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Chapter 24: The Digestive
System
Copyright 2009, John Wiley & Sons, Inc.
2 groups of organs compose the digestive
system


Gastrointenstinal (GI) tract or alimentary canal
– mouth, most of pharynx, esophagus,
stomach, small intestine, and large intestine
Accessory digestive organs – teeth, tongue,
salivary glands, liver, gallbladder, and
pancreas
Copyright 2009, John Wiley & Sons, Inc.
Parotid gland
(salivary gland)
Submandibular gland
(salivary gland)
Mouth (oral cavity)
contains teeth and tongue
Sublingual gland
(salivary gland)
Pharynx
Esophagus
Liver
Duodenum
Gallbladder
Jejunum
Ascending colon
Ileum
Cecum
Appendix
Stomach
Pancreas
Transverse colon
Descending colon
Sigmoid colon
Rectum
Anal canal
Anus
(a) Right lateral view of head and neck and anterior view of trunk
6 functions of the digestive system
1.
2.
3.
4.
Ingestion
Secretion of water, acid, buffers, and
enzymes into lumen
Mixing and propulsion
Digestion


5.
6.
Mechanical digestion churns food
Chemical digestion – hydrolysis
Absorption – passing into blood or lymph
Defecation – elimination of feces
Copyright 2009, John Wiley & Sons, Inc.
Layers of the GI tract
Wall of GI tract from lower esophagus to anal
canal has same basic 4 layers
Mucosa – inner lining

1.



Epithelium protection, secretion, absorption
Lamina propria – connective tissue with blood and
lymphatic vessels and mucosa-associated lymphatic
tissue (MALT)
Muscularis mucosae – thin layer of smooth muscle
making folds to increase surface area
Submucosa
2.



Connective tissue binding mucosa to muscularis
Contains many blood and lymphatic vessels
Submucosal plexus
Copyright 2009, John Wiley & Sons, Inc.
Layers of the GI tract
3.
Muscularis


Voluntary skeletal muscle found in mouth, pharynx, upper
2/3 of esophagus, and anal sphincter
Involuntary smooth muscle elsewhere


4.
Arranged in inner circular fibers and outer longitudinal fibers
Myenteric plexus between muscle layers
Serosa



Outermost covering of organs suspended in
abdominopelvic cavity
Also called visceral peritoneum
Esophagus lacks serosa – has adventitia
Copyright 2009, John Wiley & Sons, Inc.
Duct of gland outside
tract (such as
pancreas)
Submucosal plexus
(plexus of Meissner)
Gland in
mucosa
Mucosa-associated
lymphatic tissue
(MALT)
Lumen
Mesentery
Vein
Glands in
submucosa
Artery
Nerve
MUCOSA:
Epithelium
Lamina propria
Muscularis
mucosae
Myenteric plexus
(plexus of Auerbach)
SUBMUCOSA
MUSCULARIS:
Circular muscle
Longitudinal muscle
SEROSA:
Areolar connective tissue
Epithelium
Neural innervation

Enteric nervous system (ENS)
 Intrinsic set of nerves - “brain of gut”
 Neurons extending from esophagus to anus
 2 plexuses



Myenteric plexus – GI tract motility
Submucosal plexus – controlling secretions
Autonomic nervous system
 Extrinsic set of nerves
 Parasympathetic stimulation increases secretion and
activity by stimulating ENS
 Sympathetic stimulation decreases secretions and
activity by inhibiting ENS
Copyright 2009, John Wiley & Sons, Inc.
ENTERIC NERVOUS SYSTEM
To ANS and
CNS neurons
Myenteric plexus
Interneuron
Submucosal plexus
Motor neuron
Motor neuron
Longitudinal and
circular smooth muscle
layers of the muscularis
Sensory neuron
Mucosal epithelium
Peritoneum


Largest serous membrane of the body
Divided into


Parietal peritoneum – lines wall of cavity
Visceral peritoneum – covers some organs



Also called serosa
Space between is peritoneal cavity
5 major peritoneal folds


Greater omentum, falciform ligament, lesser omentum,
mesentery, and mesocolon
Weave between viscera binding organs together
Copyright 2009, John Wiley & Sons, Inc.
Peritoneum Cont.




Greater omentum – largest “fatty apron”
Falciform ligament – attaches liver to anterior abdominal wall
Lesser omentum – suspends stomach and duodenum from
liver – passage for hepatic portal vein and artery and common
bile duct
Next two loosely hold the intestines allowing for movement.


Mesentery – bind jejunum and ilieum of Small Intestines to
posterior abdominal wall
Mesocolon - bind transverse and sigmoid colon to posterior
abdominal wall
Copyright 2009, John Wiley & Sons, Inc.
Diaphragm
Liver
LESSER OMENTUM
Pancreas
Stomach
MESOCOLON
Duodenum
MESENTERY
Transverse colon
GREATER OMENTUM
Jejunum
Ileum
PARIETAL PERITONEUM
Sigmoid colon
Uterus
VISCERAL PERITONEUM
Urinary bladder
PERITONEAL CAVITY
Rectum
Pubic symphysis
POSTERIOR
ANTERIOR
(a) Midsagittal section showing the peritoneal folds
Midsagittal
plane
FALCIFORM LIGAMENT
Liver
Stomach
Transverse colon
GREATER OMENTUM
Urinary bladder
(b) Anterior view
Gallbladder (reflected
upward)
LESSER OMENTUM
Liver
(reflected upward)
Stomach
Duodenum
Transverse
colon
Descending colon
Ascending
colon
Sigmoid colon
(c) Lesser omentum, anterior view
(liver and gallbladder lifted)
GREATER OMENTUM
(reflected upward)
Transverse colon
Jejunum (pulled laterally)
MESENTERY
Descending colon
Ileum (pulled laterally)
Sigmoid colon
Urinary bladder
(d) Anterior view (greater omentum lifted
and small intestine reflected to right side)
Mouth




Oral or buccal cavity
Formed by cheeks, hard and soft palates, and
tongue
Oral cavity proper, is a space that extends from
gums and teeth to fauces (opening between oral
cavity and oropharynx)
Salivary glands release saliva



Ordinarily, just enough is secreted to keep mouth and
pharynx moist and clean
When food enters mouth, secretion increases to lubricate,
dissolve and begin chemical digestion
3 pairs of major salivary glands secrete most of the saliva

Parotid, submandibular, and sublingual
Copyright 2009, John Wiley & Sons, Inc.
Superior lip (lifted upward)
Superior labial frenulum
Gingivae (gums)
Palatoglossal arch
Hard palate
Fauces
Soft palate
Palatopharyngeal arch
Uvula
Cheek
Palatine tonsil
(between the arches)
Molars
Tongue (lifted upward)
Lingual frenulum
Opening of duct of
submandibular gland
Premolars
Cuspid (canine)
Incisors
Oral vestibule
Gingivae (gums)
Inferior labial frenulum
Inferior lip (pulled down)
Anterior view
Central incisor(8–12 mo.)
Lateral incisor(12–24 mo.)
Cuspid or canine (16–24 mo.)
D E FG
H
C
B
I
A
J
First molar (12–16 mo.)
Second molar (24–32 mo.)
Upper Teeth
Lower Teeth
Second molar (24–32 mo.)
T
K
S
L
R
M
Q P ON
First molar (12–16 mo.)
Cuspid or canine (16–24 mo.)
Lateral incisor(12–15 mo.)
Central incisor(6–8 mo.)
(a) Deciduous (primary) dentition; teeth are designated by letters
(with times of eruption)
Central incisor (7–8 yr.)
Lateral incisor(8–9 yr.)
7 8
6
5
4
Cuspid or canine (11–12 yr.)
9 1011
12
13
3
First premolar or bicuspid (9–10 yr.)
Second premolar or bicuspid
(10–12 yr.)
14
2
15
Upper Teeth
1
First molar (6–7 yr.)
16
Second molar (12–13 yr.)
Third molar or wisdom tooth
(17–21 yr.)
Lower Teeth
32
17
31
Third molar or wisdom tooth
(17–21 yr.)
18
30
Second molar (11–13 yr.)
29
28
2726
19
First molar (6–7 yr.)
20
21
Second premolar or bicuspid
(11–12 yr.)
22
252423
First premolar or bicuspid (9–10 yr.)
(b) Permanent (secondary)
dentition; teeth are designated by
numbers (with times of eruption)
Cuspid or canine (9–10 yr.)
Lateral incisor(7–8 yr.)
Central incisor(7–8 yr.)
Parotid duct
Zygomatic arch
PAROTID GLAND
Opening of parotid duct
(near second maxillary molar)
Second maxillary molar tooth
Tongue (raised in mouth)
Lingual frenulum
Sublingual ducts
Lesser sublingual duct
Submandibular duct
SUBMANDIBULAR GLAND
SUBLINGUAL GLAND
Mylohyoid muscle
(a) Location of salivary glands
The three major
salivary glandsparotid, sublingual,
and submandibular
Saliva

Saliva
 Mostly water 99.5%
 0.5% solutes – ions, dissolved gases, urea, uric acid,
mucus, immunoglobulin A, lysozyme, and

salivary amylase (acts on starch)
Not all salivary glands produce the same saliva
Salivation
 Controlled by autonomic nervous system
 Parasympathetic stimulation promotes secretion of
moderate amount of saliva
 Sympathetic stimulation decreases salivation


Copyright 2009, John Wiley & Sons, Inc.
Tongue and Teeth

Tongue





Accessory digestive organ
Skeletal muscle covered by mucous membrane
Maneuvers food for chewing, shapes mass, forces food
back for swallowing
Lingual glands secrete salivary lipase
Teeth or dentes




Accessory digestive organ
3 major regions – crown, root, and neck
Dentin of crown covered by enamel
2 dentitions – deciduous and permanent teeth
Copyright 2009, John Wiley & Sons, Inc.
Sagittal
plane
Enamel
Dentin
CROWN
Gingival sulcus
Gingiva (gum)
Pulp in pulp
cavity
NECK
Cementum
Root canal
Alveolar bone
ROOT
Periodontal
ligament
Apical foramen
Nerve
Blood supply
Sagittal section of a mandibular (lower) molar
Digestion in the mouth

Mechanical digestion in the mouth




Chewing or mastication
Food manipulated by tongue, ground by teeth, and mixed
with saliva
Forms bolus
Chemical digestion in the mouth

Salivary amylase secreted by salivary glands acts on
starches



Only monosaccharides can be absorbed
Continues to act until inactivated by stomach acid
Lingual lipase secreted by lingual glands of tongue acts on
triglycerides

Becomes activated in acidic environment of stomach
Copyright 2009, John Wiley & Sons, Inc.
Pharynx


Passes from mouth into pharynx
3 parts

Nasopharynx


Oropharynx


Functions only in respiration
Digestive and respiratory functions
Laryngopharynx

Digestive and respiratory functions
Copyright 2009, John Wiley & Sons, Inc.
Esophagus




Secretes mucous, transports food – no enzymes
produced, no absorption
Mucosa – protection against wear and tear
Submucosa
Muscularis divided in thirds





Superior 1/3 skeletal muscle
Middle 1/3 skeletal and smooth muscle
Inferior 1/3 smooth muscle
2 sphincters – upper esophageal sphincter (UES) regulates
movement into esophagus, lower esophageal sphincter
(LES) regulates movement into stomach
Adventitia – no serosa – attaches to surroundings
Copyright 2009, John Wiley & Sons, Inc.
Histology of the esophagus
Copyright 2009, John Wiley & Sons, Inc.
Deglutition




Act of swallowing
Facilitated by secretions of saliva and mucus
Involves mouth, pharynx, and esophagus
3 stages



Voluntary – bolus passed to oropharynx
Pharyngeal – involuntary passage through pharynx into
esophagus
Esophageal – involuntary passage through esophagus to
stomach

Peristalsis pushes bolus forward
Copyright 2009, John Wiley & Sons, Inc.
Nasopharynx
Hard palate
Soft palate
Bolus
Uvula
Oropharynx
Tongue
Epiglottis
Laryngopharynx
Larynx
Esophagus
(a) Position of structures before swallowing
(b) During pharyngeal stage
of swallowing
Esophagus
Relaxed muscularis
Circular muscles
contract
Longitudinal muscles
contract
Relaxed muscularis
Lower esophageal
sphincter
Bolus
Stomach
(c) Anterior view of frontal sections of peristalsis in esophagus
Stomach


Serves as mixing chamber and holding reservoir
4 main regions


Cardia, fundus, body, pylorus
Same 4 layers




Mucosa – gastric glands open into gastric pits
 3 types of exocrine gland cells – mucous neck cells
(mucus), parietal cells (intrinsic factor; V-B12 and HCl),
and chief cells (pepsinogen and gastric lipase)
 G cell – endocrine cell – secretes H’ gastrin
Submucosa
Muscularis – additional 3rd inner oblique layer
Serosa – part of visceral peritoneum
Copyright 2009, John Wiley & Sons, Inc.
Esophagus
Lower esophageal
sphincter
FUNDUS
Serosa
CARDIA
Muscularis:
Longitudinal layer
BODY
Circular layer
Lesser curvature
PYLORUS
Oblique layer
Greater curvature
Duodenum
Pyloric
sphincter
Rugae of mucosa
PYLORIC ANTRUM
PYLORIC CANAL
(a) Anterior view of regions of stomach
Esophagus
Duodenum
PYLORUS
Pyloric sphincter
PYLORIC CANAL
Lesser
curvature
PYLORIC ANTRUM
FUNDUS
CARDIA
BODY
Rugae of mucosa
Greater curvature
(b) Anterior view of internal anatomy
Lumen of stomach
Gastric pits
Surface mucous
cell
Lamina propria
Lymphatic nodule
Mucous neck cell
Parietal cell
Chief cell
Gastric gland
G cell
Muscularis mucosae
MUCOSA
SUBMUCOSA
Lymphatic vessel
MUSCULARIS
Venule
Arteriole
Oblique layer of muscle
Circular layer of muscle
SEROSA
Myenteric plexus
Longitudinal layer
of muscle
(a) Three-dimensional view of layers of stomach
Gastric pit
Surface mucous cells
Gastric
pit
Lamina
SEM
40x
propria
Stomach mucosa
Gastric
glands
Muscularis
mucosae
Surface mucous cell
(secretes mucus)
Mucous neck cell
(secretes mucus)
Parietal cell (secretes
hydrochloric acid
and intrinsic factor)
Chief cell (secretes
pepsinogen and
gastric lipase)
G cell (secretes the
hormone gastrin)
Submucosa
(b) Sectional view of stomach mucosa showing gastric glands and cell types
Mechanical and Chemical Digestion

Mechanical digestion


Mixing waves – gentle, rippling peristaltic movements –
creates chyme
Chemical digestion


Digestion by salivary amylase continues until inactivated by
acidic gastric juice
Acidic gastric juice activates lingual lipase


Digest triglycerides into fatty acids and diglycerides
Parietal cells secrete H+ and Cl- separately but net effect is
HCl

Kills many microbes, denatures proteins
Copyright 2009, John Wiley & Sons, Inc.
Chemical Digestion

Chemical digestion (cont.)

Pepsin secreted by chief cells digest proteins



Secreted as pepsinogen
Gastric lipase splits triglycerides into fatty acids
and monoglycerides
Small amount of nutrient absorption

Some water, ions, short chain fatty acids, certain
drugs (aspirin) and alcohol
Copyright 2009, John Wiley & Sons, Inc.
Pancreas


Lies posterior to greater curvature of stomach
Pancreatic juice secreted into pancreatic duct and
accessory duct and to small intestine


Pancreatic duct joins common bile duct and enters
duodenum at hepatopancreatic ampulla
Histology

99% of cells are acini



Exocrine
Secrete pancreatic juice – mixture of fluid and digestive
enzymes
1% of cells are pancreatic islets (islets of Langerhans)


Endocrine
Secrete hormones glucagon, insulin, somatostatin, and
pancreatic polypeptide
Copyright 2009, John Wiley & Sons, Inc.
Falciform ligament
Diaphragm
Right lobe of liver
Right hepatic
duct
Coronary ligament
Left lobe of liver
Left hepatic duct
Common hepatic duct
Gallbladder:
Neck
Body
Round ligament
Cystic duct
Fundus
Common bile duct
Pancreas
Tail
Body
Duodenum
Pancreatic duct (duct of
Wirsung)
Accessory duct
(duct of Santorini)
Head
Hepatopancreatic ampulla
(ampulla of Vater)
Uncinate process
Jejunum
(a) Anterior view
Common bile duct
Pancreatic duct
(duct of Wirsung)
Hepatopancreatic ampulla
(ampulla of Vater)
Mucosa of
duodenum
Major
duodenal
papilla
Sphincter of the hepatopancreatic
ampulla (sphincter of Oddi)
(b) Details of hepatopancreatic ampulla
Right hepatic
duct
Left hepatic
duct
Common hepatic
duct from liver
Cystic duct from
gallbladder
Common bile duct
Pancreatic duct
from pancreas
Key:
Liver
Sphincter
Gallbladder
Pancreas
Duodenum
(c) Ducts carrying bile from liver and gallbladder and
pancreatic juice from pancreas to duodenum
Pancreatic juice


1200-1500ml daily (1.2-1.5 qt)
Mostly water


Sodium bicarbonate – buffers acidic stomach chyme
Enzymes




Pancreatic amylase (starch)
Proteolytic enzymes – trypsin (secreted as trypsinogen),
chymotrypsin (chymotrypsinogen), carboxypeptidase
(procarboxypeptidase), elastase (proelastase)
Pancreatic lipase (triglyceride)
Ribonuclease and deoxyribonuclease (nucleic acids)
Copyright 2009, John Wiley & Sons, Inc.
Liver and gallbladder


Liver is the heaviest gland of the body
Liver is composed of
 Hepatocytes – major functional cells of liver


Wide variety of metabolic, secretory, and endocrine functions –
secrete bile (excretory product and digestive secretion)
Bile canaliculi – ducts between hepatocytes that collect bile

Exits livers as common hepatic duct, joins cystic duct from
gallbladder to form common bile duct
Hepatic sinusoids – highly permeable blood capillaries receiving
oxygenated blood from hepatic artery and deoxygenated nutrientrich blood from hepatic portal vein
3 different ways to organize units
 Hepatic acinus – preferred method



Hepatocytes arranged in 3 zones around short axis with no sharp
boundaries
Copyright 2009, John Wiley & Sons, Inc.
Liver
Inferior vena cava
Hepatic artery
Hepatic portal vein
Connective
tissue
Portal triad:
Bile duct
Hepatocyte
Hepatic
laminae
Branch of
hepatic artery
Branch of
hepatic portal vein
Central vein
Hepatic sinusoids
(a) Overview of histological components of liver
Central
vein
To hepatic
vein
Hepatic
sinusoids
Bile canaliculi
Portal triad:
Bile duct
Branch of hepatic
portal vein
Branch of hepatic
artery
Hepatic laminae
Hepatocyte
Stellate
reticuloendothelial
(Kupffer) cell
Connective tissue
Hepatic sinusoid
(b) Details of histological components of liver
Central vein
Portal
triad
Hepatic lobule
Portal lobule
Hepatic acinus
(d) Comparison of three units of liver structure and function
Portal triad
Central
vein
Central
vein
Zone 3
Zone 2
Zone 1
(e) Details of hepatic acinus



Zone 1 Cells – 1st to receive oxygenated
blood, nutrients and any toxins, 1st to take up
glucose and store as glycogen after a meal.
Also 1st to show change if there is a bile duct
obstruction and last to die (poor circulation)
and 1st to regenerate.
Zone 3 Cells – last to show effect of bile duct
obstruction or toxin, 1st to show effect from
poor circulation and last to regenerate, 1st to
show effect of fat accumulation.
Zone 2 Cells
Copyright 2009, John Wiley & Sons, Inc.
Gallbladder



Contraction of smooth muscle fibers eject
contents of gall bladder into cystic duct
Functions to store and concentrate bile
produced by the liver until it is needed in the
small intestine
Absorbs water and ions to concentrate bile up
to ten-fold
Copyright 2009, John Wiley & Sons, Inc.
Hepatic blood flow



Liver receives blood from
Hepatic artery carrying oxygenated blood
Hepatic portal vein carrying deoxygenated blood
with newly absorbed nutrients and possibly drugs,
microbes or toxins from GI tract
Copyright 2009, John Wiley & Sons, Inc.
Oxygenated blood
from hepatic artery
1
Nutrient-rich,
deoxygenated blood from
hepatic portal vein
2
Liver sinusoids
3
Central vein
4
Hepatic vein
5
Inferior vena cava
6
Right atrium of heart
Hepatic
Inferior vena cava
Stomach
Spleen
HEPATIC PORTAL
Short gastric
Left gastric
Liver
Pancreas (behind stomach)
Cystic
Gallbladder
Pancreati
c
Left gastroepiploic
Duodenum
Right gastric
Pancreaticoduoden
al
Pancreas
SPLENIC
Transverse colon
SUPERIOR
MESENTERIC
Middle
colic
Right colic
Jejunal
and ileal
Ascending colon
Right gastroepiploic
Left colic
Inferior
mesenteric
Descending colon
Ileocolic
Cecum
Sigmoidal
Appendix
Ileum
Drain into superior mesenteric
vein
Drain into splenic vein
Drain into inferior mesenteric
vein
(a) Anterior view of veins draining into hepatic portal vein
Sigmoid colon
Superior rectal
Rectum
Inferior vena cava
Heart
Hepatic veins
Liver
Abdominal
aorta
Proper
hepatic
artery
Splenic
vein
Tributaries from portions of
stomach, pancreas, and
portions of large intestine
Superior
mesenteric
vein
Tributaries from small intestine
and portions of large intestine,
stomach, and pancreas
Hepatic
portal
vein
(b) Scheme of principal blood vessels of hepatic portal circulation and
arterial supply and venous drainage of liver
Role and composition of bile



Hepatocytes secrete 800-1000mL of bile daily
Mostly water, bile salts, cholesterol, lecithin, bile
pigments and several ions
Bilirubin – principal bile pigment



Derived from heme of recycled RBCs
Breakdown product stercobilin gives feces brown color
Bile salts play role in emulsification

Also aid in absorption of lipids following digestion
Copyright 2009, John Wiley & Sons, Inc.
Functions of the Liver p.995







Carbohydrate metabolism and blood glucose
homeostasis.
Lipid and protein metabolism and produce plasma
proteins
Process drugs and proteins
Excretion of bilirubin a waste from recycles RBC and
excreted in feces
Synthesis of bile salts – emulsification and absorption of
lipids,
Storage – glycogen, A, B12, D, E, K, iron and copper See
Table 25.5 and 25.6,
Phagocytosis – reticuloendothelial cells, RBC, WBC
Copyright 2009, John Wiley & Sons, Inc.
Small intestine


3 regions – duodenum (10”), jejunum (3’), and ileum
(6’) – Pyloric sphincter to the ileocecal sphincter
Same 4 layers
1.
Mucosa


Absorptive cells (digest and absorb), goblet cells (mucus),
intestinal glands (intestinal juice), Paneth cells (lysozyme),
and enteroendocrine cells
Abundance of MALT
Submucosa
2.

Duodenal glands secrete alkaline mucus
Muscularis
Serosa
3.
4.

Completely surrounds except for major portion of duodenum
Copyright 2009, John Wiley & Sons, Inc.
Anatomy of the small intestine
Copyright 2009, John Wiley & Sons, Inc.
Special structural features increase surface
area for digestion and absorption
Circular folds



Permanent ridges of mucosa and submucosa
Cause chyme to spiral
Villi



Fingerlike projections of mucosa
Contains arteriole, venule, blood capillary, and lacteal
Microvilli



Projects of apical membrane of absorptive cells
Brush border with brush border enzymes
Copyright 2009, John Wiley & Sons, Inc.
Circular folds
(plicae circulares)
Circular folds
Villi
Submucosa
(a) Relationship of
villi to circular folds
Circular layer of
muscle
Longitudinal
layer of muscle
Serosa
Lumen of small intestine
Blood
capillary
Lacteal
Villi
Opening of
intestinal gland
Absorptive cell
Goblet cell
Lacteal
Lamina propria
MUCOSA
Enteroendocrine cell
Paneth cell
Lymphatic nodule
Muscularis mucosae
SUBMUCOSA
Arteriole
Venule
Lymphatic vessel
MUSCULARIS
Circular layer of muscle
Myenteric plexus
Longitudinal layer
of muscle
(b) Three-dimensional view of layers of the small intestine showing villi
SEROSA
Microvilli
Absorptive cell
(absorbs nutrients)
Blood
capillary
Lacteal
Mucosa
Goblet cell
(secretes mucus)
Lamina
propria
Intestinal
gland
Enteroendocrine cell
(secretes the hormones
secretin,
cholecystokinin, or GIP)
Muscularis
mucosae
Arteriol
Submucosa
Venule
Paneth cell (secretes
lysozyme and is capable
of phagocytosis)
Lymphatic
vessel
Muscularis
(c) Enlarged villus showing lacteal,
capillaries, intestinal glands, and cell type
Histology of the duodenum and ileum
Copyright 2009, John Wiley & Sons, Inc.
Intestinal juice and brush-border enzymes

Intestinal juice




1-2L daily
Contains water and mucus, slightly alkaline
Provide liquid medium aiding absorption
Brush border enzymes



Inserted into plasma membrane of absorptive cells
Some enzymatic digestion occurs at surface rather than
just in lumen
Carbohydrate = α-dextrinase, maltase, sucrase, lactase,
Protein = aminopetidase, dipeptidase, nucleotide =
nucleosidases and phosphatases
Copyright 2009, John Wiley & Sons, Inc.
Mechanical Digestion


Governed by myenteric plexus
Segmentations



Localized, mixing contractions
Mix chyme and bring it in contact with mucosa for
absorption
Migrating motility complexes (MMC)


Type of peristalsis
Begins in lower portion of stomach and pushes food
forward
Copyright 2009, John Wiley & Sons, Inc.
Chemical digestion

Carbohydrates




Pancreatic amylase
α-dextrinase, sucrase, lactase, maltase in brush border
Ends with monosaccharides which can be absorbed
Proteins


Trypsin, chymotrypsin, carboxypeptidase, and elastase
from pancreas – proteins to peptides
Aminopeptidase and dipeptidase in brush border
Copyright 2009, John Wiley & Sons, Inc.
Lipids and Nucleic Acids

Lipids



Pancreatic lipase most important in triglyceride digestion
Emulsification by bile salts increases surface area of
lipids.
Nucleic acids


Ribonuclease and deoxyribonuclease in pancreatic juice
Nucleosidases and phosphatases in brush border
Copyright 2009, John Wiley & Sons, Inc.
Absorption of:

Monosaccharides




All dietary carbohydrates digested are absorbed
Only indigestible cellulose and fibers left in feces
Absorbed by facilitated diffusion or active transport into
blood
Amino acids, dipetides and tripeptides


Most absorbed as amino acids via active transport into
blood
½ of absorbed amino acids come from proteins in
digestive juice and dead mucosal cells
Copyright 2009, John Wiley & Sons, Inc.
Lipids



All dietary lipids absorbed by simple diffusion
Short-chain fatty acids go into blood for transport
Long-chain fatty acids and monoglycerides

Enter lacteals to eventually enter blood
Copyright 2009, John Wiley & Sons, Inc.
Glucose and
galactose
Fructose
Secondary active
transport with Na+
Facilitated
diffusion
Facilitated
diffusion
Amino acids
Active transport or
secondary active
transport with Na+
Dipeptides
Tripeptides
Small short-chain
fatty acids
To blood
capillary of a
villus
Amino acids
Diffusion
Secondary active
transport with H+
Simple diffusion
Large short-chain
and long-chain
fatty acids
Micelle
Monosaccharides
Diffusion
Triglyceride
Simple
diffusion
To lacteal
of a villus
Monoglycerides
Chylomicron
Lumen of small
intestine
Microvilli (brush
border) on apical
surface
Basolateral surface
Epithelial cells
of villus
(a) Mechanisms for movement of nutrients through absorptive epithelial cells of villi
Left subclavian
vein
Villus (greatly enlarged)
Chylomicron
Heart
Liver
Hepatic
portal
vein
Thoracic duct
Small
shortchain
fatty
acid
Blood
capillary
Amino acid
Lacteal
Monosaccharide
Arteriole
Venule
Blood
Lymphatic vessel
Lymph
(b) Movement of absorbed nutrients into blood and lymph
Absorption of:

Electrolytes




Vitamins



From GI secretions or food
Sodium ions (Na+) reclaimed by active transport
Other ions also absorbed by active transport
Fat-soluble vitamins A, D, E, and K absorbed by simple
diffusion and transported with lipids in micelles
Most water-soluble vitamins also absorbed by simple diffusion
Water




9.3L comes from ingestion (2.3L) and GI secretions (7.0L)
Most absorbed in small intestine, some in large intestine
Only 100ml excreted in feces
All water absorption by osmosis
Digestion Video – See Video
Chapters 1-2
http://www.pbs.org/wgbh/nova/body/can
-i-eat-that.html
Copyright 2009, John Wiley & Sons, Inc.
INGESTED AND SECRETED
ABSORBED
Saliva (1 liter)
Ingestion of liquids
(2.3 liters)
Gastric juice
(2 liters)
Bile (1 liter)
Pancreatic juice
(2 liters)
Intestinal juice
(1 liter)
Small intestine
(8.3 liters)
Total ingested
and secreted
= 9.3 liters
Large intestine
(0.9 liters)
Total absorbed
= 9.2 liters
Excreted in feces
(0.1 liter)
Fluid balance in GI tract
Large intestine





Overall function to complete absorption, produce
certain vitamins, and form and expel feces
4 major regions – cecum, colon, rectum, and anal
canal
Ileocecal sphincter between small and large
intestine
Colon divided into ascending, transverse,
descending and sigmoid
Opening of anal canal (anus) guarded by internal
anal sphincter of smooth muscle and external anal
sphincter of skeletal muscle
Copyright 2009, John Wiley & Sons, Inc.
TRANSVERSE COLON
Left colic (splenic)
flexure
Right colic
(hepatic) flexure
Teniae coli
ASCENDING
COLON
Teniae
coli
DESCENDING COLON
Ileum
Omental
appendices
Mesoappendix
Haustra
Ileocecal
sphincter
(valve)
CECUM
RECTUM
VERMIFORM APPENDIX
SIGMOID
COLON
ANAL CANAL
ANUS
(a) Anterior view of large intestine showing major regions
Rectum
Anal canal
Internal anal sphincter
(involuntary)
External anal sphincter
(voluntary)
Anus
Anal column
(b) Frontal section of anal canal
Large Intestine


Same 4 layers
Mucosa – mostly absorptive and goblet cells




Submucosa
Muscularis



No circular folds or villi
Does have microvilli
Longitudinal muscle modified to form teniae coli
Forms haustra – pouches
Serosa
Copyright 2009, John Wiley & Sons, Inc.
Digestion of the Large Intestine

Mechanical digestion




Haustral churning
Peristalsis
Mass peristalsis – drives contents of colon toward rectum
Chemical digestion

Final stage of digestion through bacterial action



Ferment carbohydrates, produce some B vitamins and vitamin
K
Mucus but no enzymes secreted
Remaining water absorbed along with ions and
some vitamins
Copyright 2009, John Wiley & Sons, Inc.
Lumen of large intestine
Openings
of intestinal glands
Absorptive cell
Goblet cell
MUCOSA
Intestinal gland
Lamina propria
Lymphatic nodule
Muscularis mucosae
Lymphatic vessel
Arteriole
SUBMUCOSA
MUSCULARIS
Venule
Circular layer of muscle
SEROSA
Myenteric plexus
Longitudinal layer of
muscle
(a) Three-dimensional view of layers of large intestine
Openings of
intestinal glands
Lamina propria
Microvilli
Intestinal gland
Absorptive cell
(absorbs water)
Goblet cell (secretes mucus)
Muscularis
mucosae
Lymphatic
nodule
Submucosa
(b) Sectional view of intestinal glands and cell types
Histology of the large intestine
Copyright 2009, John Wiley & Sons, Inc.
Phases of digestion

Cephalic phase


Gastric phase


Smell, sight, thought or initial taste of food
activates neural centers – prepares mouth and
stomach for food to be eaten
Neural and hormonal mechanisms promote gastric
secretion and motility
Intestinal phase



Begins when food enter small intestine
Slows exit of chyme from stomach
Stimulates flow of bile and pancreatic juice
Copyright 2009, John Wiley & Sons, Inc.
Food entering
stomach disrupts
homeostasis by
Increasing
pH of gastric juice
Distention (stretching)
of stomach walls
Receptors
Chemorecepto
rs and stretch
receptors in
stomach
detect pH
increase and
distention
Input
Control center
Nerve
impulses
Submucosal plexus
Output
Return to homeostasis
when response brings pH
of gastric juice and
distention of stomach
walls back to normal
(pre-eating status)
Nerve impulses
(parasympathetic)
Effectors
Parietal cells
secrete HCI and
smooth muscle in
stomach wall
contracts more
vigorously
Increase in acidity of
stomach chyme; mixing
of stomach contents;
emptying of stomach
HCI
Defecation reflex

Mass peristaltic movement from sigmoid
colon to rectum, distension triggers reflex.


Diarrhea
Constipation

Exercise, stress, fluid intake, dietary fiber
Copyright 2009, John Wiley & Sons, Inc.