This course is not led by an instructor
Module 3 /Digestive System
Describe the digestive system: list the major
organs and structures, describe the major functions, and use anatomical planes and
directional terms to identify organs and their relationships to each other.
Digestive System – Function
Also called the gastrointestinal
system, the digestive system breaks down eaten material into
nutrient molecules, absorbs water and ions, and eliminates undigested residue.
The digestive system is a continuous tube (the digestive tract or alimentary
canal). Areas along this tube are specialized to perform different functions
related to getting the nutrients from your food to the cells that need them.
Accessory organs add secretions into different areas along the tube.
Your cells can’t use the pizza you had for lunch in pizza form. It needs to be
broken down into molecules that are small enough to be absorbed. As the pizza
travels along the digestive tract, each organ along the way breaks it down
further. Muscles in the walls of the digestive tract keep things moving along,
and glands in the tissues secrete digestive juices—mostly enzymes and acids—that
break up the larger substances in the pizza into smaller molecules. Food is
physically broken into smaller pieces in a process termed mechanical digestion.
These pieces are then chemically broken down into smaller units in a process
termed chemical digestion. Proteins are broken down into amino acids.
Carbohydrates are broken down into simple sugars. Fats are broken down into
molecules like fatty acids and cholesterol. It is important that the large
particles are broken into their smallest units so they can be absorbed from the
digestive tract into the bloodstream. Therefore, the main functions of the
digestive system are to ingest, break down, and absorb the nutrients from our
food. It also eliminates the wastes (anything not absorbed) as feces.
Digestive System – Organs
The specialized organs of the digestive tract extend in a roughly superior to
inferior direction from the mouth (where food goes in) to the anus (where waste
comes out) in the following order:
- Small intestine (including the duodenum, jejunum, and ileum)
- Large Intestine (including the cecum, colon, and rectum )
Accessory organs in the digestive system are connected to the digestive tract and
secrete additional digestive juices.
The salivary glands produce saliva containing (among others) amylase, an enzyme that breaks
The pancreas secretes a variety of enzymes that break down fats, carbohydrates, and
proteins, as well as bicarbonate ions that neutralize stomach acids. It is
important to note that this function corresponds to the exocrine portion of the
The liver produces bile, which aids in fat digestion and
The gall bladder stores and concentrates bile and secretes it into
the small intestine.
The stomach is a sort of muscular sac that can expand to hold a
large meal. Glands in the walls of the stomach secrete enzymes and acids that
break down food. Muscles in the walls of the stomach churn the food and
digestive juices together. Although the stomach can receive a large amount of
food at a time, it releases its contents gradually into the small intestine, so
that the intestine can better perform its function.
Digestion continues in the small intestine, with additional
digestive juices produced by the pancreas, liver, and the walls of the small
intestine itself. The walls of the small intestine have numerous tiny folds,
which increase its surface area, allowing for efficient absorption of nutrients
into the circulatory system, which in turn takes the nutrients to all the cells
of the body.
Excess water is reabsorbed in the large intestine, and the
undigested portion of your pizza leaves
body. Resident microbes of the large intestine (gut
microbiota) can digest substances that our cells cannot.