The Digestive SystemBiology 12

Intro The human body’s “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

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

Physical Digestion The breakdown of food into small pieces

without chemically changing them into different substances

Helps facilitate chemical digestion by increasing surface area Increased surface area exposes food pieces to

enzymes and other substrate molecules and increases the rate of metabolic reactions

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

Chemical Digestion The breakdown of food through chemical

reactions into small soluble molecules that cells can absorb Lipids are broken down through hydrolysis

reactions – assisted by enzyme activity:

lipid + water glycerol + fatty acids

(lipase enzyme)

Chemical Digestion Carbohydrates are also hydrolyzed with the help

of enzymes into dissacharides…and then into glucose

Starch + water maltose

(amylase enzyme) Proteins are broken into shorter polypeptides by

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

More on these processes later…

Essential nutrients Amazingly, our body can construct most of

the molecules necessary for proper function However, there are a number of substances

that the body cannot synthesize and must ingest Essential Amino Acids (8) Essential Fatty Acids (2) Essential Vitamins (4 water and 5 fat soluble) Trace Elements (many…)

Part 1: The Pathway to the Stomach

Stomach

Esophagus

Mouth

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 mouth

and under tongue) Moistens food to allow “taste” sensations and lubricate

it for transport through the digestive system

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 coat epithelial 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)

Swallowing Food entering the

mouth is transported to the stomach by swallowing

Swallowing is a very complex process controlled by nerve and muscle tissue coordination

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

contractions in the pharynx2. 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 peristalsis6. The trachea is ultimately blocked by a small flap

called the epiglottis

Part 2: The Stomach The stomach is a stretchable, muscular sac

J-shaped organ (0.5 – 2L capacity) 3 layers of muscle tissue Upper section dotted with deep depressions called

gastric pits – which secrete “gastric juice” and mucus (~500mL after a large meal!)

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

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…

Stomach Wall Secretions Peptic or Chief Cells

Secrete the enzyme pepsinogen Pepsinogen’s shape is changed to an “active”

state by HCl (pH denatures the protein-based enzyme into the desired form!)

The active form of the enzyme, pepsin, catalyzes the chemical breakdown of protein into short amino acid chains

No effect on carbs and fats!

Stomach Wall Secretions 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)

Stimuli in the Stomach The nervous system controls the digestive

activity of the stomach (secretions, muscle contractions) by releasing the hormone gastrin when protein-rich food is sensed or an inhibitor when the stomach is empty

Other hormones stimulate the release of bile and pancreatic juices…

So…where are we? It takes the bolus about 9 seconds to get from

your mouth to your stomach In the stomach, the partially digested food

mass 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…

Part 3: The Small Intestine “Small” refers to diameter, not length

About 6m long in an adult human Tapers from ~3cm diameter at the pyloric

sphincter to ~2cm diameter at the entrance to the large 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)

Big Events in the Small Intestine

The acid chyme is neutralized in the small intestine by the bicarbonate ions in pancreatic juices

A host of enzymes from the pancreas are released in 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

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!

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

pancreatic duct This juice contains pancreatic amylase that helps hydrolyze

starch into disaccharides like maltose, lactose, and sucrose Additional enzymes such as maltase, lactase, and sucrase

further reduce disaccharides into simple sugars – namely, glucose

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

The liver converts some glucose into fat, and some into glycogen (and vice versa) in the hours between meals – maintaining blood sugar levels

Protein Digestion and Absorption Pancreatic juice contains (trypsin – active form of

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

The juice also contains peptidases which help hydrolyze short polypeptides into amino acids

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

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

The break down of amino acids into glucose releases toxic ammonia…which is converted to urea…which is transported to the kidneys for excretion!

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 and

concentration (up to five-fold concentration!) Bile salts are able to form micelles to “dissolve”

fat-soluble vitamins Many waste products (500mg of cholesterol per

day) are eliminated from the body by secretion into bile…if not gall stones!

Fat Digestion and Absorption Bile emulsifies fats (mechanically) into

droplets that are broken down further into fatty acids and glycerol by pancreatic lipase enzymes

Fatty acids are lipid soluble and pass through the villi membranes into lacteals (central lymph vessel of the villi)

Solution called “lymph” is then carried to the heart and then distributed to the body’s cells through blood circulation

Nucleic Acid Digestion and Absorption Pancreatic juice released into the small

intestine contains nuclease enzymes that help break down RNA and DNA into nucleotides

Nucleotidase enzymes secreted by the small intestine itself further aid in the break down nucleotides into phosphate, sugars, and nitrogen bases

These nucleotide components are then absorbed into capilliaries and taken to the liver

Part 4: Accessory Organs The Pancreas

An elongated, flattened organ that lays partially behind the stomach – deep 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!): Insulin – released when blood glucose high Glucagon – released when blood glucose low

Part 4: Accessory Organs The exocrine (digestive) aspect of the pancreas is

stimulated when chyme enters the duodenum The hormones secretin (in response to acid) and

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

Pancreatic juice is produced and released into the duodenum 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!)

Part 4: Accessory Organs 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 to

the liver through the hepatic portal vein As we have seen…

Liver produces bile (emulsifies fats) Liver converts glucose glycogen in response to

pancreatic hormones

Part 4: Accessory Organs The liver also…

Converts toxic ammonia (from amino acid and nucleotide breakdown) into urea – excreted 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 (regulates

osmotic pressure of blood) and fibrinogen (aids in blood clotting)

Destroys old red blood cells (recycles iron) and converts hemoglobin into bilirubin/biliverdin (bile pigments)

Part 5: The Large Intestine The last section of the gastrointestinal tract is large

in diameter (6.5cm) and extend ~1.5m from the end of 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

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

Defecation – contraction of muscles in the rectum that forces open the anal sphincter (anus)

10x more bacteria in

your gut than cells in your

body!

Digestive System IssuesYour turn! Put together a 5 minute presentation

of a digestive issue of interest to you. Your presentation should meet the following criteria:

1. Have clear linkage to digestion (approved)

2. Provide a single page handout (1 side)

3. Make use of visuals

4. Be referenced

Ideas… Ulcers Indigestion Heartburn Cirrhosis Jaundice Diarrhea Constipation Flatulence Incontinence

Gall stones Diabetes Polyps Hepatitis Dysphasia Hernia Hemorrhoids Irritable Bowel

Syndrome