the-digestive-system-1212775140756645-9.ppt

7/27/2019 the-digestive-system-1212775140756645-9.ppt

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The Digestive SystemBiology 12


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Intro The human bodys gastrointestinal tract or

alimentary canal is up to 10m long

Along this pathway:

Ingestion Digestion Absorption Egestion

About 95% of ingested food is absorbed and

made available to our body, the rest is egested


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Digestion Defined Digestion is the breakdown of substances

(other organisms!) into small molecules that

can be absorbed by cells There are two classes of digestion:

Physical

Chemical


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Physical Digestion The breakdown of food into small pieces

without chemically changing them intodifferent substances

Helps facilitate chemical digestion byincreasing surface area

Increased surface area exposes food pieces to

enzymes and other substrate molecules andincreases the rate of metabolic reactions

Examples: teeth, stomach (HCl), intestines (bile)


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Chemical Digestion The breakdown of food through chemical

reactions into small soluble molecules that

cells can absorb Lipids are broken down through hydrolysis

reactionsassisted by enzyme activity:

lipid + water glycerol + fatty acids

(lipase enzyme)


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Chemical Digestion Carbohydrates are also hydrolyzed with the help

of enzymes into dissacharidesand then into

glucose

Starch + water maltose

(amylase enzyme)

Proteins are broken into shorter polypeptides by

the enzyme pepsin, and then cleaved into aminoacids by other protease enzymes

More on these processes later


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Essential nutrients Amazingly, our body can construct most of

the molecules necessary for proper function

However, there are a number of substancesthat the body cannot synthesize and mustingest

Essential Amino Acids (8)

Essential Fatty Acids (2)

Essential Vitamins (4 water and 5 fat soluble)

Trace Elements (many)


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Part 1: The Pathway to the Stomach

Stomach

Esophagus

Mouth


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The Mouth The mouth has a number of digestive features:

Teeth

Omnivores, like ourselves, have incisors (rip/shred),canines (hold/tear), premolars and molars (grind)

Teeth mechanically break down food into pieces

Saliva

Secreted into mouth by salivary glands (roof of mouthand under tongue)

Moistens food to allow taste sensations andlubricate it for transport through the digestive system


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The Mouth Dissolves substances into aqueous solutions (water)

for higher reaction rates

Contains the enzyme amylase which converts starch

into dissacharides Mucus

Secreted into mouth (and other organs) to coatepithelial tissues/surfaces

Made of water, mucins (glycoproteins), and salts Lubricates the movement of food

Serves as a barrier against noxious substances

Holds food together in a bolus (ball of food)


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Swallowing

Food entering the

mouth is transported

to the stomach by

swallowing

Swallowing is a very

complex process

controlled by nerveand muscle tissue

coordination


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Swallowing (Deglutition)1. Pressure of bolus on the pharynx stimulates muscle

contractions in the pharynx

2. The soft palate seals off the nasopharynx to prevent

food from entering the nose3. The larynx (voice box) move up to stop the passage

of food through the trachea (throat)

4. This stops breathing and widens the esophagus for

passage of the bolus5. Food passes the trachea to the stomach through

muscle contractions called peristalsis

6. The trachea is ultimately blocked by a small flapcalled the epiglottis


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Part 2: The Stomach

The stomach is a stretchable, muscular sac

J-shaped organ (0.52L capacity)

3 layers of muscle tissue Upper section dotted with deep depressions called

gastric pitswhich secrete gastric juice andmucus (~500mL after a large meal!)

Has two sphincter portals: (like draw strings) Entrance: cardiac sphincter

Exit: pyloric sphincter


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Stomach Wall Secretions

Parietal Cells

Secretes roughly 2L of concentrated (pH ~1-3)

HCl each day HCl mechanically breaks down tissues and

proteins into smaller pieces

Acidic secretions make the stomach inhospitable

to most bacteria (exception: heliobacter pyloria)

Activates pepsinogen


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Peptic or Chief Cells

Secrete the enzyme pepsinogen

Pepsinogens shape is changed to an activestate by HCl (pH denatures the protein-basedenzyme into the desired form!)

The active form of the enzyme, pepsin, catalyzes

the chemical breakdown of protein into shortamino acid chains

No effect on carbs and fats!


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Epithelial Mucus Cells

Secrete mucus from gastric pits in stomach wall

Lubricates and facilitates food transport Protects walls from abrasive food particles

Protects the protein-based walls of the stomach

from autodigestion by gastric juices!

Failure of mucus results in ulcers (bacteria, stress,

and diet causes)


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Stimuli in the Stomach

The nervous system controls the digestive

activity of the stomach (secretions, muscle

contractions) by releasing the hormone gastrinwhen protein-rich food is sensed or an

inhibitor when the stomach is empty

Other hormones stimulate the release of bileand pancreatic juices


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Sowhere are we?

It takes the bolus about 9 seconds to get from

your mouth to your stomach

In the stomach, the partially digested foodmass is called chyme

This acid chyme remains in the stomach for

2-6 hours (depending on the type of food)before release by the pyloric sphincter into the

small intestine


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Part 3: The Small Intestine

Small refers to diameter, not length

About 6m long in an adult human

Tapers from ~3cm diameter at the pyloricsphincter to ~2cm diameter at the entrance to thelarge intestine (ileocecal valve)

Three major sections of the small intestine:

Duodenum (fixed C-shaped section ~25-30cm) Jejunum (4-5m long)

Ileum (1-1.5m)


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Big Events in the Small Intestine

The acid chyme is neutralized in the small intestine

by the bicarbonate ions in pancreatic juices

A host ofenzymes from the pancreas are releasedin the first two sections of the small intestine to

break down all four basic nutrients

Bile salts are released from the liver

gall bladder intestine to emulsify fats (mechanical digestion)

Most digestion occurs in the duodenum

Most absorption occurs in latter intestinal sections


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Specialized Structure

The small intestine has a number of special features

that allows it to absorb nutrients:

Length (increases surface area)

Folding (slows the passage of food and increases surface

area)

Villi (finger-like projections, ~30 per mm2, ~1mm long,

increases surface area)

Microvilli (villi epithelial cell projections increase surface

area)

The surface area of the small intestine is about the size

of a tennis court!


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Starch Digestion and Absorption Pancreatic juice is released into the duodenum through the

pancreatic duct

This juice contains pancreatic amylase that helps hydrolyzestarch into disaccharides like maltose, lactose, and sucrose

Additional enzymes such as maltase, lactase, and sucrasefurther reduce disaccharides into simple sugarsnamely,glucose

Glucose is actively transported (against its concentrationgradient) into villi capillaries and taken directly to the liver

The liver converts some glucose into fat, and some intoglycogen (and vice versa) in the hours between mealsmaintaining blood sugar levels


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Protein Digestion and Absorption Pancreatic juice contains (trypsinactive form of

trypsinogen) which, like pepsin in the stomach, aidsthe break down of proteins into shorter polypeptides

The juice also contains peptidases which helphydrolyze short polypeptides into amino acids

Amino acids are actively transported by proteins inthe villi wall and taken to the liver by capilliaries for

processing

The liver breaks down some amino acids to producecarbohydrates, fats, blood proteins, bile, etc.

The break down of amino acids into glucose releasestoxic ammoniawhich is converted to ureawhich

is transported to the kidneys for excretion!


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Fat Digestion and Absorption

The liver makes ~1L of bile each day

Contains water, electrolytes, bile acids,cholesterol, phospholipids, and bilirubin

Green color due to bilirubin

Released into gall bladder for storage andconcentration (up to five-fold concentration!)

Bile salts are able to form micelles to dissolvefat-soluble vitamins

Many waste products (500mg of cholesterol perday) are eliminated from the body by secretion

into bileif not gall stones!


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Fat Digestion and Absorption

Bile emulsifies fats (mechanically) intodroplets that are broken down further intofatty acids and glycerol by pancreatic lipaseenzymes

Fatty acids are lipid soluble and pass throughthe villi membranes into lacteals (central

lymph vessel of the villi) Solution called lymph is then carried to the

heart and then distributed to the bodys cellsthrough blood circulation


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Nucleic Acid Digestion and Absorption

Pancreatic juice released into the smallintestine contains nuclease enzymes that helpbreak down RNA and DNA into nucleotides

Nucleotidase enzymes secreted by the smallintestine itself further aid in the break downnucleotides into phosphate, sugars, and

nitrogen bases These nucleotide components are then

absorbed into capilliaries and taken to theliver


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Part 4: Accessory Organs

The Pancreas

An elongated, flattened organ that lays partially

behind the stomachdeep in the chest cavity Acts as both an endocrine (hormone releasing)

gland and exocrine (digestive processes) gland

Two key hormones released into blood from the

islets of Langerhans (2000 of them!):

Insulinreleased when blood glucose high

Glucagonreleased when blood glucose low


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The exocrine (digestive) aspect of the pancreas isstimulated when chyme enters the duodenum

The hormones secretin (in response to acid) and

cholecystokinin or CCK ( in response to proteinand fat) are sent by duodenum cells to pancreas

Pancreatic juice is produced and released into theduodenum in response to these hormones

A host of enzymes are released

HCO3- (bicarbonate ions) in the juice change the

chyme pH from ~2-3 to ~7-8 (slightly basic!)


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The Liver

The largest internal organ! (about 1.5 kg)

Lobed (large right lobe, small left lobe),triangular shape

All blood from the intestines is brought back tothe liver through the hepatic portal vein

As we have seen Liver produces bile (emulsifies fats)

Liver converts glucose glycogen in response topancreatic hormones


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The liver also

Converts toxic ammonia (from amino acid andnucleotide breakdown) into ureaexcreted in urine

by the kidneys Detoxifies blood by breaking down: alcohol,

hormones, antibiotics, drugs, excess vitamins, etc.

Stores some vitamins

Synthesizes blood proteins like albumin (regulatesosmotic pressure of blood) and fibrinogen (aids in

blood clotting)

Destroys old red blood cells (recycles iron) andconverts hemoglobin into bilirubin/biliverdin (bile

pigments)


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Part 5: The Large Intestine

The last section of the gastrointestinal tract is largein diameter (6.5cm) and extend ~1.5m from the endof the ileum to the anus

The entrance to the large intestine is called the ileo-caecal valve (another sphincter)

There are four sections of the large intestine:

Caecum and Ascending Colon

Transverse Colon

Descending Colon

Rectum


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Large Intestine Functions

Movement and compaction of undigested food (soon

to be called feces) toward the anus

Absorption of water (about 10L of water per day!)

and some salts (sodium, etc.)

E. coli bacteria present in large intestine

Produces feces (60% bacteria!)

Synthesizes vitamin K for liver

Defecationcontraction of muscles in the rectum

that forces open the anal sphincter (anus)

10x more

bacteria in

your gut than

cells in your

body!


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Digestive System Issues

Your turn! Put together a 5 minute presentation

of a digestive issue of interest to you. Your

presentation should meet the followingcriteria:

1. Have clear linkage to digestion (approved)

2. Provide a single page handout (1 side)

3. Make use of visuals

4. Be referenced


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Ideas

Ulcers

Indigestion

Heartburn

Cirrhosis

Jaundice

Diarrhea

Constipation Flatulence

Incontinence

Gall stones

Diabetes

Polyps

Hepatitis

Dysphasia

Hernia

Hemorrhoids Irritable Bowel

Syndrome

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