Digestive System

Get started:
How do nutrients get
from food to the blood
Today we will:
• Intro to digestive
• Notes
• Color/label diagram
• The 1. alimentary canal (or gastrointestinal [GI]
tract) is made of several organs that serve to break
down and absorb nutrients from food and drink
and remove the leftover material
• It is a twisting, hollow tube that is open at both
• It consists of the mouth, esophagus, stomach,
small intestine, and large intestine
• The mouth, or 2. oral cavity, is where food enters
• It is lined with 3. mucous membranes
• The anterior of the mouth is protected by the 4.
lips, the lateral walls are formed by the 5. cheeks,
the roof is formed by the 6. hard palate and 7. soft
palate, and the bottom is filled primarily with the
8. tongue, which is secured to the bottom of the
mouth by the 9. lingual frenulum
• The space between the lips and cheeks is called the
10. vestibule
• The teeth start the process of 11. mechanical
digestion (physical breakdown of food)
• 12. Saliva also serves to begin some 13. chemical
digestion (chemical breakdown of food), but only
of carbohydrates
• The pharynx is also considered a digestive system
organ (although the nasopharynx is only part of
the respiratory system)
• The walls of the pharynx contain 14. two layers of
skeletal muscle:
• The fibers of the internal layer are 15.longitudinal
while the fibers of the external layer are 16.
concentric. This arrangement allows for a special
type of contraction called 17. peristalsis, which will
be discussed later
• The esophagus is a muscular tube that extends
from the laryngopharynx through a hole in the
diaphragm where it connects to the stomach
• The wall of the esophagus (as well as the stomach
and both intestines) consist of 4 layers:
• 1) The 18. mucosa is the innermost layer that is
made of epithelial cells (stratified squamous for
esophagus to protect from friction, simple columnar
for the rest). It also contains mucus-producing
glands that secrete into the hollow cavity (lumen) of
the alimentary canal
• 2) The 19. submucosa is next. It contains blood
vessels, nerves, lymph vessels and nodes, and
additional mucosal glands
• 3) The 20. muscularis externa layer contains two
layers of smooth muscle (an inner circular layer and
an outer longitudinal layer) that will assist with
mechanical digestion and propulsion through the
• 4) The 21. serosa is the outermost layer. It is a
single layer of serous-producing cells and is also
called the visceral peritoneum or adventitia. The
visceral peritoneum is continuous with the parietal
peritoneum, which lines the abdominopelvic cavity
along with several membrane extensions called the
22. mesenteries.
No digestion occurs in either the pharynx or the
esophagus. They just move the food along.
• The stomach is a J-shaped organ located in the
upper left abdominal quadrant
• The stomach has several portions:
• a. The 23. cardiac region is named because of its
proximity to the heart. It connects to the
esophagus via a circular muscle called the
24. cardioesophageal sphincter. This muscle
contracts to prevent food from moving back into
the esophagus
• b. The 25. fundus is the expanded portion lateral to
the cardiac region
• c. The 26. body is the mid-portion of the stomach
• d. The funnel-shaped 27. pylorus is the terminal
end and connects to the small intestine via another
circular muscle called the pyloric sphincter
• 28. The stomach is about 25 cm long, and when
full can expand to hold about 1 gallon.
• When empty it collapses inward into folds called
29. rugae.
• The convex lateral surface is the greater curvature
and the concave medial surface is the lesser
• The muscularis externa of the stomach not only
moves food along, but it mixes and churns it with
several enzymes to turn it into a thick, creamlike
substance called 30. chyme.
• Thus, mechanical and chemical digestion 31. both
occur in the stomach
Digestive System, Part 1: Crash Course A&P #33
Digestive System, Part 2: Crash Course A&P #34
Digestive System, Part 3: Crash Course A&P #35
• The small intestine is the 32.major digestive organ.
Much of 33. chemical digestion and almost all
34. absorption occurs here.
• It is a 2-4 m (7-13 ft) long twisting section of the
alimentary canal that extends from the pyloric
sphincter to the 35. ileocecal valve that connects it
to the large intestine.
• The coils are held together and anchored to the
posterior abdominal wall by the mesenteric
• The small intestine has three sections:
• 1) The 36. duodenum is about 25 cm (10 in) long
and curves around the head of the pancreas
• 2) The 37. jejunum is about 2.5 m(8 feet) long and
extends from the duodenum to the ileum
• 3) The 38. ileum is about 3.6m (12 feet) long and
extends from the jejunum to the ileocecal valve,
where it connects to the large intestine
• Most chemical digestion occurs in the small
• The intestinal cells produce an important mucus,
and the 39. pancreas and gall bladder both empty
important enzymes into the duodenum to aid in
the chemical breakdown of foods
• The 40. pyloric sphincter controls the amount of
food that enters the small intestine as it can only
handle a small amount of food at a time
• The small intestine is also where almost all
absorption takes place, and the small intestine is
well suited for that function
• The wall has three structures that aid absorption:
• 1) 41. Microvilli are tiny projections of the mucosal
cells that give the surface a fuzzy appearance
• 2) 42. Villi are fingerlike projections that give it a
velvety appearance and feel. They also include a
capillary bed that allow nutrients to get into the
• 3) 43. Circular folds are deep folds of the mucosal
and submucosal layers
• Unlike the rugae of the stomach, the circular folds
do not disappear during distension
• All three of those structures increase 44. surface
area to improve absorption
• The structures 45. decrease in number toward the
end of the small intestine to help prevent
absorption of bacteria from the leftover 46.waste
at the end of the small intestine
• The large intestine is much 47. larger in diameter
than the small intestine, but 48. shorter in length
• It extends from the ileocecal valve to the anus and
frames the small intestine on three sides
• Its major function is to dry out undigested food be
reabsorbing 49. water, and eliminate waste as
• It has several sections as well:
• 1) The 50. cecum is a saclike section after the
ileocecal valve
• 2) The 51. appendix hangs from the cecum, and is a
potential site for bacteria to accumulate and
multiply, causing a condition called appendicitis
• 3) The 52. colon is divided into 4 sub-sections: the
ascending colon travels up the right side of the
abdominal cavity; the transverse colon continues
across the abdomen from right to left; the
descending colon extends from the upper left to
lower left of the abdomen; and the sigmoid colon is
an S-shaped region that brings the colon back to the
midline of the body
Get started:
What effect does gravity
have on the propulsion
of food through the
digestive system?
• Three pairs of salivary glands secrete saliva into
the mouth.
• 1) The 55. parotid glands are located anterior to the
ears and secrete into the upper lateral portion of
the mouth
• 2) The 56. submandibular glands and
• 3) the 57. sublingual glands secrete through tiny
ducts into the floor of the mouth
• Saliva is a mixture of serous and mucous fluid and
has several functions
• The saliva dissolves food so it can be tasted, as well
as binds it together into a mass called a 58. bolus
• It contains an enzyme called salivary 59. amylase
that starts the chemical digestion of carbohydrates
• It also contains a substance called lysozyme and
antibodies that help to inhibit bacteria that was
present on food
• Teeth are responsible for 60. mastication, or
• They are aided by the tongue, which helps to move
food between the teeth
• They are responsible for beginning 61. mechanical
digestion by tearing and grinding food into smaller
pieces that are more easily digested later in the
• Humans produce two separate sets of teeth
• The first set (deciduous teeth) begins to erupt
from the gums around 6 months, and the full set
(20 teeth) is there by age 2.
• As the second set (permanent teeth) forms and
develops, the deciduous teeth fall out. The new
set has come in by the end of adolescence. There
are 32 permanent teeth, although in many people
the last set of molars (wisdom teeth) do not erupt
or are absent altogether
Why might they be absent all together?
62. Like vestigial structures – structures that
disappear due to the evolution of species
The teeth are classified into several groups:
1) The 63. incisors are chisel-shaped and are adapted
for cutting (4 on top, 4 on bottom)
2) The fanglike 64. canines are for tearing and piercing
(2 on top, 2 on bottom)
3) The 65. premolars (bicuspids) and
4) 66. molars have rounded crowns and flatter
centers and are suited for grinding food
Teeth have two main regions: the exposed 67.crown
and the hidden 68.root
• The crown is covered in 69. enamel, a hard smooth
substance that protects the tooth
• 70. Dentin, a bone-like substance, underlies the
enamel and forms the bulk of the tooth
• The dentin surrounds the pulp cavity, which
contains blood vessels, connective tissue, and
nerve fibers (collectively called 71. pulp).
• Where the pulp extends into the root, it becomes
the 72. root canal, which provides a route for the
pulp structures to enter the tooth
• The pancreas is a soft, pink, triangular organ
extending from the spleen to the duodenum
• Its endocrine function has already been covered,
but it also produces a very important substance for
digestion called 73. pancreatic juice (the details
will be discussed later)
• It secretes the pancreatic juice into the duodenum
in an alkaline fluid (to counteract the acidity of the
• The 74. liver is the largest gland in the body
located under the diaphragm in the upper right
quadrant of the abdomen
• It has four lobes and is suspended from the
diaphragm and the abdominal wall by the
75. falciform ligament
• Its digestive function is to produce 76. bile
• 77. Bile is a yellow-green solution containing bile
salts, pigments (primarily bilirubin), cholesterol,
phospholipids, and electrolytes
• The bile salts will 78. emulsify fats by breaking
large fat globules into smaller ones, allowing fatdigesting enzymes to work better
• The liver secretes the bile through the 79. common
hepatic duct and it enters the 80. duodenum
through the bile duct
• If food digestion is not occurring, the bile backs up
through the cystic duct into the 81. gallbladder, a
small pouched organ located under the liver
• If the bile is stored too long in the gallbladder, the
cholesterol can crystallize into 82. gallstones,
which can cause tremendous pain
• If the gallstones block the ducts leading to the
small intestine, the bile accumulates and backs up
into the liver, which then causes the bile salts to
enter the blood stream
• The end result is a condition that yellows the
tissues of the body called 83. jaundice, which can
also be a symptom of hepatitis or cirrhosis of the
• The activities of the GI tract include six processes:
• 1) Ingestion – an active, voluntary process of food
entering the mouth
• 2) Propulsion – the moving of food from one organ
to the next. The main process is a wavelike
contractive process called peristalsis. A second
movement pattern called segmentation moves
food back and forth within the walls of the
digestive system
• 3) Mechanical digestion – mixing of food in the
mouth, chewing, churning in the stomach, and
segmentation in the small intestine all serve to
physically break down food into smaller pieces
• 4) Chemical digestion – the sequence of steps in
which large food particles are broken into smaller
ones by enzymes
• 5) Absorption – transport of digested food
particles into the bloodstream
• 6) Defecation – the elimination of undigested
substances from the body
• Control of digestive functions occurs primarily in
response to change. The stimuli involved include:
• 1) the stretch of an organ caused by food
• 2) the pH of the contents
• 3) the presence of breakdown chemicals (like
• When the stimuli occur, they activate or inhibit
1) the glands that secrete digestive juices or
2) the smooth muscles involved in mixing and
1) Ingestion – clearly ingestion of food occurs in the
2) Mechanical digestion – the teeth are involved in
breaking food into smaller pieces
The tongue also mixes the food with saliva
3) Chemical digestion – salivary amylase is involved in
breaking down carbohydrates
4) Propulsion – swallowing occurs to move food from
the oral cavity down the pharynx and esophagus
• Swallowing involves several reflexes:
1) the soft palate raises to seal off the
2) the tongue seals off the oral cavity to force food down
the pharynx,
3) the epiglottis seals the trachea, and
4) the walls of the pharynx and esophagus contract to force
food downward
• Peristalsis continues to move the food toward the
cardioesophageal sphincter
1) Mechanical digestion – the smooth muscles in the
wall of the stomach will mix and churn food to break
it down
2) Chemical digestion – stomach glands produce
gastric juice. In addition, the presence of food causes
the release of the hormone gastrin, which stimulates
even more release of mucus, hydrochloric acid, and
enzymes. The hydrochloric acid is needed to activate
the enzyme pepsin, which is a major enzyme for
protein digestion.
• The alkaline mucus produced helps protect the
stomach itself from being digested by the acidic
environment and enzymes
• The start of protein digestion is the only chemical
digestion that occurs in the stomach, and virtually
no absorption takes place (except for aspirin and
• 3) Propulsion – once the food has been mixed,
peristalsis begins in the lower half of the stomach.
The pylorus acts to regulate the amount of chyme
that enters the small intestine
• Each contraction of the pylorus only squirts about
3 ml of chyme into the duodenum, so the rest is
pushed back into stomach for more mixing
• It usually takes about 4 hours for the stomach to
empty completely (up to 6 hours for a high-fat
• If the stomach is irritated (or overdistended) and
contracts prematurely, some of the high acid
chyme can move back into the esophagus where
mucus does not protect the wall and cause a
burning sensation (heartburn)
• If the stomach has severe irritation, it may activate
the emetic center of the brain located in the
medulla. This causes forceful contraction of the
stomach (and even the small intestine), as well as
contraction of the abdominal muscles and
diaphragm. The result is a reverse peristalsis called
• The emetic center can also be activated by
problems with equilibrium (see: motion sickness)
• 1) Chemical Digestion – the majority of chemical
digestion occurs in the small intestine, although
intestinal juice itself does not contain many
enzymes (although the mucus it secretes helps in
• Pancreatic juice contains several important enzymes
that go to work in the small intestine. They include
pancreatic amylase (carbohydrate), trypsin,
chymotrypsin, and carboxypeptidase (protein),
lipases (fats), and nucleases (nucleic acids)
• Pancreatic juice also contains bicarbonate, which
helps neutralize the acidic chyme
• The small intestine mucosal cells also produce two
hormones (secretin and cholecystokinin), when
chyme enters which will stimulate pancreatic juice
release as well as bile production
• The bile salts are important to chemically emulsify
large fat molecules so the lipases from pancreatic
juice can work more effectively
• 2) Absorption – almost all absorption occurs in the
small intestine as nutrients are moved through
active transport or diffusion into the capillary beds
that surround the villi. At the end of the ileum, all
that remains is some water, indigestible food (like
cellulose), and bacteria
• 3) Propulsion – peristalsis moves the food through
the long small intestine (which needs to be long to
allow time for all the digestion and absorption).
The process takes 3-6 hours on average.
• The residue that arrives at the large intestine still
has about 12-24 hours to spend there
• 1) Chemical Digestion – Although the large
intestine does not contain any digestive enzymes,
the bacteria that live there play a role in chemical
digestion as they break down the waste and
produce vitamin K and some B vitamins
• 2) Absorption – absorption is limited to the
vitamins produced as well as most of the
remaining water
• 3) Propulsion – two types of movements occur in
the large intestine.
• The first is a sluggish form of peristalsis, which
contributes little to propulsion but does help pack
the feces more tightly.
• The second is called mass movements, which are
long, slow, powerful contracting waves that force
the contents toward the rectum. They occur 3 or 4
times daily, usually during or just after a meal.
• 4) Defecation – the elimination of solid waste only
occurs in the large intestine
• Defecation is stimulated by the presence of feces in
the rectum, and consists of a reflex that contracts the
sigmoid colon and rectum as well a relaxation of the
anal sphincters
• Fiber in the diet helps to soften the stool and increase
contraction strength to make defecation easier
• Diarrhea occurs when irritation causes the mass
movements to occur too frequently before enough
water has been absorbed
• Constipation is the opposite condition, when too
much water is absorbed and feces become dry and
difficult to eliminate
• A nutrient is a substance used by the body to
promote normal growth, maintenance, and repair
• There are six categories of nutrients:
carbohydrates, proteins, lipids, vitamins,
minerals, and water
• Water is important as a solvent and for many body
functions (such as transport and temperature
• Almost all carbohydrates we consume are from
plants (except for lactose in milk and a small
amount of glycogen in meat)
• Sugars come mainly from fruits, sugar cane, and
• Starches are found in grains, legumes, and root
vegetables (beets, potatoes, etc.)
• Cellulose from plants is not a digestible
carbohydrate for humans, but provides fiber that
aids in waste removal
• Carbohydrates are the preferred source of energy
for body cells because they are easily broken down
into glucose, which is the main fuel used in the
formation of ATP in cellular respiration
• If blood glucose levels are high because of too
much carbohydrate consumption, some of it is
stored as glycogen in the liver and in muscle cells
• If it is still too high, the remainder is stored as fat
• Most dietary fats are triglycerides (saturated and
unsaturated fats)
• Saturated fats come from animal products such as
meat, dairy, and eggs (some are from plants like
coconuts and avocados)
• Unsaturated fats are mainly from plants like seeds,
nuts, and most vegetable oils
• The liver produces all the cholesterol we need, but
we consume it like saturated fats in animal foods
• The liver metabolizes most fat that goes into the body.
It uses some to make ATP, some to make
thromboplastin (for clotting), and some to make
cholesterol; however, it releases the rest into the
bloodstream in the form of small fat-breakdown
• The cells use those products to build cell membranes,
create steroid hormones, and build myelin sheaths
around neurons
• The rest is stored in the body. Although some
subcutaneous fat is important for temperature
maintenance, excess storage inhibits the
cardiovascular system
Proteins come in two forms: complete and incomplete
• Complete proteins contain all of the essential
amino acids needed to for tissue maintenance and
growth. They are found in eggs, milk, and most
meat products
• Incomplete proteins – such as those found in
legumes, nuts, and starches – are incomplete
because they lack one or more of the essential
amino acids needed. Different types of incomplete
proteins must be consumed to meet nutritional
• Proteins are broken down into their amino acids
which are transported via the bloodstream to the
cells to absorb and use for protein synthesis to
make body tissues, antibodies, enzymes,
hormones, spindle fibers, and mucus
• Of the 20 amino acids, cells can produce 12 of
them. The other 8 essential amino acids must be
consumed in the diet. Amino acids are only used
to make ATP if there is an overabundance of them
• The liver is important in protein metabolism
because it detoxifies ammonia (a by-product of
protein metabolism) by combining it with carbon
dioxide to make urea. Urea is not toxic and is
flushed out of the body as a component of urine
• Why would a urine test show if you are positive for
drug use?
• In addition to this (and maintaining sugar
balance in the blood), the liver also detoxifies all
medicines and alcohol in a similar manner
• It also synthesizes the non-essential amino acids
that are used by the cells
• The synthesis of cholesterol in the liver is
important because cholesterol is the basis of
steroid hormone production and vitamin D
• The liver is also responsible for making bile which,
as was discussed earlier, is responsible for
emulsifying large fat molecules in the small
intestine to begin chemical digestion of fats
• The basal metabolic rate (BMR) is the amount of
heat produced by the body per unit of time when
it is at rest
• Put another way, it is the amount of energy used
by a person to perform life-supporting activities
like breathing, heartbeat, and kidney function
BMR can be influenced by several factors:
• 1) Surface area to volume ratio – the higher the
ratio, the higher the BMR
• 2) Gender – males typically have a higher BMR
• 3) Thyroxine production – thyroxine produced by
the thyroid influences oxygen consumption and
ATP use, increasing BMR
• 4) Age – BMR typically gets lower as humans age
• 5) Strong emotions – high stress, fear, anger, and
also infections increase BMR
• Outside of BMR, a person’s activity level requires
different amounts of fuel
• Total metabolic rate (TMR) is the amount of
kilocalories the body must consume to meet the
fuel demands of all activities
• In a well-conditioned person, the TMR jumps
significantly with just a few minutes of vigorous
activity (and stays elevated for hours afterward)
• Weight is controlled using a simple formula:
Calories Consumed vs. Calories Needed
• Vitamins are small organic molecules produced by
living things that assist in many body functions
• They come in two categories: water-soluble and
• Fat-soluble vitamins include vitamins
A, D, E, and K.
• Water-soluble vitamins include
vitamin C and the B vitamins
• One difference is that fat-soluble vitamins can be
stored, while water-soluble vitamins must be
consumed daily to as they are flushed out through
• The other difference is in their ability to resist heat;
water-soluble vitamins are destroyed in high
temperatures, so cooked fruits and vegetables
lose those benefits.
• Vitamin A - maintaining healthy eyes, skin, and
bones; hormone synthesis
• The B vitamins have a range of functions but most
center on aiding metabolism
• Vitamin C is important in helping the body absorb
iron for hemoglobin and in synthesis of collagen
(for healthy skin and gums)
• Vitamin D helps the body absorb calcium and
phosphorus to maintain strong bones and teeth.
• Vitamin E is an antioxidant that helps maintain cell
• Vitamin K is essential in the process of coagulation
during wound healing
• Minerals are also small molecules needed for body
process, but they are NOT made by living
organisms and are instead found in nature
• The main important minerals include calcium,
phosphorus, potassium, iron, sulfur, sodium,
chloride, iodine, and magnesium
• Calcium – bone and teeth formation; needed for
muscle contraction
• Phosphorus – bone and teeth formation
• Potassium – involved in nervous system function
and muscle contraction
• Sodium – maintains fluid balance and is involved in
nerve impulse propagation
• Chloride – involved in acid-base balance and
formation of gastric juice
• Iron – main component of hemoglobin in red
blood cells
• Sulfur – component of cartilage, tendons, and
most proteins
• Magnesium – component of enzymes
• Iodine – needed for thyroid hormones
Medline Plus – Health Topics
Web MD
Health Line
Clear Health from NIH
1 – Share your patient’s
scenario with the class
2 – Explain your
hypotheses and why you
ruled out first guesses
3 – Give your final
diagnosis or opinion (may
be more than one) and
the advice/information
you would give this
patient/next of kin
At the Clinic
(29) Mary Maroon – vegetarian diet
(30) Mr. Ashe – heartburn
(33) A woman – severe pain left iliac
(36) Zena - body fat, crash diets, lack of exercise
(38) A 21 year old man – result of untreated appendicitis
(32) Bert – heat
Dairy issues
You should have completed this:
Copy this list: Protein, saturated
fat, carbohydrates/sugars, calcium, iron, Vitamin A, Vitamin C,
Vitamin D, Vitamin E, Potassium
Research these nutrients (macronutrient, vitamin, or mineral)
and how deficiencies and excesses of that nutrient would impact
the body in terms of disease or diminished quality of life.