Download Ch. 24 – The Digestive System

Ch. 24 – The Digestive System
• The overall idea is to obtain
nutrients from the environment
• There are 2 general types of
digestive organs involved:
– 1. Parts of the digestive tract
(= a long muscular tube from
the mouth to the anus)
• This is a.k.a. the gastrointestinal
(GI) tract or alimentary canal
• It’s made up of the oral cavity
(mouth), pharynx (throat),
esophagus, stomach, small
intestine, and large intestine
• Food passes through these
– 2. Accessory organs = the teeth,
tongue, salivary glands, liver,
gallbladder, and pancreas
• Food does not pass through these
Fig. 24-1, p. 882
The general functions of the digestive system
• 1. Ingestion = eating
• 2. Mechanical processing (e.g. by the
teeth and stomach) = crushing,
shearing, and mixing
– This increases the surface area on food
particles for attack by enzymes
– Smaller pieces are easier to move
• 3. Digestion = chemical breakdown
(usually by enzymes)
– Proteins → amino acids
– Triglycerides → glycerol + fatty acids
– Polysaccharides → monosaccharides
• 4. Secretion of water, acids,
enzymes, buffers, and ions
• 5. Absorption of nutrients across
the digestive tract epithelium, and
eventually into the blood
• 6. Excretion = the removal of waste
(via defecation)
Fig. 24-1, p. 882
1
An overview of specific digestive organ functions
Fig. 24-1, p. 882
The
peritoneum
and
mesenteries
•
•
We first learned about the
parietal peritoneum, visceral
peritoneum (serosa), and peritoneal
cavity in BIOL& 242
Mesenteries = double sheets of
peritoneal membrane that connect
the parietal peritoneum to the visceral
peritoneum
– Function: stabilize and provide a route
for N.A.V.a.L to the abdominopelvic
organs
Fig. 24-2bd, p. 884
2
The general histology of the GI tract
• Most parts of the tube have most (if not all) of these features
• Additional details are found in the textbook and on p. 24 of
the Lab Manual
Lumen (interior)
Exterior
Fig. 24-3, p. 885
Fig. 24-4, p. 887
The movement of
digestive materials
• The muscular layers of the GI tract
have gap junctions between cells
and pacesetter cells that
spontaneously depolarize
– They can also be stimulated
autonomically by the CNS and ENS
(see the next slide)
• Peristalsis = waves of rhythmic
contractions that propel the bolus
(= a soft compacted mass of
digestive materials) forward through
the tract
• Segmentation (not shown here) =
irregular contractions in the
intestines that mix/churn/fragment
the bolus
3
The
regulation
of digestive
activities
These short reflexes are part of the enteric
nervous system (ENS), the lesser known third
division of the ANS
E.g. prostaglandins
and histamine
Fig. 24-5, p. 888
E.g. gastrin, secretin, etc. We’ll
discuss some of the main ones
later in the chapter.
The oral (buccal) cavity and tongue
•
•
The oral (buccal) cavity (mouth) is lined by the oral mucosa, which has a
protective stratified squamous epithelium (some parts of it are keratinized)
Functions of the tongue include:
–
–
–
–
Mechanical processing and manipulation of food for chewing and swallowing
Sensation (taste, touch, and temperature)
Secretion of mucins (= mucus glycoproteins)
Secretion of lingual lipase, which initiates the digestion of fats
Fig. 24-6, p. 889
4
Fig. 24-7, p. 891
•
•
•
•
Salivary glands
There are 3 pairs: parotid, sublingual, and
submandibular
Functions of saliva: moisten and lubricate food, rinse/flush the mouth,
dissolve chemicals for taste bud stimulation, initiate the chemical digestion
of complex carbos (by salivary amylase)
Composition: saliva is 99.4% water; the rest is solutes, including ions,
salivary amylase, buffers (so pH ~ 7.0), waste products, IgA antibodies,
lysozyme, and mucus (which helps with the lubrication of food)
Secretion is stimulated by:
–
–
–
–
Parasympathetic activity
The smell, sight, or sound of food
Food in the mouth, the taste of food, and chewing
Stomach or small intestine irritation (in order to dilute, rinse, or buffer the
unpleasant stimulus)
• Periodontal ligament =
– Dense connective tissue
(CT) that, along with
cementum, anchors the
tooth in its alveolus (bony
tooth socket)
Teeth
• Function: mechanical processing
– Mastication = chewing
• Tooth composition:
– Dentin = the bulk of
a tooth
• It’s similar to bone
(but it’s acellular)
– The pulp cavity
within the crown
• Contains pulp
(= N.A.V.a.L.), which
exits via the root
canal and apical
foramen
– Enamel (the
hardest biologicallymade substance)
• = crystalline calcium
phosphate
• Covers and protects
the dentin of the crown
Fig. 24-8a, p. 892
5
Fig. 24-9, p. 894
Types
of teeth
•
Deciduous (primary) teeth = 20;
typically appear between 6-24 months of
age
– Per quadrant: 2 incisors, 1 cuspid
(canine), and 2 molars
•
Permanent (secondary) teeth = 32;
most appear between 6-12 years of age
– Per quadrant: 2 incisors, 1 cuspid
(canine), 2 bicuspids (premolars),
and 3 molars
Fig. 24-8b, p. 892
•
•
•
= a common passageway for
solid food, liquids, and air
Has underlying skeletal muscle
for swallowing
Has 3 regions:
The pharynx (throat)
– 1. The nasopharynx
(respiratory only) – superior to
the uvula of the soft palate
– 2. The oropharynx
(respiratory and
digestive) – from the
uvula to the base of the
tongue (or tip of the
epiglottis)
– 3. The laryngopharynx
(respiratory and digestive)
– from the base of the
tongue to the junction
of the esophagus and
larynx
•
The oropharynx and
laryngopharynx are lined and
protected by a protective
stratified squamous epithelium
Fig. 24-6a, p. 889
6
The esophagus
Fig. 24-10, p. 895
•
•
= a hollow muscular tube that functions to transport solid food and liquids
from the pharynx to the stomach
Histology notes:
– The mucosa includes a protective stratified squamous epithelium
– The submucosa contains esophageal (mucous) glands for lubrication of the
bolus
• The mucosa and submucosa are folded when the esophagus is empty, allowing for
expansion during swallowing
– The muscularis externa contains a mixture of skeletal muscle and smooth
muscle
– Adventitia (not serosa) = fibrous CT for attachment
Swallowing (deglutition)
• The entire process takes ~ 9
seconds for a typical bolus
• It consists of 3 phases:
– 1. The buccal phase – the tongue
moves the bolus from the oral cavity to
the oropharynx, and the soft palate
and uvula move upward (to seal off the
nasopharynx)
– 2. The pharyngeal phase – the bolus
stimulates receptors in the posterior
oropharynx, which triggers the
swallowing reflex:
• The larynx moves up, the epiglottis
folds over and covers the superior
opening of the larynx (directing the
bolus toward the esophagus), and
there is inhibition of the respiratory
centers (breathing stops for less than a
second)
– 3. The esophageal phase – peristalsis
pushes the bolus toward the
stomach
Fig. 24-11, p. 896
7
Stomach terminology
• Fundus = the dome-shaped region superior to the esophageal
opening
• Body = the main central region
• Pylorus = the region that connects with the duodenum via the
pyloric sphincter (a sphincter is a circular muscular valve)
• Rugae = folds of
mucosa that allow
the stomach to
stretch
• Chyme =
a viscous,
acidic,
soupy
mix of
partially
digested
food and
gastric juice
Fig. 24-12b, p. 898
Functions of the stomach
•
•
•
•
Temporary storage of ingested food
Mechanical processing
Chemical digestion via acid and enzymes
Production of intrinsic factor = a glycoprotein needed for the
intestinal absorption of vitamin B12
Fig. 24-12a, p. 898
8
Note that the:
•
•
Mucosa is
folded into
gastric pits
that open
into deeper
gastric
glands
Muscularis
externa has 3
layers of
smooth
muscle
Histology of
the stomach
lining
– An inner
oblique
layer is
present in
addition to
the circular
and
longitudinal
layers
Fig. 24-13a, p. 899
The stomach mucosa
• Gastric pits and glands contain the
following cells (which together
secrete gastric juice that mixes
with ingested food to form chyme):
Under the ‘scope in lab:
Parietal cells = “fried eggs”; central
round nucleus
Chief cells = not “fried eggs”; grainy; in
clusters with nuclei toward the outer
edge of the cluster; more numerous in
the deeper region of the gastric glands
– Parietal cells secrete HCl (hydrochloric
acid) and intrinsic factor
– Chief cells secrete the inactive
proenzyme pepsinogen…
• Which is converted by HCl to the active
enzyme pepsin, which begins protein
digestion
– Mucous cells secrete an alkaline
mucus (to protect the stomach’s
epithelium from the HCl)
– G cells secrete gastrin (a hormone),
which:
• ↑ Stomach motility
• ↑ The secretion of gastric juice
Fig. 24-13b, p. 899
9
HCl secretion by parietal cells
•
•
Keeps the stomach contents at a pH of about 1.5-2.0
Functions:
–
–
–
–
Kill most microbes
Denature the proteins (including enzymes) in food
Help break down plant materials and the CT in meat
Help convert inactive pepsinogen to active pepsin
Fig. 24-14, p. 900
Regulation of gastric activity
• Gastric acid and enzyme secretion can be
controlled/regulated by:
– 1. The CNS
– 2. Short reflexes via the stomach wall
• These are part of the enteric nervous system (ENS)
– 3. Hormones secreted by the GI tract
• There are 3 specific phases of gastric secretion:
– 1. The cephalic phase
– 2. The gastric phase
– 3. The intestinal phase
On the next two slides, for each phase, note the differences with respect to
the stimulus (what causes the phase to begin), some of the key responses,
and the overall function
10
The 3
phases of
gastric
secretion
(continued on the next
slide…)
Fig. 24-15, p. 902
Fig. 24-15, p. 903
The 3
phases
of gastric
secretion
Additional notes:
•
(continued from the
previous slide)
•
CCK, GIP, and secretin also
target the pancreas; CCK
also targets the gallbladder;
secretin also targets the liver
(more details later)
There are also two central
gastric reflexes (described
above) that stimulate activity
of the small intestine
11
Digestion and absorption
in the stomach
Digestion
• The digestion of carbohydrates and lipids (by
salivary amylase and lingual lipase, respectively)
continues until the pH of the stomach contents falls
below 4.5
• The digestion of proteins by pepsin begins
– This is the preliminary digestion of proteins (small
polypeptides and peptides)…further breakdown into
individual amino acids will be completed in the small
intestine
Absorption
• No nutrient absorption occurs in the stomach, but
there is some absorption of alcohol, aspirin, and
other lipid-soluble drugs
The small intestine
•
•
Is the site of most of the chemical digestion and nutrient absorption that occurs in
the GI tract
Has 3 segments:
– 1. Duodenum – has duodenal (Brunner’s or submucosal) glands in the submucosa
that secrete an alkaline mucus to help neutralize incoming chyme
– 2. Jejunum
– 3. Ileum – has many Peyer’s patches (lymphoid tissue) that protect the small intestine
from the large intestine’s resident bacteria
• It connects to the cecum of the large intestine at the ileocecal valve (not shown here)
Fig. 24-16, p. 904
12
Small intestine histology
•
The wall of the small intestine is modified
to increase surface area for secretion and
absorption; there are 3 levels of folding:
– 1. Circular folds (plicae circulares) =
relatively large (macroscopic), transverse,
permanent folds
• These folds contain submucosa (which
is useful for lab slides)
– 2. Intestinal villi = fingerlike
projections of
mucosa
• They contain
lacteals
(= lymphatic
capillaries),
which absorb fatty
acids, and transport
them as chylomicrons (lipid +
protein) to the blood
via the lymphatic
system
Fig. 24-17ab, p. 905
Small intestine
histology
– 3. Microvilli = microscopic projections
of the epithelial plasma (cell) membrane
(they’re so tiny that they’re barely visible
in this figure)
Fig. 24-17cd, p. 905
• They form the brush border and contain
brush border (digestive) enzymes
• Thanks to the circular folds, villi, and
microvilli, the total increase in
surface area (for digestion and
absorption) compared to the wall of a
simple, unfolded tube with smooth
walls is 600X! (~ 2200 ft2 total)
13
•
Intestinal glands
Are a.k.a. intestinal crypts or crypts of Lieberkuhn, and contain…
– Mucous cells that, like duodenal glands, secrete an alkaline mucus to help
neutralize incoming chyme
– Stem cells that replace the epithelial cells that are shed into the lumen
– Enteroendocrine cells that secrete gastrin, GIP, secretin, and CCK
•
Intestinal juice consists of water (due to osmosis from the mucosa to the
relatively concentrated chyme in the lumen), plus the secretions from the
intestinal glands
Fig. 24-17b, p. 905
Fig. 24-18a, p. 908
The
pancreas
•
•
Is a mixed gland; i.e.,
it has both exocrine
and endocrine
functions
Exocrine ducts:
– The pancreatic
duct (duct of
Wirsung) joins the
common bile duct
and empties into the
duodenum at the
duodenal papilla
– In 3-10% of the
population, the
pancreatic duct
branches to form the
accessory
pancreatic duct
(duct of Santorini),
which also empties
into the duodenum
14
Pancreatic
histology
• 1. The exocrine portion =
acini and ducts (= ~ 99%
of the pancreas)
– The acini secrete digestive
enzymes
– Duct cells secrete water and
buffers (which help neutralize
chyme)
Fig. 24-18b, p. 908
• Pancreatic juice = an
alkaline mixture of the
enzymes, H20, and buffers
• 2. The endocrine portion =
pancreatic islets (islets of
Langerhans)
– These secrete (mostly) the
hormones glucagon and
insulin
More on pancreatic juice
• Its secretion is controlled by the vagus nerve (CN X) as well
as duodenal hormones (which are released when chyme
enters the duodenum), especially…
– Secretin, which targets pancreatic duct cells to secrete water and
buffers
– Cholecystokinin (CCK), which targets pancreatic acini to secrete
pancreatic enzymes
• Its enzymes include (see Table 24-1 for much more FYI
detail)…
–
–
–
–
Pancreatic alpha-amylase (a carbohydrase)
Pancreatic lipase
Nucleases
Proteolytic enzymes (= proteases and peptidases)
• These are secreted as inactive proenzymes (to protect the cells of the
pancreas itself)
– E.g. trypsinogen, chymotrypsinogen, procarboxypeptidase, and proelastase
are converted into active trypsin, chymotrypsin, carboxypeptidase, and
elastase, respectively
15
The liver
• = the largest visceral organ in the body
• Has 4 lobes (the right and left are the largest ones)
• The falciform ligament separates the right and left
lobes
• The round ligament = a thickening in the falciform
ligament that is the remnant of the fetal umbilical vein
Fig. 24-19, p. 910
•
•
The liver is divided by CT into
roughly six-sided lobules
Portal areas (portal or
hepatic triads) at the corners
of each lobule contain
branches of the:
Liver histology
– 1. Hepatic portal vein
– 2. Hepatic artery proper
– 3. Bile ducts
•
Hepatocytes (= liver cells)
surround sinusoids (= large
leaky capillaries)
– Arterial and portal blood
empties into the sinusoids
– The sinusoids empty into the
central veins
•
Kupffer cells = fixed
macrophages…
– Which phagocytize worn out
RBCs, bacteria, debris, etc.
•
Hepatocytes secrete bile into
the bile duct system (part of
which is shown here in green),
which carries bile out of and
away from the liver
Fig. 24-20ab, p. 911
16
Some functions of the liver
It has over 200 functions! Here are just a few…
1. Metabolic regulation
– Carbohydrate metabolism (e.g. glycogenolysis, gluconeogenesis, and
conversion into lipids)
– Lipid metabolism (regulation of blood levels of triglycerides, fatty
acids, and cholesterol)
– Amino acid metabolism (e.g. protein synthesis and conversion into
lipids or glucose)
– Waste removal (e.g. conversion of ammonia → less harmful urea)
– Vitamin storage (B12 and fat-soluble vitamins A, D, E, and K)
– Mineral storage (iron)
– Drug inactivation
2. Hematological regulation
–
–
–
–
Phagocytosis and antigen presentation (by Kupffer cells)
Synthesis of plasma proteins
Removal of circulating hormones and antibodies
Removal (or storage) of toxins
3. The synthesis and secretion of bile (see the next slide for
the functions of bile)
Bile
• Contains:
– Mostly water, and minor
amounts of cholesterol,
bile salts (derived from
cholesterol), pigments
(bilirubin), and ions
• Functions:
– The emulsification of fats
• Bile salts have hydrophilic and hydrophobic ends, so they surround large
fat globules and break them into small lipid droplets in a watery
environment
– This increases the surface area for digestion by lipases, and aids absorption
– The excretion of bilirubin
• Remember (see Ch. 19), bilirubin is a pigment derived from the heme of
hemoglobin
• The bilirubin in bile is ultimately converted to urobilins and stercobilins,
which are eliminated in feces
• Note: > 90% of the bile salts in bile are reabsorbed (mostly
by the ileum) for recycling back to the liver
17
The gallbladder
Fig. 24-21, p. 912
Functions:
•
•
•
1. Bile storage
2. Bile modification – via the reabsorption of
water, concentrating the bile
3. Bile release – the presence of chyme in the
duodenum (especially if the chyme is lipid-rich)
causes the release of CCK, which causes the
hepatopancreatic sphincter (sphincter of
Oddi) to relax and the gallbladder to
contract
The main
actions of
the major
digestive
tract
hormones
Fig. 24-23, p. 916
18
The
large
intestine
•
•
Consists of the cecum, colon, and rectum
Connects with the ileum of the small
intestine via the ileocecal valve (which is
a sphincter)
Fig. 24-24a, p. 918
•
•
•
•
Histology of the large intestine
No villi are present, and no digestive enzymes are produced
The mucous (goblet) cells of the intestinal glands (crypts) produce mucus for lubrication
Lymphoid nodules are present in the mucosa and submucosa
The outer, longitudinal layer of the muscularis externa has been reduced to 3 distinct bands
called taeniae coli; the muscle tone of these bands creates haustra (= pouches)
Fig. 24-25, p. 919
19
Physiology of the large intestine
• Functions of the large intestine include:
– Absorption of:
• Water (forming feces)
• Ions (electrolytes)
• Vitamins produced by normal resident bacteria (e.g. vitamin K,
biotin, and vitamin B5)
• Bile salts
• Certain organic waste products and bacterial toxins
– Storage of feces prior to defecation
• Movements of the large intestine include:
– Peristalsis (which is typically slow)
– Segmentation movements (= haustral churning)
– Mass movements = powerful peristalsis a few times a day
• The stimulus = the stretch of the stomach or duodenum
• These move feces from the transverse colon onward into the rectum
– Defecation (see the next slide)
The defecation
reflex
•
•
•
Fig. 24-26 p. 921
The internal anal
sphincter is made of
smooth muscle
(involuntary)
The external anal
sphincter is made of
skeletal muscle
(voluntary)
The distal anal canal is
lined by a protective
stratified squamous
epithelium
Fig. 24-24c, p. 918
20
Carbohydrate
digestion
and absorption
• Amylases break down complex
carbohydrates into trisaccharides
and disaccharides
• Brush border enzymes break
down trisaccharides and
disaccharides into
monosaccharides (simple
sugars), such as glucose and
fructose
• Monosaccharides are absorbed
via facilitated diffusion and Na+linked cotransport
Fig. 24-27, p. 923
Lipid digestion
and absorption
•
•
•
•
•
Bile salts in the small intestine emulsify fats
(triglycerides)
Lipases break down triglycerides into
monoglycerides and fatty acids
Bile salts interact with monoglycerides and
fatty acids to form tiny micelles, which aid
absorption (via simple diffusion)
Monoglycerides and
fatty acids are
reassembled into
triglycerides and
coated with
proteins to form
chylomicrons
Chylomicrons are
secreted via
exocytosis
and diffuse
into lacteals
Fig. 24-27, p. 923
21
Protein digestion
and absorption
• Is complex and time-consuming
• Each protease targets specific
types of peptide bonds and
breaks down polypeptides into
smaller peptides
• Peptidases (including brush
border enzymes) break down
small peptides into individual
amino acids
• The mechanism of amino acid
absorption is similar to that of
monosaccharides (refer back 2
slides)
Fig. 24-27, p. 923
The secretion and
absorption of water
• Fun fact: you lose only ~ 2%
of the total water in the GI
tract to the feces!
– Total water ingested +
secretions = 9200 mL
– Water lost in feces = 150 mL
• Most absorption of water
occurs in the small intestine
• Water is absorbed
osmotically following the
absorption of solutes (e.g.
nutrients and ions)
– See Table 24-2 for more FYI
info on the absorption of
vitamins and ions
Fig. 24-28, p. 926
22