Topic 2.4: The Digestive System

the fate of an apple

* time in the mouth → %%<1 minute%%
* esophagus → %%\~1-2 hours%%
* stomach → small intestine, \~ %%7-8 hours%%
* whatever that hasn’t been absorbed → %%large intestine%%
* large intestine → %%12-14 hours%%

continuous state of renewal

* bones
* heart
* blood vessels
* nerves

**Each day _____% of our** %%**body weight**%% **is replaced with new tissue**

5

%%Taste cells%% are replaced every _____ days.

7

%%Intestinal tract lining cells%% are replaced every _____ days

3

Skin cells are replaced every _____.

month

Red blood cells turn over every _____ days.

120

digestion

break food down into its components

aspects of digestion

* mechanical digestion
* chemical digestion

mechanical digestion

* chewing
* action of the GI tract %%(gastrointestinal tract)%%

chemical aspects

* gastric juices

digestion is complete when

* carbohydrates (CHO), protein (PRO), and fat are %%reduced%% to substances that can be %%absorbed%%
* when vitamins and minerals are %%released%%

Peristalsis

* %%wavelike muscular contractions%% of the GI tract (esophagus to the stomach) that push its contents along

stomach action

* circular, longitudinal and diagonal muscles
* %%chyme%% released %%3 times/min i%%nto intestine

segmentation

* forward/backward movement to allow for greater contact between food and digestive juices

sphincter contractions

* prevent reflux

absorption

* take it in
* end products of digestion taken up by the %%lymphatic system%% and %%circulatory system%%

circulation

* move it around
* provide all cells with nutrients for %%growth%% and %%regeneration%%

overview of the digestion system

* %%25-30 ft%% of muscular tube and secretory organs; inner space is called the %%lumen%%
* secreted juices break foods into %%absorbable pieces%%
* carbs → *glucose* and *fructose*, *monosaccharides*
* proteins → *amino acids* (broken down)
* fats → *fatty acids* and *glycerol*

gastric juice

* contains %%water%%, %%enzymes%%, and %%hydrochloric acid%%
* %%pH = 2,%% which is 100 000 times more acidic than water which has pH of 7
* %%epithelial cells%% in the stomach must secrete mucus to protect stomach cell lining

\

%%oral cavity%% → tongue = mastication and mixing of food with saliva

%%salivary glands%% → produce enzymes that help break down starch and fats (salivary amylase and salivary lipase)

%%esophagus%% → transfers food to the stomach

%%stomach%% → secretes enzymes that break down proteins (pepsin) and fats (fat tends to separate and float on top of mixture— last to leave)

* __mixes and liquefies food__
* solid food—1 hour
* liquids—min

%%liver%% → produces bile that aids fat digestion

%%gallbladder%% → stores and secretes bile that aids in fat digestion

%%bile duct%% → conducts bile to small intestine

%%pancreas%% → Secretes enzymes that break down carbohydrates

• Pancreatic amylase

• Pancreatic lipase (enhanced by bile)

• Trypsin

• Bicarbonate for neutralization

%%pancreatic duct%% → conducts pancreatic juice into small intestine

%%large intestine (colon)%% → site of absorption of water, site of most intestinal bacteria—undigested CHO and some fibre may be digested by these bacteria

%%small intestine%% → secretes enzymes that breakdown carbohydrates, proteins, and fats, site of most nutrient absorption

%%rectum%% → stores waste products for elimination

%%anus%% → holds rectum closed, opens to allow elimination

cephalic phase

* the %%first step%% of digestion
* it is your first thought, small, sight of what you want to eat 
* %%hunger and appetite%% work together to prepare the GI tract to digest food

**absorption and digestion is enhanced by _____ that lies in the intestinal wall**

* villi lining

structures of the intestines

* %%675 square feet%% (1/3 football field)
* much of this need (%%50% in small intestine and 80% in large intestine%%) met by food that is being digested

blood vessels

arteries and veins infiltrate villi:

* arteries %%pick up nutrients%% (CHO & PRO breakdown products)
* veins %%take away wastes%%

lymphatic system

* vessels infiltrate villi
* %%fat dig by-products%% mostly absorbed by lymph vessels
* lymphatic fluid has substances to bind fat and carry it to blood stream near heart where systems merge to one vessel

fibre in the digestive tract

* humans have %%no enzymes%% break down fibre
* %%high fibre meals%% pass through the large intestine faster than low fibre meals
* some fibre is digested (ferment) in the large intestine by bacteria

3 key functions of the large intestine

* %%water, sodium,%% and %%some bacterial%% by products absorbed here
* home to %%many bacteria%%

→ use undigested fibre and complex carbohydrates

* unabsorbed substances collect in the large intestine then %%excreted in the stool%%

digestion and absorption

* %%99%%% of carbohydrates are digested and absorbed
* %%95%%% of proteins are digested and absorbed
* %%92%%% of fat are digested and absorbed

digestive disorders

* are one of the leading %%causes of hospitalization%% in the US and Canada
* adequate fibre intake in many cases offers %%prevention%% and %%protection%%

Irritable Bowel Syndrome (IBS)

* common symptoms include: gas production, bloating, diarrhea, and constipation
* prevention (mild cases): low fat diet, fibre, probiotics and medications

Constipation and Hemorrhoids

* due to: diets with little fibre and lack of physical activity

prevention: fibre intakes of %%25 - 30 g/day%% & %%plenty of fluids%%

Ulcers and Heartburn

* fatty foods, coffee, alcohol citrus fruits, soft drinks…
* prevention: %%high fibre diets%%

diarrhea

* leading cause: %%bacterial or viral contamination%% of food/water
* %%vitamin A, zinc%% or other nutrient deficiency
* %%> 2 weeks%% = dehydration, heart and kidney malfunction, possibly death
* world's children 5 and under = %%3.5 million deaths%%

prevention of diarrhea

* food and water %%sanitizations, immunizations, and adequate diets%%
* %%oral rehydration fluid%% (pedialyte and rehydralyte) shorten duration
* once rehydration (%%4-6 hours%% post rehydration fluid)
* yogurt, milk, chicken, potatoes and other vegetables, dried beans, and rice and other cereals provide nutrients needed for the repair of the intestinal tract
* %%avoid sugary drinks%%

diverticulosis

* intestinal walls develop bulges in weakened areas
* bulging pockets can %%worsen constipation%%
* diverticulosis develops when pockets:

\-%%entrap feces%%

%%-become infected and inflamed%%

prevention of diverticulosis

* %%increase%% dietary fibre
* fibre %%increases fecal mass%%
* increased fecal mass %%stimulates muscle contractions%%
* contractions strengthen intestinal muscles

flatulence

* most people produce %%1/2 to 1 1/2 litres%% and pass gas about %%14 times/day%%

gas in the digestive tract comes from

1. %%swallowed air%%
2. breakdown of certain %%undigested foods%% by %%colonic flora%%

bacterial production of gas

* gas related to dried beans, broccoli, cauliflower, Brussels sprouts, onions, corn, and other vegetables that contain %%complex carbohydrates%% that bacteria, not humans, break down
* bacteria produce gas as byproduct of %%carbohydrate digestion%%
* heartburn, gastrointestinal tract disorders, and some medications associated with gas production

lactose intolerance

* reduced digestion of lactose due to the %%unavailability of the enzyme lactase%%
* genetically determined low production of enzyme lactase
* lactose in the large intestine = lactose intolerance = GI symptoms

symptoms of lactose intolerance

1. nausea
2. bloated feeling
3. diarrhea
4. gas
5. abdominal cramping (excess gas by-product of bacteria digestion of lactose)

incidence of lactose intolerance

* many individuals have %%partial inability%% to digest
* %%yogurt%% (cultured buttermilk, aged cheese etc.) do not produce as many problems
* bacteria used to culture yogurt can digest ≥ 1⁄2 of the lactose

managing lactose intolerance

* fortified orange juice, soy or rice milk
* %%low-lactose%% cow’s milk
* milk pre-treated with %%lactase drops%%
* yogurt
* %%lactase tablets%% 30 min prior

mouth (carbohydrates)

* begin with: sugars, fibre, and %%polysaccharides%%
* salivary amylase
* left with: sugars, small starches, and fibre

small intestine (carbohydrates)

* begin with: small starches, sugars, and fibre
* pancreatic amylase: left with fibre and %%disaccharides%%
* brush border enzymes: left with fibre and monosaccharides

large intestine (carbohydrates)

* begin with: fibre and %%monosaccharides%%
* bacertial enzymes (microbiota): leads to %%gas and acid%%
* feces →bacteria, dead cells, undigested food, and fibre

protein (stomach)

* begin with protein
* hydrochloric acid and pepsin
* left with: %%polypeptides%%

small intestine (protein)

* begin with: polypeptides
* %%trypsin%% and various enzymes
* left with: %%amino acids%%

mouth (fat)

* begin with: lipids
* %%salivary lipase%%
* left with: lipids

stomach (fat)

* begin with: lipids
* %%gastric lipase%%
* %%bile%% (coming from the gallbladder)
* left with: lipids

smalle intestine (fat)

* begin with: lipids
* %%pancreatic lipase%%
* left with: fatty acids