Chapter 24
THE DIGESTIVE SYSTEM
I. INTRODUCTION
A. Food contains substances and energy the body needs to construct all cell components. The food must
be broken down through digestion to molecular size before it can be absorbed by the digestive
system and used by the cells.
B. The organs that collectively perform these functions compose the digestive system.
C. The medical professions that study the structures, functions, and disorders of the digestive tract are
gastroenterology for the upper end of the system and proctology for the lower end.
II. OVERVIEW OF THE DIGESTIVE SYSTEM
A. Organization
1. The two major sections of the digestive system perform the processes required to prepare
food for use in the body.
2. The gastrointestinal tract is the tube open at both ends for the transit of food during
processing. The functional segments of the GI tract include the mouth, esophagus, stomach,
small intestine, and large intestine.
3. The accessory structures that contribute to the food processing include the teeth, tongue,
salivary glands, liver, gallbladder, and pancreas.
B. Digestion includes six basic processes.
1. Ingestion is taking food into the mouth (eating).
2. Secretion is the release, by cells within the walls of the GI tract and accessory organs, of
water, acid, buffers, and enzymes into the lumen of the tract.
3. Mixing and propulsion result from the alternating contraction and relaxation of the smooth
muscles within the walls of the GI tract.
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Digestive System Regulation
•
Nervous regulation
– Involves enteric nervous system
•
Types of neurons: sensory, motor, interneurons
– Coordinates peristalsis and regulates local reflexes
•
Chemical regulation
– Production of hormones
•
Gastrin, secretin
– Production of paracrine chemicals
•
Histamine
•
Help local reflexes in ENS control digestive environments as pH levels
4. Digestion
a. Mechanical digestion consists of movements of the GI tract that aid chemical
digestion.
b. Chemical digestion is a series of catabolic (hydrolysis) reactions that break down
large carbohydrate, lipid, and protein food molecules into smaller molecules that are
usable by body cells.
5. Absorption is the passage of end products of digestion from the GI tract into blood or lymph
for distribution to cells.
6. Defecation is emptying of the rectum, eliminating indigestible substances from the GI tract.
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III. LAYERS OF THE GI TRACT
A. The basic arrangement of layers in the gastrointestinal tract from the inside outward includes the
mucosa, submucosa, muscularis, and serosa (visceral peritoneum).
B. The mucosa consists of an epithelium, lamina propria, and muscularis mucosa.
1. The epithelium consists of a protective layer of non-keratinized stratified cells, simple cells
for secretion and absorption, and mucus secreting cells, as well as some enteroendocrine cells
that put out hormones that help regulate the digestive process.
2. The lamina propria consists of three components, including loose connective tissue that
adheres the epithelium to the lower layers, the system of blood and lymph vessels through
which absorbed food is transported, and nerves and sensors.
a. The lymph system is part of the mucosa-associated lymph tissues (MALT) that
monitor and produce an immune response to pathogens passing with food through the
GI tract.
b. It is estimated that there are as many immune cells associated with the GI tract as in
all the rest of the body.
3. The muscularis mucosa causes local folding of the mucosal layer to increase surface are for
digestion and absorption.
C. The submucosa consists of aerolar connective tissue. It is highly vascular, contains a part of the
submucosal plexus (plexus of Meissner), and contains glands and lymphatic tissue.
1. The submucosal plexus is a part of the autonomic nervous system.
2. It regulates movements of the mucosa, vasoconstriction of blood vessels, and innervates
secretory cells of mucosal glands.
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D. Muscularis
1. The muscularis of the mouth, pharynx, and superior part of the esophagus contains skeletal
muscle that produces voluntary swallowing. Skeletal muscle also forms the external anal
sphincter.
2. Through the rest of the tract, the muscularis consists of smooth muscle in an inner sheet of
circular fibers and an outer sheet of longitudinal fibers.
3. The muscularis also contains the major nerve supply to the GI tract - the myenteric plexus
(plexus of Auerbach), which consists of fibers from both divisions of the ANS. This plexus
mostly controls GI tract motility.
E. The serosa is the superficial layer of those portions of the GI tract that are suspended in the
abdominoplevic cavity.
1. The esophagus is covered by an adventitia.
2. Inferior to the diaphragm, the serosa is also called the visceral peritoneum.
IV. PERITONEUM
A. The peritoneum is the largest serous membrane of the body.
1. The parietal peritoneum lines the wall of the abdominal cavity.
2. The visceral peritoneum covers some of the organs and constitutes their serosa.
3. The potential space between the parietal and visceral portions of the peritoneum is called the
peritoneal cavity and contains serous fluid.
4. Some organs, such as the kidneys and pancreas, lie on the posterior abdominal wall behind
the peritoneum and are called retroperitoneal.
5. The peritoneum contains large folds that weave between the viscera, functioning to support
organs and to contain blood vessels, lymphatic vessels, and nerves of the abdominal organs.
6. Extensions of the peritoneum include the mesentery, meoscolon, falciform ligament, lesser
omentum, and greater omentum.
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B. Peritonitis is an acute inflammation of the peritoneum.
V. MOUTH
A. Introduction
1. The mouth (oral or buccal cavity) is formed by the cheeks, hard and soft palate, lips, and
tongue.
2. The vestibule of the oral cavity is bounded externally by the cheeks and lips and internally by
the gums and teeth.
3. The oral cavity proper is a space that extends from the gums and teeth to the fauces, the
opening between the oral cavity and the pharynx or throat.
B. Salivary Glands
1. The major portion of saliva is secreted by the salivary glands, which lie outside the mouth
and pour their contents into ducts that empty into the oral cavity; the remainder of saliva
comes from buccal glands in the mucous membrane that lines the mouth.
2. There are three pairs of salivary glands: parotid, submandibular (submaxillary), and
sublingual glands.
3. Saliva lubricates and dissolves food and starts the chemical digestion of carbohydrates. It
also functions to keep the mucous membranes of the mouth and throat moist.
4. Chemically, saliva is 99.5% water and 0.5% solutes such as salts, dissolved gases, various
organic substances, and enzymes.
5. Salivation is entirely under nervous control.
6. Mumps is an inflammation and enlargement of the parotid salivary glands caused by
infection with the mumps virus (myxovirus). Symptoms include fever, malaise, pain, and
swelling of one or both glands. If mumps is contracted by a male past puberty, it is possible
to experience inflammation of the testes and, occasionally, sterility.
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Saliva and the Salivary Glands
•
Saliva
•
Saliva moistens the mouth, digests a small amount of starch and fat, cleanses the teeth,
inhibits bacterial growth, dissolves molecules so they can stimulate taste buds, and
moistens food and binds particles together to aid in swallowing. It is secreted by the
salivary glands.
•
Saliva is a hypotonic solution composed of 97% to 99.5% water and the following
solutes: salivary amylase, lingual lipase, mucus, lysozyme, immunoglobulin A, and
electrolytes.
•
Saliva has a pH of 6.8 to 7.0.
Salivation
•
The extrinsic salivary glands secrete 1.0 to 1.5 L of saliva per day.
•
Food stimulates tactile, pressure, and taste receptors, which transmit signals to the salivatory
nuclei in the medulla oblongata and pons. These nuclei also receive input from higher brain
centers so even the odor, sight, or thought of food stimulates salivation.
•
The salivatory nuclei send autonomic signals: Sympathetic stimulation reduces saliva output, and
parasympathetic stimulation causes the production of thinner saliva with more salivary amylase.
•
Salivary amylase begins to digest starch as the food is chewed, while the mucus of saliva binds
food particles into a soft, slippery, easily swallowed mass called a bolus.
C. Teeth
1. The teeth project into the mouth and are adapted for mechanical digestion.
2. A typical tooth consists of three principal portions: crown, root, and neck.
3. Teeth are composed primarily of dentin, a calcified connective tissue that gives the tooth its
basic shape and rigidity; the dentin of the crown is covered by enamel, the hardest substance
in the body, which protects the tooth from the wear of chewing.
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a. The dentin of the root is covered by cementum, another bone-like substance, which
attaches the root to the periodontal ligament (the fibrous connective tissue lining of
the tooth sockets in the mandible and maxillae).
b. The dentin encloses the pulp cavity in the crown and the root canals in the root.
c. The branch of dentistry that is concerned with the prevention, diagnosis, and
treatment of diseases that affect the pulp, root, periodontal ligament, and alveolar
bone is known as endodontics. Orthodontics is a dental branch concerned with the
prevention and correction of abnormally aligned teeth. Periodontics is a dental branch
concerned with the treatment of abnormal conditions of tissues immediately around
the teeth.
4. There are two dentitions, or sets of teeth, in an individual’s lifetime: deciduous (primary),
milk teeth, or baby teeth; and permanent (secondary) teeth.
5. There are four different types of teeth based on shape: incisors (used to cut food), cuspids or
canines (used to tear or shred food), premolars or bicuspids (absent in the deciduous
dentition and used for crushing and grinding food), and molars (also used for crushing and
grinding food).
E. Mechanical and Chemical Digestion in the Mouth
1. Through mastication (chewing), food is mixed with saliva and shaped into a bolus that is
easily swallowed.
2. The enzyme salivary amylase converts polysaccharides (starches) to disaccharides (maltose).
This is the only chemical digestion that occurs in the mouth.
VI. PHARYNX
A. The pharynx is a funnel-shaped tube that extends from the internal nares to the esophagus posteriorly
and the larynx anteriorly.
1. It is composed of skeletal muscle and lined by mucous membrane.
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2. The nasopharynx functions in respiration only, whereas the oropharynx and laryngopharynx
have digestive as well as respiratory functions.
B. Deglutition, or swallowing, moves a bolus from the mouth to the stomach. It is facilitated by saliva
and mucus and involves the mouth, pharynx, and esophagus.
1. Deglutition consists of a voluntary state, pharyngeal stage (involuntary), and esophageal
stage (involuntary).
2. Receptors in the oropharynx stimulate the deglutition center in the medulla and the lower
pons of the brain stem.
VII. ESOPHAGUS
A. The esophagus is a collapsible, muscular tube that lies behind the trachea and connects the pharynx
to the stomach.
B. The wall of the esophagus contains mucosa, submucosa, and muscularis layers. The outer layer is
called the adventitia rather than the serosa due to structural differences.
C. Physiology of the Esophagus
1. The esophagus contains an upper and a lower esophageal sphincter.
2. During the esophageal stage of swallowing progressive contractions of the muscularis push
the bolus onward. There propulsive contractions are termed peristalsis.
D. Gastroesophageal reflux disease occurs when the lower esophageal sphincter fails to close
adequately after food has entered the stomach, resulting in stomach contents refluxing into the
inferior portion of the esophagus. HCl from the stomach contents irritates the esophageal wall
resulting in heartburn.
VIII. STOMACH
A. Introduction
1. The stomach is a J-shaped enlargement of the GI tract that begins at the bottom of the
esophagus and ends at the pyloric sphincter.
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2. It serves as a mixing and holding area for food, begins the digestion of proteins, and
continues the digestion of triglycerides, converting a bolus to a liquid called chyme. It can
also absorb some substances.
B. Anatomy of the Stomach
1. The gross anatomical subdivisions of the stomach include the cardia, fundus, body, and
pyloris.
2. When the stomach is empty, the mucosa lies in folds called rugae.
3. Pylorospasm and pyloric stenosis are two abnormalities of the pyloric sphincter that can
occur in newborns. Both functionally block or partially block the exit of food from the
stomach into the duodenum and must be treated with drugs or surgery.
C. Histology of the Stomach
1. The surface of the mucosa is a layer of simple columnar epithelial cells called mucous
surface cells.
a. Epithelial cells extend down into the lamina propria forming gastric pits and gastric
glands.
b. The gastric glands consist of three types of exocrine glands: mucous neck cells
(secrete mucus), chief or zymogenic cells (secrete pepsinogen and gastric lipase), and
parietal or oxyntic cells (secrete HCl).
c. Gastric glands also contain enteroendocrine cells which are hormone producing cells.
G cells secrete the hormone gastrin into the bloodstream.
2. The submucosa is composed of areolar connective tissue.
3. The muscularis has three layers of smooth muscle: longitudinal, circular, and an inner
oblique layer.
4. The serosa is a part of the visceral peritoneum.
a. At the lesser curvature, the visceral peritoneum becomes the lesser omentum.
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b. At the greater curvature, the visceral peritoneum becomes the greater omentum.
5. Endoscopy refers to visual inspection of any cavity of the body using an endoscope, an
illuminated tube with lenses. Endoscopic examination of the stomach is called gastroscopy.
In addition to diagnostic use for ulcers and other stomach disorders, it may be employed to
halt bleeding, biopsy tumors, and remove foreign objects.
D. Mechanical and Chemical Digestion in the Stomach
1. Mechanical digestion consists of peristaltic movements called mixing waves.
2. Chemical Digestion
a. Chemical digestion consists mostly of the conversion of proteins into peptides by
pepsin, an enzyme that is most effective in the very acidic environment (pH 2) of
the stomach. The acid (HCl) is secreted by the stomach’s parietal cells.
b. Gastric lipase splits certain molecules in butterfat of milk into fatty acids and
monoglycerides and has a limited role in the adult stomach.
3. The stomach wall is impermeable to most substances; however, some water, electrolytes,
certain drugs (especially aspirin), and alcohol can be absorbed through the stomach lining.
Gastric Secretions
•
The gastric glands produce 2–3 L of gastric juice daily, composed mainly of water,
hydrochloric acid, and pepsin.
•
Hydrochloric Acid
•
Gastric juice has a high concentration of hydrochloric acid (HCl) and a pH as low as 0.8.
•
HCl secretion does not affect the pH of the parietal cells that manufacture it because H+
is pumped out as fast as it is generated. The bicarbonate ions are exchanged for chloride
ions from the blood plasma, and the Cl- is pumped into the lumen of the gastric gland.
HCl accumulates in the stomach while bicarbonate ions accumulate in the blood.
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
Stomach acid has several functions:
•
It activates the enzymes pepsin and lingual lipase.
•
It breaks up connective tissues and plant cell walls.
•
It converts ferric ions to ferrous ions.
•
It contributes to nonspecific disease resistance by destroying ingested bacteria
and other pathogens.

Intrinsic Factor
•
Parietal cells also secrete a glycoprotein called intrinsic factor that is essential to the
absorption of vitamin B12 by the small intestine.
•
The secretion of intrinsic factor is the only indispensable function of the stomach.
Pepsin
•
Several enzymes are secreted as inactive proteins called zymogens. Chief cells secrete the
zymogen called pepsinogen.
•
Hydrochloric acid removes some of the amino acids from pepsinogen and converts it to pepsin.
The function of pepsin is to digest dietary proteins to shorter peptide chains.

Other Enzymes
•
a. In infants, the chief cells also secrete gastric lipase and rennin.
Protection of the Stomach
•
The living stomach is protected in three ways from the harsh chemical of its interior.
•
It has a highly alkaline mucous coat.
•
Epithelial cells are replaced every 3–6 days.
•
Tight junctions between epithelial cells prevent gastric juice from seeping between cells
and digesting connective tissue.
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E. Regulation of Gastric Secretion and Motility
1. Gastric secretion is regulated by nervous and hormonal mechanisms. Stimulation occurs in
three overlapping phases: cephalic (reflex), gastric, and intestinal.
2. Cephalic Phase
a. The cephalic phase consists of reflexes initiated by sensory receptors in the head.
b. The cephalic phase stimulates gastric secretion and motility.
3. Gastric Phase
a. The gastric phase begins when food enters the stomach.
b. When the stomach walls are distended or when pH increases because proteins have
entered the stomach and buffered some of the stomach acid, the stretch receptors and
chemoreceptors are activated resulting in waves of peristalsis and continual flow of
gastric juice.
c. Hormonal negative feedback also regulates gastric secretions during the gastric phase.
1) Chemoreceptors and stretch receptors stimulate the ANS to release
acetylcholine which stimulates the release of gastrin by G cells.
2) Gastrin stimulates growth of the gastric glands and secretion of large amounts
of gastric juice. It also strengthens contraction of the lower esophageal
sphincter, increases motility of the stomach, and relaxes the pyloric and
ileocecal sphincters.
3) Acetylcholine released by parasympathetic fibers and gastrin secreted by G
cells stimulate parietal cells to secrete more HCl when histamine is present. In
other words, histamine enhances the effects of acetylcholine and gastrin.
4. Intestinal Phase
a. The intestinal phase is due to activation of receptors in the small intestine.
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b. When partially digested food enters the small intestine, it triggers the enterogastric
reflex and the secretion of gastric inhibitory peptide (GIP), secretin, and
cholecystokinin (CCK) by the intestinal mucosa. The effect is inhibition of gastric
secretion.
5. Regulation of Gastric Emptying
a. Gastric emptying is the periodic release of chyme from the stomach into the
duodenum.
b. Gastric emptying is stimulated by two factors: nerve impulses in response to
distention of the stomach and stomach gastrin in response to the presence of certain
types of foods.
c. Most food leaves the stomach 2-6 hours after ingestion. Carbohydrates leave earliest,
followed by proteins and then fats.
d. Gastric emptying is inhibited by the enterogastric reflex and by the hormones CCK
and GIP.
e. Vomiting is the forcible expulsion of the contents of the upper GI tract (stomach and
sometimes duodenum) through the mouth. Prolonged vomiting, especially in infants
and elderly people, can be serious because the loss of gastric juice and fluids can lead
to disturbances in fluid and acid-base balance. (Clinical Application)
IX. PANCREAS
A. The pancreas is divided into a head, body, and tail and is connected to the duodenum via the
pancreatic duct (duct of Wirsung) and accessory duct (duct of Santorini).
B. Pancreatic islets (islets of Langerhans) secrete hormones and acini secrete a mixture of fluid and
digestive enzymes called pancreatic juice.
C. Pancreatic Juice
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1. Pancreatic juice contains enzymes that digest starch (pancreatic amylase), proteins (trypsin,
chymotrypsin, and carboxypeptidase), fats (pancreatic lipase), and nucleic acids
(ribonuclease and deoxyribonuclease).
2. It also contains sodium bicarbonate which converts the acid stomach contents to a slightly
alkaline pH (7.1-8.2), halting stomach pepsin activity and promoting activity of pancreatic
enzymes.
3. Inflammation of the pancreas is called pancreatitis and can result in trypsin beginning to
digest pancreatic cells.
D. Pancreatic secretion is regulated by nervous and hormonal mechanisms.
The Pancreas
•
The pancreas is a soft, spongy gland posterior to the greater curvature of the stomach and outside
the peritoneal cavity. It has both endocrine and exocrine functions.
•
Its endocrine part is the pancreatic islets, which secrete insulin and glucagon.
•
Most of the pancreas is exocrine tissue that secretes 1,200–1,500 mL of pancreatic juice
per day. This secretion ends up in the main pancreatic duct running lengthwise through
the gland. It empties into the small intestine through the hepatopancreatic ampulla or by
way of a smaller accessory pancreatic duct in some people.
•
Pancreatic juice is an alkaline mixture of water, enzymes, zymogens, sodium bicarbonate, and
other electrolytes.
•
The acinar cells secrete the enzymes and zymogens, whereas the gland ducts secrete the
sodium bicarbonate. Bicarbonate buffers the hydrochloric acid from the stomach.
•
Pancreatic zymogens are trypsinogen, chymotrypsinogen, and procarboxypeptidase.
Other pancreatic enzymes include pancreatic amylase, pancreatic lipase, ribonuclease,
and deoxyribonuclease
•
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X. LIVER AND GALLBLADDER
A. The liver is the heaviest gland in the body and the second largest organ in the body after the skin.
B. Anatomy of the Liver and Gallbladder
1. The liver is divisible into left and right lobes, separated by the falciform ligament. Associated
with the right lobe are the caudate and quadrate lobes.
2. The gallbladder is a sac located in a depression on the posterior surface of the liver.
C. Histology of the Liver and Gallbladder
1. The lobes of the liver are made up of lobules that contain hepatic cells (liver cells or
hepatocytes), sinusoids, stellate reticuloendothelial (Kupffer’s) cells, and a central vein.
2. The mucosa of the gallbladder is simple columnar epithelium arranged in rugae. There is no
submucosa. The smooth muscle of the muscularis ejects bile into the cystic duct. The outer
layer is the visceral peritoneum. Functions of the gallbladder are to store and concentrate bile
until it is needed in the small intestine.
D. The liver receives a double supply of blood from the hepatic artery and the hepatic portal vein. All
blood eventually leaves the liver via the hepatic vein,
E. Hepatic cells (hepatocytes) produce bile that is transported by a duct system to the gallbladder for
concentration and temporary storage.
1. Bile is partially an excretory product (containing components of worn-out red blood cells)
and partially a digestive secretion.
2. Bile’s contribution to digestion is the emulsification of triglycerides.
F. The rate of bile secretion is regulated by nervous and hormonal mechanisms as well as by volume of
hepatic blood flow and the concentration of bile salts in the blood .
G. The liver also functions in carbohydrate, lipid, and protein metabolism; removal of drugs and
hormones from the blood; excretion of bilirubin; synthesis of bile salts; storage of vitamins and
minerals; phagocytosis; and activation of vitamin D.
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H. The fusion of individual crystals of cholesterol is the beginning of 95% of all gallstones. Gallstones
can cause obstruction to the outflow of bile in any portion of the duct system. Treatment of
gallstones consists of using gallstone-dissolving drugs, lithotripsy, or surgery.
Functions of the Liver
•
•
•
•
Carbohydrate metabolism.
•
glycogenesis
•
glycogenolysis
•
Gluconeogenesis
Lipid metabolism.
•
Stores some triglycerides.
•
Synthesizes lipoproteins.
Protein Metabolism.
•
Deaminates amino acids – ammonia – urea.
•
Alpha protein synthesis.
•
Beta protein synthesis.
•
Albumin synthesis.
•
Prothrombin synthesis.
•
Fibrinogen synthesis.
Processing of drugs and hormones.
•
Alcohol.
•
Penicillin.
•
Erythromicin.
•
Sulfanamides.
•
Thyroid Hormones.
•
Steroid Hormones.
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•
Phenobarbitol.
•
Breakdown of old erythrocytes.
•
Synthesis of bile salts.
•
•
Emulsification of lipids.
•
Absorption of lipids.
Storage of vitamins (A, B12, D, E, K) and minerals (iron and copper).
The Gallbladder and Bile
•
The gallbladder is a sac about 10 cm long.
•
When bile is not needed for digestion, the hepatopancreatic sphincter is closed. Bile then
fills up the bile duct and spills over into the gallbladder, which absorbs water and stores
the bile for later use.
•
The liver produces 500–1,000 mL of bile per day. It is a yellow-green fluid containing
minerals, bile pigments, bile salts, cholesterol, neutral fats, and phospholipids.
•
The principal bile pigment is bilirubin, derived from the decomposition of hemoglobin.
•
Bile acids are steroids synthesized from cholesterol. Bile acids are phospholipids that aid
in fat digestion and absorption. They are not excreted in the feces but are reabsorbed in
the ileum and returned to the liver.
XII. SUMMARY OF DIGESTIVE HORMONES
A. The effects of the four major digestive hormones - gastrin, secretin, CCK, and GIP - are to regulate
gastric secretion and motility, as well as secretion of the pancreas, liver, and gallbladder.
1. Gastrin promotes secretion of gastric juice and increases gastric motility.
2. Secretin promotes secretion of bicarbonate ions into pancreatic juice and bile. It inhibits
secretion of gastric juice and promotes normal growth and maintenance of the pancreas. It
enhances the effects of CCK. Overall, it causes buffering of acid in chyme.
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3. CCK stimulates secretion of pancreatic juice rich in digestive enzymes and ejection of bile
into the duodenum . It also slows gastric emptying.
B. There are other hormones secreted by and having effects on the GI tract. They include motilin,
substance P, bombesin, vasoactive intestinal polypeptide, gastrin-releasing peptide, and
somatostatin.
XIII. SMALL INTESTINE
A. Introduction
1. The major events of digestion and absorption occur in the small intestine.
2. The small intestine extends from the pyloric sphincter to the ileocecal sphincter.
B. Anatomy of the Small Intestine
1. The small intestine is divided into the duodenum, jejunum, and ileum (Figure 24.21).
2. Projections called circular folds, or plicae circularies, are permanent ridges in the mucosa
that enhance absorption by increasing surface area and causing chyme to spiral as it passes
through the small intestine.
The Small Intestine
•
Nearly all chemical digestion and nutrient absorption occur in the small intestine. The term
"small" applies to its diameter, not its length. Circular folds, villi, and microvilli all serve to
increase the surface area inside the small intestine.
•
Gross Anatomy
•
The small intestine is divided into three regions.
•
The duodenum constitutes the first 25 cm. It receives the stomach contents,
pancreatic juice, and bile. Stomach acid is neutralized here, pepsin is inactivated
by the elevated pH, and pancreatic enzymes take over the job of chemical
digestion.
•
The jejunum comprises the next 2.5 m.
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•
The ileum forms the last 3.6 m and ends at the ileocecal junction.
C. Histology of the Small Intestine
1. The mucosa forms fingerlike villi which increase the surface area of the epithelium available
for absorption and digestion.
a. Embedded in the villus is a lacteal (lymphatic capillary) for fat absorption.
b. The cells of the mucosal epithelium include absorptive cells, goblet cells,
enteroendocrine cells, and Paneth cells.
c. The free surface of the absorptive cells feature microvilli, which increase the surface
area . They form the brush border which also contains several enzymes.
d. The mucosa contains many cavities lined by glandular epithelium. These cavities
form the intestinal glands (crypts of Lieberkuhn).
2. The submucosa of the duodenum contains duodenal (Brunner’s) glands which secrete an
alkaline mucus that helps neutralize gastric acid in chyme. The submucosa of the ileum
contains aggregated lymphatic nodules (Peyer’s patches).
Microscopic Anatomy
•
The largest folds of the intestinal wall are transverse to spiral ridges called circular folds. They
occur from the duodenum to the middle of the ileum, where they cause the chyme to flow in a
spiral path along the intestine. This slows its progress, allows more contact with the mucosa, and
promotes more thorough mixing and nutrient absorption.
•
The mucosa also possesses villi, which have fingerlike shapes. The largest villi are in the
duodenum, and they become progressively smaller in more distal regions of the small intestine.
•
A villus is covered with two kinds of epithelial cells— columnar absorptive cells and mucussecreting goblet cells.
•
The core of the villus is filled with areolar tissue containing a capillary network, an arteriole, a
venule, and a lymphatic capillary called a lacteal.
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•
Each epithelial cell of a villus has a border of microvilli containing brush border enzymes that
carry out contact digestion.
Intestinal Secretion
•
The intestinal crypts secrete 1–2 L of intestinal juice per day, especially in response to acid,
hypertonic chyme, and intestinal distension.
Intestinal Motility
•
Contractions of the small intestine serve three functions: (1) to mix chyme with intestinal
juice, bile, and pancreatic juice; (2) to churn chyme and bring it in contact with the mucosa
for digestion and absorption; and (3) to move residue toward the large intestine.
•
Segmentation is the most common type of movement of the small intestine. Ringlike
constrictions appear at several places along the intestine and then relax while constrictions
occur elsewhere. The effect is to churn the contents of the intestine.
D. Intestinal Juice and Brush Border Enzymes
1. Intestinal juice provides a vehicle for absorption of substances from chyme as they come in
contact with the villi.
2. Some intestinal enzymes (brush border enzymes) break down foods inside epithelial cells of
the mucosa on the surfaces of their microvilli.
3. Some digestion also occurs in the lumen of the small intestine.
E. Mechanical Digestion in the Small Intestine
1. Segmentation, the major movement of the small intestine, is a localized contraction in areas
containing food.
2. Peristalsis propels the chyme onward through the intestinal tract.
F. Chemical Digestion in the Small Intestine
1. Carbohydrates are broken down into monosaccharides for absorption.
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a. Intestinal enzymes break down starches into maltose, maltotriose, and alpha-dextrins
(pancreatic amylase); alpha-dextrins into glucose (alphadestrinase); maltose to
glucose (maltase); sucrose to glucose and fructose (sucrase); and lactose to glucose
and galactose (lactase).
b. In some individuals, there is a failure of the intestinal mucosal cells to produce the
enzyme lactase. This results in lactose intolerance, the inability to digest the sugar
lactose found in milk and other dairy products. It may be a temporary or long-lasting
condition and is characterized by diarrhea, gas, bloating, and abdominal cramps after
ingestion of diary products.
2. Protein digestion starts in the stomach.
a. Proteins are converted to peptides by trypsin and chymotrypsin. Also, enzymes break
peptide bonds that attach terminal amino acids to carboxyl ends of peptides
(carboxypeptidases) and peptide bonds that attach terminal amino acids to amino ends
of peptides (aminopeptidases).
b. Finally, enzymes split dipeptides to amino acids (dipeptidase).
3. Most lipid digestion, in an adult, occurs in the small intestine.
a. Bile salts break the globules of triglycerides (fats) into droplets, a process called
emulsification.
b. Pancreatic lipase, due to the increase exposed surface area of the droplets, can
hydrolyze more triglycerides into fatty acids and monoglycerides.
4. Nucleic acids are broken down into nucleotides for absorption.
G. Regulation of Intestinal Secretion and Motility
1. The most important mechanism for regulating small intestinal secretion is the action of local
reflexes in response to the presence of chyme.
2. Hormones (vasoactive intestinal polypeptide or VIP) also assume a role.
21
3. Parasympathetic impulses increase motility; sympathetic impulses decrease motility.
H. Absorption in the Small Intestine
1. Absorption is the passage of the end products of digestion from the GI tract into blood or
lymph and occurs by diffusion, facilitated diffusion, osmosis, and active transport.
2. Absorption of Monosaccharides
a. Essentially all carbohydrates are absorbed as monosaccharides.
b. They are absorbed into blood capillaries.
3. Absorption of Amino Acids, Dipeptides, and Tripeptides
a. Most proteins are absorbed as amino acids by active transport processes.
b. They are absorbed into the blood capillaries in the villus.
4. Absorption of Lipids
a. Dietary lipids are all absorbed by simple diffusion.
b. Long-chain fatty acids and monoglycerides are absorbed as part of micelles,
resynthesized to triglycerides, and formed into protein-coated spherical masses called
chylomicrons.
1) Chylomicrons are taken up by the lacteal of a villus.
2) From the lacteal they enter the lymphatic system and then pass into the
cardiovascular system, finally reaching the liver or adipose tissue.
c. The plasma lipids - fatty acids, triglycerides, cholesterol - are insoluble in water and
body fluids.
1) In order to be transported in blood and utilized by body cells, the lipids must
be combined with protein transporters called lipoproteins to make them
soluble.
2) The combination of lipid and protein is referred to as a lipoprotein.
3)
22
5. Absorption of Electrolytes
a. Many of the electrolytes absorbed by the small intestine come from gastrointestinal
secretions and some are part of digested foods and liquids.
b. Active transport mechanisms are primarily used for electrolyte absorption.
6. Absorption of Vitamins
a. Fat-soluble vitamins (A, D, E, and K) are included along with ingested dietary lipids
in micelles and are absorbed by simple diffusion.
b. Water-soluble vitamins (B and C) are absorbed by simple diffusion.
7. Absorption of Water
a. Figure 24.25 reviews the fluid input to the GI tract.
b. All water absorption in the GI tract occurs by osmosis from the lumen of the
intestines through epithelial cells and into blood capillaries.
c. The absorption of water depends on the absorption of electrolytes and nutrients to
maintain an osmotic balance with the blood.
XIV. LARGE INTESTINE
A. Anatomy of the Large Intestine
1. The large intestine (colon) extends from the ileocecal sphincter to the anus.
2. Its subdivisions include the cecum, colon, rectum, and anal canal.
3. Hanging inferior to the cecum is the appendix.
a. Inflammation of the appendix is called appendicitis. (Clinical Application)
b. A ruptured appendix can result in gangrene or peritonitis, which can be lifethreatening conditions.
4. The colon is divided into the ascending, transverse, descending, and sigmoid portions.
23
The Large Intestine
•
Gross Anatomy
•
The large intestine begins with the cecum, a blind pouch inferior to the ileocecal valve.
Attached to its lower end is the vermiform appendix.
•
The appendix is densely populated with lymphocytes and is an important source
of immune cells.
•
The ascending colon begins at the ileocecal valve and passes up the right side of the
abdominal cavity near the right lobe of the liver, where it becomes the transverse colon.
This passes horizontally across the upper abdominal cavity, turns 90o downward, and
becomes the descending colon.
•
At the pelvic inlet, the colon turns medially and downward, forming a roughly S-shaped portion
called the sigmoid colon. Within the pelvic cavity, the colon straightens and becomes the rectum.
•
The rectum has three internal folds called rectal valves that enable it to retain feces while
passing gas.
•
The final 3 cm of large intestine is the anal canal, which passes through the levator ani muscle
and terminates at the anus. Here the mucosa forms longitudinal ridges (anal columns) with
depressions between them (anal sinuses).
•
Large hemorrhoidal veins form superficial plexuses in the anal columns and, since they
lack valves, are especially subject to distention, leading to hemorrhoids.
•
The anus is regulated by two sphincters—an internal anal sphincter composed of smooth
muscle, and an external anal sphincter composed of skeletal muscle.
Histology of the Large Intestine
1. The mucosa of the large intestine has no villi or permanent circular folds. It does have a
simple columnar epithelium with numerous globlet cells.
24
2. The muscularis contains specialized portions of the longitudinal muscles called taeniae coli,
which contract and gather the colon into a series of pouches called haustra.
C. Mechanical movements of the large intestine include haustral churning, peristalsis, and mass
peristalsis.
D. The last stages of chemical digestion occur in the large intestine through bacterial, rather than
enzymatic, action. Substances are further broken down and some vitamins are synthesized by
bacterial action and absorbed by the large intestine.
Microscopic Anatomy
•
The mucosa of the large intestine has a simple columnar epithelium in all regions except the anal
canal, where it is stratified squamous.
•
There are no circular folds or villi in the large intestine, but there are intestinal crypts, which are
deeper than in the small intestine and have a greater density of goblet cells. Mucus is their only
significant secretion.
Bacterial Flora and Intestinal Gas
•
The large intestine is densely populated with several species of bacteria referred to collectively
as the bacterial flora. They ferment cellulose and other undigested carbohydrates and
synthesize B vitamins and vitamin K, which are absorbed by the colon.
•
The average person expels about 500 mL of flatus per day. It is composed of nitrogen,
carbon dioxide, hydrogen, methane, hydrogen sulfide, and the amines indole and skatole.
Absorption and Motility
•
In the large intestine, food undergoes no further chemical digestion, but its volume is reduced as
it passes through. The feces consist of 75% water and 25% solid matter, of which 30% is bacteria
and 30% undigested fiber.
•
The most common type of colonic motility is a type of segmentation called haustral contractions,
which occur about every 30 minutes.
25
•
Stronger contractions, called mass movements, occur one to three times a day, last about 15
minutes each, and occur especially within an hour after breakfast.
E. Absorption and Feces Formation in the Large Intestine
1. The large intestine absorbs water, electrolytes, and some vitamins.
2. Feces consist of water, inorganic salts, sloughed-off epithelial cells, bacteria, products of
bacterial decomposition, and undigested parts of food.
3. Although most water absorption occurs in the small intestine, the large intestine absorbs
enough to make it an important organ in maintaining the body’s water balance.
F. Defecation Reflex
1. The elimination of feces from the rectum is called defecation.
2. Defecation is a reflex action aided by voluntary contractions of the diaphragm and abdominal
muscles. The external anal sphincter can be voluntarily controlled (except in infants) to allow
or postpone defecation.
3. Diarrhea refers to frequent defecation of liquid feces. It is caused by increased motility of the
intestine and can lead to dehydration and electrolyte imbalances.
4. Constipation refers to infrequent or difficult defecation and is caused by decreased motility
of the intestines, in which feces remain in the colon for prolonged periods of time. It may be
alleviated by increasing one’s intake of dietary fiber and fluids.
5. Dietary fiber may be classified as insoluble (does not dissolve in water) and soluble
(dissolves in water). Both types affect the speed of food passage through the GI tract and
may produce a number of benefits in the GI tract as well as elsewhere in the body. There is
evidence that insoluble fiber may help protect against colon cancer and that soluble fiber may
help lower blood cholesterol level.
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Defecation
•
In the intrinsic defecation reflex, stretch signals travel by the mesenteric nerve plexus to the
muscularis of the descending and sigmoid colons and the rectum. This triggers a peristaltic wave
that drives the feces downward, and it relaxes the internal anal sphincter. Defecation occurs only
if the external anal sphincter is voluntarily relaxed at the same time.
•
The intrinsic reflex is relatively weak and usually requires the cooperative action of a stronger
parasympathetic defecation reflex involving the spinal cord. This intensifies the peristaltic
response and helps relax the internal anal sphincter.
Chemical Digestion and Absorption
Carbohydrates
•
Most digestible dietary carbohydrate is starch.
•
Carbohydrate Digestion
•
Starch is digested first to oligosaccharides two to eight glucose residues long, then into
the disaccharide maltose, and finally to glucose, which is absorbed by the small intestine.
•
The process of starch digestion begins in the mouth. Salivary amylase hydrolyzes starch
into oligosaccharides and functions best at the pH of the mouth cavity. It is denatured
quickly upon contact with stomach acid.
•
Starch digestion resumes in the small intestine when the chyme mixes with pancreatic
amylase that entirely converts it to oligosaccharides and maltose within 10 minutes. Its
digestion is completed by the brush border enzymes maltase, sucrase, and lactase..
•
Carbohydrate Absorption
•
In the plasma membrane adjacent to the brush border enzymes, there are transport
proteins that absorb monosaccharides as soon as they are produced. Most of the
absorbed sugar is glucose, which is taken up by a sodium-glucose transport protein
(SGLT).
27
•
After a high-carbohydrate meal, two to three times as much glucose is absorbed by
solvent drag as by SGLT.
Lipids
•
Fats are digested by enzymes called lipases.
•
Lingual lipase from the intrinsic salivary glands is activated by stomach acid, where it
digests as much as 10% of the ingested fat.
•
Most fat digestion occurs in the small intestine through the action of pancreatic lipase.
•
In the small intestine, fats are first broken up into smaller emulsification droplets by
lecithin and bile acids in the bile. When lipase digests fats, the products are two free
fatty acids (FFAs) and a monoglyceride. Bile salts coat these and other lipids and form
droplets called micelles.
•
Micelles pass between the microvilli of the brush border, and upon reaching the surface of the
epithelial cell, they release their lipids. The lipids diffuse freely across the phospholipid plasma
membrane.
•
Within the cell, the FFAs and monoglycerides are resynthesized into triglycerides. They are
coated with a thin film of protein, forming droplets called chylomicrons.
•
Although some free fatty acids enter the blood capillaries, chylomicrons are too large to do so
and much be first transported in the lymphatic lacteal.
Proteins
•
Enzymes that digest proteins are called proteases or peptidases. They are absent from the saliva
but begin their job in the stomach.
•
In the stomach, pepsin hydrolyzes any peptide bond between tyrosine and phenylalanine,
thus digesting 10–15% of the dietary protein into shorter polypeptides.
•
In the small intestine, the pancreatic enzymes trypsin and chymotrypsin take over
protein digestion by hydrolyzing polypeptides into even shorter oligopeptides. Finally
28
these are taken apart one amino acid at a time by carboxypeptidase, aminopeptidase,
and dipeptidase. All three of these enzymes are found on the brush border.
•
Amino acid absorption is similar to that of monosaccharides. There are several sodiumdependent amino acid cotransporters for different classes of amino acids.
Nucleic Acids
•
Nucleic acids are generally present in small quantities compared to the other polymers. The
nucleases of pancreatic juice (deoxyribonuclease and ribonuclease) hydrolyze these to their
component nucleotides. Nucleosidases and phosphatases of the brush border further break them
down, and the products are transported across the intestinal epithelium by membrane carriers.
Vitamins
•
Vitamins are absorbed unchanged.
•
The fat-soluble vitamins (A, D, E, and K) are absorbed with other lipids.
•
Water-soluble vitamins are absorbed by simple diffusion, with the exception of vitamin
B12, an unusually large molecule that can only be absorbed if it binds to intrinsic factor
from the stomach.
Minerals
•
Minerals (electrolytes) are absorbed along the entire length of the small intestine.
•
Iron and calcium are unusual in that they are absorbed in proportion to the body's need,
whereas other minerals are absorbed at fairly constant rates, regardless of need.
Water
•
The digestive system is one of several systems involved in water balance.
•
The digestive tract receives about 9 L of water per day—0.7 L in food, 1.6 L in drink, 6.7 L in
gastrointestinal secretions. About 8 L of this is absorbed by the small intestine and 0.8 L by the
large intestine. Water is absorbed by osmosis.
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•
Diarrhea occurs when the large intestine absorbs too little water from the feces.
•
Constipation occurs when fecal movement is slow, too much water is reabsorbed, and the
feces become hardened.
XV. DEVELOPMENTAL ANATOMY OF THE DIGESTIVE SYSTEM
A. The endoderm of the primitive gut forms the epithelium and glands of most of the gastrointestinal
tract.
B. The mesoderm of the primitive gut forms the smooth muscle and connective tissue of the GI tract.
XVI. AGING AND THE DIGESTIVE TRACT
A. General changes associated with aging of the digestive system include decreasing secretory
mechanisms, decreasing motility of the digestive organs, loss of strength and tone of digestive
muscular tissue and its supporting structures, changes in neurosecretory feedback, and diminished
response to pain and internal sensations.
B. Specific changes include reduced sensitivity to mouth irritations and sores, loss of taste, periodontal
disease, difficulty in swallowing, hiatal hernia, cancer of the esophagus, gastritis, peptic ulcer,
gastric cancer, duodenal ulcers, appendicitis, malabsorption, maldigestion, gallbladder problems,
cirrhosis, acute pancreatitis, constipation, cancer of the colon or rectum, hemorrhoids, and
diverticular disease of the colon.
XVII. DISORDERS: HOMEOSTATIC IMBALANCES
A. Dental caries, or tooth decay, is started by acid-producing bacteria that reside in dental plaque, act
on sugars, and demineralize tooth enamel and dentin with acid.
B. Periodontal diseases are characterized by inflammation and degeneration of the gingivae (gums),
alveolar bone, periodontal ligament, and cementum.
C. Peptic ulcers are crater-like lesions that develop in the mucous membrane of the GI tract in areas
exposed to gastric juice. The most common complication of peptic ulcers is bleeding, which can lead
to anemia if blood loss is serious. The three well-defined causes of peptic ulcer disease (PUD) are
30
the bacterium Helicobacter pylori; nonsteroidal anti-inflammatory drugs, such as aspirin; and
hypersecretion of HCl.
D. Diverticula are saclike outpouchings of the wall of the colon in places where the muscularis has
become weak. The development of diverticula is called diverticulosis. Inflammation within the
diverticula, known as diverticulitis, may cause pain, nausea, vomiting, and either constipation or an
increased frequency of defecation. High fiber diets help relieve the symptoms.
E. Tumors, both benign and malignant, may occur in any portion of the GI tract. One of the most
common and deadly malignancies is colorectal cancer, second only to lung cancer in males and third
after lung and breast cancer in females. Screening for colorectal cancer includes fecal occult blood
testing, digital rectal examination, sigmoidoscopy, colonoscopy, and barium enema.
F. Hepatitis is an inflammation of the liver and can be caused by viruses, drugs, and chemicals,
including alcohol.
1. Hepatitis A (infectious hepatitis) is caused by hepatitis A virus and is spread by fecal
contamination. It does not cause lasting liver damage.
2. Hepatitis B is caused by hepatitis B virus and is spread primarily by sexual contact and
contaminated syringes and transfusion equipment. It can produce cirrhosis and possibly
cancer of the liver. Vaccines are available to prevent hepatitis B infection.
3. Hepatitis C is caused by the hepatitis C virus. It is clinically similar to hepatitis B and is
often spread by blood transfusions. It can cause cirrhosis and possibly liver cancer.
4. Hepatitis D is caused by hepatitis D virus. It is transmitted like hepatitis B and, in fact, a
person must be coinfected with hepatitis B before contracting hepatitis D. It results in severe
liver damage and has a high fatality rate.
5. Hepatitis E is caused by hepatitis E virus and is spread like hepatitis A. It is responsible for a
very high mortality rate in pregnant women.
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G. Anorexia nervosa is a chronic disorder characterized by self-induced weight loss, body-image and
other perceptual disturbances, and physiologic changes that result from nutritional depletion. The
disorder is found predominantly in young, single females and may be inherited. Individuals may
become emaciated and may ultimately die of starvation or one of its complications. Treatment
consists of psychotherapy and dietary regulation.
DIGESTIVE NOTES
I.
Nutrients
Polymers
A. Carbohydrates
B. Lipids
C. Proteins
D. Nucleic Acids
II.
Monomers
A. monosaccharides
B. monoglycerides and fatty acids
C. amino acids
D. nucleotides
Digestive Functions and Processes
A. Four Functions
1. ingestion
2. digestion
a. mechanical
b. chemical
3. absorption
4. defecation
B. Three Principal Processes in Digestive Process
1. Motility - peristalsis, mixing waves, segmentation, mass peristalsis, haustral churning
2. Secretion – enzymes, hormones
3. Membrane Transport (absorption)
III.
Mouth (pH 6.35 – 7.0)
A. Mechanical Digestion – Mastication
B. Chemical Digestion
1. Salivary Amylase – breaks starch into maltose and alpha dextrins.
2. Lingual Lipase – breaks fats into monoglycerides and fatty acids.
Bolus
IV.
Pharynx
A. Deglutition
1. Buccal Phase
2. Esophageal Phase
32
B. Peristalsis
V.
Stomach (pH 0.8-1.5)
A. Mechanical Digestion – Mixing Waves
B. Chemical Digestion
1. Pepsin – (Pepsinogen + HCL)
Breaks proteins into polypeptides.
2. Gastric Lipase – Lipids into monoglycerides and fatty acids.
3. Rennin – Curdles milk.
C. Phases of Stomach Activity
1. Cephalic.
2. Gastric
a. pH
b. Stretching
c. Chemicals
1. Acetylcholine
2. Histamine.
3. Gastrin.
3. Intestinal
a. Secretin.
b. Cholicystokinin.
c. Gastric inhibiting peptide.
VI.
Liver
A. Carbohydrate Metabolism
1. glycogenesis
2. glycogenolysis
3. gluconeogenesis
B. Lipid Metabolism
1. Stores some triglycerides.
2. Forms lipoproteins.
C. Protein Mebabolism
1. Deaminates amino acids – ammonia – urea.
2. alpha proteins.
3. beta proteins.
4. albumin.
5. prothrombin.
6. fibrinogen.
D. Processing drugs and Hormones
1. alcohol
2. penicillin
3. erythromycin
4. sulfanamides
5. thyroid hormones
6. steroid hormones
33
E. Breakdown of old ethrocytes
F. Synthesis of bile salts
1. emulsification of lipids.
2. absorption of lipids.
G. Storage
1. Vitamin A, B12, D, E, K.
2. Iron and Copper.
H. Phagocytosis
VII.
Pancreas
A. Secretions
1. Alkaline secretions. (pH 7.4-7.8)
2. pancreatic amylase - starch to maltose and alpha dextrins.
3. trypsinogen – trypsin – proteins to polypeptides.
4. chymotrypsinogen – chymotrypsin – proteins to polypeptides.
5. procarboxypeptidase – carboxypeptidase - forms dipeptides
6. pancreatic lipase – triglycerides to monoglycerides and fatty acids.
7. ribonuclease – breaks down RNA into nucleotides.
8. deoxyribonuclease – Breaks down DNA into nucleotides.
CCK and Secretin
VIII.
Small Intestine (pH 7.4 – 7.8)
A. Surface area increased by
1. circular folds
2. villi
3. microvilli (brush border)
4. length
B. Regions
1. duodenum (25 centimeters)
2. jejunum (2.5 meters)
3. ileum (3.6 meters)
C. Mechanical Digestion - segmentation
D. Chemical Digestion
1. Carbohydrates
a. pancreatic amylase
b. dextrinase - breaks down alpha dextrins.
c. Maltase – breaks down maltose
d. Sucrase – breaks down sucrose
e. Lactase – breaks down lactose
2. Lipids
a. bile – emulsifies lipids.
b. Pancreatic lipase
34
3. Proteins
a. trypsin
b. chymotrypsin
c. carboxypeptidase
d. aminopeptidase – forms amino acids
e. dipeptidase – forms amino acids
f.
4. Nucleic Acids
a. ribonuclease
b. deoxyribonuclease
c. nucleodidase – breaks down nucleotides
d. phosphatases – breaks down nucleotides
IX.
Large Intestine (pH 7.4 – 7.8)
A. Mechanical Digestion – haustral churning
B. Chemical Digestion – Bacterial
C. Defecation
1. diarrhea
2. constipation
X.
Hormones
A. Absorptive Hormones - Insulin – glycogenesis, lipogenesis
B. Postabsorptive Hormones
1.
2.
3.
4.
glucagons – glycogenolysis, gluconeogenesis, lipolysis
cortisol – gluconeogenesis, protein breakdown
growth hormone - lipolysis
epinephrine – glycogenolysis, lipolysis
VII. METABOLIC ADAPTATIONS
A. Your metabolic reactions depends on how recently you have eaten. During the absorptive state,
which alternates with the postabsorptive state, ingested nutrients enter the blood and lymph from the
GI tract, and glucose is readily available for ATP production.
35
1. An average meal requires about 4 hours for complete absorption, and given three meals a
day, the body spends about 12 hours of each day in the absorptive state. (The other 12 hours,
during late morning, late afternoon, and most of the evening, are spent in the postabsorptive
state.)
2. Hormones are the major regulators of reactions during each state.
B. Metabolism During the Absorptive State
1. Several things typically happen during the absorptive state.
a. Most body cells produce ATP by oxidizing glucose to carbon dioxide and water.
b. Glucose transported to the liver is converted to glycogen or triglycerides. Little is
oxidized for energy.
c.
Most dietary lipids are stored in adipose tissue.
d. Amino acids in liver cells are converted to carbohydrates, fats, and proteins.
2. Regulation of Metabolism During the Absorptive State
a. Soon after eating, gastric inhibitory peptide and the rise in blood glucose
concentration stimulate insulin release from pancreatic beta cells. In several ways,
insulin stimulates absorptive state metabolism.
C. Metabolism During the Postabsorptive State
1. During the postabsorptive state, absorption is complete, and the energy needs of the body
must be satisfied by nutrients already present in the body.
a. The major concern of the body during the postabsorptive state is to maintain normal
blood glucose level (70 to 110 mg/100 ml of blood).
b. Homeostasis of blood glucose concentration is especially important for the nervous
system and red blood cells.
1) The dominant fuel molecule for ATP production in the nervous system is
glucose because fatty acids are unable to pass the blood-brain barrier.
36
2) Red blood cells derive all of their ATP from glycolysis of glucose because
they lack mitochondria and thus lack the Krebs cycle and electron transport
chain.
2. Postabsorptive State Reactions
a. Reactions that produce glucose are the breakdown of liver glycogen, gluconeogenesis
using lactic acid, and gluconeogenesis using amino acids.
b. Reactions that produce ATP without using glucose are oxidation of fatty acids,
oxidation of lactic acid, oxidation of amino acids, oxidation of ketone bodies, and
breakdown of muscle glycogen.
3. Regulation of Metabolism During the Postabsorptive State
a. The hormones that stimulate metabolism in the postabsorptive state sometimes are
called anti-insulin hormones because they counter the insulin effects that dominate
the absorptive state. The most important anti-insulin hormone is glucagon.
b. A low blood glucose level also activates the sympathetic branch of the ANS.
D. Metabolism During Fasting and Starvation
1. Fasting means going without food for many hours or a few days whereas starvation implies
weeks or months of food deprivation or inadequate food intake.
a. Catabolism of stored triglycerides and structural proteins can provide energy for
several weeks.
b. The amount of adipose tissue determines the lifespan possible without food.
2. During fasting and starvation, nervous tissue and red blood cells continue to use glucose for
ATP production.
3. During prolonged fasting, large amounts of amino acids from tissue protein breakdown
(primarily from skeletal muscle) are released to be converted to glucose in the liver by
gluconeogenesis.
37
4. The most dramatic metabolic change that occurs with fasting and starvation is the increase in
formation of ketone bodies by hepatocytes.
a. Ketogenesis increases as catabolism of fatty acids rises.
b. The presence of ketones actually reduces the use of glucose for ATP production,
which in turn decreases the demand for gluconeogenesis and slows the catabolism of
muscle proteins.
5. Alcohol absorption is slower in the stomach than in the small intestine due to the increased
surface area for absorption in the small intestine.
VIII. HEAT AND ENERGY BALANCE
A. A normal body temperature is maintained by a delicate balance between heat-producing and heatlosing mechanisms.
B. Metabolic Rate
1. The overall rate at which heat is produced is termed the metabolic rate.
a. Measurement of the metabolic rate under basal conditions is called the basal
metabolic rate (BMR).
b. BMR is a measure of the rate at which the quiet, resting, fasting body breaks down
nutrients to liberate energy.
c. BMR is also a measure of how much thyroxine the thyroid gland is producing, since
thyroxine regulates the rate of ATP use and is not a controllable factor under basal
conditions.
3. Heat is a form of kinetic energy that can be measured as temperature and expressed in units
called calories.
a. A calorie, spelled with a little c, is the amount of heat energy required to raise the
temperature of 1 gram of water from 140C to 150C.
b. A kilocalorie or Calorie, spelled with a capital C, is equal to 1000 calories.
38
C. Body Temperature Homeostasis
1. If the amount of heat production equals the amount of heat loss, one maintains a constant
core temperature near 370C (98.60F).
a. Core temperature refers to the body’s temperature in body structures below the skin
and subcutaneous tissue.
b. Shell temperature refers to the body’s temperature at the surface, that is, the skin and
subcutaneous tissue.
c. Too high a core temperature kills by denaturing body proteins, while too low a core
temperature causes cardiac arrhythmias that can result in death.
2. Heat Production
a. The production of body heat is influenced by metabolic rate and responses that occur
when body temperature starts to fall.
b. Factors that affect metabolic rate include exercise, hormones, the nervous system,
body temperature, ingestion of food, age, and other factors such as gender, climate,
sleep, and malnutrition.
c. Heat conservation mechanisms include vasoconstriction, sympathetic stimulation,
skeletal muscle contraction (shivering), and thyroid hormone production.
3. Heat is lost from the body by radiation, evaporation, conduction, and convection.
a. Radiation is the transfer of heat from a warmer object to a cooler object without
physical contact.
b. Evaporation is the conversion of a liquid to a vapor. Water evaporating from the skin
takes with it a great deal of heat. The rate of evaporation is inversely related to
relative humidity.
c. Conduction is the transfer of body heat to a substance or object in contact with the
body, such as chairs, clothing, jewelry, air, or water.
39
d. Convection is the transfer of body heat by a liquid or gas between areas of different
temperature.
4. Hypothalmic Thermostat
a. The hypothalmic thermostat is the preoptic area.
b. Nerve impulses from the preoptic area propagate to other parts of the hypothalamus
known as the heat-losing center and the heat-promoting center.
5. Several negative feedback loops work to raise body temperature when it drops too low or
raises too high.
6. Hypothermia refers to a lowering of body temperature to 350C (950F) or below. It may be
caused by an overwhelming cold stress, metabolic disease, drugs, burns, malnutrition,
transection of the cervical spinal cord, and lowering of body temperature for surgery.
XIV. REGULATION OF FOOD INTAKE
A. Two centers in the hypothalamus related to regulation of food intake are the feeding (hunger)
center and satiety center. The feeding center is constantly active but may be inhibited by the satiety
center.
B. Among the stimuli that affect the feeding and satiety centers are glucose, amino acids, lipids, body
temperature, distention of the GI tract, and choleocystokinin.
X. NUTRITION
A. Guidelines for healthy eating include eating a variety of foods; maintaining healthy weight; choosing
foods low in fat, saturated fat, and cholesterol; eating plenty of vegetables, fruits, and grain products;
using sugar only in moderation; using salt and sodium only in moderation; and drinking alcohol only
in moderation or not at all.
1. The Food Guide Pyramid shows how many servings of the five major food groups to eat
each day.
40
2. Foods high in complex carbohydrates serve as the base of the pyramid since they should be
consumed in largest quantity.
B. Minerals are inorganic substances that help regulate body processes.
1. Minerals known to perform essential functions include calcium, phosphorus, sodium,
chlorine, potassium, magnesium, iron, sulfur, iodine, manganese, cobalt, copper, zinc,
selenium, and chromium.
C. Vitamins are organic nutrients that maintain growth and normal metabolism. Many function in
enzyme systems as coenzymes.
1. Most vitamins cannot be synthesized by the body. No single food contains all of the required
vitamins – one of the best reasons for eating a varied diet.
2. Based on solubility, vitamins fall into two main groups: fat-soluble and water-soluble.
a. Fat-soluble vitamins are emulsified into micelles and absorbed along with ingested
dietary fats by the small intestine. They are stored in cells (particularly liver cells) and
include vitamins A, D, E, and K.
b. Water-soluble vitamins are absorbed along with water in the GI tract and dissolve in
the body fluids. Excess quantities of these vitamins are excreted in the urine. The
body does not store water-soluble vitamins well. They include the B vitamins and
vitamin C.
c. Vitamins C, E, and beta-carotene (a provitamin) are termed antioxidant vitamins
because they inactivate oxygen free radicals.
3. Most physicians do not recommend taking vitamin or mineral supplements except in special
circumstances, and instead suggest being sure to eat a balanced diet that includes a variety of
food.
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X. DISORDERS: HOMEOSTATIC IMBALANCES
A. Body Temperature Abnormalities
1. Fever is an elevation of body temperature that is due to resetting of the hypothalamic
thermostat. The most common cause of fever is a viral or bacterial infection.
2. Heat Cramps, Heat Exhaustion, and Heatstroke
a. Heat cramps are painful skeletal muscle contractions which are due to profuse
sweating that removes water and salt from the body.
b. Heat exhaustion (heat prostration), due to fluid and electrolyte loss, results in a
normal or below normal body temperature, profuse perspiration, nausea or vomiting,
cramps, and dizziness or fainting. Blood pressure may also decrease as a result.
c. Heatstroke (sunstroke) occurs when the temperature and relative humidity are high,
making it difficult for the body to lose heat by normal means. It results in decreased
blood flow to the skin, reduced perspiration, and sharply rising body temperature.
Immediate cooling of the body must occur to prevent permanent brain damage.
B. Obesity is defined as a body weight more than 20% above desirable standard as the result of
excessive accumulation of fat.
1. Even moderate obesity is hazardous to health.
2. Obesity is implicated as a risk factor in cardiovascular disease, hypertension, pulmonary
disease, non-insulin dependent diabetes mellitus (type II), arthritis, certain cancers (breast,
uterus, and colon), varicose veins, and gallbladder disease.
C. Malnutrition refers to a state of poor nutrition.
1. Causes of malnutrition include undernutrition, which can be caused by fasting, anorexia
nervosa, deprivation, cancer, gastrointestinal obstructions, inability to swallow, renal disease,
and poor dentition.
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2. Other causes of malnutrition are imbalance of nutrients, malabsorption of nutrients, improper
distribution of nutrients, inability to use nutrients (as in diabetes mellitus), increased nutrient
requirements (due to fever, infections, burns, fractures, stress, and exposure to heat or cold),
increased nutrient losses (diarrhea, bleeding, glycosuria, and fistula drainage), and
overnutrition (excess vitamins, minerals, and calories).
3. Protein-calorie undernutrition may be classified into two types based on which factor is
lacking in the diet.
a. Kwashiorkor results from a deficient protein intake despite normal or nearly normal
calorie intake.
b. Marasmus is caused by inadequate intake of both protein and calories.

Anaerobic fermentation – The conversion of pyruvic acid to lactic acid. This process does not
require oxygen. It is anaerobic. This process produces a net of 2 ATPs.

Aerobic respiration – The metabolic pathway which breaks down pyruvic acid to carbon dioxide
and water and generates up to 36 ATPs from each glucose molecule. Requires oxygen.

Chemiosmosis – The process which occurs at the inner mitochondrial membrane and uses energy
captured during glycolysis, the oxidation of pyruvic acid (formation of acetyl CoA), and the
Kreb’s cycle to pump hydrogen and create a hydrogen gradient. The flow of hydrogen is then
used to generate ATP

Glycogenesis – The formation of glycogen from glucose.

Glycogenolysis – The breakdown of glycogen to glucose.

Gluconeogenesis – The formation of glucose from a noncarbohydrate source such as amino acids
and lipids.

Lipogenesis – The formation of lipids (fat).

Lipolysis – The breakdown of lipids (fat).
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
Beta oxidation – The catabolism of fatty acids involving the removal of two carbon atoms at a time
from the fatty acid.

Ketone bodies – Breakdown components of fatty acids which accumulate during oxidation of fats.

Deamination – The removal of the amino group from an amino acid.

Uric Acid – A substance formed during the breakdown of nucleic acids.
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