Digestion, Absorption, and Transport Chapter 3

Chapter 3 Objectives Explain how foods move through the digestive system, describing the actions of the organs, muscles, and digestive secretions along the way. List the segments of the digestive tract in order from the mouth to the colon. Explain the mechanical processes of digestion in order of occurrence in the digestive tract. List the five organs and their secretions which assist in the breakdown of food. List the secretions that break down carbohydrates. List the secretions that break down protein. List the secretions that break down fat. Describe the anatomical details of the intestinal cells that facilitate nutrient absorption. Explain how nutrients are routed in the circulatory systems from the GI tract into the body and identify which nutrients enter the blood directly and which must first enter the lymph. Describe how bacteria, hormones, and nerves influence the health and activities of the GI tract. Identify the hormones involved in digestive and absorptive processes. Outline strategies to prevent or alleviate common GI problems. Apply the concepts presented in the chapter to explain common digestive tract problems.

Digestion Breaking down foods into nutrients Challenges of digestion Prepare for absorption Challenges of digestion Tasks of the mouth (eat, breathe, drink) Diaphragm (blockade) Steady movement (pacing) Lubrication of food (fluidity) Digestive enzyme functions (surface area) Excretion of waste (periodic)

Anatomy of the Digestive Tract Gastrointestinal (GI) tract Flexible and muscular tube Path Mouth → esophagus → stomach → small intestine → large intestine → rectum → anus Lumen = inner space within the GI tract) Continuous Not “inside” the body

The Gastrointestinal Tract Fig 3-1 (pg 71)

Anatomy of the Digestive Tract Mouth Process of digestion begins Mastication Four basic taste sensations Sweet, sour, bitter, salty (savory/umami) Aroma (75-95%), texture, and temperature Must be in solution Pharynx – digestion and respiratory tube Bolus – swallowed food

Anatomy of the Digestive Tract Esophagus Two sphincters (each end) Upper esophageal sphincter Lower esophageal sphincter Stomach Movement of bolus Chyme – semiliquid mass Pyloric sphincter Sphincter is circular muscle surrounding, and able to close, a body opening. Sphincters are found at specific points along the GI tract and regulate the flower of food particles.

Anatomy of the Digestive Tract Small intestine Common bile duct Pancreas and gall bladder Three segments (duodenum, jejunum, ileum) Large intestine (colon) Ileocecal valve Withdrawal of water Rectum and anus

Opening from small intestine to large intestine Descending colon Transverse colon Ascending colon End of small intestine Opening from small intestine to large intestine Figure 3-2 The Colon The colon begins with the ascending colon rising upward toward the liver. It becomes the transverse colon as it turns and crosses the body toward the spleen. The descending colon turns downward and becomes the sigmoid colon, which extends to the rectum. Along the way, the colon mixes the intestinal contents, absorbs water and salts, and forms stools. Descending colon Appendix Rectum Sigmoid colon Anus Stepped Art Figure 3-2 p72

Muscular Action of Digestion Peristalsis Circular and longitudinal muscles working together Rate and intensity of contractions vary Stress, meds, conditions may interfere with peristalsis Stomach action Strongest, thickest muscles – circular, longitudinal, and diagonal muscles Timing the release of chyme (3x/min)

Stomach Muscles

Muscular Action of Digestion Segmentation Contraction of circular muscles in small intestine Mix chyme Promote contact with absorption cells Sphincter contractions Periodically open and close Control pace of GI tract contents

An Example of a Sphincter Muscle

Secretions of Digestion Five organs Salivary glands Stomach Pancreas Liver (gall bladder) Small intestine Secretions Water Enzymes

Secretions of Digestion Enzymes Protein that facilitates a chemical reaction A catalyst – remains unchanged Facilitate hydrolysis – addition of water to breakdown molecules Look for –ase in the name

Secretions of Digestion Salivary glands = Saliva Moisten food for easy passage Protective role Carbohydrate digestion Stomach = Gastric juice HCl, along with water and enzymes Protein digestion Mucus to protect pH units

The Salivary Glands

The pH Scale

Secretions of Digestion Pancreas = Pancreatic juice Released via ducts into duodenum Enzymes act on all three energy nutrients Sodium bicarbonate Small Intestine = Intestinal enzymes Liver/Gallbladder = Bile Liver produces Gallbladder excretes Bile = emulsifier, not enzyme Emulsifier – brings fats into suspension in water so that enzymes can break them down into their component parts

Summary of Digestive Secretions & Their Major Actions

The Final Stage Undigested residues Colon Fiber carries some minerals, bile acids, additives, contaminants out of the body Exercise intestinal muscles Fiber retains water  stool consistency Colon Intestinal bacteria ferment some fibers  gas Recyclable materials retrieved Water and dissolved salts

Absorption Small intestine Majority of absorption Ten feet long Surface area – villi, microvilli Absorption techniques Simple diffusion – with gradient Facilitated diffusion – carrier cell Active transport – against gradient

FACILITATED DIFFUSION Carrier loads nutrient on outside of cell . . . Carrier loads nutrient on outside of cell . . . Outside cell Energy Cell membrane . . . and then releases it on inside of cell. . . . and then releases it on inside of cell. Inside cell Some nutrients (such as water and small lipids) are absorbed by simple diffusion. They cross into intestinal cells freely. SIMPLE DIFFUSION FACILITATED DIFFUSION Some nutrients (such as the water-soluble vitamins) are absorbed by facilitated diffusion. They need a specific carrier to transport them from one side of the cell membrane to the other. (Alternatively, facilitated diffusion may occur when the carrier changes the cell membrane in such a way that the nutrients can pass through.) ACTIVE TRANSPORT Some nutrients (such as glucose and amino acids) must be absorbed actively. These nutrients move against a concentration gradient, which requires energy. Figure 3-8 A bsorption of Nutrients Absorption of nutrients into intestinal cells typically occurs by simple diffusion, facilitated diffusion, or active transport. Occasionally, a large molecule is absorbed by endocytosis—a process in which the cell membrane engulfs the molecule, forming a sac that separates from the membrane and moves into the cell. Stepped Art Figure 3-8 p77

Anatomy of the Absorptive System Villi Select and regulate nutrients absorbed Microvilli Enzymes and “pumps” Crypts In the “valleys” Secretion of intestinal juices Goblet cells Mucus secretion

A Closer Look at the Intestinal Cells Villi cells Regulate nutrient absorption based on needs Microvilli Enzymes and ‘pumps’ Specialized cells Duodenum not the same as ileum Food combining Enhanced use in the body Ex: vit C and iron

A Closer Look at the Intestinal Cells Once in villus, divergent transport pathways Bloodstream Water-soluble nutrients and smaller products of fat digestion Liver Lymphatic system Larger fats and fat-soluble vitamins Chylomicrons (ch 5) Bypass liver at first

The Vascular System Closed system of vessels Blood Heart pump Delivers oxygen and nutrients Removes carbon dioxide and wastes Blood flow Special routing for digestive system Liver

The Vascular System

The Liver

The Lymphatic System One-way route Entry into bloodstream No pump Circulation between cells Entry into bloodstream Subclavian vein (near heart) Nutrients in lymphatic vessels Bypass liver at first

Health and Regulation of the GI Tract Gastrointestinal bacteria – flora Most are not harmful Benefits of gastrointestinal bacteria Factors influencing bacteria presence Diet – probiotics Prebiotics Digestion of fibers and complex proteins Vitamin production

Gastrointestinal Hormones and Nerve Pathways Homeostatic regulation Endocrine system Nervous system Feedback mechanisms Examples GI hormones Gastrin  stimulates HCl secretion Secretin  stimulates bicarb secretion Cholecystokinin (CCK)  stimulates bile secretion

An Example of a Negative Feedback Loop

The Primary actions of Selected GI Hormones

The System at Its Best Sensitive and responsive to environment Immunity against intestinal diseases Defense against foreign invaders Health of digestive system Healthy supply of blood Lifestyle factors Types of foods eaten Balance, moderation, adequacy, and variety

Common Digestive Problems Highlight 3

Choking Food slips into trachea International sign for choking Cuts off breathing International sign for choking Heimlich maneuver Foods commonly associated with choking Prevention of choking

Normal Swallowing and Choking

First Aid for Choking

Vomiting Vomiting Adaptive mechanism of the body Medical treatment Dehydration Self induced

Diarrhea Diarrhea Symptom of medical conditions & treatments Treatment Irritable bowel syndrome (IBS) Colitis Treatment

Irritable Bowel Syndrome and Colitis Characterization Cause Unknown Triggers Colitis Inflammation of large intestine

Celiac Disease Autoimmune disease Gluten-containing foods Symptoms Inflammation of small intestine Gluten-containing foods Symptoms Treatment Rehydration

Constipation Not a disease Symptoms of constipation Causes Prevention Lifestyle Side effect of medications Prevention Fiber and water Physical activity

Constipation Hemorrhoids Diverticulosis Laxatives Harmful practices Colonic irrigation

Belching and Gas Belching Intestinal gas Swallowed air Prevention Consumption of certain foods Individually determined

Heartburn and “Acid Indigestion” Gastroesophageal reflux (heartburn) Causes Prevention Indigestion Antacids and acid controllers Damage to esophagus Barrett’s esophagus

Gastroesophageal Reflux

Ulcers Peptic ulcers Causes Ulcer treatment regimen Gastric ulcers Duodenal ulcers Causes Bacterial infection Anti-inflammatory drugs Excessive gastric acid secretion Ulcer treatment regimen

Strategies to Prevent or Alleviate Common GI Problems