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NUR 1023 Exam 1,2 & 3 Study Guide 2022 Fluid and Electrolytes
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NUR 1023 Exam 1,2 & 3 StudyGuide 2022 Fluid and Electrolytes
Definition
Fluid and electrolytes: refers to the process of regulating the extracellular fluid volume, body fluid
osmolality, and plasma concentrations of electrolytes.
Fluid is water plus the substances dissolved and suspended in it.
o Important characteristics of fluid are its volume (amount) and its degree of concentration
(osmolality).
Electrolytes are substances that are charged particles (ions) when they are placed in water.
o Examples of electrolytes are sodium ions (Na +), potassium ions (K+), calcium ions (Ca2+), and
magnesium ions (Mg2+).
In health care settings, people often omit the word “ion” when discussing electrolytes, referring to
potassium ions as “potassium,” for example.
All body fluids contain electrolytes; body fluids in different locations normally contain different
concentrations of electrolytes that are necessary for optimal function.
Maintaining physiological balance of body fluid and electrolytes is a dynamic interplay between three
processes: intake and absorption, distribution, and output.
o Intake and absorption: refers to the addition of fluid and electrolytes to the body (intake) and
their movement into the blood (absorption).
o Distribution: is the process of moving fluid and electrolytes between the various body fluid
compartments.
These fluid compartments include inside the cells (intracellular) and outside the cells
(extracellular).
The extracellular fluid (ECF) compartment includes fluid between the cells
(interstitial) and fluid inside blood vessels (vascular).
o Output is removal of fluid and electrolytes from the body, through normal or abnormal routes.
Intake of fluid and electrolytes influences output to some degree, but intake can easily become less
than or more than output.
Optimal fluid and electrolyte balance keeps the volume, osmolality, and electrolyte concentrations of
fluid in the various body fluid compartments within their normal physiological ranges.
Scope
Ranges from optimal balance to imbalance
Fluid and electrolyte imbalances can be too little, too much, or misplaced
Conceptually, fluid balance has two aspects: extracellular volume and osmolality.
Electrolyte balance requires separate consideration.
Extracellular fluid volume, body fluid osmolality, and plasma electrolyte concentrations each can be
visualized as a continuum with three categories: optimal balance and two types of imbalances
A. Scope of extracellular fluid volume
B. Scope of body fluid osmolality balance
C. Scope of electrolyte balance
,Normal Physiological Process
The three physiological processes whose interplay creates fluid and electrolyte balance were defined
previously: intake and absorption, distribution, and output. This discussion describes these processes
in more detail.
Optimal balance occurs when both of the following characteristics are present:
o Intake and absorption of fluid and electrolytes match the output of fluid and electrolytes.
o Volume, osmolality, and electrolyte concentrations of fluid in the various body fluid
compartments are within their normal ranges.
To maintain optimal fluid and electrolyte balance, output must be matched by appropriate intake, and
the intake must be absorbed
Intake and Absorption
The most common route of fluid and electrolyte intake is oral.
Other routes include intravenous (IV) administration and less common avenues such as insertion into
the rectum, introduction through nasogastric or other tubes into the gastrointestinal (GI) tract,
instillation into body cavities, and infusion into subcutaneous tissues (hypodermoclysis) or bone
marrow (intraosseous).
Habit and thirst are strong influences on oral fluid intake.
o The most important stimulus to thirst is increased osmolality of body fluids, although dry oral
mucous membranes, angiotensin II, angiotensin III, and arterial baroreceptor stimulation
during severe hypovolemia also trigger thirst.
Fluid and electrolytes that enter the body by all routes except IV must be absorbed into the
bloodstream.
o IV fluid and electrolytes do not need absorption because they enter the bloodstream directly.
Absorption is especially important to consider for oral intake of the electrolytes Ca 2+ and Mg2+, which
have specialized absorption mechanisms in the intestines.
o Ca2+ absorption in the duodenum, its most important absorptive site, is dependent on adequate
availability of vitamin D.
o A healthy intestinal epithelium in the terminal section of the ileum is necessary for Mg 2+
absorption.
o If fluid and electrolytes are not absorbed, they remain in the GI tract and leave the body in the
feces.
Distribution
After fluid and electrolytes enter the blood, they are distributed to various body compartments.
Normal fluid and electrolyte distribution is necessary for optimal function.
The process of filtration distributes the ECF between the two major extracellular compartments:
vascular and interstitial.
, The process of osmosis distributes water between ECF and cells because of osmotic forces.
Numerous factors distribute electrolytes between the ECF and electrolyte pools.
Fluid Distribution Between Vascular and Interstitial Compartments
Fluid distribution between the vascular and interstitial compartments occurs by filtration, the net
result of simultaneous opposing forces at the capillary level.
Two forces tend to move fluid out of capillaries and two other forces tend to move fluid into
capillaries. The forces that are stronger at any particular time determine which way the fluid moves.
o Hydrostatic pressure pushes fluid out of its compartment.
Capillary blood hydrostatic pressure (a relatively strong force) pushes fluid out of the
capillaries; interstitial fluid hydrostatic pressure (a relatively weak force) pushes fluid
out of the interstitial compartment back into capillaries.
o Colloid osmotic pressure, caused by large protein particles in the fluid, pulls fluid into its
compartment.
Thus, capillary blood colloid osmotic pressure (normally a relatively strong force)
pulls fluid into the capillaries; interstitial fluid colloid osmotic pressure (normally a
very weak force) pulls fluid out of the capillaries into the interstitial compartment.
o With normal fluid distribution, the net result of these opposing forces filters some of the ECF
from capillaries into the interstitial compartment at the arterial end of capillaries, bringing
oxygen and nutrients to cells.
o At the venous end of the capillary, some interstitial fluid filters back into the capillary,
carrying waste products to be excreted.
Water Distribution Between ECF and Intracellular Fluid
The structure of cell membranes allows water to cross the membrane readily but Na + enters with
difficulty. This is the reason why cell membranes are called semipermeable.
The process of osmosis is movement of water across a semipermeable membrane that separates
compartments with different concentrations of particles.
o As noted previously, the osmolality of a fluid is its degree of concentration.
Technically, it is the number of particles per kilogram of water.
o Osmosis occurs almost instantaneously when the osmolality changes on one side of the
semipermeable cell membrane.
o This rapid equilibration keeps the ECF and intracellular fluid at the same osmolality.
Electrolyte Distribution
With the exception of Na+, which has a high ECF concentration that reflects osmolality, electrolytes
have low concentrations in the ECF compared to their concentrations in electrolyte pools.
o The K+ pool is inside cells, which contain almost 98% of total body potassium.
o Bone is an important Ca2+ pool.
o Mg2+ pools include inside cells and bones.
o Physiologically inactive forms of Ca2+ and Mg2+ bound to albumin or organic anions such as
citrate also can be considered electrolyte pools.
o Numerous opposing factors influence distribution of electrolytes between the ECF and the
electrolyte pools.
For example, two hormones influence Ca2+ distribution: parathyroid hormone (PTH)
from the parathyroid glands and calcitonin from the thyroid.
Calcitonin moves Ca2+ into bone; PTH shifts Ca2+ from bone into the ECF.
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