The digestive system converts food to nutrients which the body
uses for growth, energy, and cell repair.
ORGAN
FUNCTION
Mouth
Chewing; Breaks down starches with saliva;
Moves food to esophagus (through pharynx)
Esophagus
Swallowing; Transports food to stomach
Stomach
Upper muscle relaxes and lets food enter;
lower muscles mix food with digestive juices;
breaks down protein with stomach acid;
Absorbs some fat-soluble substances;
releases food into duodenum (beginning of
small intestine) as chyme
Small
Propels food at a slow enough rate for
intestine
digestion and absorption; absorbs
breakdown products of carbohydrates,
proteins, lipids, and nucleic acids, along with
vitamins, minerals, and water
Liver
Breaks down fats with bile acids
Gallbladder
Stores, concentrates, and releases bile
Pancreas
Produces digestive enzymes and bicarbonate;
breaks down starches, fats, and proteins with
pancreatic juice
Large
Further breaks down food residues; absorbs
intestine
water; remaining waste material is stored as
feces
Colon
Part of large intestine; reabsorbs fluids;
prepares waste for its elimination
Anus
Opening at lower end of digestive tract;
controls expulsion of feces
SPECIAL CELLS/TISSUES:
Intestinal cells: in small intestine secrete, hydrolytic enzymes (break down macromolecules like carbohydrates); Villi/microvilli:
increase the surface area of the small intestine (absorbs more nutrients); Mucosa: surrounds the lumen, single layer of epithelial
cells (secrete mucus and enzymes—protect organs and break down food); Submucosa tissue layer: blood and lymph vessels,
carry absorbed nutrients to the rest of the body; Circular muscle layer: surrounds the submucosa, constricts the gut;
Longitudinal muscle layer: surrounds the submucosa, shortens the gut; Sphincter muscles: help the passage of contents through
the gut (in the stomach, small and large intestines, and anus); Peritoneum: surrounds the gut, secretes a fluid (lubricates organs
so they easily slide against each other); Epiglottis tissues: moves down to cover the entrance to the larynx when food is
swallowed; Chief cells (secrete pepsinogen/inactive pepsin) and parietal cells (secrete hydrochloric acid which kills
microorganisms, helps decompose food, and activates the pepsinogen to pepsin) are in the gastric pit.
HOW IT WORKS:
A mouth takes in food; saliva begins breaking down the food. The food moves through the esophagus to a storage sac, usually
called a stomach. The stomach physically (and sometimes enzymatically) breaks down the food, and produces a slurry of small
particles to the midgut, ready for digestion. Food travels through the midgut, often called a small intestine (the primary site of
digestion and absorption). Here, hydrolytic enzymes are secreted by intestinal cells and associated glands, and macromolecules
are broken down into monomers that are absorbed into the blood. The hindgut (large intestine), recovers water and ions and
stores undigested wastes, or feces, until they can be expelled through what is usually called the anus.
REGULATION AND FEEDBACK:
The enteric nervous system of the digestive system communicates some information to and from the CNS, and coordinates the
movement of food through the gut. When food arrives in the stomach, the wall of the stomach secretes the hormone gastrin
which circulates in the blood and returns to the stomach, causing an increase in motility and the secretion of digestive juices.
When acidic chyme arrives in the duodenum, cells start to release secretin which stimulates the pancreas to produce and
secrete bicarbonate ions that neutralize the acidic chyme. In the postabsorptive state, a lack of insulin protects glucose supplies
for the nervous system. During the absorptive state, the liver stores fuel as glycogen and fats and synthesizes blood plasma
proteins from circulating amino acids. When fuel molecule levels in the blood decline, the liver delivers the stored nutrients into
the blood. High-density lipoproteins remove cholesterol from tissues and carry it to the liver, where it is used to synthesize bile.
Low-density lipoproteins transport cholesterol around the body for use in biosynthesis and for storage. Very low-density
lipoproteins contain mostly triglyceride fats, which they transport to fat cells. Hunger and satiety centers in the hypothalamus
are powerfully affected by several digestive hormones, including insulin, CCK, and leptin. Gastrin release begins to be inhibited
when the pH of the stomach contents falls below 3—negative feedback. Both secretin and CCK are negative feedback signals to
the stomach to slow the delivery of chyme into the duodenum.
INTERACTIONS:
Integumentary system: Vitamin D activated in the skin plays a role in
absorption of calcium from the digestive tract; Skeletal System: Bones are
important in mastication (chewing); Muscular System: Muscles are important
in mastication, swallowing, and the mixing and moving of digestion products
through the gastrointestinal tract; Nervous System: Regulates the speed at
which food moves through
the digestive tract; Endocrine
System: Regulates digestive
function by secreting
hormones (chemical
messengers); Cardiovascular
System: The bloodstream
carries absorbed nutrients to
all body cells; Lymphatic
System: The lymphatic
system plays a major role in
the absorption of fats;
Respiratory System: The
digestive system and the
respiratory system share
common anatomical
structures; Urinary System: The kidneys and liver work together to activate
vitamin D; Reproductive System: Adequate availability of nutrients, including
fats, is essential for conception and normal development.
Human
Amphibians
Fish
Reptiles
Birds
Majority of nutrient
Shorter small intestine
Most nutrient
Digestion is slower than High metabolic rates;
absorption in
than humans, most
absorption occurs in
in mammals; most
nutrient absorption is
jejunum
nutrient absorption in
the intestine
nutrient absorption
mainly in the lower
ileum
occurs in the intestine
small intestine
Use teeth for all
Teeth used to hold
Fish without teeth
Teeth are attached by
No teeth; Use beaks;
things including
things in place; Swallow swallow food whole; one side of the inner
Swallow stones to grind
chewing; one set of
prey whole; maxillary
fish with pharyngeal
surface of the jaw bone; food; also have gizzard
teeth
and vomerine teeth
teeth crush food; fish used to tear, bite, crack, to grind food
with sharp
and/or puncture food
pharyngeal teeth
shred food
Rectum and urethra
Only have cloacae
Food is excreted
Food is excreted
Food is excreted
are present
through the
through the cloacae
through the cloacae
vent/anus
Tongues attached to
Tongues connected to
Tongue is a fold in
Many have a forked
Many have harder
backs of mouths;
start of mouth; sticky
the floor of the
tongue; use to smell and tongues than humans;
gustatory and taste
end to assist prey
mouth; conducts
sense
tongue function varies
receptors
capture
water through mouth
based on food source