LESSSON 4: THE
DIGESTIVE SYSTEM
Chapter 25
The primary purpose of the digestive system is to
break nutrients down into forms that the body can use and to absorb them so
they can be distributed to the body's tissues.
DIGESTIVE
FUNCTIONS AND PROCESSES
Digestive
Functions
There are four main functions carried out by the
digestive system.
1. Ingestion: the intake of nutrients into the body
2. Digestion: the breakdown of large molecules into
smaller ones
3. Absorption: the uptake of nutrient molecules into the
cells of the digestive tract and, from there, into the bloodstream
4. Defecation: elimination of undigested residue
Digestive
Processes
These functions are carried out by means of 3
digestive processes. These are the
actual activities of organs in the digestive tract that make the functions
possible.
1. Motility: the muscle contractions that break up food,
propel it through the canal of the digestive tract, and mix it with digestive
enzymes
2. Secretion: secretion of the enzymes, hormones, and
other products that regulate or actually carry out digestion
3. Membrane
Transport: mechanisms of
absorbing nutrients and transferring them to blood or lymph
Stages
of Digestion
There are two different kinds of breakdown that
food undergoes as it is digested. One
of these is called mechanical digestion.
This involves physically breaking the food up into smaller particles to
expose more surface area to the enzymes that finish the digestive process. It is accomplished by teeth as they cut and
grind food and by contractions of the stomach and intestine as they churn the
food around.
The other is chemical digestion, which consists
pretty much of a series of hydrolysis reactions. Remember back in the first lesson when we looked at how polymers
or macromolecules are built? We said
that you take two monomer (smaller) units and tie them together by removing a
hydrogen from one and a hydroxyl group from the other. This frees up bonds in the two molecules that
can be used to hook them together. It
also frees up the hydrogen and hydroxyl, which are then united to form a
molecule of water. You will recall that
this is called dehydration synthesis because it involves removal of water to
string smaller molecules together into bigger molecules.
The molecules in foods we eat are mainly
polymers—long chains of monomers formed by dehydration synthesis. Polymers are great big molecules—too big to
be absorbed into the bloodstream from the digestive system, so they need to be
broken down by chemical digestion.
Breaking down a polymer requires reversing the dehydration synthesis
reactions that built it. This involves
breaking the bonds between monomers that hold the polymer together; and that
means we’re going to need some hydrogens and hydroxyl groups to add back to the
resulting monomers. We get these
hydrogens and hydroxyls from water molecules.
That's why this whole break-down process is called hydrolysis--hydro-
refers to water, and -lysis means break-up; so hydrolysis is breaking up a
molecule by adding water. Pretty good
name.
Here is the rundown of macromolecules the
digestive system runs into and the products into which they're made:
Polysaccharides (mostly starch) are
broken down into disaccharides and monosaccharides (sugars.)
Proteins are broken down into amino
acids.
Fats are broken down into glycerol and fatty
acids.
Nucleic acids are broken down into nucleotides.
This should look familiar because these products
are exactly what the macromolecules were made up of in the first place.
This chemical breakdown is accomplished by
digestive enzymes produced in the salivary glands, stomach, pancreas, and small
intestine. We'll talk more about how
all this works a bit later. It is
helpful to remember that a few nutrients are present in foods in a form usable
by our bodies; so they don't require any digestion. These nutrients, like vitamins, minerals, and water, just need to
get to the cells that can absorb them.
GENERAL
ANATOMY OF THE DIGESTIVE SYSTEM
This system consists of so many organs and kinds
of tissues that it takes a while to discuss each of them, even in a general
way. We'll talk about the general
subdivisions of the system, then work our way from top to bottom, using only as
much detail as we need in each region.
Subdivisions
of the Digestive System
There are two major subdivisions of the digestive
system:
1. Digestive
Tract: a tube from the mouth to the anus through which food actually
passes from intake to elimination. This
tube is sometimes called the alimentary canal or the gastrointestinal
tract. The digestive tract is about 30
feet long and is open to the outside at both ends. Because of this openness, the digestive tract is considered to be
external to the body; this means that food you eat is not considered to be
"inside" of you until it is absorbed into the cells of the digestive
tract and then into the bloodstream. As
long as it is traveling down this tube, it is outside of you.
The organs included in the digestive tract are the
oral cavity (mouth), pharynx (throat), esophagus, stomach, small intestine, and
large intestine. Remember that these
are the organs the food actually passes through.
2. Accessory
Organs: These are organs that
are part of the digestive system, but which food never enters. They include the teeth, tongue, salivary
glands, liver, gallbladder, and pancreas.
All of these contribute to the system's functions, but none of them hold
food at any time.
Upper
Digestive System
The
Mouth. The mouth is also called the oral cavity
or the buccal cavity. It is enclosed by
the cheeks, lips, palate ("roof" of the mouth), and tongue. The posterior opening into the throat is
called the fauces.
The normal adult has 32 teeth, collectively
called your dentition. You probably
know that when a baby is born, it has no teeth. In the first few years of life 20 temporary or deciduous
teeth erupt. As you grow up, these are
replaced by your permanent teeth.
That's often when you personally experience one of the difficulties
caused by human evolution; as our faces flattened and our jaws shortened
through thousands of years of evolution, there was less room in our jaws for
the 32 teeth. As our jaws shortened, we
failed to keep up by having fewer teeth.
Frequently the last set of molars, often called wisdom teeth, fail to
erupt or crowd the other teeth because there simply isn't enough room for them.
There are salivary glands near the mouth. These come in two kinds: intrinsic
salivary glands are numerous and scattered throughout the mouth. These produce saliva pretty much
continuously and are responsible for keeping the mouth moist. You have three pairs of large, ducted extrinsic
salivary glands. The parotid
glands are just anterior to the earlobes; the submandibular glands (sub
means under; mandible is the jawbone) are under the jaw; and the sublingual
glands (lingua is the word for tongue) are beneath the tongue.
Pharynx. The pharynx is the throat. It stays closed when you are not swallowing;
this keeps excess air out of the esophagus.
Esophagus. The esophagus is a straight muscular
tube about ten inches long that extends from the throat to the stomach. It passes through the diaphragm just before
it enters the stomach at its cardiac orifice.
This lower end of the esophagus is constricted in what is called the lower
esophageal sphincter. This
closes the cardiac orifice to prevent backflow of the stomach contents into the
esophagus. When there is backflow,
called gastroesophageal reflux, you experience a burning sensation in
the lower end of the esophagus. This is
what we commonly call heartburn; as you can see, heartburn has nothing whatever
to do with the heart. It got that name
because the burning sensation is near the heart; most likely it was once
thought to originate in the heart. The
burning is because the esophageal lining isn't adapted to handle the acid
present in the stomach, and it becomes irritated.
The
Stomach
The stomach is simply a muscular sac
found in the upper left quadrant of the abdomen just below the diaphragm. It is J-shaped; the J is pretty vertical in
tall people, but fairly horizontal in short people. This is an issue of how much room there is in the abdominal
cavity because the size of the stomach itself doesn't vary much from person to
person, irrespective of height. Tall
people have room for a vertical stomach; short people don't. The stomach has four regions. At the very top is the cardiac region; this is
at the very top by the cardiac orifice (orifice means
opening). It's called cardiac because
of its proximity to the heart. The fundus
is the upward bulge next to the cardiac region. The largest portion of the stomach is the body, and the narrow
bottom part is the pyloric region.
The stomach wall has many folds on the
inside. These are called rugae
and allow for expansion when the stomach fills. In fact, these folds smooth right out when the stomach is
full. Glands inside the stomach secrete
mucus, acid, enzymes, and hormones.
We'll talk more about these secretions later.
The
Liver, Gallbladder, and Pancreas
The
Liver and Gallbladder. The liver is a large reddish-brown gland
found in the right hypochondriac and epigastric region right below the
diaphragm. At about three pounds, it is
the largest gland in the body. The
liver has four lobes, and if you look at the inferior surface, you'll find the gallbladder
nestled up against the liver. Bile is
secreted in the liver and collected into tiny channels called bile
ductules. (The ending -ule means little.) These join into the right and left hepatic ducts (hepat
= liver), which then converge into the common hepatic duct. Meanwhile a duct coming from the gallbladder
called the cystic duct (cysto = bladder) joins the common hepatic duct to
form the bile duct. Later this
joins with the pancreatic duct and empties into the duodenum.
The
Pancreas. The pancreas is another large gland
found in the upper abdomen; it is on the left side, just behind the
stomach. I's duct, the pancreatic
duct joins with the bile duct from the liver and empties into the
duodenum. In addition to this, many
people have an accessory pancreatic duct which opens into the duodenum
independently.
The
Small Intestine
The small intestine lies coiled up
filling most of the abdominal cavity.
It's called small because of its diameter, but maybe we should call it
the long intestine. This is the longest
part of the digestive tract—actually much longer than the so-called large
intestine—six or seven yards relaxed.
There are three regions of the small
intestine. First is the duodenum,
which is about ten inches long; next is the jejunum, which is about
eight feet long; and last is the ileum, about twelve feet long. It ends in the ileocecal junction with
the cecum of the large intestine.
Inside, the small intestine is highly folded. The largest folds are in a sort of spiral
pattern, like the rifling in a gun barrel.
This sends the undigested food mass in a spiral pattern through the
lumen of the intestine. All over this
folded lining are small finger-like projections that give the surface a sort of
fuzzy appearance. These are called villi
(which means hairs); each
villus has a blood vessel and a lymph vessel called a lacteal. Then on the surface of each villus is another
set of even tinier projections called microvilli.
Near the end of the small intestine, you'll also
find lots of lymphocytes (white blood cells involved in fighting infections)
and patches of lymphatic tissue.
The
Large Intestine
The large intestine or colon
is wider than the small intestine, but also much shorter--only about five feet
long. It begins where it meets the
ileum of the small intestine. Now the
small intestine does not flare out to match up with the much wider large
intestine; instead, where the small diameter tube meets the large diameter
tube, there is a sort of pouch called the cecum, that hangs below the
ileocecal valve. Also hanging on the
bottom of the cecum is a short blind tube called the vermiform appendix, what
you've probably always called just the appendix. After the junction with the small intestine, the large intestine
goes up the right side of the abdomen; this part of the organ is called the ascending
colon. It then makes a sharp
left turn at the top of the abdomen right beside the liver; the turn is called
the hepatic
flexure. The transverse
colon travels across the top of the abdomen to the splenic flexure (you've
got it--right beside the spleen) and then goes down the left side as the descending
colon. Once the colon is at the
bottom of the abdominal cavity, an S-shaped curve turns it toward the center of
the body and downward. This curved
portion is called the sigmoid colon (named for the Greek letter S, sigma), and
empties into the rectum, which ends in the anal canal and anus. There are two sphincters in this location,
an internal
and an external sphincter.
PHYSIOLOGY
OF ORGANS ABOVE THE STOMACH
The
Mouth
The mouth has several digestive functions:
§
ingestion: the intake of food
§
tasting
and other sensory responses to food
§
mastication
or chewing
§
chemical digestion
The cheeks and lips serve to retain food in the
mouth and push it into the teeth for chewing.
They also make speech possible, as well as sucking and blowing
actions. The tongue also helps to
manipulate food between the teeth; and it's remarkably agile. Working so close to the dangerous teeth, it
manages to avoid being bitten most of the time. You probably know from the rare occasions when it does get bitten
that it is a very sensitive organ. The
tongue also helps with speech. The
palate (“roof” of the mouth) serves to separate the oral cavity from the nasal
cavity just above it; this keeps food out of the respiratory tract and makes it
possible to breath while chewing.
Mastication
Chewing is the first step in mechanical digestion;
it breaks food into pieces small enough to swallow and also exposes more
surface to the action of digestive enzymes.
Whole pieces of food would require many more hours--perhaps days--to be
digested. Think about animals that do
swallow food whole, for example, some snakes swallow whole live small animals
like rabbits or gophers. Then the snake
must crawl off somewhere quiet for several days until the meal is digested. We might feel like crawling off somewhere to
nap while we digest our food, but we don’t have to. Chewing is a reflex stimulated by the presence of food.
Saliva
Saliva has many functions as well:
§
moistening
the mouth
§
digestion
of some starch and fat
§
cleansing teeth
§
inhibiting bacterial growth
§
dissolving molecules,
so that they can stimulate the taste buds
§
moistening food
and binding it together for swallowing
Saliva is a hypotonic fluid with a pH of about 6.8
to 7.0. It contains many substances
that perform the functions above. Salivary
amylase (the ending -ase is a sure sign we're talking about an enzyme)
works at a neutral pH and begins the digestion of starch. Remembering that starch breaks down into
sugars, this explains why if you chew a starchy food like a cracker for a
while, it begins to taste sweeter and sweeter.
The starch is breaking down into sugars, and you can taste them. Lingual lipase begins the digestion
of fats, but this activity doesn't start in the mouth. The lipase is activated by acid, so it
doesn't get underway until food reaches the stomach. Mucus helps to bind and lubricate the food mass; this aids in
swallowing. Lysozyme (remember this
from lesson 1?) kills bacteria. Immunoglobulin
A or IgA is an antibody that inhibits bacterial growth. Saliva also contains several electrolytes,
sodium, potassium, chloride, phosphate, and bicarbonate.
The extrinsic salivary glands secrete up to a
quart or a quart and one-half of saliva every day. They are stimulated by food or even the smell, taste, or sight of
food. Sometimes even thinking about
food can stimulate these glands.
Salivation is controlled by the nervous system. The parasympathetic nervous system
stimulates the production of abundant watery saliva with lots of enzymes; the
stimulus is food. The sympathetic
nervous system (the part the operates under stress) stimulates the production
of a thicker saliva with more mucus.
That explains why, when you are frightened or crying, your mouth becomes
pasty and sticky. Salivation is
increased when the stomach or esophagus are irritated; it is decreased when you
are dehydrated (remember lesson 3).
Swallowing
Swallowing or deglutition actually consists of a
complicated set of activities coordinated by the brain. There are two stages in swallowing:
§
buccal: The tongue collects food and pushes it back
in the mouth.
§
pharyngeal-esophageal: Food is blocked from reentering the mouth or
entering the respiratory tract.
Stretching in the esophagus as the food arrives
triggers muscular activity called peristalsis, a series of overlapping
wave-like contractions that squeeze material down a tube. Very small pieces and liquids pretty much
just fall down the esophagus; they wouldn't need peristalsis to move them
along--gravity is enough. Larger pieces
of food are moved along by peristalsis.
This muscular action is also why you can swallow even when gravity isn't
helpful, for example, when you are lying down.
You can swallow while upside down if necessary; the food will move
against gravity all the way to your stomach.
The run down the esophagus doesn't take long--a
couple of seconds for liquids, a few more seconds for food. When food arrives at the bottom of the
esophagus, the lower esophageal sphincter relaxes and permits it to flow into
the stomach.
PHYSIOLOGY
OF THE STOMACH
Gastric
Secretions
Your stomach produces two to three quarts of
gastric juice a day. It has several
components.
Hydrochloric
Acid. The pH inside the stomach when empty is
about 0.8. This is more than 1000 times
more acidic than vinegar! The glands
that make acid ionize carbonic acid, releasing hydrogen ions. Then they exchange the bicarbonate produced
in this reaction for chloride ions from the blood. That makes the chloride available to react with the hydrogen to
make hydrochloric acid for secretion into the stomach. It also raises the pH of the blood leaving
the stomach (because of all that bicarbonate), a phenomenon known as the alkaline
tide.
The acid in the stomach performs a number of roles
for the body:
§
It activates pepsin and lingual lipase, digestive
enzymes.
§
It breaks up connective tissue in meats and breaks
down plant cell walls; this helps to liquify food into a pasty substance called
chyme.
§
It converts ingested ferric iron (a +3 ion) into
the absorbable and usable ferrous (+2) form.
The body can neither absorb nor use ferric iron.
§
It kills bacteria in the stomach. Think about the number of bacteria you
ingest with food, even sanitary food--hundreds of thousands or even millions in
a single meal; these need killing before they set up infectious processes in
the body. There are very few kinds of
bacteria that can survive the pH of the stomach.
Intrinsic
Factor. Intrinsic factor is a glycoprotein that is
essential to the absorption of vitamin B12. This is the one function of the stomach that
cannot be picked up by other organs if necessary.
NOTE
of interest (not on the test):
The
inability to make intrinsic factor means you can only absorb a tiny fraction of
the vitamin B12 you ingest.
The disease which results from this inability is called pernicious
anemia, and it is fatal if not treated.
People with pernicious anemia cannot make normal red blood cells, and
the ones they make don't carry oxygen properly. A long time ago, before vitamins were discovered or understood,
people with pernicious anemia simply became sicker and sicker, wasted, and
died. A physician discovered a
"cure" for pernicious anemia; if people followed his prescription
rigorously, they could live a fairly normal life. Here's the cure: eat one pound of raw ground beef liver every
day. How would you like that? It worked because liver is so high in
vitamin B12 that absorbing even a tiny fraction provided enough of
the vitamin to sustain life. Nowadays
what we do for pernicious anemia is give a monthly injection of the
vitamin. Since injected vitamin doesn't
require absorption from the digestive tract, it is taken up directly from the
muscle (where it's injected) into the bloodstream. This by-passes the faulty absorption mechanism in the digestive
tract. Like most water-soluble
vitamins, it isn't stored very well or very long, but it's stored in the liver
well enough to make a monthly injection sufficient. Pernicious anemia becomes more common with aging; apparently the
intrinsic factor-secreting cells don't do well as they age.
Pepsin. Pepsin isn't secreted in its final
form. It is secreted as a zymogen,
an inactive enzyme that requires activation after it is secreted. The zymogen form of pepsin is called pepsinogen. Pepsinogen is activated by the hydrochloric
acid in the stomacy; this converts it into pepsin, the active form. Once you have a little pepsin, it can help
with the conversion process to make more; it's called an autocatalytic (auto
means self, so self-catalyzing) enzyme because of this property. Pepsin digests protein into peptide chains,
which are then further broken down by other digestive enzymes in the small
intestine.
Other
Enzymes. Gastric lipase helps to digest
the butterfat in milk; it's more active in infants than in adults. Renin curdles milk by
coagulating its proteins, a necessary first step in their digestion. (Renin
is what cheesemakers add to milk to cause it to form curds. This renin generally comes from the stomachs
of cattle; it’s gathered in slaughterhouses.
You may see it on the grocery store shelf, labeled “Rennet.”) This enzyme is also more active in infants
because their diet consists primarily or only of milk.
Chemical
Messengers. The stomach secretes up to twenty different
messengers. Many of these are hormones,
chemicals secreted into the bloodstream which affect target tissues distant
from the site of secretion. Others are poaracrine
secretions, chemicals that diffuse a short distance and stimulate other
cells elsewhere in the gastric lining.
And some are neurotransmitters which are active in the transmission of
impulses in the nervous system.
Gastric
Motility
The stomach is essentially a muscular bag. These muscles run in various directions in
the walls of the stomach. When food
arrives, peristaltic contractions begin; they churn the food, mix it with
gastric juice (with all its goodies), and promote the physical break-down of
the food by stirring it around. Once
the stomach has mixed and churned food for a while, turning it into chyme, it
releases the chyme gradually into the duodenum. This gradual release gives the duodenum time to neutralize
stomach acid a little bit at a time and permits gradual digestion in the
duodenum. If the duodenum becomes
overfilled, it sends signals that inhibit gastric motility, postponing the
arrival of more chyme. A typical meal
is emptied from the stomach within four hours.
Emptying takes longer if the meal is high in fat, a shorter time if the
meal is mostly liquid.
NOTE
of interest (also not on the test):
Occasionally
you hear advice from various people about what to do to avoid or cure
intoxication if you're going out drinking.
One piece of advice is useful: if you're going out, eat a high-fat meal
before you go. Because only a small
percentage of the alcohol you consume is absorbed directly from the stomach and
the rest from the intestine, anything that delays stomach emptying will also
delay alcohol reaching the intestine for
absorption. That spreads out the
passage of alcohol into the bloodstream somewhat. Now if you drink way too much, you'll become intoxicated despite
what you eat; but if you're looking to buffer the effects of moderate drinking
somewhat, a large high-fat meal will help.
It will also probably reduce your drinking just because the stomach will
feel full for several hours after eating.
Vomiting
Vomiting occurs when the diaphragm and abdominal
muscles contract while the esophageal sphincter relaxes, forcing the stomach
contents up the esophagus and out the mouth.
The burning sensation you experience in any tissues contacted by the
stomach contents is due to the effects of stomach acid on the mucous
membranes. That's why drinking or
gargling with baking soda mixed with water helps relieve that feeling. Baking soda (like Arm and Hammer) is sodium
bicarbonate, a buffer. It neutralizes
the acid, which relieves the burning. (School
children often demonstrate this neutralization reaction by mixing vinegar, an
acid, with baking soda. The bubbling
they see is carbon dioxide rising from the mixture. Take a look at the bicarbonate reaction, and this will make sense
to you.) Vomiting is induced by
excessive stretching of the stomach, by psychological stimuli, and by chemical
irritants like alcohol and other toxins.
Digestion
and Absorption
Protein
and fat are partially digested in the stomach; most chemical digestion takes
place in the small intestine. The
stomach absorbs almost no nutrients. A
few drugs are absorbed directly from the stomach (aspirin is one), and a
certain percentage of alcohol is absorbed from the stomach.
Protecting
the Stomach
The stomach lining is at risk; exposure to a pH
less than 3 can be terribly damaging to tissues, and we're talking a pH less
than 1! The stomach isn't composed of
some special cast-iron sort of tissue; once a person dies, the stomach wall
begins to break down almost immediately.
It is protected in life by on-going processes. One is the mucus coat found on the stomach lining. This is thick and very alkaline, so it
neutralizes acid in direct contact with it and covers tissue with a thick
protective layer. Epithelial cells that
line the stomach and secrete the mucus are replaced every three to six days, so
there are always young tough cells available to renew the protection. And the cells have what are called tight
junctions between them, so that there is little chance any acid will seep
between cells and get into deeper tissues.
Regulating
Gastric Function
Overall, gastric motility and secretion increase
when you eat and decrease when your stomach empties. Of course, there's a little more to it than that. Gastric function occurs in three phases,
which can overlap or even occur simultaneously:
§
Cephalic Phase: activated by the sight, smell, taste, or
thought of food; brain stimulates gastric secretion and motility even before
you eat
§
Gastric Phase: most of gastric secretion; activated by
swallowed foods; stretch and the presence of specific nutrients trigger
secretion of pepsinogen and hydrochloric acid; proteins in food buffer stomach
acid, so pH rises, stimulating further secretion; as the stomach empties, pH
drops again; pH < 2 inhibits further gastric secretion
§
Intestinal Phase: as food arrives in the duodenum, it inhibits
further gastric secretion, both through effects on the nervous system and by
chemical messengers; the pyloric sphincter at the bottom of the stomach
constricts, restricting flow of chyme into the small intestine
PHYSIOLOGY
OF THE LIVER AND GALLBLADDER
Liver
Functions
The liver has many functions in addition to its
digestive one. In fact, it used to be a
joke among Clinical Chemistry students that the all-purpose answer to the test
question, “Which organ performs this function?”, no matter the function, was
"liver" because the liver does so many things you always had a good
chance of being right. In fact the
liver is active in metabolism of carbohydrates, lipids, proteins, vitamins, and
minerals; helps to stabilize blood glucose; synthesizes plasma proteins;
disposes of many drugs, toxins, and hormones; and performs phagocytosis of
bacteria and debris. This is all in
addition to its digestive function of producing bile acids that emulsify fats.
Bile
and Fat Digestion
Here's the thing with fats: they don't mix very well
in water-based solutions. You've seen
this if you've ever used oil-and-vinegar salad dressings; you know you have to
shake and pour quickly before they separate.
Remember that neutral fats are hydrophobic, so they tend to try to get
away from the water molecules. They do
this by hanging together in large globules when mixed with water; this causes
the mixture to separate as the fats get together. The problem with large pools of fat is much the same as with
large pieces of food; there's not enough surface area to give digestive enzymes
much to work on. It will take forever
for fats to be digested unless we can find a way to break them down into
smaller particles that stay suspended in the water. What we want is a solution that won’t separate—an emulsion more
like mayonnaise, which is, after all, also a mixture of oil in a water based
solution. Mayonnaise doesn’t separate
like oil-and-vinegar dressings, does it?
The way to accomplish this is to use an emulsifier, something that
breaks the fat into tiny particles and finds a way to get them to hang among
the water molecules. In mayonnaise the
emulsifier is lecithin, an effective emulsifier found in egg yolks. Take a look at recipes for homemade
mayonnaise (which, by the way, is excellent); they all have egg yolk in
them. The emulsifiers in the digestive
system are bile salts, which are synthesized in the liver from cholesterol and
are an important constituent of bile.
Bile also contains minerals, cholesterol, neutral
fats, phospholipids, bile pigments, and bile acids. The main ingredient is bilirubin a yellow pigment that
results from the breakdown of hemoglobin from old worn-out red blood cells
processed by the liver. It is mostly
responsible for the yellow-green color of bile and also for the brown color of
feces. (In the intestine, bacteria turn
the bilirubin into urobilinogen, which is brown.) If you have no bile production, your stool will be whitish-gray
in color and be streaked with fat; fat digestion suffers without bile too.
Bile produced in the liver passes down the network
of ducts we talked about earlier. The
way it gets to the gallbladder is sort of strange; when the bile duct from the
liver overfills, it backs the bile up into the gallbladder through the cystic
duct. While the bile is there, the
gallbladder stores and concentrates it by reabsorbing water and electrolytes
from it. The liver produces from a pint
to a quart of bile per day, but the gallbladder concentrates this down to less
than 8 ounces, perhaps as little as 2 ounces.
If bile becomes very concentrated, it can solidify into gallstones,
which cause pain if they make their way down the ducts.
If you’re wondering where the name, gallbladder, came from, it will help to know that bile was once called gall. You’ll see biblical references to gall, a very bitter tasting substance derived from the gallbladders of animals. Those of you who field-dress game, such as deer, probably know that if you’re saving the liver, it’s important to cut the gallbladder (which is found attached to the liver) completely away from the liver without piercing it. If you fail in this task, the entire liver acquires a very bitter taste, ruining it as food.
PHYSIOLOGY
OF THE PANCREAS
Secretion
The pancreas is a gland that functions as an
endocrine gland--one that has no ducts, but secretes its product directly into
the bloodstream for travel to distant target tissues--and an exocrine
gland--one with ducts that passes its product directly to the target tissue via
these ducts. Its endocrine function
results in secretion of insulin and glucagon which regulate carbohydrate
metabolism—more on this next semester.
Its exocrine function results in secretion of pancreatic juice, which is
full of chemicals that aid in digestion.
The pancreas makes from a quart to a quart and one-half of pancreatic
juice every day. Pancreatic juice is
alkaline and consists of water, enzymes, zymogens, sodium bicarbonate, and
electrolytes. Here's the run-down:
§
Enzymes: become active onky in
the intestine.
ü
Pancreatic Amylase
digests starch.
ü
Pancreatic Lipase
digests fats.
ü
Ribonuclease
digests RNA.
ü
Deoxyribonuclease
digests DNA.
§
Zymogens: converted to active
enzymes after secretion.
ü
Tripsinogen
converts to trypsin, then digests protein and mediates activation of other
zymogens.
ü
Chymotrypsinogen
converts to chymotrypsin, then digests protein.
ü
Procarboxypeptidase
converts to carboxypeptidase, then digests protein.
§
Bicarbonate: neutralizes
hydrochloric acid from the stomach.
Regulation
of Secretion
The nervous system regulates secretions from the
pancreas. The parasympathetic nervous
system stimulates secretions, and the sympathetic nervous system inhibits
secretions. If you think about the primary
roles of each of these divisions of the nervous system, this makes sense. The parasympathetic nervous system functions
during routine, day-to-day operations; that's when the body has time to sit
around digesting food. The sympathetic
nervous system functions during times of stress; that's when the body has to
devote energy to escaping or dealing with a serious event. At these times, digestion is put on hold
while the body copes.
Certain hormones also play a role in regulating
pancreatic secretion.
§
Cholecystokinin (CCK)
is secreted by the duodenum in response to the presence of stomach acid and
fat. It causes contraction of the
gallbladder, secretion of pancreatic enzymes, and release of bile and
pancreatic juice into the duodenum.
§
Secretin
is produced in the duodenum in response to stomach acid. It causes secretion of bicarbonate to
neutralize the acid.
§
Gastrin
is secreted by the stomach and duodenum and causes gallbladder contraction and
secretion of pancreatic enzymes.
THE
SMALL INTESTINE
Nearly all chemical digestion and nutrient
absorption take place in the small intestine.
For this, the small intestine needs a very large absorptive surface;
this is the reason for the folds, villi, and microvilli.
The
Duodenum
The duodenum receives the contents of the stomach,
pancreatic juice, and bile. Its job is
to neutralize stomach acid before it damages the linings of the intestine,
physically break-up fats by emulsification, and inactivate pepsin so that pancreatic
enzymes can take over.
Intestinal
Secretions
The duodenum produces a quart or two of juice
every day. This is produced in response
to acid, chyme, and stretching caused by received stomach contents. Duodenal secretions have a pH from 7.4 to
7.8 and contain water and mucus. Most enzymes
are contributed by the pancreas.
Intestinal
Motility
The small intestine functions to mix chyme with
intestinal juice, bile, and pancreatic juice; to churn the chyme and bring it
into contact with the villi; and to move food residue toward the colon. The spiral folds in the intestinal lining
send the chyme in a spiral pattern which serves to slow and mix it. This allows time and lots of contact with
the intestinal walls to enhance absorption.
Since each villus contains a blood vessel and a lacteal, they are the
site of absorption. Fats are absorbed
into the lacteals and other nutrients into the blood vessels. The microvilli have enzymes in their cells
that further contribute to digestion; these enzymes are not released, but stay
inside the microvilli's cells. This
means that food must actually come into contact with the microvilli's cells in
order to complete the digestion process.
This is called contact digestion.
In order to accomplish all this mixing and
movement, two kinds of muscular contraction take place.
Segmentation. Segmentation is a series of
ring-like constrictions that come and go at various locations along the
intestine. This churn the contents of
the small intestine and mixes it with digestive juices. There is a certain amount of swishing back
and forth; but since the contractions are more frequent in the proximal end and
less frequent in the distal end, the overall effect is to slowly move contents
toward the colon.
Peristalsis. Here again, we see the successive overlapping
waves of contraction which milk the contents of the intestine along. Peristalsis doesn't begin until most
nutrients have been absorbed and eventually expels food residue and any
bacteria present into the colon. It
takes about two hours for the small intestine to empty after chyme is received
from the stomach.
When residue reaches the end of the small
intestine, it passes through the ileocecal valve into the cecum of
the large intestine. This valve opens
when food enters the stomach and closes when the cecum fills, to prevent
backflow.
THE
LARGE INTESTINE
The large intestine receives about a pint of food
residue each day. It then reduces this
by more than half by absorbing excess water and salts and eliminates the
remainder in elimination. The large
intestine needs from twelve to twenty-four hours to accomplish this, but
doesn't change the residue chemically in any way. No absorption of nutrients occurs. Feces, or stool, is about three-quarters water; much of the
remaining solid matter is bacteria. The
colon contains enormous numbers of bacteria.
Bacterial
Flora
These bacteria, normally present in the colon produce some B vitamins and vitamin K, which is essential for normal blood clotting. Since most diets don't contain sufficient vitamin K, these bacteria are an important source of this nutrient.
NOTE: (not
on test) Newborn babies don’t yet have
intestinal bacteria; these are acquired gradually in the first few months of
life through eating foods that contain bacteria. (It happens, no matter how much you sterilize everything that
baby touches.) This means that newborns
do not have a source of Vitamin K for the first few months of life; they
survive on stored Vitamin K transferred to them from mom shortly before
birth. Premature infants haven’t had a
chance to acquire this supply from mom because they’re born before the transfer
occurs. This means one of the many
problems you may see in a premature infant is a tendency to bleed excessively. Nowadays we solve this by providing a Vitamin
K injection to premies.
They also ferment cellulose (from plant cell
walls) and other carbohydrates, producing gas as a by-product. Intestinal gas is called flatus;
about a pint is produced per day. Much
of it is the result of swallowed air, but some is from bacterial
fermentation. More gas is produced when
undigested nutrients are present in the colon.
There are many substances present in this gas; the odor results from indole,
skatole, and hydrogen sulfide.
CONCLUSION
TO CHAPTER 25
That's it for Chapter 25. Now you can use your
objectives to build a study guide for this chapter. When you're confident you've learned the information in the
chapter and understand the concepts presented here, request a test via
e-mail. This is the last test of this course. When you've finished it, you're done! Congratulations; you're almost there.