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Dr. Kim Wilson
Primary function of the digestive system: to bring
essential nutrients into the internal environment so
they are available to every cell in the body
Mechanisms used to accomplish the primary
function of the digestive system
◦ Ingestion: food is taken in
◦ Digestion: breakdown of complex nutrients into simple
◦ Motility of the gastrointestinal (GI) wall: physically breaks
down large chunks of food material and moves food along
the tract
◦ Secretion of digestive enzymes allows chemical digestion
◦ Absorption: movement of nutrients through the GI
mucosa into the internal environment
◦ Elimination: excretion of material that is not
◦ Regulation: coordination of the various functions of
the digestive system
The digestive tract is functionally an
extension of the external environment;
material does not truly enter the body until it
is absorbed into the internal environment
Mechanical digestion: movements of the
digestive tract
◦ Function - Breakdown large particles of food to
small particles  chemical digestion
◦ How?
 Churn contents of the GI lumen to mix with digestive
juices and come in contact with the surface of the
intestinal mucosa, facilitating absorption
 Propel food along the alimentary tract, eliminating
digestive waste from the body
◦ Mastication: chewing movements
 Reduces size of food particles
 Mixes food with saliva in preparation for swallowing
◦ Deglutition: process of swallowing; complex process
requiring coordinated, rapid movements
◦ 3 stages:
 Oral stage (mouth to oropharynx):
 voluntarily controlled
 formation of a food bolus in the middle of the
 tongue presses bolus against the palate and
food is then moved into the oropharynx
 Pharyngeal stage (oropharynx to esophagus):
 involuntary movement;
 to propel bolus from the pharynx to the
esophagus, the mouth, nasopharynx, and
larynx must be blocked;
 a combination of contractions and gravity move
bolus into esophagus
 Esophageal stage (esophagus to stomach):
 involuntary movement;
 contractions and gravity move bolus through
esophagus and into stomach
◦ Two main types of motility
produced by the smooth
muscle of the GI tract
 Peristalsis: wavelike ripple of
the muscle layer of a hollow
 Progressive motility that
produces forward movement
of matter along the GI tract
 Segmentation: mixing
 Digestive reflexes cause a
movement with a single
segment of the GI tract
 Helps break down food
particles, mixes food and
digestive juices, and brings
digested food in contact with
intestinal mucosa to facilitate
Gastric motility
◦ Takes approximately 2 to 6 hours to
empty stomach
◦ Food is churned (propulsion and
retropulsion) and mixed with gastric
juices to form chyme
 Ejected approximately every 20 seconds
into the duodenum
Gastric emptying is controlled by
hormonal and nervous mechanisms
◦ Hormonal mechanism: fats in duodenum
stimulate the release of gastric inhibitory
peptide, which decreases peristalsis of
gastric muscle and slows passage of
chyme into duodenum
◦ Nervous mechanism: enterogastric
reflex; receptors in the duodenal mucosa
are sensitive to presence of acid and
distention; impulses over sensory and
motor fibers in the vagus nerve cause a
reflex inhibition of gastric peristalsis
 Intestinal motility includes peristalsis and segmentation
 Segmentation in duodenum and upper jejunum mixes
chyme with digestive juices from the pancreas, liver,
and intestinal mucosa
 Rate of peristalsis picks up as chyme approaches end
of jejunum, moving it through the rest of the small
intestine into the large intestine
 After leaving stomach, chyme normally takes
approximately 5 hours to pass through the small
 Peristalsis: regulated in part by intrinsic stretch
reflexes; stimulated by cholecystokinin (CCK)
Chemical digestion: changes in chemical composition of food as it
travels through the digestive tract
Mechanism = hydrolysis (water is split into hydrogen ions)
◦ Digestive enzymes
 Extracellular, organic (protein) catalysts
 Principles of enzyme action
 Specific in their action
 Function optimally at a specific pH
 Most enzymes catalyze a chemical reaction in both directions
 Enzymes are continually being destroyed or eliminated from
the body and must continually be synthesized
 Most digestive enzymes are synthesized as inactive
◦ Carbohydrates are
saccharide compounds
 Polysaccharides are
hydrolyzed by amylases to
form disaccharides
 Final steps of carbohydrate
digestion are catalyzed by
sucrase, lactase, and maltase
 Found in the cell
membrane of epithelial
cells covering the villi that
line the intestinal lumen
Protein compounds are
composed of twisted
chains of amino acids
Proteases catalyze
hydrolysis of proteins
into intermediate
compounds and, finally,
amino acids
Main proteases:
◦ Pepsin in gastric juice
◦ Trypsin in pancreatic
◦ Peptidases in intestinal
brush border
◦ Fats must be emulsified
by bile in small intestine
before being digested
 Pancreatic lipase is the
main fat-digesting
◦ Residues of digestion:
some compounds of
food resist digestion and
are eliminated as feces
Emulsification involves the formation of micelles and
breaking fat drops into small droplets to make them
soluble in water.
Micelles are spherical shaped shells formed of lecithin
molecules based on their solubility in water.
Saliva: secreted by salivary glands
Gastric juice: secreted by gastric glands
◦ Mucus lubricates food and, with water, facilitates
◦ Amylase = enzyme that begins digestion of
starches; a small amount of salivary lipase is
released, its function uncertain
◦ Sodium bicarbonate increases the pH for optimal
amylase function
◦ Pepsin (secreted as inactive pepsinogen by chief
cells) is a protease that begins the digestion of
Secretions that Aid in Digestion
◦ Hydrochloric acid: secreted
by parietal cells
 Decreases the pH of chyme
for activation and optimal
function of pepsin
 Released actively into the
gastric juice by hydrogenpotassium pumps (proton
Secretions that Aid in Digestion
Intrinsic factor (secreted
by parietal cells) protects
vitamin B12 and later
facilitates its absorption
Mucus and water
lubricate, protect, and
facilitate mixing of chyme
Vesicles in the
resting parietal cell
move to the apical
surface when the cell
becomes active
Result = increased
surface area for the
process of secretion
Secretions that Aid in Digestion
Pancreatic juice: secreted by acinar and duct
cells of the pancreas
◦ Proteases (e.g., trypsin and chymotrypsin) are
enzymes that digest proteins and polypeptides
◦ Lipases are enzymes that digest emulsified fats
◦ Nucleases are enzymes that digest nucleic acids
such as DNA and RNA
◦ Amylase is an enzyme that digests starches
◦ Sodium bicarbonate increases the pH for optimal
enzyme function; its manufacture also helps restore
normal pH of blood
Where? Pancreatic duct cells
Bicarbonate (HCO3) is produced
by the dissociation of carbonic
acid (H2CO3)
Bicarbonate ions are exchanged
for Cl ions = “chloride shift”
Electrical gradient is created
from outward movement of
negative bicarbonate ions into
lumen of the pancreatic ducts
◦ Positive Na ions from interstitial
fluid are drawn into pancreatic
HCL secretion moves
hydrogen ions into the
digestive tract to decrease
the pH of chyme
Gastric parietal cells move
bicarbonate ions into the
blood to increase the pH of
blood plasma
Duct cells function to
counterbalance the effects
of HCL secretion and
bicarbonate ion transport
Duct cells secrete HCO3into GI lumen to make blood
pH return to normal
Secretions that Aid in Digestion
Bile: secreted by the liver; stored and concentrated in the
◦ Lecithin and bile salts emulsify fats by encasing them in shells to
form tiny spheres called micelles
◦ Sodium bicarbonate increases pH for optimal enzyme function
◦ Cholesterol, products of detoxification, and bile pigments (e.g.,
bilirubin) are waste products excreted by the liver and eventually
eliminated in the feces
Intestinal juice: secreted by cells of intestinal exocrine cells
◦ Mucus and water lubricate and aid in continued mixing of chyme
◦ Sodium bicarbonate increases pH for optimal enzyme function
Secretions that Aid in Digestion
Salivary secretion
◦ Only reflex mechanisms control the secretion of
◦ Chemical and mechanical stimuli come from the
presence of food in the mouth
◦ Olfactory and visual stimuli come from the smell
and sight of food
Gastric Secretion
Three phases:
1. Cephalic phase
Called psychic phase because mental factors activate the
Parasympathetic fibers in branches of the vagus nerve
conduct stimulating efferent impulses to the glands
Stimulate production of gastrin (by G cells in the stomach)
2. Gastric phase
When products of protein digestion reach the pyloric
portion of the stomach, they stimulate release of gastrin
Gastrin accelerates secretion of gastric juice, ensuring
enough enzymes are present to digest food
3. Intestinal phase
Various mechanisms seem to adjust gastric secretion as
chyme passes to and through the intestinal tract
Endocrine reflexes involving gastric inhibitory peptide,
secretin, and Cholecystokinin (CCK) inhibit gastric
Stimulated by several hormones released by intestinal mucosa
◦ Secretin evokes production of pancreatic fluid low in enzyme
content but high in bicarbonate
◦ Cholecystokinin (CCK): several functions
 Causes increased exocrine secretion from the pancreas
 Opposes gastrin, thus inhibiting gastric hydrochloric acid
 Stimulates contraction of the gallbladder so that bile is ejected
into the duodenum
Bile Secretion
◦ Bile secreted continually by the liver
◦ Secretin and CCK stimulate ejection of bile from the
Intestinal secretion
◦ Little is known about how intestinal secretion is
◦ Suggested that the intestinal mucosa is stimulated
to release hormones that increase the production of
intestinal juice
◦ Involves passage of substances through the intestinal mucosa into
the blood or lymph
◦ Intestinal villi and microvilli help increase absorption by increasing
surface area
◦ Most absorption occurs in the small intestine
For some substances such as water, absorption
occurs by simple diffusion or osmosis
Secondary active transport: how sodium is
Sodium cotransport (coupled transport): how glucose
is transported
Fatty acids, monoglycerides, and cholesterol are
transported with the aid of bile salts from the lumen
to absorbing cells of the villi
Transcellular absorption moves nutrient particle
through cells (as described above), and paracellular
absorption moves particles between cells
After food is absorbed, it travels to the liver by the
portal system
Form of secondary active
Each involves two carriers
Mechanism involves a sodium
gradient (basal side), which
permits passive transport of
sodium and another molecule
into the GI lumen via a passive
carrier molecule on the luminal
side of the cell
Small intestine
Amount of absorption of
some substances varies
depending on the site of
◦ Ex. Alcohol
 20% in stomach
 80% in small intestine
Definition: the expulsion of feces from
the digestive tract
◦ Act of expelling feces is called
 Defecation occurs as a result of a reflex
brought about by stimulation of
receptors in the rectal mucosa that is
produced when the rectum is distended
Constipation: contents of the lower part
of the colon and rectum move at a slower
than normal rate; extra water is absorbed
from the feces, resulting in a hardened
Diarrhea: result of increased motility of
the small intestine, causing decreased
absorption of water and electrolytes and
a watery stool
Primary contribution of the digestive system
to overall homeostasis is to provide a
constant nutrient concentration in the
internal environment
Secondary roles of digestive system
◦ Absorption of nutrients
◦ Teeth and tongue, along with respiratory and
nervous system, are important in producing spoken
◦ Gastric acids help the immune system by destroying
potentially harmful bacteria
Digestion – The Big Picture
To accomplish its functions, the digestive system
needs other system contributions
◦ Regulation of digestive motility and secretion requires the
nervous system and endocrine system
◦ Oxygen for digestive activity depends on proper
functioning of the respiratory and circulatory systems
◦ Integumentary and skeletal systems support and protect
the digestive organs
◦ Muscular system is needed for ingestion, mastication,
deglutition, and defecation to occur normally