The Digestive System
Purpose: to convert foods into simpler
molecules that can be absorbed and
used by the cells of the body
Why is Digestion Important?
• When we eat such things as bread, meat,
and vegetables, they are not in a form that
the body can use as nourishment.
• Our food and drink must be changed into
smaller molecules of nutrients before they
can be absorbed into the blood and carried
to cells throughout the body.
• Digestion is the process by which food and
drink are broken down into their smallest
parts so that the body can use them to build
and nourish cells and to provide energy.
In other words…
• The purpose in your eating a Big Mac,
other than simple hedonism, is to
assimilate the nutrients it represents and
make them available to build, repair and
maintain your own tissues, as well as
provide energy for studying and
occasional other pursuits.
Let’s Break It Down…
• The digestive system it is a tube running from
mouth to anus.
• This tube is like an assembly line, or more
properly, a disassembly line.
• Its chief goal is to break down huge
macromolecules (proteins, fats and starch),
which cannot be absorbed intact, into smaller
molecules (amino acids, fatty acids and glucose)
that can be absorbed across the wall of the tube,
and into the circulatory system for dissemination
around your body.
Components of the Digestive
System
The Major Players
• Mouth
• Pharynx
• Esophagus
• Stomach
• Small Intestine
• Large Intestine
The Accessory Structures
• Salivary Glands
• Pancreas
• Liver
• Teeth
• Tongue
All of these organs work together to carry out…
3 Fundamental Processes:
• Secretion: Delivery of enzymes, mucus,
ions and the like into the lumen, and
hormones into blood.
• Absorption: Transport of water, ions and
nutrients from the lumen, across the
epithelium and into blood.
• Motility: Contractions of smooth muscle in
the wall of the tube that crush, mix and
propel its contents.
The Path of Digestion
• Digestion begins in the mouth (oral cavity)
– Food is broken down by chewing
(mechanical) and enzymes in the saliva
(chemical)
– Teeth coated with enamel cut and tear food
– Saliva contains the enzyme salivary amylase
(breaks down starch)
– Food is turned into a soft ball called a bolus
– Bolus travels from the mouth to the pharynx
(throat)
The Path of Digestion
• Swallowing is a result of the combined
effort of the tongue and throat muscles
• Epiglottis (small flap of connective tissue),
prevents food from going into the trachea
(airway)
• Next, food travels Into the esophagus
a.k.a. “food tube”
• Peristalsis squeezes
food along into the
stomach
Peristalsis
The Path of Digestion
• A ring of muscles called the cardiac sphincter
closes the esophagus after food passes into the
stomach
– Ever had heartburn?
• Stomach continues chemical and mechanical
digestion
– Chemical: Glands release mucous, HCl,
pepsin (enzyme that breaks down protein)
– Mechanical: Muscles contact and churn
contents into chyme (a thin watery liquid)
Liver, Gallbladder, & Pancreas
Liver, Gallbladder, & Pancreas
Liver
• The liver performs numerous function
in the body including storing glucose,
making proteins, and breaking down
toxic substances.
• The liver produces bile, which is vital to
digesting fats.
Liver, Gallbladder, & Pancreas
• Gallbladder
– After the liver produces bile it travels
through a Y-shaped duct to the
gallbladder.
– The gallbladder is a saclike organ that
stores and concentrates bile.
– The gallbladder releases the bile through a
common bile duct into the small intestine
Liver, Gallbladder, & Pancreas
• Pancreas
– The pancreas is an organ that lies behind
the stomach. The pancreas serves two
roles in its part in the digestive system.
• The first role it plays is to increase the pH of
the stomach acid by producing sodium
bicarbonate.
• The second role it plays is to secrete enzymes
that help to further break down carbohydrates,
proteins, lipids and nucleic acids.
The Path of Digestion
• The pyloric valve controls the one-way
flow of chyme from the stomach to the
small intestine
• Small intestine (named for its diameter) is
about 6 meters long and is broken down
into 3 sections
– 1st: Duodenum
– 2nd: Jejunum
– 3rd: Ileum
The Small Intestine
• Most of the chemical digestion and
absorption of food occurs here
• Villi- folded, finger-like projections of
the inner surface of the small intestine
absorb nutrient molecules
Large Intestine
• Once absorption is complete in the small
intestine, peristalsis will move the
remaining contents into the colon or large
intestine.
• The colon has various sections, and all of
these sections work together to finish the
absorption of nutrients and water.
• The colon initiates contractions that move
the material out of the body.
Large Intestine continued…
• As nutrients and water are absorbed
from the matter in the colon, the
matter solidifies into feces.
• Feces leave the body by passing
through the rectum and the anal
canal.
Carbohydrates, Proteins & Lipids
• The three nutrients needed by the body
in the greatest amounts are
carbohydrates, proteins, and lipids.
– All of these nutrients are called organic
compounds, which are compounds that
contain the elements carbon, hydrogen,
and oxygen.
• An example of a carbohydrate is pasta;
an example of a protein is chicken; and
an example of a lipid is olive oil.
Nutrients
• Your body also needs
–Vitamins
–Minerals
–Water