Download Gastric Secretions

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
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
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
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).
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 bonelike 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.
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)
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
http://www.mefeedia.com/video/32552677
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
rd
3 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
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
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
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
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.
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
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
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.
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