ANATOMY & PHYSIOLOGY
THE DIGESTIVE SYSTEM
Digestive System
 Ingestion: receiving food
 Digestion: breakdown of food particles small enough to
pass through cell membranes
 Absorption: transfer of food into circulation
A. Alimentary Canal – The Tube from beginning
to end ~ 9m or 30ft.
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Oral Cavity
Pharynx
Esophagus
Stomach
Small Intestine
Large Intestine
Rectum/Anal Canal
Walls of Alimentary Canal – 4 layers (inside  outside)
 Mucosa – secrete mucus, protects surrounding tissue, absorbs
nutrients
 Submucosa – nourishes surrounding tissues, carry away absorbed
nutrients
 Muscular layer – move food to stomach (peristalsis)
1) Circular: changes diameter of tube
2) Longitudinal: changes length of tube
 Serous layer/serosa – produce serous fluid to prevent friction
Tube Movements
 Mixing – some sections of the tube contract to mix and
breakdown food
 Propelling (peristalsis) – wave-like movements push food
through the canal
B. Mouth/ Oral Cavity
 Cheeks – muscles for chewing
 Lips – sensory and manipulation of food
 Tongue – manipulated food; taste buds/receptors
• Frenulum connects the tongue to the bottom of the mouth
 Palate (roof of mouth) – hard
anterior, soft posterior
• Uvula is extension of
soft palate hangs down to
direct food
 Teeth
• 20 as a child, 32 as an adult
• Incisors and canines: cutting and tearing
• Premolars and molars: mastication
 Salivary Glands
• Saliva – mostly water; slightly acidic, some antibodies
1) Serous cells: secrete serous fluid with digestive enzymes
(amylase enzyme of the mouth) to lubricate food and begin
digestion
2) Mucous cells: produce mucous to clump food into a ball/bolus
3) Three Glands
a.
b.
c.
Parotid – below the ear
Submandibular – under the mandible (jaw)
Sublingual – under the tongue
Pharynx – cavity; posterior end of the mouth, leads to
the esophagus
 Step 1 – chew, breakdown, mix with saliva into bolus
 Step 2 – swallowing reflex, once near the pharynx
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Soft palate is raised to block the nasal passageway
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Hyoid bone – larynx elevated to block trachea epiglottis then covers the
opening
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Muscle relax, lower pharynx
and open esophagus
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Peristaltic wave begins
pushing food down
esophagus
Stomach – located on left portion of body, j-shaped
 Holds between 50 mL – 4L, folded when empty
 Digest proteins
 Absorbs very little – only few liquids, drugs, alcohol, and
salts
 Mechanical and chemical digestion
Regions of the Stomach
 Cardia –small region at the esophageal-stomach junction
 Fundus – formed by the upper curvature of the organ; temporary
storage area for food
 Body – main, central region; holds max 4 Liters, avg. 0.9L
 Pylorus – small region at the stomach-duodenal junction; holds 30
mL only releases 3mL
Cells of the Stomach/ Gastric Glands
 Mucous cells – secrete mucous for protection
 Chief cells – secrete digestive enzymes; pepsinogen
 Parietal cells – secrete HCL (hydrochloric acid); pH~ 2
 Pepsinogen (an inactive enzyme) is converted to Pepsin when it
reacts with HCl/acidic environment
 Pepsin breaks down proteins into amino acids
3 Layers of the Stomach Wall
 Serosal: outermost; thick connective tissue binds to other organs
 Muscularis: thick-middle; three layers of smooth muscle tissue.
 Mucosal: innermost; relatively thick and contains numerous
tubular glands called gastric pits
• Little to no digestion occurs here; alkaline (high pH) to combat HCl
• Ulcers- holes in stomach, esophagus, duodenum brought on by stress
Control of Gastric Juices
 Sight, smell, taste stimulates secretions (parasympathetic)
 Distension of the stomach and production of chemicals due to
breakdown of food
• Gastrin: hormone released with secretion also stimulates
increase of secretion
• Decrease juices if lots of fat/protein in the duodenum or if the
intestine is extended.
Digestion and Absorption in Stomach
 HCl + churning action  breaks down food
 Smaller particles does what to surface area? What does
this have to do with enzyme digestion?
– Enzymes able to work better because they can attack a higher
surface area
 Chyme: mixture of small food particles + acid + mucous
 Little absorption occurs through the walls of the
stomach; foods high in fat stay in stomach the longest
Exiting the Stomach
 Chyme: Ingested substances combine with the secretions of the
glands of the stomach, producing a viscous, highly acidic soupy
mixture of partially digested food
Accessory Organs
 Liver
 Gallbladder
 Pancreas
 Liver: Largest gland/organ of the body; produces bile that is
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stored in the gallbladder
supplies quick energy
metabolizes alcohol
makes proteins
stores vitamins and minerals
regulates blood clotting
regulates cholesterol production
detoxifies poisons
 3lbs.
 Requires 25% of cardiac
output
 Gallbladder: stores and concentrates bile that is not needed
immediately for digestion
 Bile: yellow-green, alkaline solution containing bile salts,
bile pigments (primarily bilirubin), cholesterol, neutral
fats, phospholipids, and a variety of electrolytes
• Fat emulsification via bile salts
• Bile does not usually enter the small intestine until the gallbladder
contracts
 Pancreas (exocrine)
• Digestive enzymes - break down carbs, proteins, and fats.
• Base, bicarbonate ion (alkaline).
- Neutralizes chyme from the stomach, permitting digestive
enzymes to function.
• Secretin stimulates production
of bicarbonate ions
• Cholecystokinin (CCK) stimulates
production of enzymes
Enzymes include: proteases,
pancreatic lipase, and pancreatic
amylase
Small Intestine
 Receives chyme, pancreatic and liver secretions to
complete digestion of nutrients
• Duodenum: 25cm long x 5cm wide; c-shaped, most absorption of
nutrients occurs here
• Jejunum: middle; thicker more vascular walls, greater in diameter
• Ileum: last section; suspended from abdominal wall by mesentery
Duodenum
 Pancreatic enzymes & bile enter, continuing digestion.
 Villi: microscopic hair-like structures; absorption of nutrients
• Active transport of sugars, amino acids, and fatty acids across membranes.
• Pass through the epithelial wall into the bloodstream.
 Enzymes: break down carbs, proteins, and fats into sugars, amino
acids, & fatty acids
 This is where most nutrients are absorbed in the digestive system
Large Intestine
 Large Intestine: serves to compact the solids remaining
after digestion.
• Lacks villi: little to no absorption of nutrients
• Water and a few ions (e.g. sodium) are absorbed through the
walls of intestine; only sig. secretion is mucous
• Undigested material then passes into the rectum and is finally
eliminated through the anus.
Sections of the Large Intestine
 Cecum: beginning of the large intestine;
receives chyme from the ileum, and connects to
the ascending colon; appendix attached
 Ascending Colon: begin the process of
extracting water and electrolytes; peristalsis
required to move up to transverse colon
 Transverse Colon: attached to the stomach by
a wide band of tissue called the greater
omentum; mobile section
 Descending Colon: store undigested wastes
before moving into the rectum; IBS and colon
cancer most common here
 Sigmoid Colon: s-shaped; walls are muscular
and contract to increase pressure inside the colon
to move wastes into the rectum
 Rectum: final straight portion of the large
intestine; acts as a temporary storage site for
undigested wastes; stretch receptors located in
the walls
Ailments & Diseases of the Digestive System
 Dysphagia: difficulty swallowing; common in old age or after a
stroke/nervous system malfunction
 Gastroesophageal Reflux Disease (GERD): stomach contents
leak backwards from the stomach into the esophagus irritating the
esophagus, causing heartburn
 Crohn’s Disease: type of inflammatory bowel disease (IBD) that
may affect any part of the gastrointestinal tract; primarily
causes abdominal pain, diarrhea, vomiting, or weight loss; caused
by interactions between environmental, immunological and
bacterial factors in genetically susceptible individuals
 Ulcers: Peptic ulcers are holes/breaks in the lining of the
duodenum or the stomach, areas that come into contact with
stomach acids and enzymes. Duodenal ulcers are more common
than stomach ulcers; recent theory holds that bacterial infection is
the primary cause is the bacterium Helicobacter pylori (H. pylori)
 Celiac Disease: a disease that damages the small intestine and
interferes with absorption of nutrients from food. Cannot tolerate
gluten, a protein in wheat, rye, and barley. Gluten is found mainly
in foods but may also be found in everyday products such as
medicines, vitamins, and lip balms.
 Ulcerative colitis: a chronic, long-lasting disease that causes
inflammation and sores (ulcers) in the lining of the large intestine,
especially the colon and rectum
 Irritable Bowel Syndrome (IBS): a common disorder that
affects the large intestine (colon); commonly causes cramping,
abdominal pain, bloating gas, diarrhea and constipation
 Diverticulosis: small pouches may form along the walls of the
large intestine called diverticuli; may collect and not be able to
empty fecal material which can lead to inflammation
 Jaundice: “yellow” in french; backup of bile by-products from
the blood into body tissues. May result from blockage of the ducts
draining bile into the intestines
 Gallstones: hard, pebble-like deposits that form inside the
gallbladder; may lead to inflammatory condition characterized
by retention of bile in the gallbladder
 Cirrhosis: is scarring of the liver and poor liver function. It is the
final phase of chronic liver disease; most commonly caused
by alcoholism, hepatitis B and hepatitis C
 Hepatitis: inflammation of the liver; viruses cause most cases of
hepatitis. Drug or alcohol use can also cause hepatitis. In other
cases, your body mistakenly attacks healthy cells in the liver.
• A - hepatitis A virus (HAV); usually spread by eating or drinking food or
water contaminated with infected feces, undercook shellfish, or close
contact with an infectious person. After a single infection a person is
immune for the rest of their life.
• B - hepatitis B virus (HBV); transmitted by exposure to infectious blood
or body fluids; causes liver inflammation, vomiting, jaundice, and, rarely,
death. Chronic hepatitis B may eventually cause cirrhosis and liver cancer.
• C - hepatitis C virus (HCV); spread primarily by blood-to-blood contact
associated with intravenous drug use, poorly sterilized medical equipment,
and transfusions; often asymptomatic, but chronic infection can lead to
scarring of the liver and ultimately to cirrhosis, liver failure, or liver cancer.
 Vaccines are readily available in developed countries