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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.