Interstitial fluid originates primarily as the blood's fluid portion that escapes into
the tissues from the capillaries. Part of this fluid enters the beginning of the venous
vessels. A large portion is picked up by the lymphatic system. (para 2-9)
The body's tendency to maintain a steady state is called homeostasis. As a part
of this, the body maintains appropriate concentrations of oxygen, carbon dioxide,
nutrients, electrolytes, and other substances within the tissue fluid. A system that helps
to maintain the appropriate concentrations of oxygen and carbon dioxide is the
respiratory system. A system that helps to maintain the appropriate concentration of
nutrients is the digestive system. Such organ systems are at least partially controlled by
feedback mechanisms. These mechanisms resemble the household thermostat.
The body needs a certain amount of water to function properly. Lack of fluid may
result in heart failure. Too much fluid tends to result in swelling, known as edema. To
maintain water balance, there are feedback mechanisms. (para 2-11)
If the concentration of electrolytes is greater in the tissue fluid than in the
intracellular fluid, the tissue fluid is hypertonic. If the concentrations of electrolytes is
less in the tissue fluid than in the intracellular fluid, the tissue fluid is hypotonic. If the
concentration of electrolytes are equal in the tissue fluid and intracellular fluid, the fluids
Assume that a substance is more concentrated on one side of a membrane than
the other but that it cannot pass through the membrane. The process in which water
passes to the side of greater concentration is called osmosis.
If, however, the substance can easily pass through the membrane, the process in
which the substance passes through the membrane to the area where it is less
concentrated is called diffusion. If the substance needs assistance to pass through the
membrane, we call the process facilitated diffusion.
Since the cell membrane allows only some types of substances to pass through
it, the cell membrane is semipermeable.
When energy is used to move a substance across a membrane to an area of
higher concentration, the process is called active transport.
When a minute amount of tissue fluids is engulfed by the cell membrane, the
process is pinocytosis. When larger particles are engulfed, the process is phagocytosis.