Page 177 - Cardiac Nursing

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CHAPTER
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PRINCIPLES OF FLUID BALANCE cr creased or decreased, then the osmolality of the intracellular fluid
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changes rapidly untill intraccellularr andd extracellularr fluids again
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The fluid in the bboddy serves many viit lal functions. In addition to h have thhe same osmolality. This process is discussed later in the
being the milieu in which cellular chemistry occurs, it provides “Fluid Distribution” section.
the transport medium for oxygen and other nutrients to reach the Although the osmolality of intracellular and extracellular fluids
cells and for carbon dioxide and other metabolic waste products is the same, the ion composition of the two fluids differs. Thus,
to be removed from the body. Technically, fluid is water plus the they have the same particle concentration, but the specific kinds
substances dissolved in it. of particles are different in the two fluids. Intracellular fluid has a
With aging, the amount of water in the body decreases. The higher concentration of protein and potassium, magnesium, and
body ranges from 70% water by weight (newborn infant) to 60% phosphate ions; extracellular fluid has a higher concentration of
3
(young or middle-aged adult) to 45% (older adult woman). sodium, calcium, chloride, and bicarbonate ions. Transcellular
Women have less water by weight than men because a higher per- fluids are usually hypotonic; their ion composition varies widely
centage of their weight is fat. Similarly, water is a lower percent- depending on their physiologic function.
age of body weight in obese people. One liter of water weighs
1 kg (2.2 lb). Thus, a standard 70-kg (154-lb) middle-aged man Processes Involved in Fluid Balance
(60% water) has 42 L of body water (70 kg 0.60 42 kg;
42 kg 42 L). 1 Fluid balance is the net result of fluid intake, fluid distribution,
fluid excretion, and fluid loss by abnormal routes. Fluid balance is
maintained when fluid excretion and fluid loss through any ab-
Body Fluid Compartments
normal routes are matched by fluid intake and when the fluid is
The fluid in the body lies in several compartments. The extracellu- distributed normally into its compartments. 1
lar fluid consists primarily of vascular and interstitial fluids. Some
extracellular fluid is located in bone and dense connective tissue; Fluid Intake
this fluid is not considered accessible for dynamic exchange. Intra- The major determinant of fluid intake in a healthy adult is habit.
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cellular fluid, as the name indicates, lies in the cells. Transcellular Thirst, another important determinant of fluid intake, can be
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fluid is fluid that is secreted by epithelial cells. Examples of tran- caused by several physiologic mechanisms. These include dryness
scellular fluid are cerebrospinal fluid (CSF), saliva, and intestinal of the oral mucous membranes, increase in osmolality of the body
secretions. Many of the transcellular fluids are reabsorbed by the fluids (osmoreceptor-mediated thirst), decrease in extracellular fluid
body after they have been secreted. volume (ECV) (baroreceptor-mediated thirst), and increased renin
More water is located inside the cells than outside of them. Clin- secretion (angiotensin-mediated thirst). Osmoreceptor-mediated
ically, approximately two thirds of body water in adults is consid- thirst is the most common cause of thirst in healthy adults. This
ered intracellular and one third extracellular. Thus, the 70-kg man mechanism becomes less effective with aging. Thus, older adults
who has 42 L of body water can be considered to have approxi- often have a greater need for water before they become thirsty.
mately 28 L of water inside the cells and 14 L of extracellular water. Cultural factors have an important influence on fluid intake. For
This extracellular water is approximately one third vascular and two example, intake of certain herbal teas may be considered necessary
thirds interstitial. For clinical purposes, the 70-kg man can be con- by some individuals when they become ill. Many people refuse to
sidered to have approximately 4.5 L of water in the vascular com- drink cold water when they have certain illnesses due to their cul-
partment and approximately 9.5 L in the interstitial compartment. tural beliefs. In clinical settings, health care professionals often
regulate the fluid intake. Routes of fluid intake include oral, rec-
tal, intravenous, and intraosseous, as well as through tubes into
Osmolality body cavities. Oral fluid intake includes liquids and the water
contained in food, as well as water made by cellular metabolism of
The relative proportion of water to particles in body fluid is meas-
ured as osmolality. Osmolality can be considered to be the degree ingested nutrients.
of concentration. Technically, osmolality is defined as the number
of moles of particles per kilogram of water. The normal range of Fluid Distribution
osmolality of the blood is 280 to 300 mOsm/kg (lower in normal Two types of fluid distribution operate in the body. First, fluid is
2
pregnancy). Fluids that have osmolality within this normal range distributed between the vascular and interstitial spaces, the two
are called isotonic. Extracellular and intracellular fluids have the subcompartments of the extracellular compartment. Second, fluid
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