Human Physiology
Digestive
System
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Function of Digestive system
• Break up, mix, and move food material
• Secrete enzymes into tube where digestion
occurs
• Digest (breaks down) food particles into smaller
molecules
• Absorb nutrients and fluids
• Eliminate wastes and residues
The Digestive system
The alimentary canal or gastrointestinal (GI) tract digests and absorbs
food
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Alimentary canal –
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Mouth,
Pharynx,
Esophagus,
Stomach,
Small intestine, and
Large intestine
Accessory digestive organs –
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Teeth,
Tongue,
Gallbladder,
Salivary glands,
Liver, and
Pancreas
Processes of the Digestive System: Overview
– Ingestion – taking food
into the digestive tract
– Mechanical digestion –
chewing, churning food,
segmentation
– Propulsion – swallowing
and peristalsis
– Chemical digestion –
catabolic breakdown of
food
– Absorption – movement
of nutrients from the
GI tract to the blood or
lymph
– Excretion – elimination
of indigestible solid
wastes
Human Digestive System
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A complete system with many specialized organs
About 6.5 to 8 meters long if extended
Lined with mucus-secreting epithelium
Movement is one way, from mouth
to anus
Organization of the Digestive tract
1. Mucosa lines digestive
tract
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Moistened by
glandular secretions
Lamina propria and
epithelium form
mucosa
2. Submucosa
3. Muscularis externa
4. Serosa - serous
membrane covering
most of the
muscularis externa
The Oral Cavity
• Mechanical processing by the teeth, tongue, and palatal
surfaces
• Lubrication
• Assistance in swallowing
• Limited digestion
The Teeth
• Twenty deciduous (baby) teeth are replaced by
32 adult teeth.
• Each tooth has a crown and a root.
• The tongue mixes the chewed food with saliva
and then forms the mixture into a mass called a
bolus in preparation for swallowing.
Salivary glands (3 pairs)
• Parotid, sublingual, and submandibular glands produce saliva
– Stimulated by thought of food or ingested food
– Watery solution includes electrolytes, buffers, glycoproteins,
antibodies, enzymes
• Functions include:
– Lubrication, moistening, and dissolving
– Initiation of digestion of complex carbohydrates
Ingestion and Mechanical Digestion
Food is ingested
Mechanical digestion begins (chewing)
Propulsion is initiated by swallowing
Salivary amylase begins chemical breakdown
of starch
• The pharynx and esophagus serve as conduits
to pass food from the mouth to the stomach
• Uvula guards opening to pharynx
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Pharynx
The air passage and food passage cross in the
pharynx because the trachea is ventral to the
esophagus.
Swallowing occurs in the pharynx and is a reflex
action.
During swallowing, the air passage is usually blocked
off by the soft palate and uvula, and the trachea
moves under the epiglottis to cover the glottis
opening to the windpipe.
Esophagus
• The esophagus is a muscular tube that conducts
food through the thoracic cavity and diaphragm
into the stomach.
• Peristalsis begins in the esophagus; this collapsed
tube moves the bolus of food downward after
swallowing occurs.
• No chemical digestion occurs in the esophagus.
• The entrance of the esophagus to the stomach is
marked by a constriction, called a sphincter; the
sphincter must relax in order for food to enter the
stomach.
• The sphincter prevents food from backing up into
the esophagus.
The Stomach
• The stomach expands to store food.
• Food in the stomach is churned, mixing the food
with gastric juices containing hydrochloric acid
and pepsin for the digestion of protein to
peptides.
• Proteins are digested or broken down by the
pepsin in the stomach
• Alcohol, but not food, is absorbed here.
• In 2–6 hours, the soupy chyme leaves the
stomach.
• Ulcers are usually caused by a bacterial
infection.
Stomach secretions
• Gastric glands have a
variety of secretory cells
– Mucous Neck cells– produce
mucus
– Parietal cells – produce HCl and
intrinsic factor
– Chief cells – produce
pepsinogen
• Pepsinogen is activated to pepsin by
HCl in the stomach
– Enteroendocrine cells – gastrin,
histamine, cholecystokinin (CCK
Gastric Hormones
• Gastrin
– Release is stimulated by presence of protein in
stomach
– Acts in several ways to increase secretion of HCl
and pepsinogen
– Enhances gastric motility
Digestion And Absorption In The Stomach
• Preliminary digestion of proteins - pepsin
• Permits digestion of carbohydrates
• Very little absorption of nutrients
– Some drugs, however, are absorbed
Small intestine
• Important digestive and
absorptive functions
– Secretions and buffers
provided by pancreas, liver,
gall bladder
• Three subdivisions:
– Duodenum
– Jejunum
– Ileum
• Ileocecal sphincter transition between small and
large intestine
Small intestine
• Structural modifications of the small intestine wall
increase surface area
– deep circular folds of the mucosa and submucosa
– Villi – fingerlike extensions of the mucosa
– Microvilli – tiny projections of absorptive mucosal cells’ plasma
membranes
Small intestine
• Glands of the duodenum
– Moisten chyme
– Help buffer acids
– Maintain digestive material in solution
• Hormones
– Secretin - produces alkaline buffers, increase bile by
liver and pancreas
– Cholecystokinin – increase pancreatic enzymes,
stimulates contraction of gall bladder, reduces hunger
sensation
– GIP – stimulates release of insulin, inhibits gastric
secretion and motility
The liver
• The largest gland in the body
• Performs metabolic and hematological regulation and
produces bile
• Hexagonal-shaped liver lobules are the structural and
functional units of the liver
• Composed of hepatocytes
The liver
• Hepatocytes’ functions include:
– Production of bile
– Detoxification
• Secreted bile flows between
hepatocytes toward the bile
ducts in the portal triads
Composition of Bile
• A yellow-green
• Bile salts are cholesterol derivatives that:
– Emulsify fat
– Facilitate fat and cholesterol absorption
• The chief bile pigment is bilirubin, a waste
product of heme
The Gallbladder
• Thin-walled, green muscular sac on the ventral surface of the
liver
• Stores and concentrates bile by absorbing its water and ions
• Releases bile via the cystic duct, which flows into the bile duct
The Pancreas
• Pancreatic duct penetrates duodenal wall
Composition of Pancreatic Juice
• Anatomy
– Endocrine
• Pancreatic islets
produce insulin and
glucagon
– Exocrine
• Acini (clusters of
secretory cells) contain
zymogen granules with
digestive enzymes
– Regions: Head, body,
tail
• Secretions
– Pancreatic juice
(exocrine)
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Trypsin
Chymotrypsin
Carboxypeptidase
Pancreatic amylase
Pancreatic lipases
Enzymes that
reduce DNA and
ribonucleic acid
Digestion in the Small Intestine
• As chyme enters the duodenum:
– Carbohydrates and proteins are only partially
digested
– No fat digestion has taken place
• Digestion continues in the small intestine
– Chyme is released slowly into the duodenum
– Virtually all nutrient absorption takes place in
the small intestine
Junction of the small intestine and the large intestine
Large Intestine
• The large intestine consists of the cecum, colon,
rectum and anal canal.
• The large intestine does not produce digestive
enzymes but does absorb water, salts, and some
vitamins.
• The colon includes the ascending colon, the
transverse colon, the descending colon, and the
sigmoid colon.
Large Intestine
• The appendix is an extension of the cecum.
• Indigestible material is stored in the rectum
until the anus allows defecation.
• Anaerobic bacteria in the feces break down
indigestible material and produce some vitamins.
Functions of the Large Intestine
• Other than digestion of enteric bacteria, no
further digestion takes place
• Vitamins, water, and electrolytes are reclaimed
• Its major function is propulsion of fecal
material toward the anus
The Rectum
• Last portion of the
digestive tract
• Terminates at the anal
canal
• Internal and external
anal sphincters
• Defecation reflex
triggered by
distention of rectal
walls
Digestion and Absorption of Nutrients
• Disassembling organic food into smaller
fragments
• Hydrolyzing carbohydrates, proteins, lipids
and nucleic acids for absorption
Digestive system: Nervous regulation
• Nervous regulation
– Involves enteric nervous system
– Coordinates peristalsis and regulates local
reflexes
1. Write the function of Digestive system?
2. Name the organs of alimentary canal or gastrointestinal tract?
• Alimentary canal – Mouth, Pharynx, Esophagus, Stomach, Small intestine, and
Large intestine
• Accessory digestive organs – Teeth, Tongue, Gallbladder, Salivary glands, Liver,
and Pancreas
3. Name the 3 pairs of salivary glands?
• Parotid, sublingual, and submandibular glands
• Produce saliva which lubricates and moistens food
4. Name the enzyme present in mouth? Salivary amylase begins chemical breakdown of
starch
5. What guards the opening of pharynx? Uvula
6. What is the function of sphincter?
• Sphincter is present at the entrance of the esophagus to the stomach. It
prevents food from backing up into the esophagus.
7. What is the role of stomach in digestion?
• The food is mixed with gastric juices containing hydrochloric acid and pepsin
that helps in digestion of protein to peptides.
8. Which cells in gastric glands produce HCl? Parietal cells
9. Where do you find ileocecal sphincter? between small and large intestine
10. What is villi? Villi is finger like extensions of mucosa in intestine
11. What is the function of villi? Increases the surface area of small intestine and helps in
absorption
12. Name the largest gland of the body? Liver
13. Name the structural and functional units of the liver? Hexagonal-shaped liver lobules
14. Write the function of bile?
Bile emulsify fat and facilitate fat and cholesterol absorption
15. What is bilirubin?
Bilirubin is the chief bile pigment
16. Where is bile produced from?
Releases bile via the cystic duct, which flows into the bile duct