CHAPTER 3

Digestion, Absorption, and Transport

Digestion

Breaking down foods into nutrients Prepare for absorption

Challenges of digestion Tasks of the mouth Diaphragm Steady movement Lubrication of food Digestive enzyme functions

Anatomy of the Digestive Tract

• Gastrointestinal (GI) tract Flexible and muscular Path

Mouth → esophagus → stomach → small intestine → large intestine → rectum → anus

Lumen Inner space within GI tract, continuous from one end to the other

Anatomy of the Digestive Tract

Mouth **Process of digestion begins**

Mastication-process of chewing Fluids help dissolve food so tongue can taste it Four basic taste sensations (sweet, sour, bitter, & salty)

Aroma, texture, and temperature Pharynx- passed through when food is swallowed

Short tube that’s shared by both digestive system and respiratory system

Epiglottis closes to bypass entrance to lungs to prevent choking

Bolus After a mouthful of food has been chewed and swallowed

Anatomy of the Digestive Tract

Esophagus Two sphincters muscles at each end

Upper esophageal sphincter-opens during a swallow Bolus slides down the esophagus which passes through a hole in

diaphragm to stomach Lower esophageal sphincter-at entrance to stomach closes behind

the bolus so that it proceeds forward and doesn’t slip back up into esophagus

Stomach Retains bolus in upper portion Movement of bolus

Little by little food is transferred to lower portion, adds juices and grins it to a semi-liquid called Chyme

Pyloric sphincter Stomach releases chyme through which opens into the small

intestine (SI) then closes

Anatomy of the Digestive Tract

Small intestine- Common bile duct-at beginning of SI chyme bypasses

which is dripping fluids complements of Pancreas and gall bladder

Chyme travels down SI thru three segments: Duodenum, jejunum, and ileum (10feet of tubing coiled

within abdomen) Large intestine (colon)

remaining contents arrive at Ileocecal valve (beginning of LI); lower right side of ab

Withdrawal of water Leaving semisolid waste held till time to defecate

Rectal muscles relax and two sphincters of anus open and allows passage of waste

Muscular Action of Digestion

Peristalsis Circular and longitudinal muscles working together Wavelike muscular contractions of the GI tract that pushes its

contents along Rate and intensity of contractions vary Factors that may interfere with peristalsis

Stress, medicines, medical conditions Stomach action

Circular, longitudinal, and diagonal muscles Three sets of muscles work to force chyme downward from passing

duodenum of SI Timing the release of chyme

Chyme is churned and forced down, hits pyloric sphincter, and remains in stomach

Stomach releases gastric juices Chyme is liquefied, no longer resembles food

Segmentation Contraction of circular muscles in small intestine

Mix chyme Promote contact with absorption cells

Secretions of Digestion

Breakdown of food into nutrients requires secretions from five different organs Salivary glands, stomach, pancreas, liver, small intestine

Secretions enter GI tract at various points along the way, bringing abundance of Water Enzymes (Ch.6)

Protein facilitator of chemical reactions making, breaking apart, changing arrangement or exchanging parts of molecule

As a catalyst enzyme remains unchanged Hydrolysis-a chemical rxn involving the addition of water to

break down a molecule into smaller parts

Secretions of Digestion

Saliva Moisten food for easy passage Protective role

Teeth and lining of mouth, esophagus, and stomach from damage Contains water, salts, mucus, and enzymes that initiate CHO

digestionIn stomach, gastric glands secrete gastric juice

Mixture of water, enzymes, and hydrochloric acid (HCL) Acts primarily in protein digestion Heartburn seen with reflux Prevents bacterial growth *& kills most bacteria

Mucus To protect stomach from HCL Thick, slippery, white substance that coats cells

pH units Measurement determining strength of acid Gastric pH = below 2

Secretions of Digestion

By time food leaves stomach, digestion of CHO, fat, pro has begun & action gains momentum in SI

There pancreas contributes digestive juicesThe pancreatic juice

Released via ducts into duodenum Enzymes act on all three energy nutrients Sodium bicarbonate

Basic/alkaline (opposite of stomach acid) Neutralizes acidic chyme

Bile flows into duodenum Liver-continuously produces bile Gallbladder-squirts bile as fat arrives in duodenum Not an enzyme, emulsifier-brings fats into suspension in

water so that enzymes can break down

Secretions of Digestion

The Final Stage

At this point, CHO, fat, pro have been digested and ready to be absorbed

Vitamin and mineral absorptionUndigested residues (some fibers)

Not absorbed & continue thru the digestive tract, carrying some minerals, bile acids, additives, and contaminates out of body.

Retention of water seen in fiber is responsible for consistency of stools

Colon Intestinal bacteria

Ferment some fibers, producing water, gas, and small fragments of fat that provide E for cells of colon

Recyclable materials colon itself retrieves Water and dissolved salts

Absorption

Small intestine Traps and absorbs majority of nutrient molecules Ten feet long Absorption techniques

Simple diffusion Facilitated diffusion Active transport

Absorption

Anatomy of the Absorptive System

Folds in SI contain Villi Small fingerlike projections Select and regulate nutrients absorbed

Microvilli- Tiny, hair-like projections on each cell of every villus

that can trap nutrient particles and transport them into cells

Crypts Tubular glands that lie between the intestinal villi and

secrete intestinal juices into the SIGoblet cells

Cells of GI tract (and lungs) that secrete mucus

Preparing Nutrients for Transport

Nutrient molecule crossed cell of villus, enters bloodstream or lymphatic system Bloodstream

Water-soluble nutrients and smaller products of fat digestion

Guided to the liver where fate and destination will be determined

Lymphatic system Larger fats and fat-soluble vitamins are insoluble in water Larger molecules cluster together with special proteins

forming chylomicrons Bypass liver at first bc chylomicrons cannot pass into

capillaries

DELIVERS A NUTRIENT ONCE IT HAS ENTERED THE BLOODSTREAM

Circulatory System

The Vascular System

Or blood circulatory, closed system of vessels Blood flows continuously Heart serving as a pump

Blood Delivers oxygen and nutrients Removes carbon dioxide and wastes Blood flow

Special routing for digestive system Liver

The Lymphatic System

One-way route for fluid from the tissue spaces to enter the blood No pump; lymph circulates between the cells of the body

and collects into tiny vessels Lymph-clear, yellowish fluid similar to blood, except it

contains no red blood cells or platelets; transport fat & fat-soluble vit to bloodstream

Fluid moves from one portion of body to another as muscles contract and create pressure here and there

Much of lymph collects in thoracic ductThoracic duct opens subclavian vein which allows

entry into bloodstreamNutrients in lymphatic vessels

Bypass liver at first

Health and Regulation of GI Tract

GI tract health depends on adequate nutritionGastrointestinal bacteria – flora

Most are not harmful, but actually beneficial Factors influencing bacteria presence

Diet – probiotics (pro=for; bios=life): living microorganism found in foods and dietary supplements that when consumed in sufficient quantities are beneficial to health

GI health benefits include alleviate diarrhea, constipation, inflammatory bowel disease, ulcers, enhance immune function, etc

Digestion of fibers and complex proteins Vitamin production

Health and Regulation of GI Tract

Homeostasis Survival depends on body conditions staying about the

same If something causes conditions to change too much, body

must “do something” to bring back to normalHomeostatic regulation

Body’s regulation of digestion is example2 intricate and sensitive systems coordinate all

the digestive and absorptive processes: Hormonal or endocrine system Nervous system

As food travels through the GI tract, it either stimulates or inhibits digestive secretions by way of messages that are carried from one section of GI tract to another by hormones and nerve pathways

The Digestive 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

Sleep, physical activity, state of mind Types of foods eaten

Balance, moderation, adequacy, and variety