This preview shows half of the first page. to view the full 1 pages of the document.
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
o Cell membranes and epithelial tissues are selectively permeable or semipermeable.
o Total body water (TBW): total volume of fluid enclosed within the outer epithelial layer.
o Intra. fluid contains many proteins and is relatively rich in potassium. Extra. fluid
contains few proteins and is relatively rich in sodium.
o 20% of the extracellular fluid is in blood and the remainder is outside the blood. Plasma
is the portion of the blood (relatively rich in proteins). Interstitial fluid is outside the
blood. Capillaries are not permeable to proteins hence no proteins in ISF.
Homeostasis: A Central Organizing Principle of Physiology
Homeostasis: maintenance of constant condition in the internal environment.
The internal environment is regulated to remain constant means that the composition,
temperature, and volume of extracellular fluid do not change significantly under normal
NEGATIVE FEEDBACK CONTROL IN HOMEOSTASIS
Regulated variable: body temperature regulated to stay within relatively narrow limits.
Plasma concentrations of K, Na, and Ca are regulated variables because they are kept constant by
homeostatic regulatory mechanisms or the organ systems.
Negative feedback: if a regulated variable increases, the system responds by making it decrease.
If it decreases, the system responds by making it increase.
Set point: the actual value of the regulated variable and the normal value. Any difference is an
Positive feedback: the response of the system goes in the same direction as the change that sets it
You're Reading a Preview
Unlock to view full version