1 INTRODUCTION TO PHYSIOLOGY
THE OVERALL BODY PLAN: A SIMPLIFIED VIEW
THE BODY’S EXTERNAL ENVIRONMENT
o The epithelial barrier includes the skin and the linings of the lungs, gastrointestinal
system, and kidney tubules.
o The barrier is continuous, there is no read separation between the outer surface of the
skin and the inside surfaces of the lungs, gastrointestinal system, and kidney tubules.
THE BODY’S INTERNAL ENVIRONMENT
o Cells receive nutrients and oxygen from the bloodstream, which also carries carbon
dioxide and waste products away from cells.
o Most cells are not in direct contact with the blood but instead are surrounded by a
separate fluid (internal environment) that exchanges materials with the blood.
THE EXCHANGE OF MATERIALS BETWEEN THE EXTERNAL AND INTERNAL
o The blood must obtain oxygen, nutrients, and other needed materials from the external
environment and must release carbon dioxide and other unneeded materials into it.
o Absorption: water, inorganic salts, and nutrients obtained from digested food are
transported from the lumen to the bloodstream.
o Secretion: stomach uses materials from blood to produce acids and proteins that are
than transported into the lumen.
o Excretion: unabsorbed materials remain in the gastrointestinal tract are eliminated in
the form of feces.
o Filtration: in kidneys, fluid from the bloodstream first enters tubules .
o Reabsorption: needed materials are selectively transported back.
BODY FLUID COMPARTMENTS