the study of the function of body parts and the body as a whole.
Gross (macroscopic) anatomy
: the study of body parts visible to the naked eye, such as the heart or
: the study of tissues at the microscopic level.
: the study of cells at the microscopic level.
: the study of how the nervous system functions.
Organizations of living systems
Living systems can be defined from various perspectives, from the very broad (looking at the entire earth)
to the very minute (individual atoms). Each perspective provides information about how or why a living
At the chemical level, atoms, molecules (combinations of atoms), and the chemical bonds between atoms
provide the framework upon which all living activity is based.
is the smallest unit of life. Organelles within the cell are specialized bodies performing specific
cellular functions. Cells themselves may be specialized. Thus, there are nerve cells, bone cells, and
: is a group of similar cells performing a common function. Muscle tissue, for example, consists
of muscle cells.
: is a group of different kinds of tissues working together to perform a particular activity. The
heart is an organ composed of muscle, nerve, connective, and epithelial tissues.
An organ system
: is two or more organs working together to accomplish a particular task. The digestive
system, for example, involves the coordinated activities of many organs, including the mouth, stomach,
small and large intestines, pancreas, and liver.
: is a system possessing the characteristics of living things—the ability to obtain and
process energy, the ability to respond to environmental changes, and the ability to reproduce.
: a characteristic of all living systems is homeostasis, or the maintenance of stable, internal
conditions within specific limits. In many cases, stable conditions are maintained by negative feedback.
In negative feedback
, a sensing mechanism (a receptor) detects a change in conditions beyond specific
limits. A control center, or integrator (often the brain), evaluates the change and activates a second
mechanism (an effector) to correct the condition. Conditions are constantly monitored by receptors and
evaluated by the control center. When the control center determines that conditions have returned to
normal, corrective action is discontinued. Thus, in negative feedback, the variant condition is canceled, or
negated, so that conditions are returned to normal. Compare this with positive feedback, in which an
action intensifies a condition so that it is driven farther beyond normal limits. Such positive feedback is