

Overview of Digestion and Absorption • Chapter 3 • Gastrointestinal tract • Overview of Digestion (details discussed with each nutrient lecture) – Mechanical – Chemical • Overview of Absorption (details discussed with each nutrient lecture) • Circulation of nutrients • Regulation and influences on Digestion and Absorption 2

The Gastrointestinal Tract • Organization – Mouth anus – Accessory organs • Salivary glands, liver, pancreas, gallbladder • Functions – – – Ingestion Transport Secretion Digestion Absorption Elimination 3

The Digestive Tract Wall

Mouth: • chews and mixes food with saliva Salivary Glands: • secrete enzymes & mucous Pharynx & Esophagus: • passages for food Liver: • manufactures bile (required for fat digestion) Gallbladder: • stores and releases bile Small Intestine: • primary site of digestion and absorption ES Pyloric Sphincter Stomach: • adds acid & enzymes • mixes, churns and grinds food • regulates entry of food into small intestine Pancreas: • secretes enzymes and buffers • secretes hormones Large Intestine: • reabsorbs water and minerals • contains bacteria which digest some food and synthesize some vitamins 5

Overview of Digestion • Digestion - the mechanical and chemical breakdown of foods into smaller units that can be taken across the intestinal epithelium into the body • Physical movement – Peristalsis • Waves of muscle contraction propelling food forward – Segmentation • Mixing contractions that do not propel food forward 6

Chemical Digestion: Enzymes Definition • proteins that act as CATALYSTS to facilitate (speed up) chemical reactions • change molecules while remaining unchanged themselves • digestive enzymes break down molecules into smaller components via HYDROLYSIS “hydro” = water ENZYME “lysis” = to burst or break apart 7

8

Chemical Digestion: Enzymes cont. Examples prefix indicates molecule the enzymes acts on amylase sucrase lactase maltase digest carbohydrates lipase digests lipids suffix “ase” indicates an enzyme pepsin trypsin peptidase digest proteins 9

Secretions of the Digestive Tract Salivary Secretions – water, mucous, enzymes, buffers, lysozyme Stomach (gastric) Secretions – hydrochloric acid, intrinsic factor, pepsinogen, gastric lipase, mucous, HCO 3 -, hormones (gastrin) Liver & Gallbladder – bile 10

Secretions cont. Pancreatic Secretions – exocrine: enzymes & buffers (HCO 3 -) – endocrine: insulin, glucagon, & secretin, others Small Intestine – exocrine: enzymes, mucous – endocrine & paracrine: CCK, motilin, others 11

Absorption - movement of small units of food particles (after digestion) from the interior of the gut (lumen) into the blood stream or lymphatic system. - Requires particles to be broken down by digestion, a large surface area to interact with particles and in some cases specialized transportation mechanisms to move particles across cell membranes 12

Large surface area facilitates absorption

Mechanisms of Absorption • passive (simple) diffusion – the unassisted movement of substances in or out of cells down a concentration gradient 14

Mechanisms of Absorption • facilitated diffusion – carrier proteins (transporters) in the cell membrane help move substances in or out of cells down a concentration gradient 15

Mechanisms of Absorption cont. • active transport – the movement of molecules into or out of cells against a concentration gradient using a transport protein – requires energy (ATP) 16

Mechanisms of Absorption cont. • endocytosis – the uptake of material by a cell through indentation and pinching off of part of the membrane to form a vesicle 17

Circulation of Nutrients • Vascular system • Lymphatic system 18

Transport of Nutrients • blood – direct absorption of glucose, fructose, galactose, & proteins, and small lipid molecules – travels directly to the liver • lymph – larger lipid molecules are too large to enter the blood directly – absorbed first into the lymph – enter the blood at the inferior vena cava 19

Transport of nutrients Some nutrients travel freely in the blood (e. g. glucose, proteins). Others are bound to protein transporters (e. g. lipids, vitamins, and minerals) 20

Regulation of GI Activity • Nervous system – Regulates GI activity • Local system of nerves • Central nervous system • Hormonal system – Increases or decreases GI activity 21

Regulation of GI Activity cont. Endocrine (Hormonal) System 22

Influences on Digestion & Absorption • Psychological – taste, smell, presentation of food – Cephalic phase (see next slide) • Chemical – processing of foods and food preparation, medications • Bacterial – some may cause gastris, ulcers; others are beneficial 23

Cephalic Phase Responses 24

Nutrition and GI Disorders • Constipation – Hard, dry, infrequent stools – Reduced by high fiber, fluid intake, exercise • Diarrhea – Loose, watery, frequent stools – Symptom of diseases/infections – Can cause dehydration • Diverticulosis – Pouches along colon – High fiber diet reduces formation 25