Chapter 14 The Digestive System
Children have a special fascination with the workings of the digestive system: They relish
crunching a potato chip, delight in making "mustaches" with milk, and giggle when their
stomach "growls." As adults, we know that a healthy digestive system is essential for good
health, because it converts food into the raw materials that build and fuel our body's cells.
Specifically, the digestive system takes in food (ingests it), breaks it down physically and
chemically into nutrient molecules (digests it), and absorbs the nutrients into the
bloodstream. Then it rids the body of the indigestible remains (defecates).
Part I: Anatomy and Physiology of the Digestive System
Anatomy of the Digestive System
 Name the organs of the alimentary canal and accessory digestive organs and identify each on an
appropriate diagram or model.
o See figure 14.1 on page 433
 Identify the overall function of the digestive system as digestion and absorption of foodstuffs, and
describe the general activities of each digestive system organ.
o Function of the digestive system is to take in food (ingest it), break it down physically and
chemically into nutrient molecules (digest it), to absorb the nutrients into the bloodstream,
and to rid the body of the indigestible remains (defecate)
o The alimentary canal (gastrointestinal (GI) tract) is a continuous, coiled, hollow,
muscular tube that is open at both ends made up of many parts
o The walls of the GI tract from the esophagus to the large intestine are made up of the same
four basic tissue layers, or tunics
 The mucosa – innermost layer – a moist membrane that lines the cavity, or lumen,
of the organ – surface epithelium, plus a small amount of connective tissue (lamina
propria) and smooth muscle layer – beyond the esophagus, which has frictionresisting stratified squamous epithelium, the epithelium is mostly simple columnar
 The submucosa – just beneath the mucosa – soft connective tissue layer containing
blood vessels, nerve endings, lymph nodules, and lymphatic vessels
 The muscularis externa – muscle layers typically made up of an inner circular layer
and an outer longitudinal layer of smooth muscle cells
 The serosa – outermost layer of the wall – single layer of flat serous fluid-producing
cells, the visceral peritoneum (continuous with the slick, slippery parietal peritoneum,
which lines the abdominopelvic cavity by way of membrane extension, the
mesentery)
o General activities of each digestive system organs
 Mouth – oral cavity – food is mixed with saliva and masticated (chewed) – chemical
and mechanical digestion begins
 Pharynx – common pathway for food and air and continuous with the esophagus –
contractions (peristalsis) propels food down the tube
 Esophagus – gullet – runs from the pharynx through the diaphragm to the stomach
that conducts food from to the stomach via peristalsis
 Stomach – acts as a temporary storage tank for food as well as a site for food
breakdown via chemical (gastric juices) and mechanical processes to produce a
heavy cream-like substance called chyme
 Cardioesophageal sphincter – controls food entering the stomach from the
esophagus
 Fundus – expanded part
 Body – midportion
 Pylorus – terminal part
 Pyloric sphincter – controls the food entering the small intestine
 Small intestine – the body’s major digestive organ where chemical digestion
completes the breakdown of the food and nearly all food absorption occurs via the
microvilli
 Three subdivisions – duodenum, junum, ileum
 Ends at the ileocecal valve to join the large intestine
 Large intestine – absorption of water dries the indigestible food residue so it can
finally be eliminated from the body as feces through the anus
 Five subdivisions – cecum, appendix, colon (ascending, transverse,
descending), sigmoid rectum, anal canal, which ends at the anus
 The anal canal has an external voluntary sphincter made of skeletal
muscle and an internal involuntary sphincter formed by smooth
muscle – both closed except during defecation
 Goblet cells produce mucus that acts as a lubricant to ease the passage of
feces to the end of the digestive tract
 Bacteria in the large intestine make some vitamins (vitamin K and some B),
which are absorbed along with some ions and most of the remaining water
 Pancreas – produces enzymes that break down all categories of digestible foods and
are secreted into the duodenum in an alkaline fluid, which neutralizes the acidic
chyme coming in from the stomach
 Endocrine function is to produce the hormones insulin and glucagon
 Liver – produces bile which leaves the liver through the common hepatic duct and
enters the duodenum through the bile duct
 Bile is a yellow-to-green, watery solution containing bile salts, bile pigments
(bilirubin, a breakdown product of hemoglobin), cholesterol, phospholipids,
and electrolytes – bile salts emulsify fats by physically breaking large fat
globules into smaller ones providing more surface area for the fat-digesting
enzymes to work
 Gallbladder – stores bile when digestion is not occurring – bile backs up the cystic
duct and enters the gallbladder where is it concentrated by the removal of water
 Describe the composition and function(s) of saliva.
o A mixture of mucus and serous fluids
 Mucus moistens and helps to bind the food together into a mass called a bolus, which
makes chewing and swallowing easier
 The clear serous portion contains salivary amylase, which begins the process of
starch digestion in the mouth
 Lysozyme and antibodies (IgA) inhibit bacteria
o Saliva also functions to dissolve food chemicals so they can be tasted
 Name the deciduous and permanent teeth and describe the basic anatomy of a tooth.
o See figure 14.9 on page 442
o The function of teeth is to masticate, or chew, the food – incisors and canines cut and
tear the food while molars grind the food
o Deciduous teeth – baby teeth – milk teeth – erupt around six months and a complete
set of 20 teeth is present by age two
o Permanent teeth – second set of teeth set deeper under the deciduous teeth – as they
enlarge and develop, the roots of the baby teeth are reabsorbed causing them to loosen and
fall out around ages 6 – 12 years with all of the permanent teeth but the third molars
(wisdom teeth, which erupt around age 17 - 25) having erupted by the end of adolescence
ending in a set of 32 teeth although sometimes the wisdom teeth are absent or fail to erupt
o A tooth consists of two major regions – crown and the root
 Crown – enamel-covered and exposed above the gingival, or gum
 Enamel heavily mineralized with calcium salts
Dentin – underlies the enamel and forms the bulk of the tooth and surrounds
the central pulp cavity, which contains connective tissue, blood vessels, and
nerve fibers collectively called the pulp, which supplies nutrients to the tooth
tissues and provides for tooth sensations
 Where the pulp cavity extends into the root, it becomes the root
canal, which provides a rout for the blood vessels, nerves, and other
pulp structures to enter the pulp cavity of the tooth
 Root – portion of the tooth embedded in the jawbone and connected to the crown via
the neck
 Outer surface covered by cementum, which attaches the tooth to the
periodontal membrane (ligament) that holds the tooth in place in the bony
jaw
 Explain how villi aid digestive processes in the small intestine.
o Three structures that increase the absorptive surface in the small intestine are the microvilli,
villi, and circular folds that function to increase the surface area for digestion and
absorption – decrease in number toward the end of the small intestine while local collections
of lymphatic tissue called Peyer’s patches increase and function to prevent bacteria from
entering the bloodstream
 Microvilli – are tiny projections of the plasma membrane of the mucosa cells that
give the cell surface a fuzzy appearance called the brush border
 Villi – fingerlike projections of the mucosa that give it a velvety appearance and feel
– within each is a rich capillary bed and a modified lymphatic capillary called a lacteal
 The digested foodstuff is absorbed through the mucosa cells into both
capillaries and the lacteal
 Circular folds – plicae circulars – deep folds of both mucosa and submucosa layers
o Absorption – via active or passive transport processes into the mucosal cells

Functions of the Digestive System
 Describe the mechanisms of swallowing, vomiting, and defecation.
o Swallowing – deglutition – involves the coordinated activity of the tongue, soft palate,
pharynx, and esophagus and involves two major phases
 The voluntary Buccal phase – food is voluntarily chewed, mixed with saliva, and
formed into a bolus (food mass) then forced into the pharynx by the tongue where it
is then processed via involuntary reflex activity
 The involuntary pharyngeal-esophageal phase – transports food bolus through
the pharynx and esophagus under the control of the parasympathetic division of the
autonomic nervous system (the vagus nerve)
 The tongue blocks off the mouth, the soft palate blocks off the nasal passages,
the larynx rises so that the epiglottis covers the respiratory tract so the bolus
must enter the esophagus and move down via wavelike peristaltic
contractions of the muscular walls – first the longitudinal muscles then the
circular muscles contract to allow swallowing of the bolus
 At the end of the esophagus, the food presses against the cardioesophageal
sphincter, causing it to open and allow the food into the stomach
 Gravity plays no part in swallowing – automatic process
o Vomiting – reverse peristalsis occurring in the stomach and possibly the small intestine,
accompanied by contractions of the abdominal muscles and the diaphragm, which increases
the pressure on the abdominal organs
 Caused by local irritation of the stomach or disturbance of the equilibrium apparatus
of the inner ear that may activate the emetic center in the brain (medulla), which
then causes vomiting
Defecation – the elimination of indigestible substances from the body via the anus in the
form of feces
 The defecation reflex is initiated when feces are forces into the rectum by mass
movement (long, slow-moving but powerful contractile waves that move over large
areas of the colon that force the contents toward the rectum) and the walls of the
rectum are stretched
 Spinal reflex (sacral region) that causes the walls of the sigmoid colon and
the rectum to contract and the anal sphincters to relax
 Defecation can be delayed temporarily voluntarily – the reflex contractions end and
the rectal walls relax until the next mass movement and the process begins again
 Describe how foodstuffs in the digestive tract are mixed and moved along the tract.
o Peristalsis is the major means of propelling food through the digestive tract – involuntary
waves of contraction that move along the length of the intestine, followed by waves of
relaxation – net effect is to squeeze the food along the tract
o The mixing of food in the mouth by the tongue, churning of food in the stomach, and
segmentation in the small intestine are examples of physical processes of mechanical
digestion
 Describe the function of local hormones in the digestive process.
o See table 14.2 on page 453
o Gastrin – stimulates release of gastric juice, stimulates mobility of small intestine, relaxes
ileocecal valve
o Histamine – activates parietal cells to secrete hydrochloric acid
o Somatostatin – inhibits secretion of gastric juice and pancreatic juice and inhibits emptying
of stomach and gallbladder
o Secretin – increases output of pancreatic juice, increases bile output, inhibits gastric
mobility and gastric gland secretion
o Cholecystokinin (CCK) – increase output of pancreatic juice, stimulate gallbladder, relaxes
sphincter or duodenal papilla to allow bile and pancreatic juice to enter the duodenum
o Gastric inhibitory peptide (GIP) – inhibits gastric mobility and secretion of gastric juice
 List the major enzymes or enzyme groups produced by the digestive organs or accessory glands
and name the foodstuffs on which they act.
o Chemical digestion – sequence of steps in which large food molecules are broken down
into their building blocks by enzymes – these reactions are called hydrolysis reactions
because a water molecule is added to each bond to be broken
o Salivary amylase in the mouth – starch is broken down into maltose – produces by
salivary glands, which include the parotid gland, sublingual gland, and submandibular gland
o Pepsin (pepsinogens are activated into pepsin by hydrochloric acid found in gastric
juices) in the stomach – breaks down proteins into amino acids – produced by chief cells in
stomach lining
o Rennin in the stomach of infants – breaks down milk protein and converts it into sour
milk-like substance – not typically found in adults
o Brush border enzymes in the small intestine – break down double sugars and complete
protein digestion
o Pancreatic juice entering the small intestine – completes the breakdown of starch (via
pancreatic amylase), does half of the protein digestion (via trypsin, chymotrypsin,
carboxypeptidase), and fat digestion (via lipases), and digestion of nucleic acids (via
nucleases)
 Pancreatic juices also contain bicarbonate, which makes it very basic causing it to
neutralize the acid chyme coming in from the stomach in order to activate the
enzymes in the small intestine
o
Release of pancreatic juices is stimulated by the vagus nerve and local hormones
produced by mucosa cells – secretin and cholecystokinin (CCK) stimulate the
pancreas, liver, and gallbladder
 Name the end products of protein, fat, and carbohydrate digestion.
o See table 14.1 on page 447
 Carbohydrates – disaccharides and polysaccharides are broken down into
monosaccharides (simple sugars such as glucose, fructose, and galactose) – only
carbohydrates our system breaks down into simple sugars are sucrose, lactose,
maltose, and starch – cannot digest cellulose because we lack the enzymes
necessary
 Proteins – broken down into amino acid building blocks
 Fats – lipids – broken down into fatty acids and glycerol
o Most substances are absorbed through the intestinal cell plasma membranes by active
transport – they enter the capillary bed in the villus to be transported in the blood to the
liver via the hepatic portal vein
 Lipids (fats) are absorbed passively by the process of diffusion and enter both the
capillary bed and the lacteal in the villus and are carried to the liver by both blood and
lymphatic fluids
 State the function of bile in the digestive process.
o Bile is not an enzyme but it acts like a detergent to emulsify, or mechanically break down
large fat globules into thousands of tiny ones, providing a greater surface area for the
pancreatic lipases to work on
o Bile is also necessary for the absorption of fats and fat-soluble vitamins K, D, and A from
the intestinal tract

Part II: Nutrition and Metabolism
Nutrition
 Define nutrient and calorie.
o Nutrient – a substance in food that is used by the body to promote normal growth,
maintenance, and repair
o Calorie – the energy value of foods is measured in units called kilocalories (kcal) or “large
calories” (C) – unit of heat; the large calorie is the amount of heat required to raise 1 kg of
water 1 degree C
 List the six major nutrient categories. Note important dietary sources and the principal cellular uses
of each.
o Carbohydrates – except for milk sugar (lactose) and glycogen in meats, all sugars and
starches come from plants – the polysaccharide cellulose is not digestible by humans but
provides fiber, which increases the bulk of the stool and aids in defecation – sugars,
especially glucose are broken down to make ATP
o Lipids – mostly neutral fats such as triglycerides and some cholesterol and phospholipids –
animal products such as meats and dairy foods and a few plants such as coconut –
unsaturated fats are present in seeds, nuts, and most vegetable oils while major sources of
cholesterol are egg yolk, meats, and milk products – fats are used to build cell membranes,
make myelin sheaths, and insulate the body with a fatty cushion and for making ATP when
sugars are unavailable
o Proteins – animal products such as eggs, milk, meats are complete proteins that meet all of
the body’s amino acid requirements for tissue maintenance and growth – legumes, nuts, and
cereals are nutritionally incomplete because they are low in one or more of the essential
amino acids – proteins are the major structural materials used for building cell structures
o
o
o
Vitamins – organic nutrients of various forms that the body requires in small amounts – no
one food contains all the required vitamins – most vitamins work as coenzymes (parts of
enzymes) where they act with the enzyme to accomplish a particular type of catalysis
Minerals – inorganic substances including calcium, phosphorous, potassium, sulfur, sodium,
chloride, magnesium and trace amounts of others – the most mineral-rich foods are
vegetables, legumes, milk, and some meats
Water – 60% of the volume of the food we eat – important as a solvent for reactions
Metabolism
 Define enzyme, metabolism, anabolism, and catabolism.
o Enzyme – usually act as catalysts that speed up reactions by lowering the level of activation
energy required for the reaction
o Metabolism – a broad term referring to all chemical reactions that are necessary to
maintain life – it involves anabolism and catabolism
o Anabolism – larger molecules are built from smaller ones and energy is stored
o Catabolism – larger molecules are broken down into simpler ones and energy is released
and captured to make ATP
 Describe the metabolic roles of the liver.
o Liver cells detoxify drugs and alcohol, degrade hormones, make many substances vital to the
body as a whole such as cholesterol, blood proteins such as albumin and clotting proteins,
and lipoproteins, and process nearly every class of nutrient
 As blood circulates through the liver, liver cells remove amino acids, fatty acids, and
glucose from the blood and stores them for later or processes them while phagocytic
cells remove and destroy bacteria that have passed from the digestive tract to the
blood
 The liver is vitally important in helping to maintain blood glucose levels – after a
meal, glucose molecules are removed from the blood and combined to form
glycogen, which is then stored in the liver in a process called glycogenesis – when
glucose levels in the blood drop, liver cells break down the glycogen molecules into
glucose and release them into the blood in a process called glycogenolysis – if
glycogen and glucose are unavailable, the liver can make glucose from
noncarbohydrate substances such as fats and proteins in a process called
gluconeogenesis
 Some fats and fatty acids are used to make ATP for the liver cells but the rest are
broken down into simpler substances such as acetic acid and acetoacetic acid and
released into the blood or stored as fat reserves in the liver
 The liver also makes cholesterol and secretes cholesterol’s breakdown products in bile
 All blood proteins made by the liver are built from the amino acids its cells pick up
from the blood and the completed proteins are released back into the blood
 Albumin – holds fluids in the bloodstream – the lack of albumin causes fluid
to leave the bloodstream and accumulate in the tissues causing edema
 Clotting proteins – allow blood to clot
 The liver also makes nonessential amino acids and detoxifies ammonia by converting
it to urea
 Recognize the sources of carbohydrates, fats, and proteins and their uses in cell metabolism.
o See figure 14.19 on page 461
o Carbohydrate metabolism – glucose (blood sugar) is the major breakdown product of
carbohydrate digestion and is the preferred fuel to produce cellular energy (ATP) through the
process of cellular respiration – glycolysis, Krebs cycle, the electron transport chain – see
figure 14.17 on page 459
Fat metabolism – the liver cells use some fats to make ATP for their use, some to make
lipoproteins, thromboplastin (A clotting protein), and cholesterol, and then releases the rest
to the blood in the form of small, fat-breakdown products and the body cells remove the fat
products and cholesterol from the blood and build them into their membranes or steroid
hormones as needed – fats can also be used to form myelin sheaths of neurons and fatty
cushions around the body organs or stored as a concentrated source of energy, which can
later be used to make ATP when glucose levels are too low
o Protein metabolism – proteins are broken down into the amino acids and processed
through the liver first then the remaining amino acids are released into the bloodstream and
are taken up by body cells to use to build proteins to use and export – amino acids are taken
up by active transport and stock piled for later use – essential amino acids are those that
cannot be made by the cells and must be obtained through diet
 Amino acids are only used to make ATP when proteins are overabundant and/or
carbohydrates and fats are not available – their amine groups are removed as
ammonia, which is toxic to cells and must be combined with carbon dioxide in the
liver to form urea, which is then flushed from the body in urine
 Explain the importance of energy balance in the body and indicate consequences of energy
imbalance.
o Energy intake = total energy output (heat + work + energy storage)
 Energy intake is the energy liberated during food oxidation (cellular respiration) while
energy output includes the energy lost as heat (~60% of the total), plus that used to
do work (driven by ATP), plus energy that is stored in the form of fat or glycogen
o When energy intake and energy outflow are balanced, body weight remains stable and when
they are not balanced, weight is gained or lost
o Food intake may be controlled by several factors – rising or falling blood levels of nutrients
(glucose and amino acids), hormones (insulin, glucagon, and leptin), or body temperature
(rising temperatures is inhibitory), and psychological factors
o When the total amount of calories consumed is equal to the total metabolic rate,
homeostasis is maintained and our weight stays the same – excess calories appear in the
form of fat deposits while too few leads to weight loss
 List several factors that influence metabolic rate, and indicate the effect of each.
o See table 14.4 on page 465
o The basal metabolic rate (BMR) – the amount of heat produced by the body per unit of
time when it is under basal conditions (at rest) – reflects the energy supply a person’s body
needs just to perform essential life activities such as breathing, maintaining the heartbeat,
and kidney function
o The total metabolic rate (TMR) – refers to the total amount of kilocalories the body must
consume to fuel all ongoing activities
 Surface area – larger surface area in relation to body volume increases metabolic
rate
 Sex – metabolic rates are higher in males than females
 Thyroxine production – metabolic rates increase with production
 Age – young, rapidly growing individuals have a higher rate
 Strong emotions and infections – increase the metabolic rate
 Describe how body temperature is regulated.
o Most of the energy (~60%) released as foods are oxidized escapes as heat that is used to
warm tissues and the blood, which then circulates to all body tissues, keeping them at
homeostatic temperatures, which allows metabolism to occur efficiently
o Body temperatures reflect the balance between heat production and heat loss – the body’s
thermostat is in the hypothalamus of the brain, which through autonomic nervous system
pathways, continually regulates body temperature around a set point by initiating heat-loss
or heat-promoting mechanisms
o


Vasoconstriction of blood vessels of the skin or shivering – skin vasculature
constricts and blood is rerouted to the deeper, more vital, body organs – when simple
constriction of skin capillaries is not enough, shivering begins, which is the
involuntary shudder-like contractions of the voluntary muscles, which releases large
amounts of heat
Radiation or evaporation – blood vessels serving the skin dilate and capillary beds
in the skin become flushed with warm blood and as a result, heat radiates from the
skin surface – when radiation is not enough, the evaporation of perspiration off the
skin becomes necessary
Developmental Aspects of the Digestive System and Metabolism
 Name important congenital disorders of the digestive system and significant inborn errors of
metabolism.
o Defects that interfere with feeding – all can be corrected surgically
 Cleft palate/cleft lip defect – most common congenital defect that interfere with
feeding with the cleft palate being the most severe as it interferes with the infant’s
ability to suck properly
 Tracheoesophageal fistula – a connection between the esophagus and the trachea
and the esophagus often ends in a blind sac and does not connect to the stomach
leaving the baby to choke, drool, and become cyanotic during feeding because food is
entering the respiratory passageways
o Defects that interfere with metabolism
 Cystic fibrosis – primarily affects the lungs but it also impairs the activity of the
pancreas – huge amounts of mucus is produced, which blocks the passages of
involved organs such as blockage of the pancreatic duct that then prevents pancreatic
fluid from reaching the small intestine and as a result, fats and fat-soluble vitamins
are not digested or absorbed, and bulky, fat-laden stools result – corrected by
administering pancreatic enzymes with meals
 Phenylketonuria (PKU) – involves the inability of tissue cells to use
phenylalanine, an amino acid present in all protein foods leading to brain damage
and retardation unless a special diet low in phenylalanine is prescribed
 Describe the effect of aging on the digestive system.
o All through childhood and into adulthood, the digestive system operated with relatively few
problems unless there are abnormal interferences such as contaminated food or extremely
spicy or irritating foods or inflammation of the appendix
o Between middle age and early old age, the metabolic rate decreases by 5 to 8 percent in
every 10-year period
o Two middle-age digestive problems are ulcers and gallbladder problems such as
inflammation of the gallbladder or gallstones
o During old age, the activity of the GI tract declines and fewer digestive juices are produced,
and peristalsis slows, taste and smell become less acute, periodontal disease often develops
(decay of the teeth), and diverticulosis and cancer of the GI tract are also fairly common
problems