THE DIGESTIVE

SYSTEM

BASICSDigestion

Breakdown of ingested foodAbsorption of nutrients into the blood

MetabolismProduction of cellular energy (ATP)Constructive and degradative cellular

activities

A.DIGESTIVE ORGANSTwo main groups

Alimentary canal – continuous coiled hollow, muscular tube (aka gastrointestinal [GI] tract) open at both endsPerforms all digestive functions

Ingest, digest, absorb, defecate

Accessory digestive organsAssist process of digestion

Figure 14.1

If it’s where food goes through, it’s the GI tractIf it’s attached to GI tract, it’s an accessory organ

ALIMENTARY (GI) ORGANSOrgan Function1. Mouth Physical: break food into smaller particles;

Chemical: breakdown starches with amylase

2. Pharynx Passageway from mouth to esophagus; contractions

3. Esophagus AKA gullet, long passageway linking pharynx & stomach; contractions

4. Stomach Physical: break food into smaller fragments; Chemical: breakdown of proteins with gastric juice; contractions

5. Small intestine Chemical digestion of food, absorption of nutrients; contractions

6. Large intestine Absorption of water ; contractions

7. Anus End of GI tract, expulsion of waste

Mechanical breakdown Food is physically broken down by mastication (chewing)

Chemical digestion Food is mixed with saliva which breaking of starch into

maltose by enzyme salivary amylase Initiation of deglutition (swallowing ) by the tongue Allowing for the sense of taste

5 tastes: Sweet Sour Salty Bitter Umami (savory/tasty)

1. MOUTH (ORAL CAVITY)

ANATOMY OF MOUTH Lips (labia) Cheeks Hard palate – forms

the anterior roof Soft palate – forms

the posterior roof Uvula – fleshy

projection of the soft palate; no function

Vestibule – space between lips and teeth and gums

Figure 14.2a

Oral cavity – behind the teeth

Tongue – attached at hyoid and styloid, and by lingual frenulum

TonsilsPalatine tonsilsLingual tonsil

Part of lymphatic (immune) system

Figure 14.2a

2. PHARYNXServes as a passageway for air and foodFood movement via alternating

contractions of the muscle layers (peristalsis)

PHARYNX ANATOMY Nasopharynx – not part of the digestive

system; up to the nose Oropharynx – posterior to oral cavity;

back of throat Laryngopharynx – below the oropharynx

and connected to the esophagus

3. ESOPHAGUS Runs from pharynx to

stomach through the diaphragm

Conducts food by peristalsis (slow rhythmic squeezing)

Passageway for food only (respiratory system branches off after the pharynx)

Timeline: 5-8 seconds!

4. STOMACH Acts as a storage tank for food

Can hold 4 L (1 gal) of food!! Site of physical food breakdown Chemical breakdown of protein begins

Lined with columnar epithelium with …Mucus neck cells– mucusChief cells – pepsinogen (digest protein)Parietal cells – HCl (hydrochloric acid, pH~1.5)Enteroendocrine cells – gastrin Gastric cells – gastrin juice

Delivers chyme (processed food) to the small intestine

PEPSI & PEPSIN Caleb Bradham of North Carolina was a

pharmacist. His most popular beverage was something he

called "Brad's drink" made of carbonated water, sugar, vanilla, rare oils, pepsin and kola nuts.

He later on renamed his beverage Pepsi Cola and it advertised it as “Exhilarating, invigorating, aids digestion.”

DO NOT COPY

STOMACH ANATOMY Located on the left side of the abdominal cavity Food enters at the cardioesophageal sphincter –

circular muscle that acts as “gatekeeper” Food empties into the small intestine at the pyloric

sphincter Timeline: 2-6 hours

Contains rugae – internal folds of the mucosa Layers of peritoneum attached to the stomach

Lesser omentum – attaches the liver to the lesser curvature

Greater omentum – attaches the greater curvature to the posterior body wall

Contains fat to insulate, cushion, and protect abdominal organs (see Dr. Oz on Oprah)

OPRAH AND OMENTUM

Figure 14.4a

STRUCTURE OF THE STOMACH MUCOSA

Figure 14.4b–c

5. SMALL INTESTINE body’s major digestive organ – averages 7m

(>23 feet) long! Site of nutrient absorption into the blood

Timeline: 3-5 hours Muscular tube extending from the pyloric

sphincter to the ileocecal valve Suspended from the posterior abdominal wall

by the mesenteryDouble-layer of peritoneum

Divided up into three sections:Duodenum

Attached to the stomach

Curves around the head of the pancreas

JejunumAttaches anteriorly to the duodenum

IleumExtends from jejunum to large intestine

Many chemicals involved in digestion in small intestineEnzymes from intestinal cellsEnzymes from pancreasBile from gall bladder

Absorption is done through many villiFingerlike structures

formed by the mucosaGive the small

intestine more surface area

Microvilli on absorptive cells of villi add for super absorptionSmall projections of

the plasma membraneFound on absorptive

cells

Absorption into bloodstream carried out byAbsorptive cellsBlood capillariesLacteals

(specialized lymphatic capillaries)

Figure 14.7b

6. LARGE INTESTINE Larger in diameter, but shorter than the

small intestine – 1.5 m long Frames the internal abdomen Absorption of water Eliminates indigestible food from the body

as feces Does NOT participate in digestion of food Goblet cells produce mucus to act as a

lubricant

Divided up into Colon

Ascending – upTransverse – acrossDescending – downS-shaped sigmoidal

RectumAnal canal

• Cecum – saclike projection, hangs from first part• Appendix – twisted section that often traps

bacteria & gets infected (appendicitis)• Timeline: 4-72 hours!

7. ANUS Ending of anal canal Contains two sphincters which work to control

passage of fecal matter Internal involuntary sphincter

Signals us that it’s time to expel fecesExternal voluntary sphincter

Control opening of sphincter until ready

•ACCESSORY DIGESTIVE ORGANS

1. Salivary glands2. Teeth3. Pancreas4. Liver5. Gall bladder

1.SALIVARY GLANDSSaliva-producing

glandsParotid glands –

located anterior to ears

Submandibular glands - under mandible

Sublingual glands – under tongue

Produce salivaMixture of mucus and serous fluidsHelps to form a food bolus (ball of

masticated food)Contains salivary amylase to begin

starch digestionDissolves chemicals so they can be

tasted

2. TEETH role is to masticate (chew)

food Humans have two sets of

teethDeciduous (baby or milk) teeth20 teeth fully formed by age

two Permanent teeth

Replace deciduous teeth beginning ages of 6 to 12

full set is 32 teeth, some people do not have wisdom teeth

3. PANCREASProduces a wide spectrum

of digestive enzymes that break down all categories of food

Enzymes secreted into duodenum

Alkaline fluid introduced with enzymes neutralizes acidic chyme

Endocrine products of pancreas:InsulinGlucagon

4. LIVERLargest gland in the bodyLocated on the right side of the body

under the diaphragmConsists of four lobes suspended

from the diaphragm and abdominal wall by the falciform ligament

Connected to the gall bladder via the common hepatic duct

Produces bile – highly bitter green liquid containing:Bile saltsBile pigment (mostly green bilirubin

from the breakdown of hemoglobin)CholesterolPhospholipidsElectrolytes

5. GALL BLADDER Sac found in hollow fossa of liver Stores bile from the liver by way of the

cystic duct Bile is introduced into the duodenum in the

presence of fatty food Gallstones can cause blockages

B. SIX PROCESSES OF DIGESTIVE SYSTEM

1. Ingestion – getting food into the mouth2. Mechanical Digestion – physically

breaking food down3. Propulsion – moving foods from one

region of the digestive system to another

4. Chemical Digestion – chemically breaking down food

5. Absorption – getting nutrients/water into blood stream

6. Defecation – expelling wastes

Figure 14.11

1. Ingestion• Voluntary process of getting food into mouth

2. Mechanical Digestion Mixing of food in the mouth by the tongue Churning of food in the stomach Segmentation in the small intestine

Movement of food back & forth serving to mix it with digestive juices

3. Propulsion• Food is processed by more than

one digestive organ so must be propelled from one to another. Done via peristalsis

o Peristalsis – involuntary action where alternating waves of contraction & relaxation of smooth muscles squeeze food along GI tract

PERISTALSIS

Food must first be well mixed Rippling peristalsis occurs in lower stomach pylorus meters out chyme into the small

intestine (30 ml at a time) stomach empties in four to six hours

Figure 14.15

4. Chemical DigestionEnzymes break down food molecules into

their building blocksEach major food group uses different enzymes

Carbohydrates simple sugarsProteins amino acidsFats fatty acids and alcohols

5. AbsorptionEnd products of digestion absorbed in blood

or lymphFood must enter mucosal cells, then into blood or lymph capillaries

6. DefecationElimination of indigestible substances as feces

CHEMICALS OF DIGESTIONChemical Type Produced Function

Amylase Enzyme Salivary glands, pancreas

Breakdown starch into maltose

Bile Compound

Liver Breakdown fats

Cholecystokinin (CCK)

hormone Small intestine

Stimulate pancreas to release secretions; gallbladder to release bile

Gastrin Hormone stomach Stimulates production of gastric juice

Glucagon Enzyme Pancreas Converts glycogen to glucose

HCl Acid Stomach Activate pepsinogen

Insulin Hormone Pancreas Directs cells to uptake sugar; converts excess glucose to glycogen

Lipase Enzyme Pancreas Breakdown lipids/fats

Pepsin Enzyme Pepsinogen Breakdown proteins

Pepsinogen Enzyme Stomach Precursor of pepsin

Rennin Enzyme Stomach Breakdown milk protein

Secretin Hormone Small intestine

Stimulate pancreas & liver to release secretions

Trypsin Enzyme Pancreas Breakdown protein

C. CONTROL OF DIGESTIVE ACTIVITY

Mostly controlled by reflexes Chemical and mechanical receptors are

located in organ walls that trigger reflexes Stimuli include:

Stretch of the organpH of the contentsPresence of breakdown products

Reflexes include:Activation or inhibition of glandular

secretionsSmooth muscle activity

D. ACTIVITIES BY GI ORGANS Mouth-Esophagus (Deglutition = Swallowing)

Buccal phaseVoluntary; occurs in mouth; food formed into bolus

Bolus forced into pharynx by tonguePharyngeal-esophageal phase

Involuntary transport of bolusAll passageways blocked

Tongue blocks off mouthSoft palate (uvula) blocks nasopharynxEpiglottis blocks larynxCardioesophageal sphincter opened when bolus presses against it

DEGLUTITION (SWALLOWING)

Figure 14.14

StomachGastric juice regulated by neural and

hormonal factorsPresence of food and/or falling pH (due to HCl)

causes release of gastrinGastrin causes stomach glands to produce protein-digesting enzymes pepsin & renninHCl activates pepsinogen to become pepsin for protein digestion

HCl provides hostile environment for microorganisms (except Helicobacter pylori)

Only absorption occurring stomach is alcohol and aspirin

Small intestineEnzymes from brush border break double

sugars (lactose) into simple sugars (galactose & glucose)Complete some protein digestion

Pancreatic enzymes play the major digestive functionHelp complete digestion of starch (pancreatic amylase)

Carry out about half of all protein digestion (trypsin, etc.)

Responsible for fat digestion (lipase)Digest nucleic acids (nucleases)Alkaline content neutralizes acidic chyme

Release of pancreatic juice stimulated by:Vagus nerveLocal hormones

Secretin: causes liver to produce more bile; pancreas to release more alkaline juice

Cholecystokinin (CCK): stimulates gall bladder to release more bile; pancreas to release more enzymes

Figure 14.16

LARGE INTESTINENo digestive enzymes are producedResident bacteria digest remaining

nutrients (Escherichia coli)Produce some vitamin K and BRelease gases

Water and vitamins K and B are absorbed

Remaining materials are eliminated via feces

Sluggish peristalsisMass movements

Slow, powerful movementsOccur three to four times per day

Presence of feces in the rectum causes a defecation reflexInternal anal sphincter is relaxedDefecation occurs with relaxation of

the voluntary (external) anal sphincter

E. NUTRITIONNutrient – substance used by the body

for growth, maintenance, and repairCategories of nutrients

Carbohydrates – most from plants (except lactose)

Lipids – fats (meat, nuts, oils)Proteins – meats, milk, legumesVitamins – act with enzymesMineral – essentials in body (Ca2+)Water

F. METABOLISM Chemical reactions necessary to maintain life

Catabolism – substances broken down to simpler substances; energy released

Anabolism – larger molecules built from smaller ones; energy used

Fat metabolism (9 cals/g)Handled mostly by liver

Use some fats to make ATPRelease breakdown products of fatty acids to the blood

Body cells remove fat and cholesterol to build membranes and steroid hormones

Carbohydrate metabolism (4 cals/g)Carbohydrates are broken down into simple

sugars or monosaccharidesMonosaccharide – one sugar molecule. Only three are found in our diet that make it into blood Fructose (fruit), galactose (milk), glucose (ubiquitous)

Disaccharide – two sugar moleculesSucrose (table sugar) = glucose + fructoselactose (milk) = galactose + glucosemaltose (malt) = glucose + glucose

Polysaccharide – many sugar moleculesStarch (breads), cellulose (plant walls – fiber)

Protein metabolism (4 cals/g)Proteins conserved by body cells as they

are used for most cellular structuresIngested proteins broken down to amino

acidsCells remove amino acids to build proteins

Synthesized proteins are actively transported across cell membranes

Amino acids used to make ATP only when proteins are overabundant or there is a shortage of other sources

Cholesterol metabolismFunctions of cholesterol

Serves as a structural basis of steroid hormones (testosterone & estrogen) & vitamin D

Is a major building block of plasma membranesMost cholesterol is produced in liver and is not

from dietCholesterol & fatty acids can’t freely circulate

in bloodstreamAre transported by lipoproteins (lipid-protein

complexes)Low-density lipoproteins (LDLs) transport to body cells

High-density lipoproteins (HDLs) transport from body cells to liver (“good”, since it gets stored in liver)

G. ROLE OF LIVER IN METABOLISM

Several roles in digestionDetoxifies drugs & alcoholDegrades hormonesProduce cholesterol, blood proteins

(albumin and clotting proteins)Plays a central role in metabolism

Glucose = useable sugarGlycogen = stored sugar

Glycogenesis: making glycogenGlucose molecules converted to

glycogen when in excessGlycogen molecules stored in liver

Glycogenolysis: breaking glycogenGlycogen converted into glucose

when not enough, released from liver Gluconeogenesis: making new glucose

Glucose is produced from fats and proteins in emergencies

Figure 14.21

Glucose ($)

Glycogen (€)

blood

liver

glycogenesis

Fats/protein (£)

glu

coneog

enes

is

InsulinConverts

glucose to glycogen

Helps cells absorb glucose

GlucagonConverts

glycogen back to glucose

H. HOMEOSTATIC IMBALANCES (DISEASES)

Hyperglycemia = diabetes mellitusHigh (hyper) sugar in bloodLiterally from Latin “something sweet

is being siphoned from body”High levels of glucose stimulate

release of hormone insulin (produced by pancreas) to assist all cells in absorbing glucose level decreases

As glucose level decreases, insulin no longer released

Normal fasting glucose level is 70-100 mg/dL

Diabetes glucose level is > 126 mg/dLExcess sugar is not being absorbed by cells for one of two reasons:Type I Diabetes: insulin is not produced (genetic)

Type II Diabetes: insulin receptors unresponsive (adult-onset)

Glucose flushed from body in urineBody still needs glucose, so pulls it from fats &/or protein

Saudi ArabiaUnited Arab EmiratesOman

HypoglycemiaLow (hypo) sugar in bloodHigh levels of glucose stimulate excess

insulin to be released from pancreasToo much glucose absorbed by cells fasting glucose level < 70 mg/dL in bloodMay lead to diabetes in future

Peptic ulcerCraterlike erosion in mucosa of any GI tract

organs that gets exposed to gastric juiceEsophagus – esophageal ulcerStomach – gastric ulcerDuodenum – duodenal ulcer (most common)

Definitive cause still unknownMay be triggered by:

Excess HCl and pepsin due to stress, dietAcid-resistant bacteria Helicobacter pylori (70-90% people with ulcers have it)

Overuse of OTC (over-the-counter) pain killers like aspirin, naproxen, ibuprofen (acetaminophen ok)

DiarrheaWatery stools as a result of any condition that

rushes food residue through large intestine before it has chance to absorb waterMost commonly result of bacteria (food poisoning), viruses (cold/flu), food intolerances, or reaction to medications

ConstipationHardened stools as a result of any condition

that prolongs food residue’s time in large intestine, leading to excess water being absorbedMost commonly result of lack of fiber (poor diet), poor bowel habits (holding it too often), laxative abuse, not enough water

AppendicitisInflammation of the appendix due to a

blockageUsually blocked by stool, but a foreign body or cancer can prevent proper drainingAs result, bacteria in large intestine multiply in appendix, causing swelling

If appendix ruptures (pops open), bacteria in peritoneum (abdominal cavity) cause peritonitis which can be fatal

SymptomsPain in lower

right abdomenLoss of appetiteNausea/

vomitingAbdominal

swellingFever of 99º F

to 102º F

JaundiceNot a disease but a condition

signaling complications in liver and/or gallbladder as a result of bile salts and bilirubin pigments entering bloodstreamBlockage of hepatic or bile ducts prevents bile from entering small intestine. May be fromGallstonesHepatitis (inflammation of liver)Cirrhosis (scarring of liver)

DiverticulosisDiet devoid of fiber will

force colon to contract more forcefully to move stool

Too much force causes pouch-like diverticula to herniate or pop outward.

Condition is called diverticulosis, which may lead to diverticulitis

Diverticulitis Inflammation of diverticulaMay be fatal if diverticulum

ruptures

HemorrhoidsWhen too much force is

needed to expel feces, veins in anus bulge causing hemorrhoidsCan be internal (rectum) or external (anus)

May have to be surgically removed if inflammation causes defecation to be painful or impossible internalinternal

externalexternal

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