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Chapter 24
The Digestive System: An Overview
▪Works in concert with respiratory, cardiovascular, and lymphatic systems to
provide body tissues with appropriate nutrients and oxygen.
▪Consists of the digestive tract (gastrointestinal tract or alimentary canal) and
accessory organs.
▪Functions of the Digestive System
▫Ingestion▫Mechanical processing▫Digestion▫Secretion▫Absorption▫Excretion▫Lining of the digestive tract also functions in protection of surrounding
tissues.
▪The Digestive Organs and the Peritoneum
▫The peritoneal cavity is lined by a serous membrane that consists of a
superficial mesothelium covering a layer of areolar tissue.
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▫Mesenteries- double sheets of peritoneal membrane that suspend
portions of the digestive tract, connects the parietal and visceral
peritoneums.
-Lesser omentum-Falciform ligament-Greater omentum-Mesentery proper-Mesocolon▪Histological Organization of the Digestive Tract
▫The mucosa is the inner lining of the digestive tract, is a mucous
membrane (epithelium and lamina propria)
▫The digestive epithelium is either simple or stratified, depending on
location and common stresses.
▫The lamina propria consists of a layer of areolar tissue that contains
blood vessels, sensory nerve endings, lymphatic vessels, smooth muscle
cells, and scattered lymphoid tissue.
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▫The submucosa is a layer of dense irregular CT that surrounds the
muscularis mucosae.
▫The muscularis externa is a region dominated by smooth muscle cells,
found outside the submucosa.
▫The serosa is a serous membrane that surrounds most portions of the
digestive tract, it is basically the visceral peritoneum.
▪The Movement of Digestive Materials
▫Peristalsis▫Segmentation-
▪Control of the Digestive Function
▫ Local Mechanisms▫ Neural Mechanisms▫ Hormonal Mechanisms-
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The Oral Cavity
▪The Tongue- manipulates materials inside the mouth and is occasionally used to
bring foods into the oral cavity. The primary functions of the tongue include:
1) mechanical processing by compression, abrasion, and distortion
2) manipulation to assist in chewing and prepare for swallowing
3) sensory analysis in touch, temperature, and taste
4) secretions of mucins and lingual lipase
▫The tongue can be divided into the body and the root.
▫Epithelium covering the inferior surface of the tongue is thinner and more
delicate than that of the dorsum.
-Lingual frenulum▫Tongue epithelium contains glands that secrete water, mucins, and
lingual lipase
▫The tongue contains both extrinsic and intrinsic skeletal muscles.
-Extrinsic muscles-Intrinsic muscles-Both groups of muscles are innervated by the hypoglossal nerve.
▪Salivary Glands- three pairs of salivary glands secrete into the oral cavity
▫Parotid salivary glands- large glands that lie inferior to the zygomatic arch
and under the skin that covers the mandible.
▫Sublingual salivary glands- located on the floor of the mouth, covered by
mucous membrane
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▫Submandibular salivary glands- located within the mandibular goove.
▫Saliva is 99.4% water, with the remaining 0.6% a mixture of electrolytes,
buffers, glycoproteins, antibodies, enzymes, and waste products.
▫Control of salivary secretion is normally controlled by the ANS
▪The Teeth- Adults can have up to 32 teeth
▫Occlusal surfaces▫Dentin▫Pulp cavity▫Root canal▫Periodontal ligament▫Cementum▫Enamel▫Types of teeth- there are four types of teeth:
1. incisors
2. cuspids
3. bicuspids
4. molars
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▫Dental succession▫Mastication-
The Pharynx
▪A common passageway for food and air, composed of nasopharynx, oropharynx,
and laryngopharynx.
The Esophagus
▪The esophagus is a hollow muscular tube about 25 cm long, and a diameter of 2
cm.
▪Its primary function is to carry food and liquids to the stomach
▪The esophagus enters the abdominopelvic cavity through the esophageal hiatus,
an opening in the diaphragm
▪Histology of the Esophagus
▫The wall of the esophagus contains mucosal, submucosal, and muscularis
layers.
▪Swallowing, also called deglutition, is a complex process that starts voluntarily,
then proceeds automatically. Divided into the buccal, pharyngeal, and
esophageal phase.
▫Buccal phase▫Pharyngeal phase▫Esophageal phase6
The Stomach- the stomach has four major functions
1)
2)
3)
4)
▪Anatomy of the Stomach- is the shape of an expanded J
▫Lesser and greater curvature forms the shape of the stomach
▫The stomach can be divided into four regions; cardia, fundus, body,
pylorus.
-Cardia-Fundus-Body-Pylorus▫While relaxed, the mucosa is thrown into prominent folds called rugae,
allows for stomach to expand when food enters.
▫Contains circular, longitudinal, and oblique layers of smooth muscle
▫Simple columnar epithelium lines the entire stomach, provides a carpet of
mucus that covers the interior surfaces, alkaline property protects from
acids and enzymes.
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▫Gastric Glands-
-Parietal cells- secrete intrinsic factor, facilitates the absorption of
vitamin B12, also secrete HCl, which has four important functions:
1)
2)
3)
4)
-Chief Cells- secrete pepsinogen which is converted to pepsin by
acids in the lumen of the stomach.
-Pyloric Glands- mainly produces mucous secretions, along with
some endocrine hormones
▪Regulation of Gastric Activity
▫Can be controlled by the CNS, regulated by short reflexes of the enteric
nervous system, or regulated by hormones of the digestive tract.
▫There are three phases of gastric control:
-Cephalic phase-
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-Gastric phase- begins with the arrival of food in the stomach.
-Neural response-
-Hormonal response-
-Local response-
-Intestinal phase- begins when chyme starts to enter the small
intestine.
-Neural response-
-Hormonal response▪Digestion and Absorption in the Stomach
▫Carbohydrates and lipids continue being digested in the stomach, until
the pH goes below 4.5
▫Proteins begin being broken down by pepsin at and below pH of 2
▫Absorption does not take place because:
1)
2)
3)
4)
▫Alcohol and aspirin can be absorbed in the stomach
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The Small Intestine and Associated Glandular Organs
▪The Small Intestine- plays a key role in the digestion and absorption of nutrients
▫90% of nutrient absorption occurs in the small intestine, the rest occurs
in the large intestine
▫Averages about 6 meters (20 feet), consists of the duodenum, jejunum,
and the ileum.
-Duodenum-
-Jejunum-
-Ileum-
▪Histology of the Small Intestine
▫Circular folds▫Intestinal Villi-Lacteal▫Intestinal Glands- goblet cells located between the columnar epithelial
cells eject mucins onto the intestinal surfaces.
-Brush border enzymes-Duodenal glands-
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▫Regional specializations▫Intestinal secretions▪Intestinal Movements
▫Short reflexes control weak peristaltic movements from the duodenum to
the jejunum
▫Some smooth muscle cells contract periodically, even without stimulation,
establishing a basic contractile rhythm that then spreads from cell to cell.
▫Stimulation of the parasympathetic system increases the sensitivity of the
reflexes, and accelerates both local peristalsis and segmentation.
▫Gastroenteric reflex▫Gastroileal reflex▪The Pancreas- located posterior to the stomach, and laterally from the
duodenum toward the spleen.
▫Histological Organization-The surface of the pancreas has a lumpy,
lobular texture
-Primarily an exocrine organ, producing digestive enzymes and
buffers
-Pancreatic duct-
-Accessory pancreatic duct-
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-Pancreas is separated into lobules, contains pancreatic acini
-Pancreatic enzymes are produced by the acinar cells
▫Physiology of the Pancreas- main function is to produce and secrete
pancreatic juice to aid in chemical digestion.
-Secretory activities are controlled primarily by hormones from the
duodenum.
-When chyme enters the duodenum, secretin is released,
secretin triggers a watery alkaline solution
-Cholecystokinin stimulates production and secretion of
pancreatic enzymes.
-Pancreatic enzymes include:
-Pancreatic alpha-amylase-Pancreatic lipase-Nucleases-Proteolytic enzymes (proteases and peptidases)
-Proenzymes▪The Liver- the largest visceral organ, one of the most versatile organs in the
body.
▫Anatomy of the liver- The liver is wrapped in a tough fibrous capsule, and
is covered by a layer of visceral peritoneum
-Falciform ligament12
-Caudate lobe-Quadrate lobe-Porta hepatis-Hepatocytes-
▫Histological organization of the liver- each lobe of the liver is divided by
CT into approximately 100,000 liver lobules, the basic functional unit of
the liver.
-At the center of each lobule is a central vein, sinusoids empty their
contents into the central vein.
-Kupffer cells-
-Portal areas-
-Blood travels from hepatic artery and hepatic portal vein, through
the sinusoids, and into the central vein, which drains into the
hepatic veins.
▫Bile duct system- bile first enters the bile canaliculi, then it enters the bile
ductules. From there, it enters the right or left hepatic ducts, then on to
the common hepatic duct.
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▫Physiology of the liver- the liver has three general categories of function;
metabolic regulation, hematological regulation, and bile production.
-Metabolic regulation- the liver is the primary organ involved in
regulating the composition of circulating blood.
Carbohydrate metabolism:
Lipid metabolism:
Amino acid metabolism:
Removal of waste products:
Vitamin storage:
Mineral storage:
Drug inactivation:
-Hematological regulation- the liver is the largest blood reservoir in
the body. As blood passes through it, it performs these functions:
Phagocytosis and antigen presentation:
Synthesis of plasma proteins:
Removal of circulating hormones:
Removal of antibodies:
Removal/ storage of toxins:
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Synthesis and secretion of bile:
-Functions of Bile:
▪The Gallbladder- a hollow organ that stores concentrated bile prior to excretion
into the small intestine.
▫Hepatopancreatic sphincter▫The gall bladder has two major functions:
1)
2)
▪The Coordination of Secretion and Absorption
▫Neural mechanisms involving the CNS deal with:
1)
2)
▫Intestinal hormones- all are peptide hormones with similar structures,
hard to identify functions and target.
Secretin: released when chyme inters the duodenum.
-Origin-Target-Effects-
Cholecystokinin (CCK): released when chyme containing lipids and
partially digested proteins enter the duodenum.
-Origin-Target-Effects-
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Gastric Inhibitory Peptide (GIP): released when chyme containing
large quantities of fats and glucose enter the small intestine.
-Origin-Target-Effects-
Vasoactive Intestinal Peptide (VIP): released when chyme enters
the duodenum
-Origin-
-Target-Effects-
Gastrin: released in response to vagus nerve stimulation of arrival
of food in the stomach, and arrival of chyme containing large
quantities of undigested protein.
-Origin-Target-Effects-.
Enterocrinin: released when chyme enters the duodenum
-Origin-Target-Effects▫Intestinal Absorption- usually takes about 5 hours for materials to pass
from the duodenum to the end of the ileum
-The movement of the small intestine along with the plicae, villi,
and microvilli cause the intestinal contents to change constantly.
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The Large Intestine
▪The Cecum- expanded pouch at the beginning of the large intestine
▫Ileocecal valve▫The cecum collects and stores materials from the ileum and begins the
process of compaction
▫Appendix- slender hollow organ attached to the posteromedial surface of
the cecum.
-The mucosa and submucosa are dominated by lymphoid nodules
▪The Colon- has a larger diameter and a thinner wall than the small intestine.
Distinctive features include:
▫Wall of the colon forms haustra (pouches), allow for expansion
▫Taeniae coli▫Omental appendices- numerous teardrop-shaped sacs of fat.
▫The colon can be subdivided into four regions:
-Ascending colon-Transverse colon-Descending colon-Sigmoid colon▪The Rectum- forms the last 15 cm of the digestive tract
▫Expandable for the temporary storage of fecal material
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▫The movement of fecal materials into the rectum triggers the urge to
defecate
▫Anal canal-The distal portion of these columns is where the columnar
epithelium changes to keratinized stratified squamous epithelium
▫Anus-Internal anal sphincter-External anal sphincter▪Histology of the Large Intestine- thinner wall than the small intestine
▫The colon lacks villi, but has an abundance of goblet cells and distinctive
intestinal glands.
-The intestinal glands are dominated by goblet cells, which produce
high quantities of mucus, which provide lubrication for fecal
material.
-Mucus is secreted in response to friction or exposure to
harsh chemicals
▪Physiology of the Large Intestine- no digestive enzymes are released by cells in
the large intestine. If any are present, they came from the small intestine, or
the normal microbiota of the large intestine.
▫Less than 10% of nutrient absorption takes place in the colon
▫Reabsorption of water is the primary function of the large intestine
▫The large intestine also reabsorbs bile salts, vitamins, and organic waste
products
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▫Bacteria in the large intestine generate three vitamins that supplement
our dietary supply:
-Vitamin K-Biotin-Vitamin B5▫Organic wastes absorbed by the large intestine include:
-Bilirubin-Peptides that remain in the feces are broken down by bacteria and
this generates ammonia (NH4+), indole and skatole, and hydrogen
sulfide (H2S).
-Ammonia and other toxins are absorbed and converted in
the liver to less toxic substances that can be released into
the blood, or excreted at the kidneys.
▫Movements of the large intestine- gastroileal and gastroenteric reflexes
move materials into the cecum.
-Movement from the cecum to the transverse colon is slow,
allowing for water reabsorption
-Peristaltic waves move along the length of the colon, and haustral
churning mix the contents of adjacent haustra.
-Mass movements occur a few times a day, are powerful peristaltic
contractions that move material from the transverse colon through
the rest of the large intestine
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-Defecation reflex- distension of the rectal wall triggers this reflex,
involves two positive feedback loops.
-Rectal stretch receptors also trigger two reflexes important in the
voluntary control of defecation. Both are long reflexes.
-The first triggers the relaxation of the internal sphincter
-The second triggers the contraction of the external
sphincter
Digestion and Absorption
▪The Processing and Absorption of Nutrients
▫Food contains large organic molecules, many of them insoluble
▫These molecules have to be broken down to be able to be absorbed.
▫The smaller products of digestion will either be used to provide energy or
used to synthesize carbohydrates, proteins, and lipids.
▫Digestive enzymes break the bonds between these large macromolecules
using a process called hydrolysis (adding a water to a molecule to break it
into smaller parts)
-These enzymes come from saliva, tongue, stomach, pancreas, and
the brush border.
▪Carbohydrate Digestion and Absorption- proceeds in two steps:
▫Salivary and pancreatic enzymes- break complex carbohydrates into
disaccharides and trisaccharides
-Salivary amylase20
-Pancreatic alpha-amylase-
▫Brush Border Enzymes-
▫Intestinal epithelium absorbs monosaccharides by facilitated diffusion and
cotransport.
▪Lipid Digestion and Absorption- involves lingual lipase and pancreatic lipase
▫The most abundant and important of dietary lipids are triglycerides.
▫Lipases break the triglycerides into monoglycerides
▫Bile salts emulsify these lipid droplets, producing micelles, a much smaller
lipid droplet.
▪Protein Digestion and Absorption- proteins are large and complex, so their
digestion is complex and time consuming.
▫Mastication and the highly acidic environment of the stomach provide
physical breakdown of food into smaller pieces so pepsin can reach the
individual proteins.
▫Once in the small intestine, pancreatic proteases can begin working.
▫Epithelial cells of the small intestine contain dipeptidases that break short
peptides into individual amino acids
*see diagram 24-27 and table 24-1 for a summary of the chemical events in digestion*
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▪Water Absorption- water cannot be absorbed or secreted, it moves by osmosis
▫Water moves to where there are higher concentrations of solutes
*see diagram 24-28 to visualize water movements into and out of the digestive tract.*
▪Ion Absorption- many regulatory mechanisms controlling the rate of ion
absorption are poorly understood.
▫Na+ may enter intestinal cells via diffusion, cotransport, or active
transport.
-High salt in food increases Na+ uptake, leading to water following
▫Ca2+ absorption involves active transport at the epithelial surface
▫K+ passively diffuses following the concentration gradient
▫Mg2+ and Fe2+ use specific transport proteins, and are moved using
active transport.
▫Cl-, I-, HCO3-, and NO3- are absorbed by diffusion or carrier-mediated
transport
▫PO43-, and SO42- enter epithelial cells only be active transport.
▪Vitamin Absorption- include fat soluble and water soluble vitamins
▫Fat soluble vitamins (A, D, E, K) can dissolve into lipids, absorbed with
micelles.
▫Water soluble vitamins (B vitamins, C), there are 9 altogether. All but one
(B12) can be easily absorbed by diffusion across the epithelial membrane.
*see table 24-2 to see the summary of ion and vitamin absorption along the digestive tract*
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