Science High School Reviewer | Grade 4 Science | Life Science | Systems of the Human Body | Lesson 14: The Digestive and Nervous Systems

The digestive system makes nutrients available to all body cells.
The nervous system controls all other systems in the body.
The Digestive System
One of your favorite foods might be pizza. It contains many
nutrients your body needs. But a whole pizza, or even a thin
slice, could not travel through your body. Even the tiniest piece
of food is too large to be used by your cells.
Any food you eat must be broken down into nutrients that
your cells can use. Your digestive system does this job, which is
called digestion. Digestion is the process that breaks food into
forms that your cells can use.
Digestion begins as soon as you bite into food. It continues
as the foods you eat travel through the organs in your
digestive system. The food changes into nutrients that your
body can use. The nutrients pass into your blood through the
thin wall of blood vessels in the lining of your intestines. Your
circulatory system carries blood with the nutrients to the cells
throughout your body.

Parts of the Digestive System (diagram Scott Foresman)

Digestion begins in your mouth. Your teeth
and chemicals in your saliva start to break
down the food.

The food you swallow enters a tube called
the esophagus. The esophagus is about
25 centimeters long. Smooth muscles in its
walls contract and move the food toward
your stomach.

Next, the partly digested food enters your
stomach. Smooth muscles in the walls of
your stomach mash the food. They mix it
with juices from your stomach walls until
it is a thick liquid. Muscles then push the
food into the small intestine.

The small intestine is a twisted, folded
tube that is about 7 meters long and
2.5 centimeters wide. Most digestion
takes place during the three to six hours
that food stays in your small intestine.
Juices made in the small intestine and
other organs break the food into nutrients.

Anything that cannot be digested and
some liquid move into the large intestine.
The large intestine removes much of the
liquid and stores the solid waste until it
leaves the body.

The Central Nervous System
Your central nervous system is the control center of
your body. It links all body systems and carries signals
from one system to another. Your brain and spinal cord
are parts of your central nervous system. Your central
nervous system controls your breathing, heart rate,
and the movements of skeletal and smooth muscles.

The central nervous system also handles information
about what is happening outside your body. It gets
much of this information from sense organs, such as
your eyes, ears, nose, and tongue. The system then
sends messages to different body systems so you can
respond. Suppose a friend yells, “Catch!” You see the
ball, reach out, and grab the ball in midair. All these
actions depend on the central nervous system sending
signals between many body parts.

The Spinal Cord
The spinal cord is the main communications path
that connects the parts of the central nervous system.
Three layers of connective tissue cover and protect the
spinal cord. Signals pass along it between your brain
and the rest of your body. Sometimes the spinal cord
decides what your body does. Your body reacts before
you think for even a split-second about what to do.
The action is quick and automatic. Reflexes such as
blinking or pulling your hand away from a hot surface
are controlled by your spinal cord.

The Brain
The brain is a very complex organ that is made
up of billions of nerve cells. It is the central nervous
system’s main switching area. The same three layers of
connective tissue that cover the spinal cord also cover
the brain. A watery fluid also surrounds the brain and
spinal cord. The skull, layers of connective tissue, and
fluid all help protect the brain from injury.

Neurons
The basic working
unit of the nervous
system is the
nerve cell, or
neuron. All
parts of your
nervous system contain
neurons. A nerve is a bundle
of neurons, and many
bundles of neurons make up
the spinal cord. Neurons are
specialized cells. They carry
information in the form of
electrical signals. A signal
to or from the brain goes
through many neurons.
Every neuron has a cell
body with a nucleus. The
cell body has two types of
extensions. One type carries
nerve signals away from
the cell body. The other type
sends them toward it.