DIGESTIVE SYSTEM



DIGESTIVE SYSTEM

Digestion is the process by which food substances are broken down

into usable, absorbable substances by mechanical and chemical means.

The digestive system consists of the following:

Alimentary Canal – mouth, pharynx, esophagus, stomach, small

and large intestines

Accessory Organs – salivary glands, liver, pancreas, gall bladder

General Characteristics of the Alimentary Canal

9 meters in length

Wall has 4 layers:

1) Mucous Membrane is the innermost layer

Has folds and tiny projections in some places and contains

glands that secrete mucus and enzymes. It protects tissues

beneath that are used for secretion and absorption.

2) Submucosa is where blood and lymph vessels for nourishment

are located and also many nerves. It nourishes surrounding

tissues and carries away absorbed substances.

3) Muscular Layer smooth muscle that initiates the movement of

the tube. There is an inner circular layer and an outer

longitudinal layer.

4) Serous Layer – visceral peritoneum (connective under

epithelium) It is the outer covering and protects the

underlying tissues and keeps them lubricated with serous

fluid which keeps the tube’s outer surface so that other

abdominal organs can slide freely against one another

Movements of the Tube

Mixing – muscle undergoes rhythmic contractions in the stomach

Propelling – wavelike motion called peristalsis

receptive relaxation – one area contracts while the next

area relaxes; this causing a pushing effect

Mouth receives the food and reduces its size by chewing and adding

saliva (mastication)

The cheeks and lips contain sensory receptors for judging texture and

temperature

Tongue is thick, muscular organ that occupies the floor of the mouth.

It is covered by a mucous membrane and is connected to the

floor of the mouth by a membranous fold called the frenulum

Papillae are rough projections that provide friction which is useful

in handling food and also contain taste buds

Root is the posterior region and is anchored to the hyoid bone and

covered with rounded masses of lymphatic tissue called the

lingual tonsils

TASTE

Taste buds are located on the surface of the tongue. They have tiny elevations

called papillae. They are also found on the roof of the mouth and walls of the

pharynx.

Each taste bud includes a group of modified epithelial cells – taste cells or

gustatory cells that function as receptors

There is an opening called the taste pore and tiny projections called

taste hairs which are the sensitive parts of the receptor cells. Nerve cells

are wrapped all around these receptor cells

Again, the food chemical must be dissolved in a watery fluid – in this case

saliva

[pic]

Taste Sensations

4 primary taste sensations

They are each concentrated in different regions of the tongue’s surface

Sweet – tip of tongue

Organic – sugars and polysaccharides

Inorganic – salts of lead and beryllium

Sour – along the sides

Acids – proportional to the amount of Hydrogen ions

Salt – tip and upper front portion of the tongue

Ionized inorganic salts

Bitter – towards the back of the tongue

Two Others:

Metallic - salts of magnesium and calcium

Alkalines – strychnine, nicotine, morphine

(persons reject – protective mechanism)

Taste receptors undergo rapid adaptation. Moving food over the tongue

helps continual stimulation

Even though receptors are close to the surface and exposed to damage, we

do not lose out taste with age because the taste cells are replaced

continually every week

[pic]

Palate forms the roof of the oral cavity. The hard anterior part is

called the hard palate. The soft posterior part is called the

soft palate which forms a muscular arch that extends down as

a cone-shaped projection called the uvula. It is drawn up with

the soft palate during swallowing to close off the nasal passage

[pic]

Palatinie Tonsils are made of lymphatic tissue around the palate

Pharyngeal Tonsils (adenoids) sometimes become enlarged and block

the nasal cavity and pharynx (posterior wall of the pharynx)

Teeth - two set form during development

Primary Teeth (deciduous) these erupt between 6 months and 2-4

years, through the gums (gingiva). There are 10 of these in each

jaw.

Beginning midline: central incisor, lateral incisor, cuspid (canine),

1st molar, and 2nd molar

These are usually shed in the same order they appeared but before

this happens their roots are reabsorbed. The teeth are then pushed

out from pressure exerted by the secondary teeth (permanent)

The secondary teeth consist of 32 teeth: central incisor, lateral

incisor, cuspid, 1st bicuspid (premolar), 2nd bicuspid, 1st molar,

2nd molar, 3rd molar (wisdom teeth).

[pic]

The permanent teeth begin to form at about the age of 6 and may

not be completed until between 17 and 25

The wisdom teeth often come in wedged in the wrong position.

They are said to be impacted if this happens.

The teeth mechanically breakdown food into smaller pieces. This

allows more surface area of the food to be exposed to the

digestive enzymes in the saliva.

Tooth functions:

Incisors are for biting

Cuspids are for grasping and tearing

Bicuspids and Molars are for grinding

Tooth Structure

Two main portions:

Crown is the part that projects above the gum line. It is covered by a

glossy, white enamel (hardest substance in the body) made of

calcium salts. It cannot be replaced and does wear down with age

The dentin is below the enamel. It is much like bone but much harder

The pulp cavity is the central portion which contains blood vessels,

nerves, and connective tissue (pulp). The blood vessels and nerves

reach this area through a tubular root canal

Root is attached to the jawbone. It is enclosed in a thin layer of bone-

like material called cementum which is surrounded by a

periodontal ligament which firmly attaches the tooth to the jaw.

The area where the crown and root meet is called the neck.

[pic]

Salivary Glands

These secrete saliva which moistens food particles and helps bind

them together and begins the digestion of carbohydrates.

It also acts as a solvent that dissolves food chemicals so that they

can be tasted

The saliva also helps cleanse the mouth and teeth and keeps the pH

level between 6.5 and 7.5 with the help of bicarbonate ions(HCO3).

These neutralize the acid concentration that builds with the

introduction of food. This pH range is favorable for the activity

of the salivary enzyme and it keeps the teeth from dissolving in

and excessively acid environment.

The Salivary Glands have two types of cells:

Serous Cells produce a watery fluid that contains a digestive

enzyme called amylase for starch digestion

Mucous Cells secrete a thick, stringy liquid called mucus which

binds food particles together and lubricates the pharynx and

esophagus during swallowing

The largest Salivary Glands are the:

Parotid Glands are in front of and just below each ear. They are

the largest of the glands and secrete a watery fluid rich in amylase.

This fluid moves through the Stensen’s Duct to the upper 2nd molar

Submandibular Glands are in the floor of the mouth and inside

of the lower jaw. The fluid secreted is thicker and moves through

the Warton’s Duct which opens under the tongue (water and mucus)

Sublingual Glands are the smallest and are located on the floor

of the mouth under the tongue. The fluid secreted is thick and

stringy. These secretions enter the mouth through many separate

ducts (primarily mucus)

[pic]

[pic]

The Pharynx connects the nasal and oral cavities with the larynx and

esophagus. It is divided into 3 parts:

Nasopharynx lies just above the soft palate. It is a passageway for

breathing and provides an area for the eustachian tube to open

Oropharynx lies behind the mouth down to the epiglottis. It is a

passageway for food moving down from the mouth and also

as a passageway for air from the nasal cavity

Layrngopharynx extends from the epiglottis to the larynx

Is the passageway to the esophagus

The pharynx has two layers of muscles:

The inner layer is circular and consists of constrictor muscles that

pull the walls inward during swallowing

The outer are longitudinal

These are both skeletal muscles but are not under voluntary control

Swallowing (deglutition) occurs in three stages



1st is initiated voluntarily. The bolus of food is forced into the

pharynx by the tongue

2nd the soft palate is raised so that the food does not go into the

nasal cavity.

The hyoid bone and larynx are elevated, preventing food

from entering the trachea

The tongue is pressed against the soft palate which seals off

the oral cavity

The longitudinal muscles contract, pulling the pharynx

upward

The inferior constrictor opens the esophagus

The superior constrictor contracts, starting peristalsis

3rd occurs when breathing stops momentarily and the food

moves through the esophagus to the stomach

Esophagus is a straight, collapsible tube behind the trachea

The esophageal hiatus is where the esophagus through the

diaphragm.

The esophagus is also lubricated by mucous glands

There is a thickened area where it enters the stomach to prevent

regurgitation (esophageal sphincter)

Stomach is a J-shaped, pouch-like organ which hangs under the

diaphragm in the upper left portion of the abdomen

It has a capacity of about one liter or more inner lining has thick

folds (rugae) which disappear when its wall is stretched out.

The food is mixed with gastric juices

The digestion of proteins is initiated here

Carries on a limited amount of absorption (drugs and alcohol) and

moves food into the small intestine

Parts of the Stomach

Cardiac Region is near the esophageal opening (cardia)

Fundic Region is the bulge above the cardiac region. It is a

temporary storage area and is sometimes filled with swallowed air.

Body Region is the main part

Pyloric Region narrows into the pyloric canal and leads to the

small intestine

Pyloric Sphincter is the valve that regulates the passage of material

into the small intestine – it prevents a backup of food

[pic]

Gastric Secretions

The mucous membrane lining of the stomach is thick and studded

with gastric pits which are located at the ends of the gastric glands

The structure and secretion composition of the gastric glands vary in

different parts of the stomach. They generally contain three types

of secretion cells:

Mucous (Goblet) Cells – these are located in the neck of the gland

near the pit opening and secrete mucus. Mucus is an

alkaline secretion which forms a protective coating on the wall

Chief Cells secrete hydrochloric acid

Parietal Cells secrete digestive enzymes and intrinsic factor

These three make up what is called gastric juice

[pic]

Gastric Juice contains several digestive enzymes

Pepsin is secreted by chief cells as pepsinogen. It gets activated

by contact with HCl. This is strong enough to digest the

stomach lining. Pepsin is a protein-splitting enzyme

Gastric Lipase is a fat-splitting enzyme (butter fat) and is

relatively weak in a low pH

Intrinsic Factor is secreted by the parietal cells. It aids in the

absorption of vitamin B12

Regulation of Gastric Secretions

Controlled neurally and hormonally and is produced continuously

although the rate varies. The parasympathetic

stimulates the release of large amounts. Also stimulates the

stomach cells to release the hormone called gastrin which

increases the secretions

Three Stages of Gastric Secretions

Cephalic Phase begins before any food reaches the stomach

Do not have to take in food. Can just taste, smell, see, or think

about food. The hungrier a person is, the greater the secretion

Gastric Phase starts when food enters the stomach and the stomach

distends. Alkaline Tide is the build up of bicarbonate ions in the

blood and urine after a meal

Intestinal Phase begins when food leaves the stomach and enters

the intetsine – stimulates more gastric secretions. As more food

enters the intestine, the gastric secretions decrease due to sympathetic

impulses triggered by acid in the upper part of the small intestine.

The presence of fats and proteins also stimulates the release of a

hormone, cholecystokinin from the intestinal wall which also causes

a decrease in the gastric activity

Gastric Absorption

Not well adapted to absorb things but small quantities of water,

glucose, certain salts, alcohol, and lipid-soluble drugs do get

absorbed

Mixing and Emptying Actions

Although the stomach enlarges, the muscles still maintain their tone

so that the internal pressure remains the same

Too much food makes the pressure increase and the pain receptors

are stimulated

The mixing movement of the stomach creates a semi-fluid paste of

food particles and gastric juice called chyme

The stomach moves chyme into the pylorus where it is moved into

the intestine a little at a time. The rate depends on the fluidity of

the chyme and the type of food – carbohydrates leave the stomach

the quickest followed by proteins, then fats. It usually takes

anywhere from 3-5 hours for solids and much faster for liquids

As the food enters the duodenum it stimulates sensory receptors

causing an enterogastric reflex which inhibits peristalsis in the

stomach to slow the food leaving the stomach

Vomiting is usually triggered by irritation or distension of the

stomach or intestine.

Response: deep breath which raises the soft palate and closes the

nasal cavity and closes the trachea; cardiac sphincter relaxes;

the diaphragm contracts which in turn pushes on the stomach;

and the contraction of the abdominal wall

As a result of these actions, the stomach is squeezed from all sides

The vomiting center (medulla oblongata) can be stimulated by

drugs, toxins in contaminated food, and sometimes rapid changes in

body motion (impulses from the inner ear reach the vomiting

center- motion sickness)

Also stimulated by sights, sounds, odors, tastes, emotional

feelings or mechanical stimulation of the back of the pharynx

Nausea occurs when stomach movements are slowed or absent

and the duodenal contents may back up into the stomach

Pancreas are closely associated with the small intestine. It lies in the

posterior abdominal wall in the C-curve of the duodenum

Pancreatic acinar cells produce pancreatic juices. These cluster

around small tubes which joint to form the …….

Pancreatic Duct is fed by many smaller tubes and extends the

length of the pancreas. It connects with the common bile duct

at a junction called the hepatophacreatic ampula (ampula of

Vater) at the end of which is the hepatopancreatic sphncter,

(sphincter of Oddi) which controls the movement of pancreatic

juices into the duodenum

[pic]

[pic]

Pancreatic Juice contains enzymes capable of digesting

carbohydrates, fats, proteins, and nucleic acids

Pancreatic Amylase converts starch or glycogen into dissacharides.

Pancreatic Lipase digests fats. Converts triglycerides into fatty

acids and monoglycerides

Tripsin, Chyomotrypsin, and Carboxypetidase

These digest proteins – no single enzyme can split all of the possible

amino acid combinations

These enzymes are stored in an inactive form and are activated

by other enzymes released from the mucosa of the small intestine

Nucleases break up nucleic acid molecules

Regulation of Pancreatic Secretion

Regulation by nerve impulses and the endocrine system

Secretin is the hormone released into the blood by the mucous

membrane when acidic chyme enters the duodenum. It travels to

the pancreas where it stimulates the release of enzymes which are

high in bicarbonate ions

Cholecystokinin is also released from the intestinal wall. It

stimulates the pancreas to release digestive enzymes in high

concentrations

Liver is located in the upper right and central portion of the abdominal

cavity. It is the largest gland.

Structure of the Liver

It is enclosed in a fibrous capsule and divided into lobes:

Large, right lobe and smaller left lobe are separated by a falciform

ligament made of connective tissue

Two minor lobes: Quadrate lobe near the gall bladder

Caudate lobe near the vena cava

[pic]

Viceral Peritoneum fastens it to the abdominal wall

Coronary Ligament attaches it to the diaphragm

Each lobe is divided into many hepatic lobules which are the

functional units

Plate-like groups of hepatic cells radiating out from a central vein, are

separated by vascular channels called hepatic sinusoids

Blood from the digestive tract is brought to the sinusoids by the

portal vein. It nourishes the hepatic cells

Kupffer Cells in the inner lining filter out bacteria in the blood

The blood then passes into the central veins and moves out of the liver

Within the lobules are tiny bile canals. These unite to form larger

ducts and finally converge to form hepatic ducts which in turn join

to form the common hepatic duct

Functions:

Carbohydrate metabolism – maintains normal blood sugar levels

(glucose (( glycogen) hormones insulin and glucagon

Oxidizes fatty acids – synthesizes lipoproteins, phospholipids,

and cholesterol

Converts carbohydrates into proteins and fats and sends to adipose

tissue for storage

Most Vital Functions:

Deaminating amino acids

Forms urea

Synthesis of blood proteins, especially those needed for blood clotting.

Converting various amino acids into other amino acids

Stores a variety of substances: glycogen, iron, vitamins A,D, and B12

(iron storage maintains blood iron homeostasis)

Destroys damaged red blood cells and foreign substances

Decomposes toxic substances (detoxification)

Secretes Bile

Stores 200-400 milliliters of blood (reservoir)

All of these functions are not related to digestion but will be

discussed as needed

Composition of Bile

Bile is a yellowish, green liquid and is secreted continuously by the

hepatic cells

It contains water, bile salts, bile pigments, cholesterol, and

electrolytes. Bile salts the most numerous and the only

substance to have a digestive function

Hepatic cell use cholesterol to produce bile salts and in the process

they release cholesterol into the blood

Bile pigment are the products of the breakdown of hemoglobin

from red blood cells

Gall Bladder lies under the right lobe of the liver and is connected to

the cystic duct which in turn joins the hepatic duct.

It has a capacity of 30-50 ml

Function: stores bile between meals

concentrates bile by reabsorbing water

releases bile into the duodenum

Common Bile Duct is formed by the union of the hepatic and

cystic ducts and leads to the duodenum where the

hepatopancreatic sphincter guards its exit. It normally remains

closed so that bile collects in the bile duct and backs up into the

cystic duct. Bile flows into the gall bladder and gets stored

The bile composition is changed while in the gall bladder. The

lining removes some water and electrolytes, leaving a

concentration of salts, pigments, and cholesterol. Sometimes

the cholesterol precipitates out as well and forms solid crystals.

If this continues, the crystals enlarge and form gallstones.

[pic]

Regulation of Bile Release

Cholecystokinin stimulates the gallbladder. This is released from

the intestinal mucosa in response to the presence of fats and

proteins in the duodenum. The sphincter relaxes when peristaltic

waves from the duodenum reach it.

Functions of Bile Salts

These do not act as digestive enzymes, instead they aid the actions of

digestive enzymes and enhance the absorption of fatty acids and

certain fat-soluble vitamins by causing fat globules to break up into

smaller droplets (emulsification) which allows them to mix better

with water so that lipase can digest fat better

They aid in the absorption of fatty acids and cholesterol by forming

complexes (micelles) that are very soluble in chyme and more

easily absorbed by epithelial cells. They also make the absorption

of fat soluble vitamins possible (A,D,E,K)

If bile salts are lacking, lipids may be poorly absorbed leading to a

vitamin deficiency

The salts get reabsorbed and sent back to the liver where they are

reabsorbed.

Small Intestine

18-20 ft relaxed (cadaver) (1 /2 this size in a living person)

Receives secretions from pancreas and liver

Complete digestion

Absorbs the products of digestion

Transports residue to large intestine

Parts of the Small Intestine

Duodenum is about 25cm lone and 5cm in diameter

Shortest and most fixed portion of the intestine

Carries out both digestion and absorption

Jejunum - digestion and absorption

Ileum – absorption

Mesentary is a double-layered fold of peritoneum which suspends

the jejunum and ileum from the abdominal wall. It contains blood

vessels, nerves, and lymphatic vessels that supply the intestinal wall

[pic]

Greater Omentum is a double-layered fold pf peritoneal membrane

that drapes like an apron from the stomach over the transverse colon

and the folds of small intestine. It prevents infections in the

alimentary canal from reaching the peritoneal cavity

Structure of the Small Intestinal Wall

Intestinal Villi line the duodenum and the proximal part of the

jejunum. They project into the passageway, or lumen, of the canal.

These villi increase the surface area of the wall and in turn increase

absorption – microvilli extend from the end of each villus. Each

also contains blood capillaries, a lymphatic capillary (lacteal) and

nerve fibers

Intestinal Glands (crypts of Lieberkuhn) lie between the bases of

the villi

[pic]

Secretions of the Small Intestine

Goblet Cells secrete mucus

Intestinal Glands are at the base of the villi and secrete a watery

substance that is reabsorbed into the villi, carrying digestive

products with it (absorption) This substance has a pH of 6.5 – 7.5

and contains no digestive enzymes

Epithelial Cells of the Mucosa secretes the following enzymes that

can break down food molecules just before absorption

1) Peptidases – peptides ( amino acids

2) Sucrase – sucrose ( glucose

Maltase – malstose ( fructose

Lactase – lactose ( galactose

3) Intestinal Lipase – fats into fatty acids and glycerol

These secretions are stimulated by mechanical and chemical stimulus

when chyme enters the small intestine

Absorption

Monosaccharides – facilitated diffusion and active transport

Amino Acids – active transport

Fatty Acids and Glycerol - diffusion

Electrolytes – diffusion and active transport

Water – Osmosis

Often there are reversals from hypertonic to hypotonic and visa versa

Movements

Mixing movements called segmentation breaks the chyme into small

parts and slows its movement

Peristalsis is usually weak and stops after pushing the chyme a short

distance.

It takes food anywhere from 3-10 hours to pass through the small

intestine.

Stimulation of the small intestine by over-distension or by severe

irritation may elicit a strong peristaltic rush that passes the entire

length of the small intestine. This sweeps the entire contents into

the large intestine quire rapidly. This prevents the normal

absorption of water, nutrients, and electrolytes – the result is

diarrhea. Defecation becomes more frequent and stools are watery

Ileocecal Valve (Sphincter) , located at the end of the ileum, is usually

closed, preventing the contents of the small intestine from entering

the large intestine and the contents of the large intestine from backing

up into the small intestine

However, after a meal, a reflex (gastroileal reflex) is started and

peristalsis in the ileum is increased. This forces some of the

contents of the small intestine into the cecum. The sphincter regulates the passage of material into the large intestine.

Large Intestine is about 1.5 meters long. It begins in the lower right

side of the abdomen, travels up and crosses over to the left and

descends into the pelvis - opens to the outside

It reabsorbs water and electrolytes and also stores and form feces

until defecation

Parts of the Large Intestine

Cecum is dilated and pouchlike and hangs below the ileocecal

opening. It has a closed en – vermiform appendix. The appendix

has no known digestive function but it contains lymphatic tissue

that can serve to resist infections

Colon (large intestine) has 4 portions:

Ascending colon

Transverse colon – longest and most mobile

Descending colon – makes an S-curve called the sigmoid colon

Rectum becomes the anal canal

Anus is the end of the anal canal. It contains internal and external

sphincters.

[pic]

Structure of the Wall

Lacks villi

Longitudinal muscles are in 3 distinct bands called the teniae coli.

These exert tension of the wall creating a series of pouches

(haustra). Small collections of fat (epiploic appendages)

Functions of the Large Intestine

Little or no digestion occurs here

Secretes a lot of mucus from goblet cells. This lubricates, holds the

fecal matter together, and helps control the pH by being alkaline

(bacteria in feces release acids)

Absorption of water and electrolytes

Bacteria digest cellulose and produce vitamins K, B12, Thiamine,

and riboflavin. Bacteria also causes gas

Movement of the Large Intestine

Mixing is very sluggish – helps absorption

Peristalsis occurs only 2 or 3 times each day and causes mass

movements. Colitis is an inflamed colon (more frequent)

Feces is composed largely of materials that were not digested or

absorbed, together with water, electrolytes, mucus, and bacteria

It is usually 75% water

The color is due to bile pigments that have been altered by bacteria

The odor is from the compounds produced by bacteria (phenol,

hydrogen sulfide, indole, skatole, and ammonia

Phenol is a white crystalline compound produced from coal tar

(C6H5OH) or by the hydrolysis of chlorobenzene and used in

making explosives. It is a strong, corrosive poison – strong odor

Dilute form of carbonic acid

Indole is a white crystalline compound (C8H7N) obtained from

indigo and the decomposition of proteins used in perfumery as a

reagent

Skatole is a foul-smelling, colorless, crystalline compound (C9H9N)

formed by the decomposition of proteins

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