FLUID, ELECTROLYTES, ACID/BASE BALANCE
CHAPTER 27 SUMMARY
G.Brady/SFCC/2014
A/P 242
Tortora, 13th ed.
Body fluid refers to body water and its dissolved substances.
Regulatory mechanisms ensure homeostasis of body fluids since their malfunction may seriously endanger nervous
system and organ functioning.
In the average adult, body fluids comprise about 60% of total body weight. Water is the main component of all body
fluids.
About two-thirds of the body's fluid is located inside cells and is called INTRACELLULAR FLUID.
The other one third is called EXTRACELLULAR FLUID. About 80% of extracellular fluid is found in between cells and
is called INTERSTITIAL FLUID. The other 20% of extracellular fluid is blood plasma.
Examples of interstitial fluid are:
Lymphatic fluid, cerebrospinal fluid, GI tract fluids, synovial fluid, fluid in the eyes (aqueous humor and vitreous body)
and ears (perilymph and endolymph), pleural, pericardial and peritoneal fluids and glomerular filtrate in the kidneys.
Selectively permeable membranes separate body fluids into distinct compartments. Plasma membranes of individual
cells separate intracellular fluid from interstitial fluid. Blood vessel walls divide interstitial fluid from blood plasma.
Although fluids are in constant motion from one compartment to another, the volume of fluid in each compartment
remains fairly stable; another example of homeostasis.
Fluid "balance" means that the various body compartments contain the required amount of water, proportioned
according to their needs.
Osmosis is the primary way in which water moves in and out of body compartments. The concentrations of solutes in
the fluids is a major determinant of fluid balance.
Most solutes in body fluids are ELECTROLYTES, compounds that dissociate into ions. Fluid balance means water
balance, but also implies electrolyte balance since the two are inseparable.
BODY WATER
Water is the most abundant molecule in the body and its percent of total body weight depends on age and the amount
of fat present.
Fluid intake (gain) normally equals fluid output (loss) so the body maintains a constant volume.
Regulation of Water Gain:
Metabolic water volume depends mostly on the level of aerobic cellular respiration, which reflects the demand for ATP
in body cells.
The main way to regulate body water balance is by adjusting the volume of water intake.
When water loss is greater than water gain, dehydration occurs. The stimulus for fluid intake (gain) is dehydration,
resulting in thirst sensations. One mechanism for stimulating the thirst center in the hypothalamus is the renin-
angiotensin II pathway, which responds to decreased blood volume (therefore, decreased blood pressure), which
results in the secretions of aldosterone from the adrenal glands which increases water reabsorption.
