The Digestive System
Lizzie Tillo
Caroline Gerhardt
Digestive Tracts
 Functions: ingests food, breaks it down, absorbs
nutrients, eliminates undigestible remains
 Not ALL animals have a digestive tract
 Ex) sponges- food vacuoles
 Most animals have a gut or digestive tract
 Food is digested into small particles that can cross the
plasma membranes
Incomplete/Complete Tracts
 Incomplete tracts have a single opening- entrance for
food/exit for wastes
 Ex) planarians
 Complete tracts have a mouth and an anus, and often many
specialized parts
 Ex) earthworms
 Typhlosole: internal fold that increases the surface area of
digestive tract in order to absorb more nutrient molecules
Continuous Feeders
 Filter Feeders
 Ex) clams
 Water is always moving into the mantel cavity through the
incurrent siphon (slitlike opening), depositing small particles
on the gills
 Larger particles moved by ciliary action to the labial palps,
which force them through the mouth & into the stomach
 Ex) baleen whale
 Baleen hangs from the roof of the mouth and filters krill
(small shrimp) from the water
Discontinuous Feeders
 Require a storage area for food, such as a crop or a
stomach
 Can be carnivores (ex. blue herons) or herbivores (ex.
elephants)
 Ex) Squid (mollusk)
 Has 2 long tentacles with toothed rings that seize prey
 Pieces pulled into mouth by radula, a tonguelike structure
 Stomach (with cecum) retains food until digestion is complete
Diet Adaptations




Filter feeders- omnivores
Land snails- herbivores
Spiders, sea stars- carnivores
Horses- sharp incisors for clipping off blades of grass;
molars for crushing the grass, disrupting the cell wall
 Bacteria called cecum is able to digest cellulose
 Cattle, sheep, deer- graze quickly & swallow partially
chewed grasses into a rumen
 Cud is formed and regurgitated; chewed and swallowed again
for complete digestion
Human Digestive Tract
 Extracellular
 2 major stages:
 Mechanical digestion: physical breakdown of food into
smaller particles
 Chemical digestion: requires enzymes secreted by the
digestive tract or by accessory glands to break down
macromolecules
Mouth
 Beginning of digestive tract
 Palate (roof of the mouth)- separates the oral cavity from the
nasal cavity
 Cheeks and lips retain food while it is chewed by teeth and
mixed with saliva
 3 major pairs of salivary glands send their juices to the mouth
through ducts
 Salivary amylase- enzyme in saliva that begins the process of
starch digestion
 Disaccharide maltose is typical end product
 Bolus- food and saliva mixture formed in preparation for
swallowing
Pharynx and Esophagus
 Pharynx- common passageway for both food intake and air
movement; located between the mouth and the esophagus
 Esophagus- tubular structure (25 cm) that takes food to the
stomach
 Sphincters- muscles that encircle tubes and act as valves
(contract- close, relax-open)
 Lower gastroesophageal sphincter- where esophagus enters
stomach
 When food enters the esophagus, peristalsis occurs- rhythmical
contraction that serves to move the contents along the tubular
organs
Stomach
 Lies on left side of body beneath the diaphragm
 Deep folds (rugae) disappear as the stomach fills to about 1 liter
 Epithelial lining contains millions of gastric pits, which have
gastric glands that produce gastric juice
 pH of about 2; kills bacteria and stops the activity of salivary
amylase
 Thick layer of mucus protects wall
 Ulcers- open sores caused by the gradual destruction of tissues
 Food + gastric juice- chyme
 Slow manner in which it enters the small intestine allows for
thorough digestion
Small Intestine
 Small in diameter, but about 6 m in length
 Duodenum- first 25 cm
 Duct brings bile from the liver and gallbladder, and pancreatic juice from the
pancreas
 Bile- emulsifies fat, causing fat droplets to disperse in water
 Slightly basic pH due to sodium bicarbonate from pancreatic juice to
neutralize chyme
 Food digestion is completed by the enzymes in pancreatic juice and
enzymes produced by the intestinal wall
 Fingerlike projections called villi- composed of microvilli; greatly increase
surface area for absorption of nutrition
 Lacteal- vessels of a villus where nutrients are absorbed
 Then carried to all cells of the body by the boodstream
Large Intestine




Includes cecum, colon, rectum, and anus
Larger in diameter but shorter in length (1.5 m)
Absorbs water, salts, and some vitamins
Stores undigestible material until it is eliminated at the anus
(**NO digestion occurs here**)
 Cecum- blind end of large intestine; has small projection
called the appendix (plays a role in fighting infections)
 Colon joins the rectum (last 20 cm of intestine)
 About 95% of water is absorbed by the small intestine, and much
of the rest is absorbed by the colon
 Diarrhea- occurs when water is not absorbed; can lead to
serious dehydration
 Large amounts of bacteria (esp E Coli) ; break down
undigestible material and produce some vitamins
 Wastes leave through anus (feces: 25% sold material, 75%
water)
 Colon is subject to polyps (small growths arising from mucosa)

Dietary fat increases likelihood of colon cancer; causes increase in bile secretion, and
intestinal bacteria convert bile salts to substances that promote the development of
cancer
Pancreas
 Deep in abdominal cavity
 Endocrine function- secretes insulin and glucagon,
which keep blood glucose level within normal limits
 Exocrine function- produce pancreatic juice with
sodium bicarbonate and digestive enzymes for food
 Neutralizes acid chyme from stomach
 Amylase digests starch, trypsin digests protein, and
lipase digests fat
Liver
 Largest gland in body; upper right section of abdominal cavity
 Triads (between the lobules-structural and functional units)
consist of bile duct, a branch of the hepatic artery, and a branch
of the hepatic portal vein
 “gatekeeper to blood”; as blood passes through the liver
removes poisonous substances and detoxifies them; also
removes and stores iron and vitamins
 Regulates quantities of cholesterol
 Maintains blood glucose level
 Produces bile stored in the gallbladder
 Bile slats emulsify fat in the small intestine, providing a larger
surface area
Liver Disorders
 Life-threatening: hepatitis and cirrhosis
 Affect the entire liver and hinder its ability to repair itself
 Jaundice- skin and whites of eyes have a yellowish tinge (bilirubin is
deposited in skin due to its large amount in the blood)
 Hepatitis: inflammation of the liver
 A: sewage-contaminated drinking water
 B: sexual contact or contaminated needles
 C: contact with infected blood- can lead to chronic hepatitis, liver cancer,
and death
 Cirrhosis: liver tissue replaced by scar tissue; often in alcoholics, as
the liver is forced to break down excessive amounts
 Can recover if rate of regeneration exceeds rate of damage
Gallbladder
 Pear-shaped, muscular sac attached to the surface of the
liver
 Stores excess bile
 Reabsorbs water so that bile becomes a thick, mucuslike
material
 can leave and proceed to the duodenum
 Cholesterol content can come out of solution and form
crystals, whose growth can lead to gallstones
 Could block common bile duct and cause obstructive jaundice