the bones of the finger are loosely joined for freedom of
movement, but both places where the bones come to-
gether are considered joints.
Let’s look at how the body is organized.
Levels of Organization Extend from
Atoms to the Human Organism
Your body has many organizational levels (
F igu re 1 .1
Like all other things, your body is made of matter; the
basic building blocks of matter are
ato m s
, such as hy-
drogen, oxygen, carbon, and nitrogen. Atoms combine
m o le cu le s
, which perform various
b io c h e m i-
energy, fats store energy, proteins carry out many func-
tions, and DNA codes information. Molecules compose
c e lls
, which distinguish living things from non-living
chemicals. Examples of cells include neurons, epithe-
lial cells, and myocytes. Cells that have similar func-
tions are organized into
tis s u e s
, such as muscle tissue,
connective tissue, and nervous tissue. Different types of
tissue join together to form structures called
o rg a n s
including the heart, kidney, and brain. Related organs
with a common function join to form a
system); body systems include, among others, the diges-
tive system, cardiovascular system, nervous system, and
respiratory system. Finally, all the systems combine to
o rg a n ism
such as a human. In anatomy and
physiology, the organism is considered the highest level
Let’s look at the various systems of the human body.
Every Body System Performs
If you think about all the things that your body must do
to survive, you will see that you have a system that covers
each one (see
on the following page).
Different tissues join together to
form structures called organs. For
example, your stomach is lined
with epithelial tissue surrounded
by a thick layer of smooth muscle
tissue, which is in turn covered by
a connective tissue membrane.
Related organs with a common function form a
system. For example, your digestive system
consists of your stomach, esophagus, liver,
pancreas, gallbladder, small intestine, and large
intestine. The function of the digestive system is
to break down and absorb the food you eat.
An organism consists of all of
the body systems combined.
An organ is composed of more than one type of