Ch.3

 Digestion: Breaking down foods into absorbable nutrients

• Nutrients are then broken down further for energy

• Digestion prepares our body for absorption

• Absorption: Uptake of these nutrients

• Nutrients are absorbed by cells of the

Small Intestine for transport into the blood or lymph

Challenges of Digestion

• Multiple tasks of the mouth

• breathing, eating & drinking: swallowing and breathing must not interfere with each other

• Diaphragm

• food must pass through the diaphragm

• Food needs consistent, steady, slow, forward movement

• Lubrication with fluids

• correct consistency is key – not too watery, not too dry. Needs to be regulated.

• Digestive enzyme function

• foods must be broken down into small particles and suspended in liquid so that each

particles is accessible by enzymes

• Digestive enzymes digest CHO, Fat, Protein

• digestive tract must be protected from digestive enzymes

• tract also made of CHO, fat, protein

• Excretion of waste

• must be periodic, not continuous© 2016 Cengage Learning. All Rights Reserved.

Anatomy of the Digestive Tract

Gastrointestinal (GI) tract

• Flexible and muscular tube

• Extends from:

• Mouth → esophagus → stomach →

small intestine (duodenum →jejunum →ileum)

→ large intestine → rectum → anus

• All parts are connected by the lumen

• Lumen: inner space of the GI tract

• Begins at mouth ends at anus

 Digestion begins in the mouth

• Chewing breaks up food = mastication

• Fluids from salivary glands blend with food and liquids to ease swallowing

• Tongue cannot taste unless food is in a solution

• Five basic taste sensations

• Sweet, sour, bitter, salty, and umami (savory… MSG)

• Aroma, appearance, texture, and temperature also affect taste perceptions

• Swallowed food passes through the pharynx as a bolus*

• Pharynx: short tube in throat - connects mouth to both the lungs and

esophagus

• Epiglottis: covers the airway (trachea) while food is being swallowed

*Bolus: a mouthful of food after it has been chewed and swallowed

 Esophagus

• 2 sphincter muscles: one at each end. They control movement

1. Upper esophageal sphincter/ UES (pharynx to esophagus)

• Opens during swallowing. Bolus enters. Passes through diaphragm.

2. Lower esophageal sphincter/LES (esophagus to stomach)

• Closes once bolus enters stomach. Stops it going back up to esophagus.

• Stomach

• Stomach churns bolus and adds digestive juices creating CHYME*

• Stomach releases chyme into small intestine through pyloric sphincter

• Pyloric sphincter: opens from stomach allowing chyme to enter small intestine

* Chyme: Semi-liquid mass of partly digested food expelled by the stomach into

the duodenum

The Small Intestine

Small Intestine (SI)

• 10 feet long

• 3 segments: Duodenum; Jejunum; Ileum

• Most absorption happens in the small intestine

• Chyme enters the SI and bypasses the common bile duct

• Common bile duct: drips fluids into small intestine from the gallbladder &

pancreas

• Chyme travels through the 3 segments

1. Duodenum

2. Jejunum

3. Ileum

Contents travel through the small intestine and arrive at the large

Intestine (colon)

• Contents enter large intestine via Ileocecal valve

• Sphincter at beginning of large intestine that connects ileum with large intestine

• Contents bypass the appendix

• Travels along…

• Ascending colon; Transverse colon; Descending colon to the rectum

• At the rectum, colon withdraws water to leave semi-solid waste

• Rectum – strong muscles – holds waste defecation

• rectal muscles relax and 2 sphincters of anus open to allow passage of waste

Gastrointestinal (GI) Motility: ability of the digestive tract to move

• Segmentation & Peristalsis

• GI tract - 2 layers of muscles coordinate to produce

• Segmentation and Peristalsis

• Segmentation:

• Inner circular muscles relax and contract

• Periodic squeezing action mixes and churns chyme

• Aids contact with digestive juices and intestine absorbing cells

• Peristalsis:

• Outer longitudinal muscles rhythmically contract

• Wavelike contractions move chyme forward

• Contractions happen at varying rates and intensity

• Altered by: Stress; drugs; illness

Stomach Action

• Stomach: thickest wall and the strongest muscles of all GI organs

• 3 muscle layers: circular; longitudinal; & diagonal

• Chyme is forced downwards and churned by stomach

• 4

• Pyloric Sphincter holds chyme in stomach until

fully liquefied & mixed with gastric juices

• Gastric juices released from stomach wall

mix with chyme

• Once chyme is completely liquefied with gastric

juices, pyloric sphincter opens and small

portions are released into the duodenum

• Chyme release is timed – about 3x per minute

Sphincter Contractions

• Sphincter muscles periodically open and close.

• Allow contents to move forward along GI tract at a controlled pace

• Sphincter muscles in action

• Upper Esophageal Sphincter: opens when swallowing

• Lower Esophageal Sphincter prevents reflux of stomach contents

• Pyloric Sphincter:

• Located at bottom of stomach

• Holds chyme in stomach long enough for it to be mixed with gastric juice & liquefied

• Also prevents intestinal contents from backing up into stomach

• Ileocecal Valve:

• Located at the end of the small intestine

• Allows contents of small intestine enter into the large intestine

• Rectal & Anal Sphincter Muscles : prevent continuous elimination

To breakdown food into nutrients we need secretions

from 5 different organs

1. Salivary glands

2. Stomach

3. Pancreas

4. Liver

5. Small intestine

Secretions enter GI tract at various points

• Deliver fluids and enzymes*

*Enzyme: protein that facilitates/catalyzes a chemical reaction. Can make, break, change, or exchange parts of a molecule while the enzyme remains unchanged.

A note on Enzymes:

• Enzymes are proteins that facilitates/catalyzes a chemical reaction.

• Can: make, break, change, or exchange parts of a molecule while the

enzyme remains unchanged.

• Digestion: enzymes facilitate hydrolysis - a chemical action that adds

water (hydro) to break (lysis) a molecule into smaller pieces.

• Words ending in ase denote enzymes

• Identified by the organ they come from

• Gastric lipase – stomach

• Pancreatic lipase –pancreas

Saliva moistens food for easy passage

• Secretes enzyme rich saliva into mouth to begin digestive process

• Contains: water, salts, mucus, and enzymes

Stomach: Gastric glands secrete Gastric juice

• Gastric juice is a mix of water, enzymes, and hydrochloric acid

• HCl, pH of 2

• HCL acts primarily in protein digestion

• Acid so strong that it can reflux of stomach contents into

esophagus causing heartburn

• Cells of stomach wall secrete mucous for protection

• Coats the cells protecting it from stomach acid,

enzymes and disease forming bacteria

By the time food leaves the stomach, digestion of all 3

macronutrients has begun

• Small intestine: Pancreatic juice released via ducts into duodenum

• Pancreatic juice has enzymes that act on all 3 macronutrients

• Amylase: amyl- starch/carbohydrate

• Protease: prot- protein

• Lipase: lip- lipids/ fat

• Pancreatic juice also contains sodium bicarbonate

• Alkaline: Neutralizes acidic chyme from stomach – why?

Bile

• Bile also flows into the duodenum

• Continuously produced by the liver then concentrated & stored in gallbladder

• Gallbladder squirts it into duodenum in when fat is present

• Bile is an Emulsifier:

• disperses fat in watery solutions so enzymes can access them

The Final Stage

• Large intestine

 Some fluids, salts and undigested material (fiber)

enter the large intestine

 Fiber brings in some fat, cholesterol, a few

minerals and water to form stool (waste matter)

• Intestinal bacteria ferment some fibers

 Produces gas, water, and small fragment of fat as

energy for the cells of the colon

 Colon reabsorbs water and some salts

• body recycles these salts

 Any excreted waste is of no use to the body

Nutrient Absorption

Within 3–4 hours of a meal, the body must find a way to

absorb the macronutrients and micronutrients

• Small Intestine: Most absorption occurs here

• Ten feet long

• Large surface area >> surface area increased by folds (to size of tennis court!!) -

villi and microvilli

• Absorption techniques- see next slide

• Simple diffusion

• Facilitated diffusion

• Active transport

Absorption happens at the lining of the Small Intestine

• Lining has hundreds of folds

• Each fold has thousands of fingerlike projections called villi

• Villi

• Select and regulate nutrient absorption

• Constant motion

• Microvilli (tiny hair-like projections)

• Provide absorptive surface that allow any nutrient that’s small enough to pass through

the body

• Enzymes and “pumps” recognize and act on different nutrients

• Specific enzymes and pumps fore each nutrient in upcoming chapters

• Crypts

• Found in the crevices between the villi

• Secrete intestinal juices into the small intestine

• Goblet cells

• Mucus secretion (for protection)

Once nutrients are absorbed through the villi, they go to one of the

body’s 2 circulatory systems - the vascular or lymphatic systems

1. Vascular System

• Water-soluble nutrients and smaller products of

fat digestion pass into the villa and go to the liver

first and then the blood stream

• Blood is mostly water

2. Lymphatic system

• Larger fats and fat-soluble vitamins are insoluble

in water

• Get assembled into chylomicrons for transport

• They pass into the villi and go to the lymph and

then bloodstream

• These systems deliver nutrients wherever needed

The Vascular System / Circulatory system

• Closed system of vessels

• Bloods flows continuously

• Heart serves as a pump

• Nutrient Transport

• Body tissues get nutrients and oxygen from blood

• Waste and carbon dioxide deposited back into blood

• Lungs exchange oxygen (delivers to all cells) and carbon dioxide (exhaling)

• Kidneys filter other waste from blood and excrete it as urine

Typical circulatory route

Heart – arteries – capillaries – veins – heart

-------------------

Routing of blood leaving digestive system

Heart – arteries – capillaries (in intestines) - hepatic

portal vein - capillaries (in liver) - hepatic vein – to heart

Liver is an accessory organ of the GI tract

• Receives almost all nutrients after absorption

• Detoxifies blood

• Plays a detox role

• Synthesizes HDL,VLDL cholesterol

One-way route from digestive tract

• No pump - lymph vessels collect fluid and

direct it towards the heart

• Moves in response to body movement

• Voluntary (skeletal muscle action)

• involuntary (smooth muscle action – breathing)

• Collects in thoracic duct behind heart

• Enters bloodstream via subclavian vein

• Bypass liver at first – then heart for circulation

• Cells remove the nutrients they need,

the liver then deals with the remnants

 Microbes in a healthy GI tract

• 100 trillion microbes – GI microbiota

• Good bacteria offer protection by “crowding-out” harmful bacteria

• Similar in people with same disease

• Differences in obese (less diverse) vs nonobese people

• Factors influencing bacteria presence

• Prebiotics: fibers not digested by the human body are food for bacteria

• Plant based eating

• Probiotics: live microbes

• yogurt, miso, kefir (fermented foods)

• Bacteria in GI tract produce vitamins

• Biotin, folate, pantothenic acid, riboflavin, thiamin, B12, B6, Vit. K

• not in sufficient quantities

• Thoughts on Antibiotics??

The body regulates digestion through Homeostasis

• Homeostatic regulation of digestion is governed by 2 systems:

1. Endocrine system (hormonal system)

2. Nervous System

• Hormones work via feedback mechanisms

• Feedback mechanisms:

1. A certain condition demands a response.

2. The response changes that condition, that change cuts off the response

3. Back to the beginning

• GI hormones include:

• Gastrin

• Secretin

• Cholecystokinin (CCK)

 Healthy Digestive System

 Adequate supply of blood

 Lifestyle factors

• Sleep, physical activity, stress

 Types of foods eaten

• Balance, moderation, adequacy, and variety

Common digestive problems

Choking

• More than 5000 die each year from choking

 Almost all are over 75yo or younger

 than 3yo

• Body's response to food blockage is to cough

 Usually clears

• True choking – person cannot cough or breathe

 No oxygen – permanent damage within 5 min / or die

• Recognize universal distress signal for choking

 Ask can you speak?

 If yes, and they are breathing coughing – do not interfere

 If no – take action, abdominal thrusts/Heimlich Maneuver

 Get help© 2016 Cengage Learning. All Rights Reserved.

Common Digestive Problems

Vomiting

• An adaptive body mechanism that can often result in dehydration

• Usually from stomach. Sometimes from the duodenum, if green bile present

• Repeated vomiting – erosion of teeth & gums, and other issues

• Diarrhea

• Categorized by frequent, loose, watery stools

• Intestinal contents move too quickly to allow water resorption

• common symptom with IBS, colitis, and celiac disease, infection, medication

• Treatment begins with rehydration and then remove cause, if possible

• 2000+ child deaths worldwide. Hand washing might reduce incidence

Constipation

• Symptoms

 Hard stool, abdominal discomfort, back aches, headaches, gas

 Infrequent or difficult bowel movement relative to the individual’s baseline

• Causes

 Lifestyle: diet, hydration, sedentary, ignoring signal to move bowels

 Side effect of medications

 Certain medical conditions

• Prevention

 Fiber: Increase to 25 – 28grams daily. Increases bulk

 Water: Lack of H2O >> colon absorbs more water >> constipation

 Physical activity: promotes GI motility

Constipation/vomiting/diarrhea: symptoms not diseases

Belching and Gas

• Belching caused by swallowed air

• Eating food fast; ill-fitting dentures; chewing gum; carbonated beverages

• Solutions: eat slowly; chew thoroughly; relax when eating

• Intestinal gas (flatulence) caused by consumption of

certain foods

• Can be embarrassing, but is normal

• Bloating is different

• Gas causing foods individually determined

• Elimination trial

• Most often sugars, starches, and fibers

• Legumes, broccoli, cabbage, onions

• Partially digested fibers reach large intestine

• Digested by bacteria

• Gas is a byproduct

 Irritable bowel syndrome (IBS)

• Frequent/severe abdominal discomfort

• Most common GI disorder

• GI contractions are stronger and last longer

• force intestinal contents through quicker

• Gas/bloating/diarrhea

• Cause: unknown. Triggers: Stress and certain foods

• Treatment: Small meals/peppermint oil/drugs/gluten free/FODMAP/probiotics etc.

• Colitis

• Inflammation of the large intestine

• May cause severe diarrhea; abdominal pain; and inflammation

• Some relief with restrictive diet; May have to resect the bowl/remove colon

• Celiac disease / Nonceliac Gluten Sensitivity

• Autoimmune condition / Intolerence

• inflammation of SI in response to gluten (protein found in wheat, barley and rye)

Peptic Ulcers

• Gastric ulcers / Duodenal ulcers

• Compromised lining of stomach or duodenum – exposed to gastric juices

• Leads to internal bleeding (iron deficiency) – 250,000 hospital cases yearly

• Causes

• Infection: Helicobacter pylori>> most common cause

• Anti-inflammatory drugs: NSAIDs

• Excessive gastric acid secretion (rare disorder)

• Treatment

• Antibiotics

• Avoid drugs (NSAID)

• Eliminate foods that cause pain

• Eliminate coffee, caffeine & alcohol beverages

Gastroesophagal Reflux aka Heartburn

 Painful sensation behind the breastbone

• lower esophageal sphincter allows stomach contents into the esophagus

 Causes: Eating or drinking too much; large fat-rich meals; excess carbonation,

tight clothing, lying down after eating; bending over; excess weight; medications;

smoking

• Indigestion

 Large, improperly chewed meals

• that upset normal peristalsis

 May taste stomach acid

 Antacids – OTC

 Persistent indigestion

• Damage to esophagus – acid

• Barrett’s esophagus

• Esophageal cancer risk