Lets get on Tract

Illustrations are from "Art Explosion 40,000," copyright Nova Development Corporation, Calabasas, CA.;
used under terms of license granted to Dr. J.V. Aliff.
 
 

Chapter 23,  Digestive System  - Marieb and Hoehn, 8th edition
 

Eating and Digesting a Bacon-Lettuce and Tomato Sandwich
See p. 883, 7th. ed.; p. 851, 8th ed.

BLT Contents

1. Protein in meat: Canadian bacon has less fat and more protein. Remember that
   protein is needed for full brain and muscle development. Protein malnutrition
   (kwashiorkor) is one of the world's most severe nutrition-related diseases.
2. Lipids: the bacon, bread and mayonnaise have fat - saturated and unsaturated.
   There is cholesterol dissolved in the bacon fat.
3. Water: in the lettuce and mayonnaise in particular.
4. Starches: occur in the bread primarily.
5. Sugars: added to bread, mayonnaise and even to preserved meat in order to
   improve taste.
6. Vitamins: some are added to the bread like niacin and riboflavin. Vitamins are
   needed to serve as parts of enzymes that must be acquired from the diet.
7. Fiber: cellulose fiber, which is indigestible, is found in vegetables and grain husks.
   Oat bran fiber is soluble, meaning it consists of small molecules semi-dissolved in water.
   Larger fibers are insoluble. Fiber does two things: first it speeds up the time of
   passage of the digesting food; second, it seems to interfere with the absorption of
   cholesterol.

Teeth do mechanical digestion (breakdown of food). Teeth are composed of a
highly uniform, crystalline calcium phosphate, particularly the enamel covering.
Teeth come in three basic models: See p. 889, 7th ed.; p. 859, 862-863 8th ed. They are anchored to the maxilla and mandible
by a layer of collagen, the periodontal ligament.

Incisors: Two pairs (upper or lower) plate-like cutting and snipping teeth.

Canines (cuspids): Count Dracula had a pretty good set of these. One pair of
canines are present, upper or lower. Carnivores have well-developed canines
used for clamping and tearing.

Premolars (bicuspids) - two pairs (upper or lower) located between the molars
and canines, in function intermediate between cutting and grinding. Premolars don't
occur in milk dentition.

Molars: grinding teeth, especially in herbivores where they are quite block-like. A
carnivore's teeth are more like sharp mountain ridges that cut like scissors. Two
pairs are included in milk dentition, three pairs in permanent dentition (upper or
lower). Bacteria, eating sugars from partly digested starches, produce acids that
attack the crystalline enamel.


Label the cross section as follows: pulp, dentin and enamel. See p. 894, 7th ed.; p. 863, 8th ed.
-

 

Label the teeth as follows: incisors, canines, premolars, and molars.
See p. 893, 7th ed.; p. 862, 8th ed.

The teeth cut and grind the food up while the mouth mixes the salivary secretions. Stimulated by the Parasympathetic N.S.,
saliva, made of mucous (thick) and serous (watery) secretions, is there to
lubricate the food. However, some minor amount of digestion of starches occurs
using amylase secreted by the salivary glands. See p. 892, 7th ed.; p. 861, 8th ed.

        Salivary Amylase
H₂O + Starch -----> Disaccharides (maltose) and dextrins (short
polysaccharides)

Salivary amylase is activated by Cl^- ions in saliva. This is an example of chemical
digestion.

SALIVARY GLANDS

1. Parotid - serous secretions include amylase and Cl^- ions that activate salivary
   amylase. Stensen's ducts empty into the mouth. The parotids swell in viral
   "mumps."
2. Submandibular - conduct their secretions via Wharton's ducts; consist of mixed
   mucous and serous secretions. Amylase and lysozyme are included.
    
3. Sublingual - mixed but mostly mucous.
    
4. Lingual - mucous.

Label as appropriate to the discussion below.

1. Mucosa - includes squamous epithelium in the esophagus and columnar
   epithelium in the stomach and gut. A lamina propria of connective tissues and blood
   vessels are under the epithelium. A muscularis mucosa underlies that.
    
   Label the mucosa, sub mucosa and muscularis mucosae.

   See pg. 887, 7th ed.; p. 857, 8th ed.

    

2. Submucosa - a thicker layer of connective tissues and blood vessels between the
   muscular is mucosa and the circular and longitudinal muscle of the gut wall.
3. Muscularis - a circular smooth muscle cells closest to the lumen, longitudinal
   muscle cells are outside.
4. Serosa -collective tissues and a serous secreting membrane covered by simple
   squamous epithelium within a serous membrane.

The tongue makes a ball (bolus) of food and "flips" it to the pharynx.
Pharyngeal constrictor muscles (superior, middle and inferior) contract in the back of the mouth to
prepare a round ball (bolus) of food and send it into the esophagus. The pharyngeal bolus stimulates
the deglutition center of the medulla to produce these swallowing movements.
The larynx is pulled under the pharynx to receive the bolus. Normally the epiglottis
prevents the food from entering the trachea as discussed before in respiration.
See pg. 889, 7th ed.; p. 867, 8th ed.

Esophagus: the esophagus is a very muscular tube in the body for its size. When it
contracts it squeezes the food into the stomach, going through the cardiac
sphincter valve. "Heartburn" or "acid reflux" is caused by acid burping into the
esophagus. Since this happens at the same thoracic level as the heart, heart attacks
and heartburn are much confused by patients. See p. 898, 7th ed; p. 867, 8th ed.  A hiatal hernia results
when the diaphragm tears away from the cardiac valve, weakening it. With prolonged acid
reflux, the lower esophageal epithelium can change from stratified squamouis to
mucous columnar (metaplasia), a precancerous condition called Barrett's esophagus.

Can a person swallow while standing on his/her head?

Chemical Digestion: the chemical, enzymatic breakdown of food.

Stomach: the stomach is lined with mucus columnar epithelial cells and therefore
are protected from acid attack. See p. 901, 7th ed.; 868-869, 8th ed.  Hydrochloric acid is produced in a
gastric gland or pit by special parietal cells. HCl serves to dissolve some food, soften
it, and activate pepsin. It is also an antiseptic bath. Parietal cells also secrete Vitamin B₁₂
intrinsic absorptive factor.

Pepsinogen, secreted by chief or zymogenic cells, is the protein precursor of
protein-digesting pepsin that is activated by H ions secreted by the parietal cells of the gastric
glands or pits. Chief cells also secrete gastric lipase that digests neutral fats into
fatty acids and glycerol.. See below.

Enteroendocrine or "G" cells secrete the hormone gastrin which
then stimulates the secretion of the gastric glands.

Pepsin digests proteins to short chains of amino acids; i.e.,

Protein + Pepsinogen + Water + HCl  -----> Short Polypeptides and
"Peptides" (one more digestion step is needed to complete protein digestion of the
short peptide chains (e.g., chain of 10 amino acids). Protein is a long polypeptide of
amino acids.) See chart on p. 906, 7th ed., p. 886-887, 8th ed.

The food in the stomach is churned together with HCl acid at a pH of 2-3. The acid
helps breakdown the food so that it is in even smaller pieces/molecules in solution.
The wall of the stomach is folded into ridge-like rugae (which aid mixing). Food exits
the stomach through the pyloric sphincter valve as chyme. In children, rennin is also
secreted to curdle milk - this is particularly important for the digestion of infants.  It prevents colic.
 

The vomiting center of the medulla oblongata receives sensory input from the senses of
the stomach wall caused by distention or irritation from bacterial toxins or mechanical
stimulation of the pharynx.
 

REGULATION OF STOMACH MOTILITY AND SECRETIONS

1. Cephalic phase - the Parasympathetic N.S. stimulates stomach motility and
   secretions after the CNS sees food. The Sympathetic N.S. inhibits.
2. Gastric phase - when stomach walls are stretched or the pH of the stomach
   increases because of entering food. Parasympathetic innervation increases
   secretions and motility. At a pH of 2, HCl secretion normally stops.

Gastrin (see p. 903, 7th ed.; p. 875, 8th ed.) is an important stimulus which directly causes increased
gastric gland secretions. Histamines enhance the effect of gastrin on target cells.
Tagamet is a drug that blocks histamine (h₂) receptors for the treatment of ulcers.
Cutting the Vagus nerve is a drastic treatment for such. Why? We know now that
70% of ulcers are caused by infection by Helicobacter  pylori. High protein chyme
stimulates more stomach acid secretion. Caffeine and stretching of the stomach does
this also! Ulcers, mostly due to infection with Helicobacter, occur in the pyloric
stomach and the proximal duodenum.
See http://www-medlib.med.utah.edu/WebPath/GIHTML/GI256.html

1. Intestinal phase - when partly digested food containing fatty acids, amino
   acids and glucose enter the small intestine, the hormones gastric inhibitory peptide,
   CCK, and secretin decrease stomach activities. In general, high fat diets or high
   protein diets slow the motility of the stomach and small intestine.

Duodenum: the first foot or so of the small intestine is the duodenum. Two major
digestive glands empty their secretions there; the liver that makes the emulsification
agent bile, and the pancreas contributes bicarbonate and enzymes.

Brunner's alkaline mucous submucosal glands help neutralize stomach acid and
protect the duodenal mucosa.

Pancreas: the pancreas empties the following products into the small intestine-
duodenum (see p. 919, 7th ed.; p. 886, 8th ed.):

 
 

Label as appropriate.

1. Sodium Bicarbonate (NaHCO₃) - this neutralizes the HCl acid from the

stomach, the pH increases from 2 or 3 to 8, e.g.,

HCl + NaHCO₃----->NaCl + H₂O + CO₂ gas

1. Amylase, as above, converts starches (amylose) to maltose disaccharides. This
   does not complete digestion of starches - one more step is needed.
2. Trypsinogen precursor to Trypsin is a protein digestion enzyme activated by OH^- hydroxyl
   cofactors and enterokinase enzyme. It has the same general products as pepsin, e.g.,

                1.  Lipase is an enzyme which digests triglycerides or neutral fats to fatty acids,
                monoglycerides, and glycerol. Fat digestion is greatly improved by emulsification with
                bile from the gall bladder. Bile is a green fluid composed of cholesterol, bile acids and
                salts derived from broken down red blood cells and hemoglobin. Bile is both a
                secretion and an excretion (cholesterol and iron). The large fat globules are broken
                into small globules. If fat emulsification does not occur, the intestine will slow down
                drastically. Reduced bulk also slows the gut.  Also, bilirububin is converted to
                urobilinogen and stercobilin that color the feces brown.

2. Disaccharidases including sucrase, maltase, and lactase break down
disaccharides into the monosaccharides which can then be absorbed by the simple
columnar cells lining the intestinal villi, then into blood. A deficiency of lactase
causes lactose intolerance as bacteria eat undigested lactose and produce acids
and gas which irritate the gut. Glucose and galactose are absorbed by active
cotransport with Na⁺ ions, fructose by facilitated diffusion.

3. Peptidases - break down peptide chains of amino acids into single amino acids
which then are absorbed. Mono-, di-, and tripeptides are absorbed by active transport
and carried into the blood.


4. Nucleases - digest DNA and RNA to nucleotides which are absorbed by active
transport.

Neutral Fat - broken down by gastric, pancreatic and intestinal mucosal lipase and absorbed as fatty acids,
glycerol, and monoglycerides. See p. . Intestinal simple columnar epithelial cells take in the products by
simple diffusion but then reconstruct them into triglycerides which are packaged into
lipoprotein carriers called chylomicrons. These are deposited in the lacteal ----->
lymphatic ducts -----> blood. Lipoprotein lipase assists the breakdown and entrance
of fats into liver , muscle and adipose cells. The liver then makes LDLs, HDLs and VLDLs.
 

Pancreatitis, common in alcoholics, results from the blockage of the pancreatic ducts, causing
the pancreas to digest itself.

Pancreatic cancer generally will not present symptoms until the tumor strangles the small
intestine, causing vomiting and pain. The tumor may have metastasized into the liver by that time.

Cystic fibrosis is caused by defective Chloride channels that results in a thick pancreatic secretion.
The feces of the patient will be very smelly due to the lack of proper digestive enzymes.

1. Gastrin is a hormone secreted by the wall of the stomach directly proportional in
   response to a high protein meal. Gastrin increases gastric gland secretion of HCl and
   pepsinogen in response to a high protein meal or high stomach pH.
2. Cholecystokinin (CCK) is made in the wall of the duodenum, in response to a
   high amount of unemulsified fats, triglycerides, and amino acids in the chyme, travels
   by the blood stream to the gall bladder and stimulated it to contract and squeeze out
   bile. CCK also stimulates secretion by the pancreas high in enzymes (e.g., lipase).
3. Secretin also stimulates pancreas secretion high in bicarbonates in response
   to a high acid chyme. It is secreted in the wall of the duodenum.
4. GIP - secretion is stimulated by glucose and fatty acids, it causes release of insulin
   and inhibits stomach motility and secretions.

The small intestine is covered by small 1-2 mm long finger-like projections, villi, which
serve to greatly increase the surface area for absorption and house glands at their
bases. Intestinal glands in the duodenum and jejunum produce these enzymes (see
p. 910, 7th ed; p. 877, 8th ed.).

Label the columnar epithelium and villi.

LIVER - Label as appropriate

Label the right and left lobes and the falciform ligament. See p. 913, 7th ed; p. 882, 8th ed.

Functions

1. Deamination and Urea Production: blood from the wall of the small intestine
   is sent directly to the liver through the hepatic portal vein system. See p. 916, 7th ed.; p. 883, 8th ed..
   The liver takes amino acids from the blood and removes the nitrogen
   containing amine group (deamination) to subsequently make ammonia and,
   then, a less toxic urea.
    
   -
2. Blood Glucose Regulation: the liver monitors the blood glucose level,
   maintaining it at 0.1% or 100 milligrams/DL. When hepatic portal blood sugar
   is too high, insulin is secreted by the islets in the pancreas. Insulin binds to receptors
   on muscle and adipose cells. It interacts with liver cells by decreasing glucose production
   and secretion. Glucose enters the brain, intestinal epithelial and liver cells without insulin's
   assistance.  The diabetogenic hormones cortisol and glucagon increase in secretion when
   hyperinsulinemia causes hypoglycemia in a type 2 diabetes mellitis syndrome. The response is
   hyperglycemia. Why?

   If the blood sugar level is too low, glucagon is secreted by other islet cells, and the
   liver responds by breaking down its starch glycogen and releasing the glucose product
   to the blood stream. Sugar diabetes or hyperglycemia (Diabetes Mellitis) occurs due to
   a lack of insulin (Type I, Insulin Dependent) or a lack of insulin receptors (Type II,
   Non-Insulin dependent). Juvenile diabetes is Type I. Adult onset diabetes may
   be Type I or Type II. (Also see the excretion chapter.) Type II diabetes mellitis
   with adult onset may be caused by either beta cell burnout caused by a life-long
   "sweet tooth" and the "wearing out" of beta cells or by receptor down regulation
   in response to high insulin levels. Both are abetted by genetic defects. Non-insulin
   dependent sugar diabetes (NIDM) is treated with a diet low in simple carbohydrates
   and high in protein. Avoiding damage to the circulatory system is based on
   preventing high "spikes" of blood glucose. Hyperperglycemia in type X sugar diabetes is
   caused by the excess release of glucose by the liver.
    

3. Detoxification: the liver detoxifies ethyl alcohol, converting it to acetaldehyde
   and then acetate for the metabolism of ATP. Ammonia is detoxified by conversion
   to urea,  and other chemicals such as anesthetics are detoxified by the cytochrome
   system of liver cells. Some antibiotics like terramycin and penicillin are excreted
   by the liver.
    
4. Production of Bile: As previously discussed, the liver makes and stores bile
   in the gall bladder. Review how hemoglobin is broken down and the resulting
   waste products are excreted.

    5. Production of Lipoproteins

    6. Production of Fetal Hemoglobin and RBCs

    7. Phagocytosis of particles in the blood passing through the liver by fixed macrophages or Kupffer cells.

The small intestine joins the large intestine with the ileum at the ileocaecal valve.

Where the two join, a short caecum leads to an appendix in humans. The appendix is
lined with lymphocytic modules which fight intestinal bacteria there. As a matter of
fact, as food proceeds down the small intestine and in the large intestine, numbers of
bacteria increase and so do the lymphocytic nodules in the intestinal wall. The colon
is lined by mucous columnar epithelium. Large intestine segments are the
ascending, transverse, descending and sigmoid. The sigmoid (s shaped) colon
joins the rectum. The anus is closed by an inner involuntary internal sphincter and
an outer voluntary sphincter muscle.

Label the segments of the colon, the ileum, colon and rectum.

Note the appendix protruding from the caecum.

To see an inflamed appendix, go to: http://www-medlib.med.utah.edu/WebPath/GIHTML/GI102.html .

The principal function of the colon is to absorb water. Stretching the intestine
stimulates the defecation reflex. The feces released can be composed of 50% bacteria.
The further down the gut, the more bacteria-fighting lymphocytic nodules occur in the
gut wall.

Why do you have large amounts of bacteria in the chyme of the lower small intestine
and in the stool? See the chapter on vitamins.

Diarrhea is the world's #1 symptomatic killer of children in undeveloped areas of the world. Through dehydration and
resulting hypovolemic shock, viruses, bacteria, intestinal worms and protozoa can all cause diarrhea. These organisms abound in untreated/purified
water.

Colon cancer risks go up with increased consumption of (1) fat, (2) preserved meat,
(3)  highly cooked or refined (soft) foods. (4)  High fat slows gut
action, increasing exposure to carcinogens. Also fat and protein feed tumors.

The gut, from the stomach to the rectum, empties by mass action - as mass
accumulates, parasympathetic reflexes occur which increase the speed at which the
chyme and feces pass down the gut. Food fiber (cellulose) adds mass and therefore
speeds gut action. Food fiber also interferes with the absorption of cholesterol.

What are the dangers of a "twinkie" diet?

Carcinogens, including nitrites, nitrates and nitrosamines (formed by the reaction
HCl + Amino Acids + NO₂^- nitrite or NO₃^-   ----->   nitrosamines) are exposed
to the gut wall.

Slowing of gut speed theoretically increases exposure, although recent studies show
that increasing fiber content in diet has no value in preventing colon cancer.

Coming from fresh fruits and vegetables, Vitamin C detoxifies nitrosamines. Some sausage
manufacturers add it to compensate for the nitrite preservatives. Also, Vitamins A and E
scavenge nitrites and nitrates.

SELECTED DISEASES OF THE DIGESTIVE TRACT

1. Hepatitis - inflammation of the liver and resulting obstruction of the bile ducts
   caused by a virus (obstructive jaundice). Hepatitis A and E are transmitted
   by sewage contamination of drinking water. B, C and D are transmitted by
   contact with blood or sexually transmitted. If the liver is inflamed or the bile ducts
   blocked, the biliary bilirubin increases in the blood, staining the skin and whites of
   the eyes yellow (jaundice). The urine may appear the color of coca-cola as large
   amounts of bilirubin and related products are excreted. Premature babies may
   be jaundiced because of the breakdown of RBC's containing fetal hemoglobin
   (hemolytic jaundice).. See http://www-medlib.med.utah.edu/WebPath/LIVEHTML/LIVER009.html
2. A ruptured appendix can produce an infection of the abdominal cavity serous
   membranes called peritonitis. See http://www-medlib.med.utah.edu/WebPath/GIHTML/GI036.html
3. Gallstones are mostly cholesterol. Why?
4. Coeliac disease - marked by destruction of intestinal villi with mal-absorption
   resulting. This occurs because of an allergic reaction to the wheat protein gluten.
   A corn or rice diet is substituted.
5. Amoebic dysentery is a third world protozoan infection which ulcerates the
   intestine. If not treated, amoebae can spread to the liver, lungs or the brain.
6. Giardiasis is among many other protozoans which cause severe mucous an
   bloody diarrhea. See http://www-medlib.med.utah.edu/WebPath/GIHTML/GI048.html
7. Diarrheas are also caused by viruses such as rotovirus and bacteria including
   some strains of Escherichia coli, Salmonella, Vibrio cholera and others. Irritation of the small
   intestine causes a peristaltic rush of liquid into the large intestine, which empties prematurely.
8. Lactose intolerance is caused by an inherited lack of lactase enzyme either in
   childhood or adulthood or both. Bacteria eat the undigested lactose and make acids
   and gas that irritate and cause pain in the lower intestine.
9. Cirrhosis of the liver is caused by long term alcoholism, certain parasites, or
   exposure to certain chemicals. The chronic inflammation causes the liver to fill with
   scar tissue and fat as the hepatocytes die. Jaundice with intense itching of the skin
   is seen. See http://www-medlib.med.utah.edu/WebPath/LIVEHTML/LIVER013.html
10. Diverticulosis of the colon - develops with age. Note the outpockets of the
    colon that get inflamed with trapped feces.
    See http://www-medlib.med.utah.edu/WebPath/GIHTML/GI082.html

1. Describe the risk factors and development of colon cancer.
2. List the digestive enzymes including the site of release into the digestive tract,
   substrate and product of each.
3. Describe the liver's functions of detoxification and urea formation.
4. Explain blood sugar regulation by the liver.
5. Explain the two types of sugar diabetes