Human physiology is the science that explains the

normal operation of the human machine. To under-

stand that operation, the body must be studied at all

levels of integration, from the submolecular to the

whole organism. Each level is a web of complex re-

lationships. Molecular biology has shown that the

ultimate components of the machine--proteins--

can be understood as machines in themselves. Cells,

long considered the basic building blocks of organ-

isms, are now understood as highly organized inter-

communicating systems. At the highest level of

complexity, the brain (probably the most complex

structure known) cannot be understood by study of

molecular interactions aone but requires a highly

integrated approach.

Today s students of human physiology may be

preparing for careers in health care delivery or

health related research or education. Others simply

are curious about how their own bodies work. For

all of our students, our goal in preparing the second

edition of Human Physiology: Foundations & Frontiers,

as it was in the firsl edition, was to provide an up-

to-date grasp of the subject as well as to confer the

excitement of its current discoveries. We included a

sense of the history of the science by giving credit to

some of its important pioneers. Respecting the in-

telligence of today s students and the awesome

complexity of the human organism, we endeavored

to present clear, complete explanations without dis-

torting oversimplification. Early in the text we dis-

tinguish between "how" and "why" explanations of

body functions and related the latter to human evo-

lution.

Elementary textbooks of science tend to present

as revealed fact what is in actuality only current

consensus. By including some of the experimental

bacleground for key insights into how the body

works, we remind students that experimentation,

including animal studies, is a primary source of in-

formation in physiology. Where important ques-

tions remain unanswered, we point this out and

give current hypotheses.

Although this text focuses mainly on the normal

function of the body, we introduce disease condi-

tions where they illustrate fundamental physiologi-

cal principles. This challenges the students to syn-

thesize information and begLl applying the course