Nutrition Science and Everyday Application - beta v 0.1

VITAMINS AND MINERALS INVOLVED IN FLUID AND ELECTROLYTE BALANCE 463
Figure 8.6. Fluid compartments in the human body. The intracellular fluid (ICF) is the fluid within
cells. The extracellular fluid (ECF) includes both the blood plasma and the interstitial fluid (IF)
between the cells.
Although water makes up the largest percentage of body volume, it is not actually pure
water, but rather a mixture of dissolved substances (solutes) that are critical to life. These
solutes include electrolytes, substances that dissociate into charged ions when dissolved
in water. For example, sodium chloride (the chemical name for table salt) dissociates into
sodium (Na + ) and chloride (Cl − ) in water. In extracellular fluid, sodium is the major positivelycharged
electrolyte (or cation), and chloride (Cl − ) is the major negatively-charged electrolyte
(or anion). Potassium (K + ) is the major cation inside cells. Together, these electrolytes are
involved in many body functions, including water balance, acid-base balance, and assisting
in the transmission of electrical impulses along cell membranes in nerves and muscles.
FLUID AND ELECTROLYTE BALANCE
One of the essential homeostatic functions of the body is to maintain fluid and electrolyte
balance within cells and their surrounding environment. Cell membranes are selectively
permeable: Water can move freely through the cell membrane, while other substances,
such as electrolytes, require special transport proteins, channels, and often energy. The
movement of water between the intracellular and extracellular fluid happens by osmosis,
which is simply the movement of water through a selectively permeable membrane from an
area where solutes are less concentrated to an area where solutes are more concentrated
(Figure 8.7).