Biology 221
Anatomy & Physiology II
Digestive System
Chapter 18
pp. 651-677
E. Lathrop-Davis / E. Gorski / S. Kabrhel
Digestive System Functions
The digestive system:
• provide nutrients in usable form; and
• removes unusable wastes from the body.
Digestive System Overview
There are two main groups of organs:
• The alimentary canal (also known as the
gastrointestinal or GI tract) is the tube through which
food passes.
– The GI tract is responsible for digestion and
absorption of food.
– Organs of the GI tract include the mouth, pharynx,
esophagus, stomach, small intestine, and large
Fig. 24.1, p. 888
Digestive System Overview
• Accessory organs are organs, glands and structures
which aid digestion but not part of GI tract itself.
– Accessory organs include the teeth, tongue,
salivary glands, pancreas, liver, and gall bladder.
Fig. 24.1, p. 888
Processes of Digestion
There are 6 main processes involved in digestion:
• Ingestion is the entrance of food and drink into the mouth.
• Mechanical digestion is the physical breakdown into smaller
• Propulsion is movement of materials through the gut.
• Chemical digestion is the breakage of bonds in molecules to
form smaller compounds.
• Absorption is the uptake of nutrients from lumen (if it’s not
absorbed, it’s not usable).
• Defecation is the removal of indigestible material (like
Fig. 24.2, p. 889
• The peritoneum is a serous membrane.
• The parietal peritoneum lines the abdominal cavity.
– Organs posterior to the parietal peritoneum are
• The visceral peritoneum (also called the serosa)
covers surfaces of most abdominal organs.
– Mesenteries are a double layer of peritoneum
extending from the body wall to certain digestive
– Intraperitoneal organs are ones found in the
See also Fig. 24.5, p. 891
Fig,. 24.30, p. 929
• The peritoneal cavity is the fluid-filled “space”
between the visceral and parietal peritoneum.
• Peritonitis is any inflammation of the peritoneum. It
makes movement within the gut more difficult and
See also Fig. 24.5, p. 891
Fig,. 24.30, p. 929
Splanchnic Circulation
Arteries serving the digestive organs are branches of the
abdominal aorta.
• The celiac trunk is the first branch of the abdominal
aorta. It is very short; gives rise to:
– hepatic artery, which serves the liver, gall bladder,
stomach, and duodenum;
– left gastric artery serving the stomach and inferior
esophagus; and the
– splenic artery serving the spleen, stomach, and
Fig. 20.22, p. 761
Fig. 20.22, p. 759
Splanchnic Circulation
• The superior mesenteric artery supplies the small
intestine and most of large intestine, and the pancreas.
• The inferior mesenteric artery serves the large intestine.
Fig. 20.22, p. 761
Fig. 20.22, p. 759
Hepatic Circulation:
Hepatic portal system
• The hepatic portal system consistes of veins draining
digestive organs and carrying nutrient-rich blood to
the liver.
– The gastric vein drains the stomach.
– The superior mesenteric vein drains the small
– The splenic vein drains spleen.
° The inferior mesenteric vein drains the large
intestine and empties into the splenic vein.
• Venous blood from the hepatic portal system mixes
with arterial blood (from the hepatic artery) in liver.
Fig. 20.27, p. 771
Hepatic Circulation: Hepatic Veins
• drain venous blood from liver into inferior vena cava
Fig. 20.27, p. 771
• The mucosa is the mucous membrane lining the gut.
• The mucosa consists of:
– an epithelial lining;
– a lamina propria of areolar connective tissue; and
– the muscularis mucosae of smooth muscle.
Mucosa: Epithelium
• Type of tissue comprising the epithelium varies
depending on location.
– Stratified squamous epithelium is found in mouth,
esophagus and anal canal where abrasion is likely.
– Simple columnar epithelium is found in stomach and
intestines where secretion and absorption are
• The epithelium secretes mucus, digestive enzymes, and
• It provides an intact barrier to protect against entry of
Mucosa: Lamina Propria
• The lamina propria is a ayer of areolar connective
• Blood capillaries nourish epithelium, absorb and
transport digested nutrients.
• Lymphatic capillaries provide drainage for interstitial
fluid and transport fats to the venous circulation.
Mucosa: Muscularis Mucosae
• The muscularis mucosae consists of smooth muscle.
• It is used for local movement and to hold mucosa in
folds (small intestine)
• The submucosa is dense connective tissue superficial
to the mucosa.
• It is highly vascularized with many lymphatic vessels.
• Lymph nodules help protect against diseases.
– MALT, or mucosa-associated lymphatic tissue, is
common especially in the small intestine (as
Peyer’s Patches) and large intestines.
Muscularis Externa (Muscularis)
• The mucularis consists of two layers in most organs
(3 in stomach).
– The circular layer runs around the tube.
– The longitudinal layer runs the length of the tube.
• Peristalsis moves material through gut by alternating
contraction of circular and longitudinal muscle.
• Segmentation is a series of ring-like contractions that
help mix material with digestive enzymes in small
Fig. 24.3, p. 890
• The visceral peritoneum, or serosa, is composed of
simple squamous epithelium (mesothelium) with
areolar CT.
• The adventitia is a dense connective tissue covering
without epithelium. It is found around the esophagus.
• Retroperitoneal organs have both a serosa (of parietal
peritoneum) and adventitia (on the side abutting body
Enteric Nervous System
Intrinsic nerve plexuses consist of enteric neurons.
– Enteric neurons are able to act independently of
central nervous system.
– They communicate with each other to control GI
• There are two main enteric plexuses.
– The submucosal nerve plexus regulates glands in
submucosa and smooth muscle of muscularis
– The myenteric nerve plexus regulates activity of
muscularis externa (with aide of submucosal nerve
Central Nervous System Control
• Enteric nerve plexuses linked to CNS by visceral
afferent (sensory) fibers
• The digestive system receives motor input from the
sympathetic and parasympathetic divisions of the
autonomic nervous system.
– Parasympathetic outflow generally increases
– Sympathetic outflow generally decreases activity.
Mouth (Oral Cavity)
• The oral oriface is the anterior opening.
• The mouth is continuous posteriorly with the
• The lips and cheeks keep food in oral cavity.
• There ar three layers of tissue in the mouth wall:
– the mucosa (stratified squamous epithelium);
– the submucosa; and
– the muscularis externa, which consists of skeletal
muscle for voluntary control.
Mouth: Palate
• The palate separates the oral and nasal cavities and is
why you can chew and breath at the same time.
• The hard palate consists of the palatine process of
maxilla and the palatine bones.
• The soft palate is composed of muscle only.
– It prevents food from entering nasopharynx during
– The uvula is the part that hangs down in middle.
Fig. 24.7, p. 895
Mouth: Arches
• The palatoglossal arch anchors the soft palate to the
• The palatopharyngeal arch anchors the soft palate
to the wall of the oropharynx.
• The fauces refers to the area between these arches.
The palatine tonsils are located in the fauces.
Fig. 24.7, p. 895
Mouth: Tongue
• The lingual tonsil sits at base of tongue and protects
against invasion by bacteria (see Topic 5 - Lymphatic
• Taste buds contain receptors for taste. They are
scattered over the tongue in papillae.
Fig. 24.8, p. 896
Mouth: Tongue
• The tongue forms a bolus, or ball, of food, making it
easier to swallow.
– The tongue also keeps the food between the teeth.
• Muscles are served by the Hypoglossal (XII) nerve (see
A&P I Unit VI).
– Intrinsic muscles within tongue (not attached to
bone) allow the tongue to change shape for
swallowing and speech.
– Extrinsic muscles attach to bone or the soft palate
and alter tongue position (protrusion, retraction,
Fig. 24.7, p. 895
Salivary Glands
• The salivary glands produce saliva.
• There are two groups of salivary glands:
– The intrinsic glands (or buccal glands) lie within the
oral cavity.
– The 3 pairs of extrinsic glands (see A&P I Unit VI
for innervation) lie outside the cavity. They are:
° the parotid glands (connected to oral cavity by
parotid duct; mumps is a viral infection of the
parotid glands);
° the sublingual glands; and
° the submandibular glands.
Fig. 24.9, p. 897
• Mucus cells in the salivary glands are less common than
serous cells and produce mucus to lubricate the food.
• Serous cells produce a watery saliva that is composed
mostly (97-99.5%) of water.
– This saliva is slightly acidic (pH ~ 6.8).
– It contains several types of solutes including:
° electrolytes (ions such as Na+, K+, Cl-, PO4=,
° metabolic wastes (urea, uric acid), and
° proteins.
Salivary Proteins
Salivary proteins include:
• mucin, the glycoprotein portion of mucus that
lubricates oral cavity;
• lysozyme, which has antibacterial properties;
• IgA antibodies that prevent antigens from attaching to
the mucus membrane;
• defensins secreted by neutrophils that act as local
antibiotic and chemotatic agents when the mucosa is
damaged; and
• salivary amylase, which begins starch hydrolysis.
Control of Salivation
• The sympathetic division of the ANS either:
– stimulates production of mucin-rich saliva, or
– inhibits salivation altogether at high levels.
• The parasympathetic division of ANS stimulates
– Chemoreceptors (excited most by acidic
substances) and baroreceptors (excited by
mechanical stimuli) send messages to the
salivatory nuclei in pons and medulla. This results
in increased parasympathetic motor output.
– Parasympathetic motor output results in increased
Control of Salivation
• The salivary nuclei are also under psychological
control, wherein they respond to visual and olfactory
stimuli, even thoughts of food, by increasing
parasympathetic output.
• Salivary nuclei are also stimulated by irritation to the
lower GI tract.
• Parasympathetic nerves that stimulate salivation are
– the facial, which goes to the submandibular and
sublingual salivary glands; and
– the glossopharyngeal, which goes to the parotid
salivary glands.
• Teeth lie in the alveoli of the mandible and maxilla
(see A&P I axial skeleton lab).
• Primary dentition refers to the 20 deciduous teeth
(also called milk or baby teeth).
– The roots are absorbed as the permanent teeth
grow in causing the baby teeth to fall out.
Fig. 24.10, p. 899
Permanent dentition refers to the 32 adult teeth. Each
jaw contains:
• 4 Incisors (central and lateral)
• 2 Canines (eyeteeth)
• 4 Bicuspids = premolars
• Molars
– 2 first molars
– 2 second molars
– 2 third molars, which are also called wisdom teeth,
and may become impacted as they grow in. When
this happens they are surgically removed.
Fig. 24.10, p. 899
Tooth Structure
• The crown is the part visible above the gum line. It is
covered by enamel (the hardest substance in body)
with dentin under it.
• The neck is the area between the crown and the root.
• The root has no enamel; its dentin is under the
– Cementum is calcified connective tissue covering
the dentin of root. It attaches the root to the
periodontal ligament which anchors tooth into the
– The pulp cavity houses blood vessels and nerves
that enter or leave via the apical foramen in the
root canal (narrowed end of the cavity). Fig. 24.11, p. 900
• Only oropharynx and laryngopharynx are involved in
digestion (nasopharynx is only respiratory).
• The pharynx is lined with nonkeratinized stratified
squamous epithelium to protect against abrasion.
• Mucus-producing glands in submucosa produce
mucus that lubricates food.
• Skeletal muscle in the wall responds to somatic
reflexes to move food quickly past laryngopharynx.
• There is no serosa or adventitia around the pharynx.
• The esophagus runs from the laryngopharynx
through the mediastinum to the stomach.
• All 4 tissue layers are present in its wall.
– The mucosa consists of stratified squamous
– The submucosa consists of dense irregular
connective tissue with mucus-secreting
esophageal glands.
• The muscularis changes muscle type as it goes
– The top 1/3 is skeletal muscle
– The middle 1/3 is a mix of skeletal and smooth
– The bottom 1/3 is smooth muscle.
• The adventitia is the dense connective tissue
covering of the esophagus.
Structures Associated with the
• The upper esophageal sphincter controls movement
of material from pharynx into esophagus.
• The esophageal hiatus is the opening in the
diaphragm that allows the esophagus to pass from the
thoracic cavity into the abdominal cavity.
• The gastroesophageal (cardiac) sphincter is a
thickening of smooth muscle at the inferior end of
the esophagus.
– It is aided by diaphragm in closing the bottom of
the esophagus.
– This helps prevent reflux of acidic gastric juice.
Esophageal Disorders
• Heartburn results from a failure of the lower
esophageal sphincter to close completely allowing
acidic gastric juice into esophagus.
• Hiatus hernia refers to the protrusion of the superior
portion of stomach above diaphragm through the
esophageal hiatus.
• An esophageal ulcer is an erosion of the esophageal
wall due to chronic reflux of stomach acid. An ulcer
represents a chemical burn to the esophageal wall.
Digestive Processes in Mouth,
Pharynx and Esophagus
• Ingestion is taking food into the mouth.
• Mechanical digestion in the mouth consists of
mastication by teeth (with aid of tongue) and
formation of a bolus.
• Chemical digestion of carbohydrates starts with
salivary amylase produced by the salivary glands.
– Amylase breaks starch and glycogen into smaller
fragments (including maltose [disaccharide] if left
long enough, which is why a cracker starts to taste
sweet if left in the mouth).
– Activity of salivary amylase continues until it
reaches acid environment of the stomach.
Digestive Processes in Mouth,
Pharynx and Esophagus
• Absorption in the mouth, pharynx and esophagus is
almost nil with the exception of some drugs, such as
• Movement of food in these areas includes deglutition
(swallowing) and peristalsis through the esophagus.
– Deglutition moves food from oral cavity to stomach.
° The process is voluntary in the oral cavity (buccal
phase) and reflexive in the pharynx.
- Think About It: What is the importance of having
deglutition be reflexive in the pharynx?
– Peristalsis occurs and is involuntary where smooth
muscle is found.
Fig. 24.13, p.904
Stomach: Gross Anatomy
• The cardiac region (cardia) is the first area into which
the food goes.
• The fundus is a temporary storage area at the upper left
of the stomach.
• The body is the main portion of the stomach.
– The greater curvature runs along the inferior surface
of the stomach;
– the lesser curvature, along the upper surface.
• The pyloric region is the distal portion leading into the
small intestine.
– The pyloric sphincter controls movement of chyme
into small intestine.
Fig. 24.14, p. 905
Stomach: Gross Anatomy
• Arterial blood is supplied by the gastric arteries and
the splenic artery. (See 20.22, p. 758)
• Venous blood is drained by the gastric veins, which
empty into the splenic vein or directly into the hepatic
portal vein.
Fig. 24.14, p. 905
Stomach Histology
• The mucosa consists of simple columnar epithelium.
– The muscularis mucosae throws mucosa into folds
called rugae.
• The submucosa consists of connective tissue.
• The muscularis has 3 layers that allow it to create
mixing waves in addition to peristalsis. The three
layers are from outside to inside:
– the longitudinal layer;
– the circular layer; and
– the oblique layer .
• The serosa covers the stomach.
Fig. 24.14, p. 905
Microscopic Anatomy
• The surface of the epithelium is composed mainly of
goblet cells that secrete mucus.
• Gastric pits are downward extensions of the
– Tight junctions between epithelial cells prevent
acidic gastric juice from reaching underlying
– Gastric pits contain gastric glands which secrete
gastric juice. Four types of cells are present.
Fig. 24.15, p. 906
Gastric Pit Cells
• Mucous neck cells secrete bicarbonate-rich mucus
• Parietal (oxyntic) cells secrete:
– HCl ( which is buffered by bicarbonate rich mucus
to protect the mucosa); and
– intrinsic factor (which is essential to absorption of
Vit. B12 by small intestine).
• Chief (zymogenic) cells secrete:
– pepsinogen (inactive form of the protease pepsin
for protein hydrolysis); and
– minor amounts of lipases (which hydrolyze lipids).
Gastric Pit Cells
• Enteroendocrine cells release hormones and hormonelike products into the lamina propria where they are
picked up by blood and carried to other digestive
– Gastrin is generally stimulatory.
– Histamine stimulates H+ secretion.
– Somatostatin is generally inhibitory.
Digestive Processes in Stomach
• Mechanical digestion in the stomach is the result of
mixing waves, which help break food into smaller
• Chemical digestion in the stomach produces chyme
with a pH of about 2.
– Acid (HCl) secreted by parietal cells breaks some
bonds and activates pepsinogen into pepsin.
– Pepsin hydrolyses proteins and is produced as
pepsinogen by chief cells.
Think About It: What would happen if pepsin were
activated in the gastric pit?
– Rennin is a protease secreted in children that acts
on milk proteins.
Digestive Processes in Stomach
• Movement in the stomach includes:
– mixing waves that mix food with acid and
enzymes; and
– peristalsis that moves material through stomach
and into small intestine.
• Absorption is limited to lipid soluble substances such
– alcohol,
– aspirin, and
– some other drugs.
Regulation of Gastric Secretion
Gastric secretion is controlled by the autonomic nervous
system and hormones.
• Hormonal control is accomplished by locally secreted
– Gastrin from the stomach stimulates secretion.
– Somatostatin, gastric inhibitory protein (GIP), and
cholecystokinin from the small intestine inhibit
secretion. Somatostatin is also produced by the
stomach itself.
See Fig. 24.16, p. 910
Regulation of Gastric Secretion
• Neural control is accomplished by the ANS and local
enteric nerve reflexes.
– Autonomic control starts in the CNS.
° The parasympathetic division works through the
Vagus (X) nerve and stimulates activity.
° The sympathetic division works through
thoracic spinal nerves and inhibits activity.
– Local enteric nerve reflexes are based on
distension of the stomach or duodenum.
° Distension of the stomach stimulates gastric
° Distension of the duodenum inhibits gastric
See Fig. 24.16, p. 910
Stimulation of Gastric Secretion
• The cephalic phase involves subconscious activity
resulting from the thought or sight of food and from
stimulation of areas of the hypothalamus by the smell or
taste of food.
– Impulses sent via the Vagus nerve stimulate secretion
of gastric juice.
• The gastric phase starts as food enters the stomach.
Distension and certain chemicals (especially caffeine
and peptides) stimulate receptors resulting in:
– release of gastrin from the stomach;
– local reflexes; and
– long reflexes involving the medulla oblongata and
Vagus nerves.
Stimulation of Gastric Secretion
• The intestinal phase starts as food enters the
duodenum (proximal portion of the small intestine).
– Initially, partially digested food entering the
duodenum causes the intestine to release enteric
gastrin, which stimulates gastric activity.
– As the duodenum is stretched by additional acidic
chyme from the stomach and as the pH decreases,
the enterogastric reflex starts and inhibits gastric
Fig. 24.16, p. 910
Inhibition of Gastric Secretion
• During the cephalic (cerebral) phase, loss of apetite
decreases parasympathetic impulses going to the
• During the gastric phase, excess acidity in the
stomach inhibits gastrin secretion. Emotional upset
triggers the fight-or-flight response, thus sending
sympathetic impulses instead of parasympathetic
• During the intestinal phase, further distension of the
duodenum and accumulation of fatty, acidic, or
hypertonic chyme results in local reflexes that inhibit
gastric activity.
Fig. 24.16, p. 910
Gastric Disorders
• Gastritis is an inflammation of underlying layers of the
stomach wall.
• Gastric ulcers are eroded areas of the stomach wall
where it has been chemically burned.
– Helicobacter infections associated with ~90% of all
ulcers, although it is uncertain as to whether it is
causitive agent or just opportunistic.
– Non-infectious ulcers are associated with persistent
inflammation and with chronic ingestion of nonsteroidal anti-inflammatory drugs like aspirin,
which are absorbed through the gastric mucosa.
Gastric Disorders (con’t)
• Emesis, also called vomiting, is usually caused by
° extreme stretching of stomach or small intestine,
° by the presence of irritants in the stomach (e.g.,
bacterial toxins, excessive alcohol, spicy foods,
certain drugs).
– The emetic center in the medulla initiates impulses:
° that lead to contraction of abdominal muscles
(which increases the intra-abdominal pressure);
° relax the cardiac sphincter; and
° raise the soft palate to closes off the nasal
– Excessive vomiting results in dehydration and
metabolic alkalosis (increased blood pH).
Small Intestine: Gross Structure
• The small intestine has a diameter of about 2.5
cm. It is approximately ~ 2-4 m (8-13’) in length
in a living person. (In a cadaver, it is 6-7 m [2021’] long because the muscle is not contracted.)
• The small intestine designed for secretion
(especially proximal end) and absorption
– It is the site of most chemical digestion.
– It is also the site of most absorption
Fig. 24.21, p. 916
Small Intestine: Gross Structure
• The pH of chyme entering the small intestine is
about 2, it is buffered to between pH 7 and pH 8
by alkaline secretions from the duodenal
(Bruner’s) glands.
• There are three areas of small intestine:
– the duodenum is the first 25 cm;
– the jejunum is the middle portion; and
– the ileum is the distal portion that empties into
the large intestine.
Fig. 24.21, p. 916
Small Intestine: Duodenum
• The duodenum receives acidic chyme from the
• The hepatopancreatic ampulla is formed from the
union of the common bile duct and the pancreatic
– The ampulla opens into the duodenum via the
major duodenal papilla.
– The hepatopancreatic sphincter (sphincter of Oddi)
controls entry of fluid from ampulla.
• Duodenal (Brunner’s) glands located in the
submucosa secrete an alkaline mucus that helps
neutralize the acidity of the chyme.
Fig. 24.20, p. 915
Small Intestine: Jejunum & Ileum
• The jejunum extends from duodenum to ileum.
Digestion and absorption continue.
• The ileum extends from the jejunum to the large
intestine. Digestion and absorption continue.
– The ileocecal valve controls movement of chyme
from the ileum of the small intestine into the
caecum (cecum) of the large intestine.
Small Intestine: Innervation
• Parasympathetic impulses supplied by Vagus nerve
stimulates activity in the small intestine.
• Sympathetic impulses supplied by thoracic splanchnic
nerves inhibit activity in the small intestine.
• Enteric nerves act locally to control movement.
Fig. 14.4, p. 517
Fig. 14.5, p. 519
Small Intestine: Blood Supply
• Arterial blood is supplied by
– the common hepatic artery, which serves
duodenum; and
– the superior mesenteric artery, which serves most
of small intestine.
• Venous drainage is provided by the superior
mesenteric vein, which drains the entire small
intestine. The superior mesenteric vein is part of the
hepatic portal system.
Fig. 20.22, p. 761
Fig. 20.27, p. 771
Small Intestine: Overview of
Special Anatomical Features
• The plicae circularis are circular folds of the
• Villi are fingerlike projections of intestinal wall
that increase overall surface area.
• Microvilli are projections of individual cell
membranes, thus increasing surface area of the
cells to increase absorption.
See Fig. 24.21, p. 916
Small Intestine: Plicae Circularis
• The plicae circularis are deep, permanent circular
folds of the mucosa and submucosa.
• Their presence forces chyme to spiral through lumen
of the small intestine. This:
– mixes chyme with intestinal juice and
– slows movement of the chyme through the small
Think About It: Why is this helpful?
Fig. 24.21, p. 916
Small Intestine: Villi
• Villi are small finger-like projections of the mucosa
(over 1 mm tall).
See also Fig. 24.22, p. 917;
• Each villus contains:
Fig. 24.21, p. 916
– a blood capillary bed;
– a lacteal to drain the tissue; and
– smooth muscle, which allows the villus to
shorten and lengthen. This
° increases contact between the villus and
chyme in the lumen, and
° “milks” the lacteal to move the lymph.
Small Intestine: Microvilli
• Microvilli are extensions of cell membrane and are
sometimes called the “brush border”.
• Functions of the microvilli include:
– secretion of brush border enzymes; and
– increasing the surface area for absorption by
individual cells.
Fig. 24.21, p. 916
See also Fig. 24.22, p. 917
Small Intestine: Mucosa
• The mucosa is renewed every 3-6 days.See Fig. 24.21, p. 916
• It consists of
– simple columnar epithelium with
° numerous goblet cells that secrete mucus; and
° absorptive cells that absorb nutrients.
- Absorptive cells are bound by tight junctions, which
means most substances
go through the cell to be
- Microvilli increase the surface area for absorption.
– a lamina propria in which blood vessels and lacteals
are found; and
– intestinal crypts (next slide).
Small Intestine: Mucosa
– Intestinal crypts (crypts of Lieberkuhn) extend
down between the villi.
° Most cells of the crypt secrete “intestinal juice”,
a watery fluid rich in enzymes.
° Paneth cells secrete lysozyme, which is an
antibacterial compound.
See Fig. 24.21, p. 916
Small Intestine: Submucosa
• Peyer’s patches, which are part of the MALT (see
Topic 5) are clusters of lymphatic tissue for protection
against disease.
• Duodenal (Brunner’s) glands are found only in the
– They secrete an alkaline mucus rich in bicarbonate
(HCO3-), which raises the pH of chyme from less
than pH 3 (highly acidic) to between pH 7 and pH 8
(slightly alkaline).
– Recall from general biology, that pH affects protein
configuration and therefore affects enzyme activity.
Small Intestine: Muscularis & Serosa
• The muscularis consists of the two layers of smooth
muscle (circular and longitudinal) and creates two
kinds of movement:
– peristalsis moves chyme through intestine; and
– segmentation mixes chyme with intestinal juice.
° Chyme moves between segments a few cm at a
° Intrinsic control occurs via intrinsic pacemaker
cells in longitudinal muscle.
– intensity altered by nervous system and hormones.
° parasympathetic impulses increase strength of
contraction; sympathetic impulses decrease it.
Small Intestine: Muscularis & Serosa
• The serosa (visceral peritoneum) is the outer layer of
the small intestine wall.
– Mesenteries are formed from visceral peritoneum
and attach the small intestine to posterior body
– Intraperitoneal organs are those that are completely
surrounded and supported by mesenteries. The
jejunum and ileum are intraperitoneal.
Small Intestine: Digestive Processes
• Mechanical digestion occurs via bile salts secreted
by the liver
– Bile salts are stored in and released from gall
bladder in response to cholecystokinin.
– Bile salts emulsify fat globules (make them into
smaller droplets) to increase the surface area
lipases have available to work on.
• Chemical digestion hydrolyses macromolecules.
All macromolecule types are broken down by
enzymes in the small intestine.
See Fig. 24.33, p. 933
Small Intestine: Lipid Digestion
• Pancreatic lipase from the pancreas hydrolyses
triglycerides in the small intestine.
• Most common lipids are neutral fats (triglycerides).
– glycerol + 1 fatty acid = monoglyceride
– glycerol + 2 fatty acids = diglyceride
– glycerol + 3 fatty acids = triglyceride
• Triglycerides cleaved into glycerol and 3 fatty acids
or monoglycerides and 2 fatty acids.
Fig. 2.14, p. 48
Small Intestine: Protein Digestion
• Pancreatic and intestinal proteases break proteins into
amino acids
• Pancreatic proteases are trypsin, chymotrypsin and
– These are secreted as inactive precoursers
(trypsinogen, chymotrypsinogen, and
procarboxypolypeptidase, respectively) to protect
the intestinal mucosa (and the pancreas) from
being digested
– These cut large proteins into small peptides.
See Fig. 2.17, p. 52
Small Intestine: Protein Digestion
• Intestinal proteases include aminopeptidase,
carboxypeptidase, and dipeptidase.
– These cleave small peptides into individual amino
acids that can be absorbed.
See Fig. 2.17, p. 52
Small Intestine:
Carbohydrate Digestion
• Starches are long chains of glucose.
Fig. 2.13, p. 46
– Starches are cleaved into short chains
(oligosaccharides) and maltose (a disaccharide) by
pancreatic amylase secreted by pancreas.
• Disaccharides hydrolyzed by intestinal (brush border)
enzymes specific to the disaccharide:
– maltase cleaves maltose;
– lactase cleaves lactose; and
– sucrase cleaves sucrose.
• Lactose intolerance results from a deficiency of
lactase. Lactose is digested by bacteria in the large
intestine instead, causing gas production.
Small Intestine:
Nucleic Acid Digestion
• Pancreatic nucleases (pancreatic ribonuclease and
pancreatic deoxyribonuclease) cleave nucleic acids
into individual nucleotides.
• Nucleosidases and phosphatases cleave the
nucleotides into their components: sugars (ribose or
deoxyribose), phosphates, and nitrogen bases.
Fig. 2.22, p. 58
Small Intestine: Absorption of
• Absorption moves nutrients from lumen into cells,
thence into interstitial fluid to blood or lymph
• Carbohydrates are absorbed as monosaccharides by:
– cotransport with Na+ (based on setting up Na+
gradient using active transport; glucose and
galactose); or
– facilitated transport (fructose).
Small Intestine: Absorption of Proteins
and Nucleic Acids
• Proteins are absorbed as amino acids by cotransport
with Na+ (based on setting up Na+ gradient using
active transport).
– Proteins rarely taken up intact (absorbed peptides
may cause food allergies that disappear as the
mucosa matures).
• Nucleic acids are actively transported into epithelial
cells as their components: sugar (ribose or
deoxyribose), phosphate, and nitrogen bases
Small Intestine: Absorption of Lipids
• Lipids combine with bile salts to form micelles, which
deliver the lipids to the microvilli.
– Lipids are absorbed passively through lipid bilayer
of the microvilli as glycerol and fatty acids or
– Once in the cell, they recombine into triglycerides,
which combine with proteins to form chylomicrons.
These are then released into the interstitial fluid.
° Most chylomicrons enter lymph through lacteals
(lymphatic capillaries) in the villi and are
transported to the subclavian veins where they
join the blood circulation.
Small Intestine: Absorption of
• Fat-soluble vitamins (DAKE) are incorporated into
micelles and absorbed in same manner as fats
(passively through lipid bilayer).
• Water-soluble vitamins (C, B complex) are mostly
absorbed by diffusion.
– The exception is B12, which must bind to intrinsic
factor produced in stomach to be actively absorbed
in the ileum (recognition of B12-intrinsic factor
complex by receptors in plasma membrane of cells
triggers active receptor-mediated endocytosis).
Small Intestine: Absorption of
• Most are actively absorbed throughout small intestine.
• Absorption based on how much is in food
• The Na+/K+ pump plays role in moving Na+ into the
epithelial cells and K+ out (into the lumen).
• K+ passively absorbed based on gradient created by
absortpion of water from the lumen.
Small Intestine: Absorption of
• Iron (Fe) and calcium (Ca) only absorbed in
duodenum depending on the needs of the body.
– Iron actively transported into cells where it
becomes bound to ferritin. In the blood iron is
carried bound to transferrin. (See topic 1)
– Calcium absorption regulated by vitamin D which
serves as cofactor in Ca transport. Vitamin D
synthesis begins in the skin. Parathyroid hormone
(PTH) stimulates the final formation of Vit. D by
the kidneys when blood calcium is low.
Small Intestine: Movement
• Peristalsis moves chyme through the intestine.
• Segmentation mixes chyme with intestinal juices.
Fig. 24.3, p. 890
Hormonal Control of
Small Intestine Activity
Gastrin secreted by stomach is stimulatory. It stimulates:
– contraction of intestinal smooth muscle; and
– relaxation of ileocecal valve.
Vasoactive intestinal peptide (VIP) from the duodenum
acts on the duodenum to stimulate secretion of
bicarbonate-rich intestinal juice to neutralize the
Somatostatin from stomach and duodenum inhibits
blood flow and absorption from the small intestine.
Nervous System Control of Small
Intestine Activity
• Sympathetic impulses decrease activity.
• The gastroileal reflex, initiated by increased activity
in stomach, is a long reflex involving brain and
parasympathetic innervation.
– Parasympathetic impulses increase activity in the
small intestine.
Accessory Glands:
Liver Gross Anatomy
• The liver is the largest gland in body and weighs
approximately 1.4 kg (a little over 3 lbs).
• It is mainly located in the upper right hypochondriac
region and extends into the epigastric region.
• There are 4 primary lobes: right, left, caudate,
• The liver is covered by serosa except for uppermost
region just under diaphragm
Fig. 24.1, p. 888
See Fig. 24.23, p. 919
Liver: Hepatic Ducts
Hepatic ducts carry bile from the liver to the small
• The right hepatic duct serves the right lobe.
• The left hepatic duct serves the other 3 lobes.
• The common hepatic duct is formed from the union
of the right and left hepatic ducts.
– The common hepatic duct joins the cystic duct of
gall bladder to form common bile duct, which
p. 915main pancreatic duct to form hepatojoins
pancreatic ampulla.
Liver: Ligaments
• The falciform ligament is a piece of mesentery that
separates right and left lobes.
– It suspends liver from diaphragm and anterior
abdominal wall.
• The round ligament (ligamentum teres) is a remnant
of the umbilical vein in the inferior portion of the
falciform ligament.
• The ligamentum venosum is a remnant of the ductus
venosus and runs through the liver.
See Fig. 24.23, p. 919
Liver: Blood supply
• The hepatic artery supplies oxygen-rich arterial
• The hepatic portal vein carries nutrient-rich venous
blood from the stomach, intestines, pancreas, and
spleen (see lab for vessels) to the liver.
• The hepatic vein drains venous blood from the liver
into the inferior vena cava.
Fig. 20.27, p. 771
Liver: Microscopic Anatomy
• The liver is designed to filter and process nutrientrich blood.
• It is composed of lobules which are hexagonal
structure with a portal triad at each corner. The portal
triad is composed of:
– a branch of hepatic artery;
– a branch of hepatic portal vein; and
– a bile duct.
Fig. 24.24, p. 921
Liver: Microscopic Anatomy (con’t)
• Sinusoids are specialized capillaries in which venous
and arterial blood mix.
– Hepatocytes (liver cells) just outside the walls of
the sinusoid perform the functions of the liver.
– The walls of the sinusoids are incomplete and
Kupffer cells are space irregularly along the wall.
These fixed macrophages remove debris, bacteria,
and worn out RBCs.
Fig. 24.24, p. 921
Liver: Microscopic Anatomy (con’t)
• A central vein drains each lobule.
– These veins join to form hepatic veins
• Bile canaliculi are channels between the hepatocytes.
– They carry bile and join to form bile ducts.
– Bile flow is counter to blood flow.
Fig. 24.24, p. 921
Liver Functions
Liver functions include:
• Processing blood-borne nutrients, including:
– storing glucose (as glycogen);
– storing fat-soluble vitamins; and
– storing iron (Fe);
• Detoxifying poisons (toxins);
• Production of plasma proteins (see Topic 1);
• Cleansiong the blood of debris, including bacteria and
worn out RBCs; and
• Production of bile and bile salts.
Liver Functions: Bile
• Bile consists of bile salts, bile pigments, cholesterol,
neutral fats, phospholipids, electrolytes in water.
• Bile salts aid digestion of fat because they:
– emulsify (break up) fat globules into droplets; and
– form micelles (which ferry fats to mucosal wall).
• Bile salts are conserved by the enterohepatic
circulation in which some is reabsorbed by the ileum
and returned to liver via hepatic portal system.
• The main bile pigment is bilirubin, which is:
– formed from breakdown of hemoglobin, and
– metabolized by bacteria in large intestine (it
becomes a brown pigment).
Control of Bile Production
Bile production is stimulated by
• bile salts returning via hepatic portal blood; and by
• secretin (a hormone secreted by the small intestine
in response to fats in chyme).
Fig. 24.25, p. 923
Liver Disorders/Disease
• Hepatitis is an inflammation of the liver, often caused
by viral infection (HVA, HVB, HVC, HVD).
– Hepatitis can be transmitted enterically (HVA) or
through blood (HVB, HVC, HVD).
– Blood-borne viruses are linked to chronic hepatitis
and cirrhosis.
• Cirrhosis is a chronic disease characterized by growth
and replacement of hepatic tissue by scar tissue.
• Jaundice is a yellowing of the skin due to a build up
of bilirubin from liver disease or excessive
destruction of RBCs (e.g., neonatal jaundice).
Accessory Glands: Gall Bladder
• The gall bladder lies in a depression on ventral
surface of liver.
• It is a thin-walled, muscular sac that holds about 50
ml of bile.
• It is designed to store and concentrate bile.
• It releases bile via the cystic duct.
• Histologically, it has 4 tissue layers:
– The mucosa has cells with microvilli to increase
reabsorption of water.
– The submucosa consists of dense CT.
– The muscularis of smooth muscle contracts to
expel bile
– The serosa is present over the ventral portion only.
Control of Bile Release
• Bile produced by the liver backs up into the gall
bladder when the hepatopancreatic sphincter is closed.
• The gall bladder releases bile into the cystic duct
when stimulated by cholecystokinin (secreted by
duodenum) and/or parasympathetic impulses.
• Release is inhibited by somatostatin produced by the
stomach and duodenum.
Fig. 24.25, p. 923
Disorders of the Gall Bladder
• Gallstones (biliary calculi) result from crystallization
of cholesterol due to excess of cholesterol or too little
bile salts. These hard structures can become lodged in
the duct system.
• Obstructive jaundice is yellowish coloration of skin
caused by build up of bile pigments resulting from
blockage of bile ducts.
Accessory Glands: Pancreas
• The pancreas is mostly retroperitoneal; the head is
encircled by duodenum, the tail abuts the spleen.
• Acinar cells (acini) secrete pancreatic juice rich in
– The enzymes are stored in inactive form in
zymogen granules within the cells until released.
– Pancreatic juice is excreted through the main and
accessory pancreatic ducts.
• The islets of Langerhans are the endocrine cells of
the pancreas.
– Beta cells secrete insulin.
– Alpha cells secrete glucagon.
– Delta cells secrete somatostatin.
See Fig. 24.20, p. 915
Composition of Pancreatic Juice
• Pancreatic juice is watery and rich in bicarbonate
– Bicarbonate makes it alkaline and neutralizes the
acidity of chyme entering the duodenum from the
• Digestive enzymes (see Small Intestine: Digestion):
– Proteases are produced and released as zymogens
(inactive precursors).
° Trypsin is released as trypsinogen and activated
by enterokinase enzyme in brush border cells.
° Carboxypeptidase & chymotrypsin are activated
from precursors by trypsin.
Pancreatic Enzymes (con’t)
Digestive enzymes produced by the pancreas include:
• pancreatic amylase, which hydrolyzes starch and
glycogen (animal “starch”) into short carbohydrate
chains and maltose;
• pancreatic lipases, which hydrolyze neutral fats into
fatty acids and glycerol (or mono- and diglycerides);
• pancreatic nucleases, which hydrolyze nucleic acids
into nucleotides; and
• pancreatic nucleosidases, which hydrolyze
nucleotides into ribose, nitrogen bases and phosphate.
Control of Pancreatic Secretion
• Secretin stimulates acini to produce juice rich in
– Secretin is released from the small intestine in
response to acidic chyme entering the duodenum.
• Cholecystokinin (CCK) is released from duodenum
in response to fatty or protein-rich chyme.
– CCK stimulates the acini to secrete juice rich in
• The Vagus nerve stimulates secretion during the
cephalic and gastric phases of digestion.
Fig. 24.28, p. 925
Pancreas’ Endocrine Role: Insulin
• Insulin is secreted when blood glucose increases.
• Insulin lowers blood sugar by:
– stimulating uptake by body cells (except liver,
kidney and brain);
– stimulating glycogen formation in liver and
skeletal muscle;
– stimulating carbohydrate metabolism in most cells;
– inhibiting gluconeogenesis (conversion of fats and
protein to glucose) in liver.
• In the absence of insulin, fat is released and
metabolized resulting in the formation of keto acids,
which lower the pH of the blood if uncompensated.
Pancreas’ Endocrine Role: Glucagon
• Glucagon is secreted in response to low blood
• Glucagon increases blood sugar by:
– promoting breakdown of glycogen
(glycogenolysis) by the liver;
– stimulating synthesis of glucose from lactic acid
and noncarbohydrate sources (gluconeogenesis) by
– stimulating release of glucose into blood by the
liver; and
– inhibiting uptake and use of carbohydrates by
skeletal muscle. Skeletal muscle can metabolize
fats and amino acids as well.
Disorders of the Pancreas
Pancreatitis is an inflammation of the pancreas.
• It is sometimes caused by excessive fat in blood.
• Pancreatitis results in release and activation of
enzymes within pancreas (pancreas digests itself).
• Most severe cases are due to alcohol abuse.
Large Intestine
• The large intestine is located primarily in abdominal
cavity, distal end is in pelvic cavity.
• The large intestine is larger in diameter, but shorter
(~1.5 m) than the small intestine.
• Modifications seen in the large intestine include:
– reduction of longitudinal layer of muscularis to form
strips called the teniae coli;
– pocket-like sacs called haustra formed by motor tone
of the teniae coli; and
– epiploic appendages, small, adipose-filled pouches of
visceral peritoneum.
Fig. 24.29, p. 928
Large Intestine: Subdivisions
• The cecum (caecum) is the first, small pouch-like
area of the large intestine; it receives chyme from the
– The vermiform appendix contains lymphatic tissue
for protection against disease.
• The “colon” forms the bulk of the large intestine and
is composed of:
– the ascending colon, that rises along the right side;
– the transverse colon, that goes across just under the
Fig. 24.29, p. 928
– the descending colon, that goes down along the left
side; and
– the sigmoid colon, that enters the pelvic cavity.
Large Intestine: Subdivisions
• The rectum is the pouch just before the anal canal. It
stores feces until they are released.
• The anal canal is the final portion of the large
intestine and serves as the passageway for removal of
Fig. 24.29, p. 928
Large Intestine: Anal Canal
• The mucosa of the anal canal is arranged as long
folds called anal columns.
– The mucosa is composed of stratified squamous
– Anal sinuses secrete mucus when compressed by
feces. The mucus helps lubricate and makes
defecation easier.
• Sphincters control defecation.
– The internal anal sphincter is composed
Fig. of
24.29, p. 928
smooth muscle and is involuntary.
– The external anal sphincter is composed of
skeletal muscle and is under voluntary control.
Large Intestine: Histology
• The mucosa is thicker than in small intestine and
contains crypts with numerous goblet cells.
– Most of the mucosa is simple columnar epithelium.
– Stratified squamous epithelium is present in the
anal canal where it protects against abrasion.
• The submucosa is thinner than in small intestine and
has less lymphatic tissue.
• The longitudinal layer of the muscularis is modified as
strips of muscle called the teniae coli, causes the colon
to form bag-like structures called haustra.
• The serosa covers all of the colon except the region in
the pelvic cavity (sigmoid colon and rectum).
See Fig. 24.31, p. 930
Intestinal Flora
• Resident bacteria present in the large intestine are
dominated by Escherichia coli (E. coli).
• These bacteria ferment some indigestible
carbohydrates resulting in mixture of irritating acids
and gases.
• They also synthesize several B vitamins and vit. K.
Digestion in Large Intestine
• There is no additional breakdown of molecules except
by bacteria.
• Reabsorption of water and electrolytes by the large
intestine are very important to water and electrolyte
– Severe diarrhea can lead to severe dehydration.
• Vitamins produced by bacteria are absorbed.
Movements in Large Intestine
Movement in the large intestine forms feces.
• Haustral churning is a slow process in which
distention of a hastrum stimulates contraction which
moves food into next haustrum.
– This process mixes food residue and aids water
• Mass peristalsis refers to long, slow movements along
the length of the large intestine, wherein feces are
forced toward the rectum.
– Mass peristlsis is stimulated by gastrocolic reflexes
based on stretching of stomach.
To release feces, the two sphincters must relax.
• Entrance of feces into the anal canal results in
parasympathetic reflex relaxation of smooth muscle
(internal) sphincter.
• The external sphincter (skeletal muscle) is controlled
by voluntary nerve impulses.
– Damage to the descending tracts of the spinal
cord or cerebral cortex result in loss of ability to
control defecation.
Fig. 24.32, p. 931
Large Intestine: Disorders
• Appendicitis is an inflammation of the appendix,
usually caused by bacterial infection. Rupture of the
• Diarrhea results when the colon is irritated and
watery stools are produced.
– Diarrhea is caused by irritants, bacterial or viral
– Loss of water and electrolytes can lead to
dehydration and electrolyte imbalances.
Large Intestine: Disorders
• Constipation refers to the condition in which hard
stools are produced due to increased time for water
– Diets low in fiber may lead to constipation.
– Constipation can also lead to electrolyte and pH
• Hemorrhoids are due to inflammation of the
superficial anal veins. They can either be external or
internal and are often painful.
• Colitis is any inflammation of the colon.
Large Intestine: Disorders
• Diverticulosis is the formation of small pouches
(called diverticula) in the mucosa of the large
– This is common in the elderly, especially in people
with poor diets low in fiber. Fiber holds water and
keeps the feces more pliable.
• Diverticulitis is an inflammation of the diverticula.
– Inflammed diverticula can rupture leading to
• Crohn’s disease is a chronic inflammation of the GI
tract, usually involving the ileum or large intestine.
Changes in the wall occur.
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