Download Learning Outcome I: The Digestive System

The Digestive System
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See fig. 12.1 p. 214; also table 12.1 p. 214 (overview of
major digestion structures)
Cardiac Sphincter
Pyloric Sphincter
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The Digestive Process
MOUTH
• Both the Mechanical and Chemical Digestion of
food begins in the mouth  saliva is present.
• The pH of the mouth (saliva) is 7 (neutral).
a. Mechanical Digestion:
-- food is chewed with TEETH (32 in total) to
increase its surface area so that chemical
digestion can occur more efficiently.
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• There are four types of teeth (see fig. 12.2 p. 215):
i. Incisors: Scraping – 2 (per quarter of mouth)
ii. Canines (Cuspids): Piercing – 1
iii. Premolars (Bicuspids): Ripping – 2
iv. Molars: Crushing – 3 (incl. “Wisdom” teeth)
• Human Dental Formula = 2123
2123
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• The tongue manipulates the food into a ‘wet ball’
called a bolus and then pushes the bolus to the back
of the throat.
• The tongue also possesses taste buds which help to
determine the quality of the food being consumed.
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b. Chemical Digestion:
-- there are three pairs of salivary glands that are
connected to the mouth by ducts (‘connector tubes’).
-- these glands SECRETE (produce and release)
saliva into the mouth at the thought, smell, or taste of
food.
Salivary Ducts
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-- saliva is comprised of three substances:
i. Water
ii. Mucus
iii. Salivary Amylase (enzyme)
-- Functions of Saliva:
1. To lubricate food making it easier to swallow
(water/mucus).
2. To coat the throat/esophagus for protection
(water/mucus).
3. To provide the digestive enzyme salivary
amylase which digests (hydrolyzes) STARCH into
the disaccharide sugar MALTOSE. Salivary
amylase functions optimally at a pH of ~7.
-- Not all of the starch that humans ingest is
hydrolyzed in the mouth, some is swallowed along 7
with the newly produced maltose.
Swallowing
Pharynx
Also: See Fig. 12.3 on p.216
• Swallowing occurs in the
pharynx (throat), a region
between the mouth and the
esophagus that receives food
from the mouth and air from
the nasal cavity.
• Once the tongue pushes the
bolus into the pharynx,
swallowing is induced
through a REFLEX action.
• Two preventative measures
also occur by reflex,
simultaneous to the swallowing reflex, in order to disallow
food from entering the air
passages:
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1. As the bolus enters the
pharynx, the soft palate
(including the uvula) closes
over the nasal passages at the
roof of the pharynx.
2. At the same time, the epiglottis
(flap of tissue) flops down over
the glottis, which is the opening
to the trachea, making the
esophagus the primary receiver
of the bolus. As well, we do
NOT breathe when we swallow.
Uvula
* The larynx is the first portion of the trachea and will be
discussed in the Respiration Unit.
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Esophagus (see fig. 12.4 p. 217)
• The esophagus is a 25-30 cm muscular (smooth
muscle) tube extending from the pharynx to the
stomach; it is dorsal to the trachea (which is ventral).
• No digestion occurs here, only the transport of the
bolus (‘wet ball’ of food) to the stomach.
• The inner lining of the esophagus (lines the central
cavity or lumen) is made up of mucosa tissue which
produces large amounts of mucus to constantly
protect the esophagus from damage due to ‘coarse’,
‘sharp’, or acidic foods.
• The bolus is squeezed rather tightly in the esophagus,
so gravity is usually insufficient for transport
(exception: liquids), but the mucus lining aids in easing
the transport process.
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• Smooth muscle contractions by esophageal
muscles, known as PERISTALSIS, move the
bolus down the esophagus by contracting
behind the bolus.
•The bolus then moves
through the Cardiac
Sphincter (‘Cardiac’
because it’s closer to the
heart) which regulates a
steady influx of food into
the stomach. This
sphincter acts as a VALVE
EXCEPTIONS: Vomiting
that, for the most part,
(Reverse Peristalsis) and
prevents the backflow of
Heartburn.
food into the esophagus.
Read p. 217 “The Wall of the Digestive Tract” to
learn a bit about tissue layers in the dig. tract.
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Stomach
• The stomach is a large, thick-(and folded)-walled, J-shaped,
distensible (able to stretch) organ that can expand to hold 2-4 L
of food (see fig. 12.5 p. 218).
• Lies on the left of the body beneath the diaphragm.
Oblique Muscle
Circular Muscle
Longitudinal Muscle
The term ‘gastric’ refers to
the stomach.
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• Both Mechanical and Chemical Digestion occur in the
stomach:
a. Mechanical Digestion
i. Food is CHURNED to break apart the bolus
(increasing the Surface Area (S.A.) of the bolus) and
to aid in its mixing with the gastric juice.
-- churning is accomplished by the contraction of the
smooth muscle lining the stomach.
-- there are three layers of smooth muscle: Circular,
Longitudinal, and Oblique (diagonal).
ii. Storage of Food:
-- the stomach regulates the release of the
‘finished’ product into the small intestine.
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b. Chemical Digestion
-- the thought, smell, or taste of food triggers a
nervous message from the brain to the stomach which
initiates secretion of gastric juice (which contains
ingredients from three different cells in the stomach).
-- the subsequent presence of food (or saliva) in the
stomach initiates the release of the hormone
GASTRIN from the cells (G cells) of the stomach wall
into the capillaries of the stomach.
-- gastrin stimulates further release of gastric juice, in
order to provide a sustained secretion that continues
until food exits the stomach.
** Nervous system STARTS the release… hormones
SUSTAIN the release…
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-- Gastric juice is made up of three substances,
each secreted from a different stomach cell
(thus, gastrin has to signal all three cell
types):
1. Mucus
-- secreted by Mucosa (or Goblet) cells.
-- serves to line the stomach lumen and
protect the walls from HCl acid and pepsin
(protein-digesting enzyme) activity
-- water is a large constituent of mucus and
serves to regulate temperature within the
stomach and to lubricate its contents.
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2. Pepsinogen:
-- an inactive enzyme BUT a precursor to the
important enzyme pepsin.
-- secreted by the Chief cells in the gastric pits
of the stomach lining.
-- is a type of zymogen (a protein-digesting
enzyme that is secreted in an inactive form).
3. Hydrochloric Acid (HCl)
-- secreted by the Parietal cells in the gastric
pits of the stomach lining.
-- has three major functions:
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i.
To dismantle the tissues in food by disrupting the
intercellular ‘glue’ (matrix) that binds cells
together in meat and plant material. Also, it
serves to denature proteins (incl. salivary
amylase). Both of these functions serve to
increase the SA of food. (eg. of Mech. Dig.).
ii. Lowers the pH of the stomach lumen to approx.
2-2.5 which kills most bacteria present in the
ingested food.
iii. Reacts with pepsinogen to convert it to the
active enzyme pepsin, which is an active
PROTEASE. Pepsin digests proteins down to
polypeptides. Hydrolysis is incomplete since
pepsin can only break peptide bonds adjacent to
certain amino acids.
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GASTRIC PITS
• The Mucosa, Chief,
and Parietal cells are
all located within
stomach wall infoldings
known as GASTRIC
PITS (otherwise known
as gastric glands).
Stomach Lumen
(secretes gastrin)
It is important that the HCl and the pepsinogen are
released from different cells because if they mixed prior
to their release into the stomach lumen, pepsin would be
produced and the cells lining the stomach would be
digested from the inside-out.
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• Pepsin’s optimal pH is 2-3
• The mucus protects the stomach wall from
autodigestion by pepsin and denaturation by HCl.
• Still, the Mucosa cells are constantly eroded, and
mitosis must regenerate enough cells to completely
replace the stomach lining every 3 days.
• If there is a lack of mucus in a localized area (caused
by stress (can inhibit mucosa cell effectiveness) or a
Helicobacter pylori bacterial infection (actually
destroys mucus lining)), an ULCER may form.
• Once food is finished with in the stomach, it is called
acid chyme (liquid), and the passage of it into the
small intestine (duodenum) is regulated by the
PYLORIC SPHINCTER one ‘squirt’ at a time.
• It takes approx. 2-6 hours for a meal to completely exit
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the stomach.
A typical stomach ulcer
There’s a hole in my stomach, dear Liza, dear Liza…
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Small Intestine
• A coiled, narrow tube about 6-7 meters in length and
about 2-5 cm in diameter (“small” = narrow).
• Divided into three sections:
i. Duodenum: approx. 25-30 cm in length.
-- lined with a thin layer of
mucosa cells for mucus
production.
-- location of almost all
chemical digestion.
ii. Jejenum
iii. Ileum
* Both the jejenum and ileum are responsible for nutrient
absorption (primarily) along with some ‘finishing touches’
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to chemical digestion.
• Most enzymatic (chemical)
digestion of macromolecules
occurs in the small intestine
relative to the stomach and
the mouth.
• Furthermore, the absorption
of the final products of
digestion occurs in the SI with
help from tiny ‘infoldings’
along its length called villi
which increase the SA for
absorption.
Lumen of SI
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Duodenum – Chemical Digestion
 The duodenal walls possess chemoreceptors
(chemical-sensitive nerve endings) that detect
when as little as a drop of acid chyme enters the
duodenum.
 Once the chyme is detected (note that the chyme
‘trickles’ into the duodenum due to the regulation of
the pyloric sphincter), three hormones are
released simultaneously from cells lining the
duodenum into the capillaries of the duodenum.
 Hormone #1: Secretin
-- travels through the blood to the pancreas (large
gland that lies between the stomach and the SI).
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 Secretin signals the pancreas to release Sodium
bicarbonate (NaHCO3) through the pancreatic
duct directly into the duodenum.
 The NaHCO3 acts as a buffer to neutralize the
acidic pH of the acid chyme in order to maintain a
duodenal pH of about 8-10 (slightly basic in
nature).
 This is important for two reasons:
i. The acid could eventually damage duodenal
tissue if left un-neutralized (would erode relatively
thin mucus layer relative to the stomach’s).
ii. All enzymes that function in the duodenum and
other regions of the SI require an optimal pH of 810.
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 Hormone #2: CCK (Cholecystokinin)
-- travels through the blood to TWO locations:
1. Gall Bladder
-- CCK signals the gall bladder to release bile into the
bile duct taking it directly to the duodenum.
-- bile is produced by the liver and is stored in the gall
bladder.
-- bile contains bile salts, which are important for the
efficient digestion of fats.
-- bile serves to EMULSIFY fats which means that
large fat globules are physically broken down into smaller
fat globules (‘spreads fats out’, so to speak).
-- this increases the SA of the fats so that enzymes
can collide (mix) with them more efficiently (more
collisions)-- bile is NOT an enzyme!!!
-- it contributes to Mechanical Digestion.
-- bile also contains pigments that are byproducts of Red Blood Cell
destruction in the liver (this allows these byproducts to exit the body
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in the feces and to not accumulate in the liver).
Emulsification of
Fats (ignore the rest
of the picture)
2. Pancreas
-- CCK also signals the
pancreas to release its
digestive enzymes in
the form of pancreatic
juice.
-- the juice is released into
the pancreatic duct
(with the NaHCO3) and
into the duodenum to
meet and ‘attack’ the
incoming chyme (food).
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The Bile and Pancreatic Ducts
Liver
Pancreas
‘Direct’ connections to the Duodenum
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Hormone #3: Enterogastrone
- acid chyme in the duodenum also stimulates the
duodenal cells to release enterogastrone (a steroid
hormone).
- enterogastrone travels to the stomach where it
inhibits muscle contractions, thereby slowing down
the entry of food into the duodenum.
- this provides a suitable amount of time for the
digestive processes to occur in the duodenum, thus
ensuring that the process is as comprehensive as it
needs to be.
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Pancreas (An Accessory Organ)
• The pancreas possesses two cell types:
1. Hormonal (Endocrine) Cells
-- these cells produce the hormones insulin
and glucagon that help to regulate bloodglucose levels (more on insulin later this unit).
2. Exocrine cells that secrete Pancreatic Juice
-- pancreatic juice is released from the
pancreas into the pancreatic duct directly into
the duodenum.
-- pancreatic juice contains the following
substances which aid in chemical digestion of
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all four major macromolecules:
a. Trypsinogen (another zymogen):
-- released into the duodenum and is converted into the
ACTIVE enzyme trypsin by a duodenal-produced
enzyme enterokinase (duodenal equivalent of HCl in
stomach, as it activates trypsinogen into trypsin)
-- Trypsin digests the polypeptides (‘mini-proteins’) that
resulted from stomach digestion into even smaller, more
manageable, polypeptides.
-- nevertheless, protein digestion is still incomplete.
b. Peptidases
-- also released as zymogens and activated by the
duodenum-released enterokinase.
-- two types:
i. Aminopeptidases
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ii. Carboxypeptidases
-- these enzymes digest the smaller
polypeptides into individual amino acids by
‘clipping’ them off one at a time.
-- Seems onerous, but…
-- increased SA of substrates
(polypeptides) because of trypsin.
-- aminopeptidase can cleave from
amino end of chain while
carboxypeptidase can cleave from the
carboxyl end (meet at the middle).
- even after this process, some dipeptides still
exist!
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c. Pancreatic Amylase
-- an enzyme that digests the remaining starch to
maltose (a disaccharide of two glucoses).
-- does exactly the same job as Salivary Amylase
except functions at a different pH.
d. Lipase
-- an enzyme that digests the emulsified (high SA)
fats (thanks to bile) into their fatty acid and glycerol
constituents.
e. Nucleases
-- various enzymes that digest DNA/RNA into
individual nucleotides that we can use during DNA
replication and RNA transcription.
- however, some non-monomers of nucleic acids still
exist even after nuclease action.
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•
Once chemical digestion is completed, the
jejenum and ileum portions of the small
intestine must serve to ABSORB the
monomers created by the digestive
process.
However, two types of dimers still exist:
•
Maltose – a disaccharide made up of two
glucose molecules.
ii. Dipeptides – two amino acids joined together.
*As well, some nucleic acids still require further
digestion.
• The digestion of these molecule-types does not
occur within the duodenum.
i.
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Small Intestine (Jejenum/Ileum)
• Topographically speaking, the lumen of the
digestive tract is ‘outside’ the body (ie. It is a
tunnel).
• To ‘enter’ the body, the nutrients (amino acids,
fatty acids, glycerol, nucleotides, glucose
(eventually), vitamins, minerals, water) must
cross the lining of the small intestine and enter
the bloodstream.
• Once in the blood, these nutrients can be
delivered to all cells in the body, and not be
eliminated as waste.
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• Food moves through the small intestine by peristalsis
(SI is lined with smooth muscle).
• The SI has a huge SA which maximizes the efficiency
of absorption (600X more absorptive than that of a
straight tube).
• The jejenum/ileum are lined with villi (singular: villus)
which are numerous infoldings/projections that increase
the SA of the SI.
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• Furthermore, each villus possesses numerous
microvilli which are further projections that serve to
further increase the SA for absorption (the
villi/microvilli both project towards the lumen of the SI).
One villus
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• Each villus has its own capillary network as well as a
lacteal (lymphatic capillary) (see fig 12.6 p. 219).
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Villi Cells
• The cells lining the villi are
equipped with numerous
mitochondria because
active transport is often
required for absorption.
• The ‘lumen’ of the villi is
the ECF between the villi
cells and the
capillaries/lacteals.
• The villi cells also produce
three enzymes and release
them into their lumen just
prior to absorption in order
to complete digestion.
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• These three enzymes are:
i. Maltase
-- digests maltose to glucose within the villi lumen.
ii. Dipeptidases
-- digest dipeptides, producing amino acids.
iii. Nucleosidases
-- digest remaining small nucleic acid chains to
individual nucleotides.
** all chemical digestion is now complete!!!
• Once fatty acids and glycerol enter the villi lumen
(ECF), they recombine to form fats which are too big
to be absorbed by a capillary, so they enter the lacteal
(lymphatic system).
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• So, fats initially enter the lymphatic system, but
eventually enter into the circulatory system at
the subclavian vein in the arm (where the
lymphatic system forms a ‘Y’ with the circ. sys.).
• What, therefore, do the blood capillaries within
a villus absorb?
– Amino acids
– Glucose
– Nucleotides
– Vitamins
– Minerals
– Water
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Large Intestine (Colon)
• Once nutrients are
absorbed, the remains
consist of undigestible
materials such as cellulose
(fiber), excess water, and
perhaps some nutrients that
were consumed in excess.
• These materials pass
through the ileo-caecal
valve into the large
intestine (colon).
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• Anaerobic bacteria called E. coli live within the
colon and feed on any unabsorbed nutrients
that may exist there.
• In return, E. coli produces various vitamins
(B12, thiamine, riboflavin, K) that are normally
deficient in the diet.
E. coli
• The major role of the colon is to reabsorb the water
that was added to the food during the digestive process,
thus solidifying the once liquid ‘stuff’.
• The vitamins produced by E. coli are reabsorbed
along with the water.
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• E. coli also begins the decomposition of the leftover food and converts it all to FECES with a
changed colour, smell, and texture.
• Read about diarrhea and constipation (p.221).
• The rectum then stores the feces until
defecation through the anus.
** the appendix is a small projection off of the
colon which fights infections by accumulating
toxins taken in through the mouth which
somehow survived the stomach’s caustic
lumen.
* the appendix may become inflamed and may
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even burst.
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