CHAPTER 24 DIGESTIVE SYSTEM
Digestion--breaking down of food into
molecules small enough to enter body cells
FIGURE 24.1 P. 897—Very
general diagram of organs of digestion
Organs of digestion are divided into 2
groups:
A. Gastrointestinal
(GI) tract (alimentary canal)--continuous tube passing through the ventral body
cavity from mouth to anus.
The GI tract contains the food while it is in the body, until it is either
absorbed or eliminated. Total length is about 30 feet.
1. Mouth
2. Pharynx
3. Esophagus
5. Small intestine (SI)
6. Large intestine (LI)
B. Accessory structures---not part of
the tube
1. Teeth---chewing
2. Tongue---handling food and swallowing
3. Salivary glands
4. Liver
Produce or store secretions, add
5. Gallbladder them to
GI tract through ducts,
6. Pancreas never
touch food
Gastroenterology--structure, function,
diagnosis & treatment of diseases of the stomach and intestines
Gastro- stomach
Entero-
intestine
6 basic activities of the digestive
system:
1. Ingestion--taking in food
2. Secretion---7 liters/day---water,
acid, buffers, enzymes
3. Mixing and movement of food (propulsion)
along the GI tract
4. Digestion--mechanical and chemical
breakdown
a. Mechanical digestion--chewing, mixing, etc.
b. Chemical digestion--splitting of ingested carbohydrate, protein and
lipid
molecules into smaller molecules that can be
absorbed and used by cells
5. Absorption--passage of digested food
into the cardiovascular or lymphatic systems for distribution to cells
6. Defecation--elimination of
indigestible material
(feces)
The wall of the GI tract from esophagus
to anal canal has the same 4 basic layers:
1. Mucosa--mucous membrane that lines the GI tract
a. Epithelium--next to food
1) Non-keratinized stratified squamous--mouth, pharynx, esophagus, anal
canal--protection
2) Simple columnar--rest of tract, absorption and secretion
b. CT layer called the lamina propria--support, provides blood supply
and contains many immune cells. The lymphatic tissue of this area is called the
mucosa-associated lymphoid tissue (MALT)
c. Muscularis mucosae—very thin layer of smooth muscle, helps form the
ridges and folds of the mucosa
2. Submucosa--areolar CT that binds the mucosa to the third layer and
contains the submucosal plexus (plexus of Meissner), the part of the enteric
nervous system
concerned with control of secretion, vasomotor
nerves, and the muscularis mucosae. This layer may also contain glands and
lymphatic tissue.
3. Muscularis
a. Skeletal muscle of the mouth, pharynx, and upper esophagus aids in
chewing and swallowing
b. Skeletal muscle of the anal canal (external sphincter) permits
voluntary control
of defecation
c. Smooth muscle throughout the rest of the tract--contractions help break down food, mix it with digestive secretions and
propel it along the GI tract. 2 layers:
1) Inner sheet of circular fibers
2) Outer sheet of longitudinal fibers
d. Also contains the other ENS nerve plexus, the myenteric plexus of
Auerbach, which is the major control of GI tract motility.
4. Serosa--outermost layer--serous membrane of simple squamous
epithelium (mesothelium) and CT.
a. Esophagus---outer covering is the adventitia, areolar CT only
b. Below the diaphragm the serosa forms the visceral peritoneum.
Peritoneum--the largest serous membrane
of the body and consists of a layer of mesothelium (simple squamous epithelium)
and underlying CT
Visceral peritoneum--covers organs, forming their serosa
Parietal peritoneum--lines the abdominal cavity
The peritoneal cavity is a potential
space between the 2 layers. Contains a small amount of serous
fluid for lubrication. (Abnormally large amount of this fluid is called
ascites). Peritonitis is inflammation of the peritoneum.
Some organs, such as the kidneys, lie
behind the peritoneum on the posterior abdominal wall (retroperitoneal).
The peritoneum forms several large
folds that bind organs to each other and to the abdominal walls, and also carry
blood vessels, lymphatics and nerves to various organs.
1. Mesentery--binds the small intestine to the abdominal wall (a loose
attachment)
2. Mesocolon--binds large intestine (much closer attachment)
3. Falciform ligament--attaches liver to the anterior abdominal wall and
the
diaphragm
4. Lesser omentum--suspends the stomach (lesser curvature) and duodenum
from
the liver
5. Greater omentum--originates from the greater curvature of the stomach
and drapes over the transverse colon and small intestine. Contains
large quantities of adipose tissue and lymphatic nodes and nodules.
FIGURE 24.4 P. 900-901
1. MOUTH--(oral/buccal
cavity)
a. Cheeks—form lateral walls and terminate in the lips. The zone where
the mucous membrane from the inside and skin from the outside meet is called
the vermilion (red part of the lips). A midline fold, the labial frenulum,
attaches lip to gum. The area from lips/cheeks to gums/teeth is the vestibule
of the oral cavity. The opening between mouth and pharynx is the fauces.
b. Roof of the mouth—formed anteriorly by the hard palate and
posteriorly by the soft palate. The uvula hangs from the soft palate and closes
off the nasopharynx during swallowing.
2. SALIVARY
GLANDS—secrete small amounts of saliva at all times and more when food is
present. Saliva lubricates, dissolves and begins the chemical breakdown of
food. Mucous membrane of the mouth and tongue contains many small salivary
glands, but most saliva comes from the 3 paired major salivary glands:
a. Parotid glands—inferior and anterior to the ears, secretion reaches
the mouth by Stenson's duct, which runs across the masseter muscle and empties
into the mouth opposite the upper 2nd molars.
b. Submandibular glands—beneath the base of the tongue--empty into the
floor of the mouth by Wharton's ducts.
c. Sublingual glands—superior to submandibulars and also empty into the
floor of the mouth by ducts of Rivinus.
Saliva is 99.5% water and 0.5% solutes
(ions, lysozyme, mucus, salivary amylase, and lingual lipase). Composition:
Parotids—watery liquid containing large amounts of salivary amylase
Submandibular—thicker & mixture of salivary amylase and mucus
Sublingual—mostly mucus
Water in saliva dissolves the food so
it can be tasted. Along with mucus, lubricates food. Lysozyme destroys some
bacteria. Chloride ions activate salivary amylase.
Secretion of saliva is under nervous
control only (no hormones). Average amount is 1000-1500 ml per day.
Parasympathetic impulses increase secretion, sympathetic decrease.
The presence of food in the mouth
causes an increase in parasympathetic impulses and an increase in secretion of
saliva. The sight, smell, thought of food may also increase secretion.
3.
TONGUE—composed of
skeletal muscle covered by mucous membrane. The lingual frenulum attaches
tongue to floor of mouth. The top and sides of the tongue are covered by
papillae. Lingual glands on top of the tongue secrete an enzyme, lingual
lipase, which begins the digestion of fats. The tongue manipulates food during
chewing and aids in swallowing.
4.
TEETH (dentes)—located
in alveolar processes of mandible & maxillae. Gums (gingivae) cover these
processes. The periodontal ligaments are CT that lines the sockets and binds
the tooth roots in place. 3 portions of a tooth:
a. Crown—exposed portion above the gum line and covered by enamel
(hardest substance in the body) composed of calcium phosphate and calcium
carbonate.
b. Neck—at gum line between crown and root
c. Root—each tooth has 1-3 roots that are embedded in the socket. The
root is covered by cementum.
Dentin and cementum are living CTs of
the tooth and have living cells associated with them. The enamel is classified
as a cellular secretion and does not contain cells.
The inside of a tooth is composed of
dentin, a calcified CT that encloses a central hollow area, the pulp cavity.
This contains the pulp, the non-calcified living tissue of the tooth. Blood
vessels, nerves, and lymphatics enter the apical foramen (an opening at the tip
of the root) and pass through the root canal to the larger pulp cavity.
FIGURE 24.7 P. 905
Humans have 2 sets of teeth:
a. Deciduous (primary, milk, baby) teeth--20 in
all and erupt from 6 months-2years of age. Lost and replaced over ages of 6-12
years.
b. Permanent (secondary) teeth--32 teeth that appear from the age of 6
years to adulthood. Includes 4 types of teeth:
1) Incisors--2 pairs on midline--chisel-shaped for cutting
2) Cuspids (canines)--1 pair with a pointed surface (cusp)--used to tear
or shred food
3) Premolars (bicuspids)--2 pairs, 2 cusps each--crushing and grinding
4) Molars--3 pairs, 4 cusps each--crushing and grinding. The 3rd molars
are commonly called the wisdom teeth. Modern human jaws often do not have room
for them to erupt (impacted wisdom teeth).
a. Mechanical—chewing (mastication) reduces the food to smaller pieces
and mixes it with saliva. A “mouthful” of food forms a soft mass called a
bolus.
b. Chemical—2 enzymes
1) Salivary amylase—initiates the breakdown of starches
2) Lingual lipase begins digestion of triglycerides (the form of fat we
eat the most of)
5.
PHARYNX—food being
swallowed passes through the pharynx (oropharynx & laryngopharynx)
Swallowing (deglutition)—moves food
from mouth to stomach in 3 stages:
a. Voluntary—bolus moved into oropharynx by movement up and back of
tongue
b. Pharyngeal stage—involuntary and moves bolus into the esophagus.
Epiglottis and uvula close off respiratory passages.
These movements are coordinated by the
deglutition center in the medulla.
c. Esophageal stage--through esophagus to stomach
6.
ESOPHAGUS—muscular
collapsible tube with typical layers except no true serosa. The outer layer is
known as the adventitia and has no epithelial layer. The functions are
transportation of food and secretion of mucus for lubrication (no enzymes, no
absorption). It penetrates the diaphragm at the esophageal hiatus. The upper
esophageal sphincter in the pharynx opens during swallowing to allow food to
enter. Peristalsis (wave-like muscle contractions) moves the food down the
esophagus. At the lower end the lower esophageal (cardiac) sphincter opens to
allow food into the stomach. If this sphincter fails to close, acid stomach
contents may leak upward into the esophagus (heartburn).
7.
STOMACH—enlargement of
the GI tract with 4 areas:
a. Cardia—surrounds esophageal opening—if this upper portion slips up
through the esophageal hiatus this is a hiatal hernia.
b. Fundus—above and to the left of the cardia
c. Body—large central portion
d. Pylorus—connects to duodenum and contains the pyloric sphincter,
which must open to allow stomach contents to move into the SI
When the stomach is empty, the mucosa
lies in thick ridges called rugae, which flatten out as it fills. The stomach
mucosa contains a layer of simple columnar cells with many gastric pits
extending down to the lamina propria and connecting to the gastric glands.
FIGURE 24.12 P. 868
Gastric glands contain 4 kinds of
cells:
a. Chief (zymogenic) cells—secrete pepsinogen and gastric lipase
b. Parietal cells—secrete HCl and intrinsic factor (necessary for the
absorption of Vitamin B12)
c. Mucous neck cells—secrete mucus
The secretion of these 3 types of cells
mixes in the duct (gastric pit) as it moves into the stomach, where it is
called gastric juice. About 2000-3000 ml per day is secreted.
d. G cells—do not contribute to gastric juice--these are endocrine cells
and secrete the hormone gastrin
The muscularis of the stomach has 3
layers:
a. Outer longitudinal
b. Middle circular
c. Inner oblique—extra layer not found in rest of tube
This allows the stomach to move in a
variety of ways to churn and mix food.
a. Mechanical—after food enters,
frequent peristaltic mixing waves pass over the stomach. Food mixes with
gastric juice and is then known as chyme. Mixing waves become gradually
stronger. Eventually the waves force chyme against the pyloric sphincter. A
little of the most liquid chyme is squirted into the duodenum as the sphincter
opens briefly. Most of the chyme remains in the stomach and undergoes further
mixing, then a little more is moved into the duodenum as the stomach is slowly
emptied. Chyme remains in the stomach 2-6 hours, depending on content. Meals
high is carbohydrate leave fastest, protein next fastest and high fat meals
remain the longest.
b. Chemical digestion
1) HCl—kills microbes, unfolds proteins (a preliminary step in
digestion) and activates pepsinogen
2) Pepsin is the protein-digesting enzyme of the stomach. It breaks
peptide bonds and splits proteins into fragments called peptides. It can act
only in a very acid pH (about 2). Pepsin is secreted in an inactive form,
pepsinogen. This protects the secreting cells themselves from digestion by the
enzyme. It becomes active only in the stomach, where lining cells are protected
by a thick layer of alkaline mucus.
3) Gastric lipase—acts in the digestion of milk fats--important in
babies, limited role in adults
3 phases:
a. Cephalic phase—initiated in the cortex by the sight, smell, taste or
thought of food. Parasympathetic impulses increase all secretions of the
gastric glands and increase motility. Emotions (fear, anger, tension) may
interfere with this by generating sympathetic impulses.
b. Gastric phase—occurs when food actually
enters the stomach.
1) Nervous—food stretches the stomach and stimulates stretch receptors
in the wall. Also chemoreceptors sense that the pH has risen as non-acid food
enters. Both of these trigger parasympathetic impulses that increase
peristalsis and secretion of gastric juice. As the pH returns to normal and
then later food moves into the small intestine, these receptors stop triggering
impulses until more food arrives.
2) Hormonal—distention of the stomach, partially digested proteins, and
caffeine all stimulate the G cells and cause the secretion of gastrin. This
hormone stimulates secretion of gastric juice, increases stomach motility and
acts on the sphincters (contraction of the lower esophageal sphincter and
relaxation of the pyloric and ileocecal sphincters).
c. Intestinal phase—this occurs when food moves to the duodenum and
results in inhibition of gastric secretion and motility. The intestinal
phase slows gastric digestion and the movement of chyme so the duodenum won't
be overloaded.
Gastric emptying is under both neural
and hormonal control.
Factors that encourage
:
1) Distention of the stomach
2) Presence in the stomach of partially digested proteins, alcohol, and
caffeine
3) Gastrin
Factors that slow:
1) Enterogastric reflex—this is initiated by the presence of food in the
SI. It inhibits parasympathetic impulses to the stomach and decreases gastric
secretion and motility.
2) Hormonal—presence of chyme containing glucose and fatty acids in the
small intestine causes enteroendocrine cells in the wall of the SI to release 2
hormones:
a) Secretin—decreases gastric secretions
b) Cholecystokinin (CCK)—inhibits stomach emptying
Only a few substances are absorbed
through the stomach wall:
Water Certain drugs (aspirin)
Electrolytes Alcohol
8.
PANCREAS—accessory
structure of major importance to digestion in the SI. Lies retroperitoneal,
posterior to the greater curvature of the stomach and consists of a head, a
body and a tail. Two large ducts carry pancreatic secretions to the duodenum:
a. Pancreatic duct (duct of Wirsung)—larger and joins the common bile
duct from the liver and gallbladder
b. Accessory duct (duct of Santorini)—directly from pancreas to duodenum
The gland is made up of small clusters
of exocrine cells called pancreatic acini which secrete pancreatic juice.
Scattered among the acini are islets of Langerhans, which are the endocrine
portion (1%).
Pancreatic juice contains mostly water,
sodium bicarbonate and a number of enzymes. About 1200-1500 ml per day is
secreted. Functions:
a. Buffers the acidic chyme to slightly alkaline
b. Stops the action of pepsin
c. Provides the following enzymes:
1) Pancreatic amylase
2) Trypsin
3) Chymotrypsin
4) Carboxypeptidase
5) Elastase
6) Pancreatic lipase
7) Ribonuclease
8) Deoxyribonuclease
The pancreatic protein digesting
enzymes are also secreted in an inactive form and activated in the SI. If these
enzymes become activated within the pancreas they would digest the gland
itself. This sometimes occurs (acute pancreatitis).
Trypsinogen—inactive form of trypsin—activated by enterokinase, an
enzyme secreted by cells of the SI
Chymotrypsinogen
Procarboxypeptidase all
activated by trypsin
Proelastase
REGULATION OF
PANCREATIC SECRETION
Nervous—during the cephalic and gastric phase of gastric digestion, the
vagus nerve also sends impulses to the pancreas to increase secretion.
Hormonal
a. Acidic chyme entering the SI causes enteroendocrine cells in the SI
mucosa to release secretin, which stimulates flow of pancreatic juice rich in
bicarbonate ions.
b. Partially digested fats and proteins entering the SI cause other
enteroendocrine cells to secrete cholecystokinin (CCK). CCK stimulates pancreatic
juice rich in enzymes.
9.
LIVER—2nd largest
organ of the body, located under the diaphragm and covered by peritoneum and
dense CT. Divided into 2 principal lobes, a larger right and a smaller left,
separated by the falciform ligament. Also 2 smaller lobes,
the caudate and quadrate.
The liver is made up of many small
functional units called lobules, which consist of hepatocytes (specialized
epithelial cells) arranged in plates around a central
vein. Instead of capillaries, blood passes through larger vessels called
sinusoids, which contain Kupffer's cells that phagocytize worn-out blood cells,
bacteria and certain toxic substances.
Bile is secreted by the hepatocytes and
enters bile caniculi that empty into bile ductules. Bile ductules merge to form
larger bile ducts and eventually form the right and left hepatic ducts, which
unite and leave the liver as the common hepatic duct. The common hepatic duct
is joined by the cystic duct from the gallbladder to form the common bile duct,
which along with the pancreatic duct enters the duodenum at the
hepatopancreatic ampulla (ampulla of Vater).
FIGURE 24.14 P. 917 (again)
Blood from both the hepatic artery and
the hepatic portal vein enters the liver sinusoids, where hepatic cells remove
oxygen, nutrients and certain poisons and Kupffer's cells also function. As the
blood travels through the liver:
Nutrients are stored or modified
Poisons are stored or detoxified
Products manufactured by hepatic cells and some nutrients are secreted
into the blood
Blood drains into the central vein of
the lobule and then the hepatic vein. Often branches of the hepatic portal
vein, the hepatic artery, and a bile duct travel together through the liver and
form a portal triad at the corners of liver lobules.
Hepatocytes secrete 800-1000 ml of bile
daily. It is a yellow, brownish or olive green liquid that consists mostly of
water with:
Bile salts Lecithin
Bile acids Bile pigments
Cholesterol Ions
Bile is both an excretory (waste)
product and a digestive secretion. Bile salts act in the emulsification and
absorption of fats. Emulsification is the breakdown of large fat globules into
tiny droplets. Bilirubin is the principal bile pigment and is a waste product
of RBC breakdown. In the intestine, bilirubin is broken down to stercobilin,
which gives the brownish color to feces.
Jaundice is a yellowish coloration of
skin, mucous membranes and the sclera due to a buildup of bilirubin.
a. Prehepatic (hemolytic) jaundice---rapid excess production of
bilirubin
b. Hepatic jaundice---liver malfunction
c. Extrahepatic (obstructive) jaundice---blockage of bile drainage
REGULATION OF BILE
SECRETION---small
amounts are produced at all times)
1. Nervous--parasympathetic impulses delivered by the vagus nerve
increase production
2. Hormonal--secretin stimulates secretion
3. Increased blood flow through the liver increases secretion
4. Presence of large amounts of bile salts in the blood increases
secretion
1. Carbohydrate metabolism—very important in maintaining normal blood
glucose level. Converts glucose to glycogen back to glucose as needed. Can also convert certain amino acids, lactic acid and other sugars
to glucose when needed.
2. Lipid metabolism--breaks down fatty acids to generate ATP. synthesizes lipoproteins and cholesterol, converts some
cholesterol to bile salts
3. Protein metabolism—loss of just this function could be fatal in a few
days. Converts ammonia, a highly toxic byproduct of protein breakdown, to urea,
which is less toxic. Urea is then removed by the kidneys. Also synthesizes most
plasma proteins (globulins, albumin, clotting factors).
4. Removal of drugs and hormones
5. Excretion of bilirubin
6. Synthesis of bile salts
7. Storage of glycogen, vitamins, minerals
8. Phagocytosis
9. Activation of Vitamin D
10.
GALLBLADDER--small sac
on the ventral surface of the liver that stores and concentrates bile by
absorbing water and ions. The wall contains smooth muscle fibers which can
contract to squeeze bile into the duodenum through the cystic duct and then the
common bile duct.
When the SI is empty, a sphincter
closes the hepatopancreatic ampulla so bile backs up into the gallbladder for
storage. Food in the SI stimulates secretin of CCK, which causes contraction of
the gallbladder and the opening of the sphincter.
FIGURE 24.14 P.
917 (AGAIN)
TABLE 24.8
P. 939
Where are they produced?
Where do they act?
What do they do?
Remember, the same hormone may affect
more than one target.
11.
SMALL INTESTINE--most
of digestion and absorption occur here. It totals about 10 feet in a living
person (21 feet in a cadaver) and is divided into 3 sections:
a. Duodenum (10 inches)—begins at pylorus
b. Jejunum (3 feet)
c. Ileum (6 feet)—joins the LI at the ileocecal valve (sphincter)
The SI is highly adapted for digestion
and absorption.
Modifications for absorption:
a. Intestinal mucosa is arranged in permanent ridges called circular
folds which cause the chyme to spiral
b. Mucosa forms villi, fingerlike projections that increase surface
area. Inside each villus are CT, blood vessels and a lacteal (special lymphatic
vessel) to pick up absorbed nutrients.
c. Microvilli—extremely tiny projections of just the plasma membrane of
the lining cells that also increase the surface area. Cannot be distinguished
individually through a light microscope but give a fuzzy appearance called the
brush border.
FIGURE 24.18 P. 923
Secretions:
a. Intestinal glands (crypts of
Lieberkuhn)—these are deep crevices in the mucosa lined with glandular
epithelium. They secrete the watery fluid called intestinal juice, which helps
dissolve and further liquefy the chyme.
b. Intestinal lining cells secrete the
following enzymes (called brush border enzymes):
1) Maltase
2) Sucrase
3) Lactase
4) a-dextrinase
5) Peptidases
a) Aminopeptidase
b) Dipeptidase
6) Nucleosidases
7) Phosphatases
b. Goblet cells of the mucosa secrete
mucus
c. Duodenal (Brunner's ) glands of the
submucosa secrete a special
alkaline mucus
d. Paneth cells in the intestinal
glands secrete lysozyme and carry on phagocytosis
e. Enteroendocrine cells secrete
digestive hormones
The lamina propria of the SI contains
large amounts of lymphatic tissue (mucosa associated lymphatic tissue or MALT)
to prevent bacteria from entering the blood. Some lymphatic nodules are in
groups called Peyer's patches, esp. in the ileum.
2 types of movements occur:
a. Segmentation is the major movement—it is a localized contraction in
areas containing food to mix chyme with digestive juices and bring material in
contact with the mucosa for absorption. It occurs due to contraction of
circular muscle fibers.
b. Peristalsis—propels chyme onward. In the small intestine, this is a
wave of peristalsis known as a migrating motility complex (MMC). Beginning just
past the stomach, each MMC travels slowly the entire length of the small
intestine. Chyme remains in the SI for 3-5 hours.
1)
Carbohydrates
a. Some of the starches in the food
are broken down by salivary amylase. Remaining digestible starches are broken
down into fragments by pancreatic amylase. The brush border enzyme a-dextrinase splits glucose units off
the starch fragments.
b. Ingested sucrose, lactose, and
maltose are split into monosaccharides by the brush-border enzymes sucrase,
lactase, and maltase. Many individuals do not produce enough lactase to
completely break down the lactose, which leads to lactose intolerance.
2) Proteins—pancreatic protein digesting enzymes take up where pepsin of
the stomach leaves off. Two brush-border enzymes,
aminopeptidase and dipeptidase, complete the process of breaking the protein
down to individual amino acids.
3) Lipids—In adults, the entire fat-digesting
process is carried out by pancreatic lipase, acting on fats emulsified by bile
salts. Triglycerides are broken down by removing 2 of the 3 fatty acids,
leaving 2 fatty acids and a monoclygeride.
4) Nucleic acids—ribonuclease and deoxyribonuclease in pancreatic juice
break down RNA and DNA into pentoses, phosphates, and nitrogenous bases.
Local reflexes in response to the
presence of chyme are most important. Intestinal distension and parasympathetic
impulses increase motility. Sympathetic impulses decrease secretion and
motility.
A hormone, vasoactive intestinal
polypeptide (VIP) stimulates production of intestinal juice.
12.
LARGE INTESTINE--extends
from ileum to anus and is about 5 feet long. It is attached to the posterior
body wall by mesocolon. The 4 regions are:
Cecum
Rectum
Anus
A sphincter (ileocecal valve) regulates
movement of chyme from the
SI to the LI. A blind pouch, the cecum, hangs down
from the junction of the ileum and LI and ends in a twisted tube, the vermiform
appendix.
The large intestine continues from this
area as the colon: ascending to hepatic flexure, transverse to splenic flexure,
descending and
sigmoid. The rectum is the last 8 inches of the LI and terminates as the anal
canal. The anus is the opening to the outside and contains 2 sphincters, the
internal involuntary and the external voluntary.
FIGURE 24.22 P. 932
Digestion is mostly complete when chyme
enters the LI, so little secretion or absorption occur.
The muscularis has 3 bands of thickened longitudinal muscles called taenia coli
which run most of the length of the LI. Contraction of these bands gathers the
colon into a series of pouches called haustra and gives it a puckered
appearance.
The ileocecal valve regulates the
passage of chyme from the ileum into the LI. Immediately after a meal the
gastroileal (ileogastric) reflex causes a strong wave of peristalsis that sends
any chyme present in the ileum into the LI.
In the LI one movement that occurs is
haustral churning. One pouch fills and then its walls contract to send the
chyme on into the net pouch. Peristalsis also occurs at a slow rate. Mass
peristalsis occurs in response to filling of the stomach (gastrocolic reflex).
This drives colon contents into the rectum.
The last stage of chemical digestion
involves the action of bacteria in the LI. Any remaining carbohydrates are
fermented with production of gas. Remaining proteins are also broken down.
Bilirubin is decomposed to urobilinogen. Bacteria produce some of the B
vitamins and vitamin K, which we absorb.
Chyme remains in the LI for 3-10 hours,
becoming solid or semisolid as much of the remaining water is absorbed. It is
then called feces and contains water, salts, sloughed-off mucosal cells,
bacteria, and undigested food. Absorbed in the LI:
Water
Vitamins
Electrolytes
About 1/2 to 1 liter of water enters
the LI daily. All but 100-200 ml of it is reabsorbed.
Mass peristalsis pushes feces from
sigmoid colon to rectum. Distension of the rectum stimulates stretch receptors
which initiate a defecation reflex, which causes contraction of the
longitudinal muscles of the rectum and relaxation of the internal sphincter. At
this point defecation occurs in infants and small children. We develop
voluntary control of the external sphincter and can delay defecation until a
convenient time as we mature. In this case feces moves back
into the sigmoid colon until another wave of mass peristalsis occurs.
Consists of indigestible plant
materials--cellulose, lignin, pectin--found in fruits, vegetables, grains and
beans.
Insoluble fiber--does not dissolve in water--fruit & vegetable
skins, wheat and corn bran. Important in keeping foods moving through the
digestive tract and may reduce the risk of colon cancer, gallstones, etc.
Soluble fiber--dissolves in water--found in beans, oats, barley,
broccoli, prunes, apples and citrus fruits. Seems to help lower cholesterol
levels (binds bile salts and prevents their reabsorption).
CHAPTER 24 SUMMARY OF
CHEMICAL DIGESTION AND ABSORPTION
Chemical
digestion is the result of various enzymes acting to chemically break down food
molecules.
TABLE 24.54 P. 927—VERY IMPORTANT—THIS TABLE WITH
BLANKS WILL BE ON YOUR HOUR EXAM
Absorption---occurs
once food molecules are broken down to a form that can pass through the cells
of the mucosa into the blood or lymphatic vessels
90% of absorption occurs in the SI
10% occurs in the stomach and LI
CARBOHYDRATES are broken down to monosaccharides and
absorbed into capillaries of the villi, some by active transport and some by
facilitated diffusion.
PROTEINS are broken down to amino acids and
absorbed by active transport. A few dipeptides and tripeptides are absorbed
into cells and broken down there.
LIPIDS are broken down to monoglycerides and
fatty acids and absorbed by simple diffusion. Most lipids enter the lacteals
and are transported in the lymphatic system to the subclavian vein. Bile salts
are necessary.
About
9.3 liters enter the SI each day, about 2.3 liters from ingested liquids and 7
liters from digestive secretions. 8 - 8.5 liters are absorbed in the SI and
most of the rest in the LI. Only about .1 liters per day is excreted from the
digestive tract. Water is absorbed by osmosis.
A. Fat-soluble vitamins (ADEK) are absorbed
along with ingested dietary fats (cannot be absorbed with no fat in GI tract).
B. Water-soluble vitamins (BC) are absorbed
by simple diffusion except for B12. It must combine with intrinsic
factor and the combination is absorbed by receptor-mediated endocytosis.