Digestive System


Breaks down food, absorbs
nutrients and eliminates
wastes
Food must be broken down
so that the pieces are small
enough to pass through the
walls of the intestinal villi
2 Forms of Breakdown:
Mechanical and Chemical:
1. MECHANICAL
 in the mouth: teeth
cut and grind the
food into small
pieces
Mechanical
2. in the stomach:
strong muscles mix
and churn the food
CHEMICAL:
1. in the mouth: saliva is
mixed with the food to
moisten and start chemical
breakdown
 - salivary amylase begins
the chemical breakdown of
starch into maltose
 (starch + H2Omaltose)
 ( saliva also contains
bacteria-killing
enzymes and antibodies to
help guard against
infection)
2. In the stomach:
four fluids are
secreted by the
stomach to aid
chemical
digestion:
Gastrin, HCl,
Pepsin, Mucus


Gastrin
(a hormone)
stimulates the
secretion of HCl
HCl

converts pepsinogen to pepsin
Pepsinogen:


an inactive form of pepsin, which
breaks down proteins (a protease)
(protein + H2O --> peptides)
Mucus

protects the stomach lining from digesting itself!
(the lining is still replaced every few days)
3. In the Small Intestine:

The PANCREAS
secretes “pancreatic
juice” which
contains sodium
bicarbonate
(NaHCO3 ) (making it
basic) and digestive
enzymes:
a.
Pancreatic Amylase:

also digests starch
(starch + H2O --> maltose) (hydrolysis)

b. Trypsin:
 also digests proteins (a “protease”) - is secreted
as trypsinogen, and converted to trypsin in the
duodenum
 (protein + H2O --> peptides)
c. Lipase



digests fat
droplets after
they have been
emulsified by bile
salts
(fat + bile salts  fat
droplets)
(fat droplets + H2O
 glycerol + fatty
acids)
d. Nuclease


digest nucleic acids (DNA
and RNA)
DNA + H2O --> nucleotides
4. In the intestines: the mucosa of the
intestinal villi secrete 2 enzymes:
a. Peptidase
 digest peptides into amino acids (finish protein
breakdown)
 (peptides + H 2O amino acids) (hydrolysis)
b. Maltase



digests maltose to glucose (finishes starch
breakdown)
( maltose + H2O glucose)
(the other disaccharides are digested in the
intestines as well - each have their own enzyme)



The digestive enzymes
are well adapted to the
environments that they
encounter.
Salivary amylase works
best in a neutral pH.
Pepsin works best in a
very acidic environment
(the stomach!) and the
pancreatic and
intestinal enzymes work
best in a basic
environment.
Enzyme
Produced by:
Acts on:
Optim
al pH
Site of
Action
Product(s)
Salivary
Amylase
Salivary
Glands
Starch
7
Mouth
maltose
Pepsin
Stomach
Protein
Acidic
Stomach
Peptides
Pancreatic Pancreas
Amylase
Starch
Basic
Small
intestine
maltose
Trypsin
Pancreas
Protein
Basic
Small
intestine
peptides
Lipase
Pancreas
Lipids
Basic
Small
intestine
Glycerol +
fatty Acids
Peptidases Small intestine
Peptides
Basic
Small
intestine
Amino
acids
Maltase
Small intestine
Maltose
Basic
Small
intestine
glucose
Nuclease
Pancreas
Nucleic acid
Basic
Small
intestine
nucleotides
The System in Brief:

Food enters via the mouth.
dig system animation

Swallowing Animation
food is cut and
ground into small
bits, mixed with
saliva, and rolled (by
the tongue) into a
bolus which is
swallowed.
Dr. Levitan's Nasoendoscopyin New Zealand

Laryngoscopic Trachea vs Esophagus
The bolus travels
down the esophagus,
(directed past the
trachea by the
epiglottis) by the
rhythmic contractions
of the smooth
muscle (called
peristalsis )

The bolus enters the
stomach through the
cardiac sphincter.


The Stomach has 3
Functions:
Temporary storage

mechanical digestion (mixing and churning)



Chemical digestion (with
the gastric juices)
slightly digested liquidy
food (called chyme)
leaves the stomach
through the pyloric
sphincter
- the pancreas secretes
digestive enzymes and
NaHCO3 (sodium
bicarbonate) into the
DUODENUM (these are
the pancreatic juices)

slightly digested
liquidy food (called
chyme) leaves the
stomach through the
pyloric sphincter


Helicobacter pylori (commonly referred to as h. pylori)is a bacterium that
infects the mucus lining of the stomach. Many peptic ulcers and some types
of gastritis are caused by H. pylori infection, although most humans who are
infected will never develop symptoms. This bacterium lives in the human
stomach exclusively and is the only known organism that can thrive in that
highly acidic environment. It is helix-shaped (hence the name helicobacter)
and can literally screw itself into the stomach lining to colonize.
H. Pylori infections

It is estimated that about two thirds of the world population are infected by
helicobacter pylori. Actual infection rates vary - the Western/Developed
nations have rates around 25% and less developed nations have much
higher rates of infections. In the United States, infection is primarily in the
older generations (about 50% for those over the age of 60 compared with
20% under 40 years) and the poorest. This is largely attributed to higher
hygiene standards and widespread use of antibiotics. However, antibiotic
resistance is appearing in H. pylori. There are already metronidazole
resistant strains present in the UK population.

There is also evidence of a higher incidence of symptoms in people who eat
spicy foods, apparently because of the effect these foods have on the
stomach lining. When the protective lining is reduced from certain foods, h.
pylori is better able to spiral into the stomach wall. This is one of the reasons
that it was (and still is) commonly thought that spicy foods caused stomach
ulcers. In actual fact, it is the h pylori that causes the stomach ulcers, but the
spicy foods creates the conditions for h. pylori to cause problems.

In the absence of any treatments, H. pylori infection apparently persists for
life; our immune systems are not able to eradicate it.
Peptic ulcer


The GALL
BLADDER secretes
bile (made in the
LIVER) into the
duodenum to
emulsify fats
Gallstones

* Most of the
chemical digestion
happens in the small
intestine, as does the
absorption of
nutrients**


the large
INTESTINE
absorbs H2O and
salts
the rectum
temporarily
stores solid waste,
which leaves the
body through the
anus.
Some Structures Up Close:
The Small Intestines:
 small in diameter but not
in length! (approximately
6 meters long)
 chemically (with
intestinal juices) and
mechanically breaks
down chyme, absorbs
nutrients and transports
undigested molecules
to the large intestines


Segmentation movements
(unsynchronized
contractions) slosh the
chyme back and forth in
the lumen of the small
intestine.
Peristalsis (not nearly as
strong as in the esophagus)
moves the chyme along to
the LARGE INTESTINE.

The first section of
the small intestine is
called the duodenum


The wall of the small
intestines are covered
in villi- small finger-like
projections
each villus contains an
outer layer of epithelium,
blood vessels and a
LACTEAL(a small
lymphatic vessel)
The Epithelium:


each cell is covered
with microvilli
the microvilli look
like a “brush border”
and secrete the
intestinal digestive
enzymes (therefore
called “brush
border” enzymes)


these finish the
digestion of the chyme
into small pieces that
can cross the cell
membranes of the
epithelium
the microvilli GREATLY
increase the surface area
for absorption
The Blood vessels:
readily pick up the
nutrients from the
epithelium cells and
transport them via the
HEPATIC PORTAL
VEIN to the liver.

The Lacteal:

will transport the
components of fat
(glycerol + Fatty
acids) away from the
intestines
The Absorption of Fats



fatty acids and glycerol are absorbed by the
epithelial cells
- inside, they are recombined along with
phospholipids, cholesterol and proteins (called a
chylomicron)
- the chylomicron then enters the lacteal of the
villus, travel through the lymphatic system and will
enter the blood at a duct near the R.
subclavian vein

the liver picks up the chylomicrons as they come
through (in the blood) and either store them or
changes them into specific lipoproteins to be
transported to other parts of the body (lipids must
be combined with protein carriers to make them
soluble so they can be transported in the blood and
utilized by the cells.)




The Large Intestine
has two sections, the
COLON and the
RECTUM
contains bacteria
(including E.coli.) that
live on unabsorbed
and undigested
nutrients
- the bacteria
ferment any
remaining
carbohydrates and
release H+, CO2, and
methane gas.
(flatus (flatulence)



by-products of the
bacterial digestion
account for the color
and the smell of the
feces.
While using our
undigested chyme as
nutrients, the bacteria
synthesize vitamin
B12, thiamin,
riboflavin and vitamin
K (essential!)
- the colon absorbs the
vitamins, water and
salts
The Liver:
Has 6 functions: (From
Mader)
1. Converts Hb for RBCs
to bilirubin and
biliverdin which are
excreted along with
bile salts in bile.
2. Produces bile (bile
salts are used to
emulsify fats), which
is stored in the gall
bladder .


3. Detoxifies blood by
removing and
metabolizing
poisonous substances.
4. Maintains
[glucose] in the
blood. (Stores glucose
as glycogen after
eating and breaks
down glycogen to
glucose to maintain
the [glucose] of blood
between eating
periods.)


5. Produces urea from
the breakdown of
amino acids
(deamination)
6. Makes blood
proteins from amino
acids.

(7. Produces
lipoproteins which transport fatty
acids, triglycerides,
and cholesterol to and
from body cells.

Nutrients travel
directly from the
intestines to the liver
via the hepatic
portal system.
Bile:
A mixture of many
things including
bilirubin (a waste
product from old
blood) and bile salts
(made form
cholesterol and amino
acids)
 - produced by the liver
 - stored and
concentrated in the
gall bladder
 - excreted into the
duodenum via the bile
duct

The bilirubin is broken
down in the intestines
and excreted as solid
waste. (responsible for
the color of the feces.

The bile salts are used
to emulsify fats. They
have a hydrophilic end
and a hydrophobic
end. The hydrophilic
end dissolves in the
lipid. They will
surround the lipid
molecules, breaking
apart the fat ”globs”.
(= emulsification)

This greatly
increases the surface
area available for
lipases to attach to
them for digestion.
The Pancreas:

Both an exocrine
gland (a gland that
secretes substances via
a duct) and an
endocrine gland (a
gland that secretes
hormones that diffuse
into the blood stream).
The Exocrine Functions:

The digestive enzymes
it produces (pancreatic
amylase, trypsin,
lipases, and nucleases)
are released into the
duodenum via the
pancreatic duct.
(which connects to the
bile duct. )
The Exocrine Functions:
The “islet cells” inside
the pancreas produce
two hormones:
insulin and glucagon.

These have opposite
functions:
Insulin
- Causes the body cells
to take up glucose
from the blood and
either use it or store it
as glycogen or fat .
(resulting in lowering
the [glucose] in the
blood.
Glucagon:

Stimulates the liver to
break down its stored
glycogen and release
the glucose into the
blood. Also stimulates
the break-down of fat
to glucose. (results:
increases the [glucose]
in the blood.

The 2 hormones work
to maintain the blood
glucose at a constant
level.

Diabetes mellitus: A
condition resulting
from a nonfunctioning insulin
system.
Mesenteric Arteries and Veins

When large numbers of chylomicrons are
being absorbed, the lymph draining from the
small intestine appears milky and the
lymphatics are easy to see. In the image
below, of abdominal contents from a coyote,
the fine white lines (arrows) are intestinal
lymphatics packed with chylomicrons. That
lymph passes through mesenteric lymph
nodes (LN) and then into larger lymphatics.
Induced fit