Most digestive system organs reside in the abdominopelvic cavity. All body cavities contain serous
membranes. Connecting the visceral and parietal peritoneums is a fused double layer of parietal
peritoneum called the mesentary. It provides a route for conducting blood vessels, lymphatics, and
nerves to the digestive viscera; helps hold the organs in place; and stores fat. In most places the
mesentery is dorsal and attaches to the posterior abdominal wall.
The hepatic portal circulation collects nutrient-rich venous blood draining from the digestive tract
and delivers it to the liver.
From the esophagus to the anal canal, the walls of every organ of the alimentary canal are made up
of the same four basic layers, or tunics.
From the lumen outward, these layers are the:
1. mucosa 3. muscularis externa
2. submucosa 4. serosa
The mucosa lines the lumen from mouth to anus. Its major functions are: Secretion of mucus,
digestive enzymes, and hormones. Absorption of end products. Protection against infectious disease.
The mucus protects certain digestive organs from being digested themselves by enzymes, and eases
food passage along the tract.
The submucosa contains blood and lymphatic vessels, lymph nodules, nerve fibers, and a rich supply
of elastic fibers.
The muscularis externa is responsible for segmentation and peristalsis. This tunic typically has an inner
circular layer and an outer longitudinal layer of smooth muscle cells. In several places along the tract,
the circular layer thickens to form sphincters that act as valves to prevent backflow and control food
The serosa is the protective outermost layer.
The mouth is the only part of the alimentary canal involved in ingestion. The mouth also begins the
propulsive process of swallowing, during which food is carried through the pharynx and esophagus
to the stomach. The tongue contains most of the taste buds and some mucous and serous glands,
and during chewing, the tongue grips the food and constantly repositions it between the teeth. It
also mixes the food with saliva and forms it into a compact mass (bolus) and then initiates
swallowing. A number of glands produce and secrete saliva. Saliva cleanses the mouth, dissolves
food chemicals so that they can be tasted, moistens food and aids in compacting it into a bolus, and
contains enzymes. Saliva is mostly water and includes electrolytes, salivary amylase, mucin, lysozyme,
and IgA and metabolic wastes.
The mouth ingests and begins mechanical digestion by chewing and initiates propulsion by
swallowing. Salivary amylase starts the chemical breakdown of polysaccharides. Essentially no
absorption occurs in the mouth.
Both the pharynx and the esophagus serve only as conduits to pass food from the mouth to the
stomach. Their single function is propulsion of food.
The stomach is a temporary "storage tank" where the chemical breakdown of protein as begins and
in which food is converted to a creamy paste called chyme.
The major regions of the stomach are: 1. Cardiac region (near the heart) - surrounds the cardiac
orifice through which food enters the stomach. 2. Fundus - dome shaped, tucked beneath the
diaphragm 3. Body - midportion 4. Pylorus - inferior terminus, continuous with the duodenum.
The stomach has an extra layer of smooth muscle in its walls that allows the stomach not only to
move food along the tract,, but also to churn, mix, and pummel the food, physically breaking it
down into smaller fragments. The lining epithelium (mucosa) produces large amounts of protective
mucus. The lining is covered with millions of deep gastric pits which lead into the gastric glands that
produce the stomach secretion called gastric juice.
The glands contain a variety of secretory cells:
1. Parietal cells - secrete HCl and intrinsic factor. HCl makes the stomach contents very acid (pH 1.
5 - 3.5), necessary for activation of pepsinogen and to kill ingested bacteria. Intrinsic factor is
required for the absorption of Vit. B12 in the small intestine.
2. Chief cells - produce pepsinogen
3. Enteroendocrine cells - release a variety of hormones including gastrin, which plays essential roles
in regulating stomach secretion and mobility.
Besides serving as a holding area for ingested food, the stomach continues the demolition job begun
in the mouth. It then delivers chyme into the small intestine at an appropriate rate.
Protein digestion is essentially the only type of enzymatic digestion that occurs in the stomach. The
most important protein-digesting enzyme produced by the gastric mucosa is pepsin.
The only stomach function essential to life is secretion of intrinsic factor which is required for
intestinal absorption of Vit. B12 needed for the production of mature red cells.
Hormonal control is largely the province of gastrin, which stimulates the secretion of both enzymes
Stimuli acting at three distinct sites -- head, stomach, and small intestines -provoke or inhibit gastric
secretory activity. The three phases of gastric secretion are called the: cephalic, gastric, and intestinal.
The cephalic phase occurs before food enters the stomach and is triggered by the sight, aroma, taste,
or thought of food. The brain gets the stomach ready for its upcoming digestive chore.
In the gastric phase, once food reaches the stomach neural and hormonal mechanisms are initiated.
About 2/3's of the gastric juice is released. Chemical stimuli provided by partially digested proteins
and caffeine directly activate gastrin-secreting cells. Gastrin's main target is the HCL-producing
parietal cells. Gastrin secretion is inhibited when the gastric contents become highly acidic (pH < 2).
When protein foods are in the stomach, the pH of the gastric contents generally rises. The rise in
pH stimulates gastrin and subsequently HCl release. This provides the acidic conditions needed for
protein digestion. The more protein in the meal, the greater the amount of gastrin and HCl released.
As proteins are digested, the gastric contents gradually become more acidic, which again inhibits the
gastrin-secreting cells. This negative feedback mechanism helps maintain an optimal pH.
The intestinal phase has two components - one excitatory and the other inhibitory. The excitatory
aspect is set into motion as partially digested food begins to fill the initial part of the small intestine.
This stimulates the release of a hormone that encourages the gastric glands to continue their
secretory activity. As the intestine is distended with chyme, the inhibitory component is triggered in
the form of the enterogastric reflex. This reflex causes the pyloric sphincter to tighten and prevent
farther food entry into the small intestine. As a result, gastric secretory activity declines.
Stomach contractions not only cause its emptying but also compress, knead, twist and continually
mix the food with gastric juice to produce chyme.
The mixing movements are accomplished by a unique type of peristalsis. Each peristaltic wave
reaching the pyloric muscle squirts 3ml or less of chyme into the small intestine. Since the
contraction closes the pyloric valve, the rest of the chyme is propelled backward into the stomach
where it is mixed further. The stomach usually empties completely within 4 hours after a meal. The
larger the meal and the more liquefied its contained food, the faster the stomach empties. Fluids
pass through the stomach very quickly; solids remain until they ewe well mixed with gastric juice and
converted to the liquid state. As a rule, a meal rich in carbohydrates moves through the duodenum
rapidly, but fats form an oily layer at the top of the chyme and are digested more slowly.
The body's major digestive organ. Digestion is completed and virtually all absorption occurs here.
The small intestine has three subdivisions:
The duodenum curves around the head of the pancreas and is about 25cm long. The bile duct joins
close to the duodenum. The jejunum is about 2.5 m long and extends from the duodenum to the
ileum. The ileum is approximately 3.6 m long and joins the large intestine at the ileocecal valve. The
small intestine is highly adapted for nutrient absorption. Its length provides a huge surface area and
its walls have three structural modifications: plicae circulares, villi, and microvilli These structures
amplify to: absorptive surface greatly.
Plicae circulares are deep, permanent circular folds that extend either entirely or part way around the
circumference of the small intestine. They force chyme to spiral through the lumen continually
mixing the chyme with intestinal juices and slowing its movement, allowing time for full nutrient
The villi are finger like projections of the mucosa; over 1 mm high. Within the core of each villus are
a dense capillary bed and a modified lymphatic capillary called a lacteal. Digested foodstuffs are
absorbed through the epithelial cells into both the capillary blood and the lacteal. The villi are large
and leaflike in the duodenum and gradually become narrower and shorter along the length of the
The microvilli are tiny projections of the plasma membrane of the absorptive cells of the mucosa
(brush border). In addition to enhancing absorption, the plasma membranes of the microvilli bear
the intestinal digestive enzymes, referred to collectively as brush border enzymes. The enzymes are
mainly disaccharidases (sucrase, lactase, maltase) and peptidases (aminopeptidase, dipeptidase),
which complete the digestion of carbohydrates and proteins.
These organs are accessory organs associated with the small intestine. The liver has many metabolic
and regulatory roles. Its only digestive function is to produce bile for export to the duodenum.
Bile is a fat emulsifier; it breaks fats into tiny particles so they can be more accessible to digestive
The liver also processes nutrient-laden venous blood delivered to it from the digestive organs. This
is a metabolic rather than a digestive role.
The gallbladder is chiefly a storage organ for bile.
Bile leaves the liver through several bile ducts that ultimately fuse to form the hepatic duct. The
hepatic duct fuses with the cystic duct draining the gallbladder to form the common bile duct.
The liver is composed of structural and functional units called liver lobules. Each lobule is a
hexagonal, roughly cylindrical structure consisting of plates of hepatocytes. The hepatocyte plates,
radiate outward from a central vein.
At each of the six corners of a lobule is a portal triad containing three basic structures -- a branch of
the hepatic artery, a branch of the hepatic portal vein and a bile duct.
Between the hepatocyte plates are enlarged blood-filled capillaries, or sinusoids. Blood from both
the hepatic portal vein and the hepatic artery percolates from the triad regions through the sinusoids
and empties into the central vein. Inside the sinusoids are hepatic macrophages (kupffer cells) which
remove debris and worn-out red cells.
Besides producing bile, the hepatocytes process the blood-borne nutrients in various ways: store
glucose and use amino acids to make plasma proteins, store fat-soluble vitamins, detoxification
(convert ammonia to urea). The blood leaving the liver contains fewer nutrients and waste materials
than the blood entering it.
Bile entering the bile ducts eventually leaves the liver via the hepatic duct and is stored in the
gallbladder when digestion is not occurring. Bile is a yellow-green, alkaline solution containing bile
salts, bile pigments, cholesterol, neutral fats, phospholipids, and a variety of electrolytes. Only the
bile salts and phospholipids aid the digestive process.
Bile's role is to emulsify fits, but it also facilitates fat and cholesterol absorption. Bile salts are
conserved by means of recycling (enterohepatic circulation). Bile salts are reabsorbed into the blood
by the distal part of the small intestine (ileum) and returned to the liver via the hepatic portal blood.
Gall Bladder - Stores and concentrates bile that is not immediately needed for digestion. When the
muscular wall contracts, bile is expelled into the cystic duct and then flows into the common bile
duct. The liver makes bile continuously and stores it in the gallbladder. The major stimulus for
release is cholecystokinin which is released into the blood when acidic, fatty chyme enters the
Pancreas - An accessory digestive organ that produces a broad spectrum of enzymes. This exocrine
product (pancreatic juice) drains from the pancreas via centrally located pancreatic duct which
generally fuses visa the common bile duct. The pancreas also has endocrine function (insulin,
glucagon). Pancreatic juice contains mainly water, enzymes and electrolytes. It has a pH of 8.0 which
enables the pancreatic fluid to neutralize the acid chyme entering the duodenum and provides the
optimal environment for the operation of intestinal and pancreatic enzymes. Pancreatic enzymes
include amylase, proteases (trypsin, chymotrypsin, carboxypeptidase), lipases, nucleases.
Frames the small intestine on three sides and extends from the ileocecal valve to the anus. Its
diameter is larger, but its length is less than the small intestine.
Its major function is to absorb water and to eliminate indigestible food residues from the body as
The large intestine has the following subdivisions:
Cecum, appendix, colon, rectum, anal canal
The sac like cecum lies below the ileocecal valve and is the first part of the large intestine. Attached
is the vermiform appendix which contains masses of lymphoid tissue.
The colon has several distinct regions. The ascending colon travels up the right side of the
abdominal cavity, the transverse colon travels across the abdominal cavity. It then turns acutely and
continues down the left side as the descending colon and enters the pelvis, where it becomes the
S-shaped sigmoid colon. The sigmoid colon joins the rectum. The rectum has three lateral curves or
bends, represented internally as three transverse folds called the rectal valves, which separate feces
from flatus (gas).
The anal canal is about 3 cm long and opens to the body exterior at the anus. The anal canal has two
sphincters which act to open and close the anus.
The wall of the large intestine differs in several ways from the small intestine. Because most food is
absorbed before reaching the large intestine, there are no plicae circulares, no villi, and virtually no
cells that secrete digestive enzymes. The mucosa is thicker and there are tremendous numbers of
goblet cells. The lubricating mucus cases the passage of feces and protects the intestinal wall from
irritating acids and gases.
The stretch of the rectal walls due to movement of feces into the rectum stimulates receptors,
sending a signal along afferent fibers to the spinal cord. A spinal reflex sends parasympathetic nerve
impulses to contract rectal walls and relax the internal anal sphincter. When ready, a voluntary motor
neurons are inhibited, allowing the external anal sphincter to relax. Rectal muscles contract along
with the increase of intra-abdominal pressure (contraction of diaphragm and abdominal wall
muscles) to expel feces from the body.
Dietary recommendation for carbohydrate intake is 45-65% of one's total caloric intake with emphasis on
complex carbohydrates (whole grains and vegetables).
Polysaccharides include starch, glycogen, cellulose, and soluble fiber. Cellulose (insoluble fiber) increases
bulk of stool and facilitates defecation. Soluble fiber reduces blood cholesterol. Starch (from plants) is used
to supply glucose molecules needed for energy. Extra glucose in our body can be stored in the liver in the
Disaccharides such as sucrose, maltose, and lactose are broken down to component monosaccharides. Nonglucose
monosaccharides are converted to glucose.
Glucose is the preferred fuel of the body; some other carbohydrates are also used to synthesize nucleic acids
and to make glyocproteins and glycolipids. Glucose is completely broken down by three processes.
Glycolysis - occurs in cytosol, 10 steps, one glucose is metabolized into two pyruvic acid molecules;
forms 2 ATP (net) and 2 molecules of reduced coenzymes
Krebs cycle - occurs in mitochondria; requires oxygen; begins with pyruvic acid being converted to
acetyl CoA which enters the 8-step cycle; each pyruvic acid yields 3 CO2
and 5 molecules of reduced
coenzymes and 1 ATP (and since each glucose yielded 2 pyruvic acid molecules the totals from the
Krebs cycle is 6 CO2, 10 molecules of reduced coenzymes, and 2 ATP)
Electron transport chain and oxidative phosphorylation - occur in the mitochondrion; begins with
the reduced coenzymes from glycolysis and the Krebs cycle; requires oxygen; hydrogens removed
from the reduced coenzymes are combined with O2
to form water after they have released energy
which is used to attach inorganic phosphate groups to ADP, forming ATP; the 12 reduced
coenzymes from one molecule of glucose provides energy to form 28 ATP (net). Thus the
maximum yield of ATP from one glucose = 4 ATP produced directly + 28 ATP from the reduced
coenzymes = 32 ATP
The average person at rest uses about 100 kcal/hour. A mole of glucose can generate 262 kcal worth of
energy stored in ATP (38% of the total energy available in the glucose molecules). 1 mole of glucose weighs
180 grams and contains 6.02 x 1023 molecules of glucose.
Glycogenesis - the process of storing excess glucose in glycogen (especially in liver and skeletal muscle)
Glycogenolysis - the process which liberates individual glucose molecules from glycogen when blood
glucose gets too low
Gluconeogenesis - the process which makes glucose from noncarbohydrate molecules (amino acids,
glycerol) when blood glucose is low and glycogen stores are depleted
Dietary recommendation for protein is a daily intake of 0.8 g per kilogram of body weight.
Complete proteins provide all the essential amino acids (those that cannot be made by the body); sources of
complete proteins are eggs, milk, fish, and most meats. Legumes, nuts, and cereals are incomplete proteins
because they are low in one or more essential amino acids.
Amino acids are used to make the body's proteins. These proteins provide the structure of many tissues,
also enzymes, transport proteins, antibodies, and some hormones. Amino acids can also be broken down
Before amino acids can be broken down for energy, the nitrogen must be removed by the process of
deamination. The resulting molecule (without the nitrogen) is converted to pyruvic acid or to a Krebs cycle
The deamination process transfers the amine group to an intermediate molecule which takes the nitrogen to
the liver. There it is removed as ammonia which combines with CO2
yielding urea and water. The urea is
released to the blood, removed by the kidney, and put into the urine.
Seven minerals are required in moderate amounts (calcium, phosphorus, potassium, sulfur, sodium,
chlorine, and magnesium). About a dozen others are trace minerals.
Minerals work with other nutrients to ensure smooth functioning. Some minerals give added strength to
tissues (calcium, phosphorus, and magnesium salts in bone and teeth). Most minerals are ionized in body
fluids or bound to organic compounds to form phospholipids, hormones, and various functional proteins.
8th EditionElaine N. Marieb
12th EditionDavid N. Shier, Jackie L. Butler, Ricki Lewis
Michelle Provost-Craig, Susan J. Hall, William C. Rose
11th EditionDavid N. Shier, Jackie L. Butler, Ricki Lewis