Download The Digestive System

The Digestive System
• Unlike plants, which can form organic molecules
using inorganic compounds such as carbon
dioxide, water, and ammonia, humans and other
animals must obtain their basic organic
molecules from food.
Nutrition
• Nutrient: substance in food used to promote
growth, maintenance, and repair
• Major nutrients:
• Carbohydrates – sugars & starches
• Lipids – saturated/unsaturated fats
• Proteins – eggs, milk, meat (complete – all
AA); legumes, nuts, cereals (incomplete)
• Vitamins – A, B, C, E, D, K
• Minerals – Ca, P, K, S, Na, Cl, Mg
The digestive process is hydrolysis of
nutrients to monomers and their
subsequent absorption
• Within the lumen of the gastrointestinal tract, large
food molecules are hydrolyzed into their monomers
(subunits).
• These monomers pass through the inner layer, or
mucosa, of the small intestine to enter the blood or
lymph in a process called absorption.
The digestion of food molecules through
hydrolysis reactions.
The digestion of food molecules through
hydrolysis reactions.
The digestion of food molecules through
hydrolysis reactions.
•The Digestion
of Carbohydrates
• Salivary amylase:
• Begins starch digestion.
• Pancreatic amylase:
• Digests starch to
oligosaccharides.
• Oligosaccharides hydrolyzed by
brush border enzymes.
• Glucose is transported by
secondary active transport with
Na+ into the capillaries.
•The Digestion of Lipids
• Arrival of lipids in the duodenum serves
as a stimulus for secretion of bile.
• Emulsification:
• Bile salt micelles are secreted into
duodenum to break up fat droplets.
• Pancreatic lipase and colipase hydrolyze
triglycerides to free fatty acids and
monglycerides.
• Colipase coats the emulsification
droplets and anchors the lipase
enzyme to them.
• Form micelles and move to brush
border.
•The Digestion of Proteins
• Digestion begins in the stomach when
pepsin digests proteins to form
polypeptides.
• In the duodenum and jejunum:
• Endopeptidases cleave peptide bonds in
the interior of the polypeptide:
• Trypsin.
• Chymotrypsin.
• Elastase.
• Exopeptidases cleave peptide bonds from
the ends of the polypeptide:
• Carboxypeptidase.
• Aminopeptidase.
To implement the digestive process, there are
three main functions of the gastrointestinal tract.
•Motility
•Secretion
•Absorption
Functions of the GI Tract
 Motility propels ingested food from the mouth
toward the rectum and mixes and reduces the size
of the food.
 The rate at which food is propelled through the
gastrointestinal tract is regulated to optimize the
time for digestion and absorption.
Functions of the GI Tract

Motility:
• Ingestion:
• Taking food into the mouth.
• Mastication:
• Chewing the food and mixing it with saliva.
• Deglutition:
• Swallowing the food.
• Peristalsis:
• Rhythmic wave-like contractions that move food through GI tract.
• Segmentation:
• churns material in the small intestine
• Defecation:
• eliminate indigestible residues (feces)
Contraction of circular
muscles behind food mass
The second component of stereotyped
peristaltic behavior is contraction of the
circular muscle in the segment behind
the advancing intraluminal contents.
The longitudinal muscle layer in this
segment relaxes simultaneously with
contraction of the circular muscle,
resulting in the conversion of this region
to a propulsive segment that propels the
luminal contents ahead, into the
receiving segment.
Longitudinal
muscle
Circular
muscle
Contraction
From
mouth
To
anus
• During peristalsis, the longitudinal muscle layer
in the segment ahead of the advancing
intraluminal contents contracts while the
circular muscle layer simultaneously relaxes.
• The intestinal tube behaves like a cylinder with
constant surface area.
• The shortening of the longitudinal axis of the
cylinder is accompanied by a widening of the
cross-sectional diameter.
• The simultaneous shortening of the longitudinal
muscle and relaxation of the circular muscle
results in expansion of the lumen, which
prepares a receiving segment for the forwardmoving intraluminal contents during peristalsis.
Contraction of circular
muscles behind food mass
Longitudinal
muscle
Circular
muscle
Contraction
From
mouth
To
anus
Contraction of longitudinal
muscles ahead of food mass
Contraction
Contraction
Contraction of circular
muscles behind food mass
Longitudinal
muscle
Circular
muscle
Intestinal segments ahead of
the advancing front become
receiving segments and then
propulsive segments in
succession as the peristaltic
complex of propulsive and
receiving segments travels
along the intestine.
Contraction
From
mouth
To
anus
Contraction of longitudinal
muscles ahead of food mass
Contraction
Contraction
Contraction of circular muscle
layer forces food mass forward
Mixing movements.
The segmentation pattern of motility is
characteristic of the digestive state.
Propulsive segments separated by
receiving segments occur
randomly at many sites along the
small intestine.
Mixing of the luminal contents
occurs in the receiving segments.
Receiving segments convert to
propulsive segments, while
propulsive segments become
receiving segments.
Functions of the GI Tract
•Secretion:
• Includes both exocrine and endocrine secretions.
• Exocrine:
• HCl, H20, HC03-, bile, lipase, pepsin, amylase, trypsin,
elastase, and histamine are secreted into the lumen of the
GI tract.
• Endocrine:
• Stomach and small intestine secrete hormones to help
regulate the GI system.
• Gastrin, secretin, CCK, GIP, GLP-1, guanylin, VIP, and
somatostatin.
Functions of the GI Tract
• Absorption:
• Process of the passage of digestion (chemical
subunits) into the blood or lymph.
Digestive System (GI)
• GI tract divided into:
• Alimentary canal.
• Accessory digestive
organs.
• GI tract is 30 ft long and
extends from mouth to
anus.
Insert fig. 18.2
Anatomy
Alimentary canal
Gastrointestinal (GI) tract
Mouth  pharynx  esophagus  stomach 
small intestine  large intestine
Accessory digestive organs
Teeth, tongue, digestive glands
• What is the Histological Organization—Four
Layers?
• Mucosa
• Epithelium
• Submucosa
• Connective tissue, nerves, blood vessels
• Muscularis externa
• Serosa (in peritoneal cavity)
Mucosa
• Lines the lumen of GI tract.
• Consists of simple columnar epithelium.
Mucosa
• Lamina propria:
• Thin layer of connective tissue containing lymph nodules.
Mucosa
• Muscularis mucosae:
• Thin layer of smooth muscle responsible for the folds.
• Folds increase surface area for absorption.
Mucosa
• Goblet cells:
• Secrete mucus.
Submucosa
• Thick, highly vascular layer of connective tissue.
• Absorbed molecules enter the blood and lymphatic vessels.
• Submucosal plexus (Meissner’s plexus):
• Provide autonomic nerve supply to the muscularis mucosae.
Muscularis
• Responsible for segmental contractions and peristaltic movement through the GI tract.
• Inner circular layer of smooth muscle.
• Outer longitudinal layer of smooth muscle.
• Contractions of these layers move food through the tract; pulverize and mix the food.
Muscularis
• Myenteric plexus located between the 2 muscle layers.
• Major nerve supply to GI tract.
• Fibers and ganglia from both sympathetic and parasympathetic nervous systems.
Serosa
• Binding and protective outer layer.
• Consists of areolar connective tissue covered with
simple squamous epithelium.
Regulation of the GI Tract
• Extrinsic innervation:
• Parasympathetic nervous system:
• Vagus and spinal nerves:
• Stimulate motility and GI secretions.
• Sympathetic nervous system:
• Postganglionic sympathetic fibers that pass through
submucosal and myenteric plexuses and innervate GI
tract:
• Reduce peristalsis and secretory activity.
Regulation of the GI Tract
• Enteric nervous system:
• Sites where parasympathetic fibers synapse with
postganglionic neurons that innervate smooth muscle.
• Submucosal and myenteric plexuses:
• Local regulation of the GI tract.
• Paracrine secretion:
• Molecules acting locally.
• Hormonal secretion:
• Secreted by the mucosa.
The Oral Cavity
Oral cavity: mechanical, chemical
digestion
• What are the Functions of the
Oral Cavity?
• Mechanical processing
using teeth, tongue, and
palate
• Lubrication of food by
mucus in saliva
• Enzymatic digestion by
enzymes in saliva
• Sensory analysis (taste,
texture)
The Oral Cavity
• What are the Functions of the
Tongue?
• Mechanical processing of
food
• Manipulation to assist
chewing and swallowing
• Sensory analysis (taste,
texture)
The Oral Cavity
• What are the Salivary
Glands?
• Three pairs of glands
• Parotid
• Sublingual
• Submandibular
Salivary glands: saliva
lubricates food
Saliva = mucus, salivary
amylase (starch
breakdown)
• Teeth
• Participate in
mastication of food
From Mouth to Stomach
Oral
phase
Nasopharynx
• Mastication (chewing):
• Mixes food with saliva which contains salivary
amylase.
• Enzyme that can catalyze the partial digestion of
starch.
Soft
palate
Bolus
Epiglottis
Trachea
•
What is Swallowing (Deglutition)—Three Steps?
1. Oral phase
Oral phase is voluntary.
•
Oral
phase
What is Swallowing (Deglutition)—Three Steps?
1. Oral phase
2. Pharyngeal phase
• Larynx is raised.
• Epiglottis covers the entrance to respiratory tract.
Pharyngeal
phase
Nasopharynx
Soft
palate
Bolus
Epiglottis
Trachea
Tongue
Pharyngeal and esophageal phases are
involuntary.
Cannot be stopped.
•
Oral
phase
What is Swallowing (Deglutition)—Three Steps?
1. Oral phase
2. Pharyngeal phase
• Larynx is raised.
• Epiglottis covers the entrance to respiratory tract.
3. Esophageal phase
• Peristalsis pushes bolus toward stomach
Pharyngeal
phase
Nasopharynx
Esophageal
phase
Peristalsis
Soft
palate
Bolus
Epiglottis
Trachea
Tongue
Esophagus
• Involuntary muscular contractions and relaxations in the mouth, pharynx,
larynx, and esophagus are coordinated by the swallowing center in the
medulla.
Oral
phase
Pharyngeal
phase
Nasopharynx
Esophageal
phase
Peristalsis
Soft
palate
Bolus enters
stomach
Thoracic cavity
Lower
esophageal
sphincter
Bolus
Epiglottis
Trachea
Tongue
Esophagus
Stomach Diaphragm
• Esophagus:
• Connects pharynx to the stomach.
• Upper third contains skeletal muscle.
• Middle third contains a mixture of skeletal and smooth muscle.
• Terminal portion contains only smooth muscle.
• Peristalsis:
• Produced by a series of
localized reflexes in response
to distention of wall by
bolus.
• Wave-like muscular
contractions:
• Circular smooth muscle
contract behind, relaxes in
front of the bolus.
• Followed by longitudinal
contraction (shortening) of
smooth muscle.
• Rate of 2-4 cm/sec.
• After food passes into
stomach, LES constricts.
Esophagus
Insert 18.4a
Stomach
• Most distensible part of GI
tract.
• Empties into the
duodenum.
The Stomach
• What are the Functions of the Stomach?
• Temporary storage of ingested food
• Mechanical breakdown of food
• Forms chyme, a liquid suspension
• Breakage of chemical bonds in food by action of
acid and enzymes
• Initiates digestion of proteins.
• Kills bacteria.
• Moves food (chyme) into intestine.
• What are the Regions of
the Stomach?
• Cardia
• Closest to heart
• Fundus
• “Hump” on top
• Body
• Shaped like the letter “C”
• Pylorus
• Pyloric sphincter guards the
exit
There are three components of gastric
motility:
• (1) relaxation of the orad region of the stomach to receive
the food bolus from the esophagus,
• (2) contractions that reduce the size of the bolus and mix
it with gastric secretions to initiate digestion, and
• (3) gastric emptying that propels chyme into the small
intestine.
• The rate of delivery of chyme to the small intestine is
hormonally regulated to ensure adequate time for
digestion and absorption of nutrients in the small
intestine.
Stomach
• Gastric mucosa has
gastric pits in the folds.
• Cells that line the folds
deeper in the mucosa,
are gastric glands.
Insert fig. 18.7
Stomach
• Gastric juice: converts meal to acidic chyme
• HCl: pH 2, kills bacteria, denatures proteins
• Pepsin: enzyme breaks down proteins
• Mucus = protects lining of stomach
The Stomach
• What are the Secretions of the Gastric
Glands?
• Parietal cells
• Secrete HCl (strong acid), intrinsic
factor
• Chief cells
• Produce pepsinogen, an inactive
enzyme
• HCl activates pepsinogen to pepsin
• Goblet cells
• Produce mucus
Gastric
Hormone secretion
• Enterochromaffin-like cells
(ECL): histamine and
serotonin.
• G cells: gastrin.
• D cells: somatostatin.
• Stomach: ghrelin.
HCl
Production
1 step
In intracellular fluid, carbon dioxide (CO2) produced from aerobic metabolism combines with
H2O to form H2CO3, catalyzed by carbonic anhydrase. H2CO3 dissociates into H+ and HCO3-.
The H+ is secreted with Cl- into the lumen of the stomach, and the HCO3- is absorbed into the
blood, as described in steps 2 and 3, respectively.
HCl
Production
2 step
At the apical membrane, H+ is secreted into the lumen of the stomach via the H+-K+ ATPase.
The H+-K+ ATPase is a primary active process that transports H+ and K+ against their
electrochemical gradients (uphill). H+-K+ ATPase is inhibited by the drug omeprazole View
drug information, which is used in the treatment of ulcers to reduce H+ secretion. Cl- follows
H+ into the lumen by diffusing through Cl- channels in the apical membrane.
HCl
Production
3 step
At the basolateral membrane, HCO3- is absorbed from the cell into the blood via a Cl--HCO3exchanger. The absorbed HCO3- is responsible for the "alkaline tide" (high pH) that can be
observed in gastric venous blood after a meal. Eventually, this HCO3- will be secreted back
into the gastrointestinal tract in pancreatic secretions.
HCl
Production
4 step
In combination, the events occurring at the apical and basolateral membranes of gastric
parietal cells result in net secretion of HCl and net absorption of HCO3-.
Three substances
stimulate H+ secretion
by gastric parietal cells:
ACh (a neurocrine),
histamine (a paracrine),
gastrin (a hormone).
HCl Functions
• Activates pepsinogen to
pepsin.
• Pepsin is more active at pH
of 2.0.
HCl Functions
• Makes gastric juice
very acidic.
• Denatures ingested
proteins (alter
tertiary structure) so
become more
digestible.
Insert fig. 18.9
Digestion and Absorption in the Stomach
• Proteins partially digested by pepsin.
• For a variable period of time after food arrives in
the stomach, starch continues the digestion that
began with salivary amylase.
• Alcohol and aspirin are the only commonly
ingested substances absorbed.
Regulation of Gastric Function
• Gastric motility and secretion are automatic.
• Waves of contraction are initiated spontaneously
by pacesetter cells.
• Extrinsic control of gastric function is divided into 3
phases:
• Cephalic phase.
• Gastric phase.
• Intestinal phase.
• The Phases of Gastric Secretion
• Stimulated by sight, smell, and taste of
food.
• Activation of vagus:
• Stimulates chief cells to secrete
pepsinogen.
• Directly stimulates G cells to secrete
gastrin.
• Directly stimulates ECL cells to secrete
histamine.
• Indirectly stimulates parietal cells to
secrete HCl.
• Continues into the 1st 30 min. of a
meal.
•The Phases of Gastric Secretion
• Arrival of food in stomach stimulates the
gastric phase.
• Gastric secretion stimulated by:
• Distension.
• Chemical nature of chyme (amino acids
and short polypeptides).
• Stimulates G cells to secrete gastrin.
• Stimulates chief cells to secrete
pepsinogen.
• Stimulates ECL cells to secrete
histamine.
• Histamine stimulates secretin of
HCl.
• Positive feedback effect.
• As more HCl and pepsinogen are
secreted, more polypeptides and
amino acids are released.
Gastric Phase
(continued)
• Secretion of HCl is also
regulated by a negative
feedback effect:
• HCl secretion decreases if
pH < 2.5.
• At pH of 1.0, gastrin
secretion ceases.
• D cells stimulate
secretion of
somatostatin.
• Paracrine regulator
to inhibit secretion
of gastrin.
Insert. Fig. 18.30
• The Phases of Gastric Secretion
• Inhibits gastric activity when chyme
enters the small intestine.
• Arrival of chyme increases osmolality
and distension.
• Activates sensory neurons of vagus and
produces an inhibitory neural reflex:
• Inhibits gastric motility and secretion.
• In the presence of fat, enterogasterone
inhibits gastric motility and secretion.
• Hormone secretion:
• Inhibit gastric activity:
• Somatostatin, CCK, and GLP-1.