Page 178 - Cardiac Nursing

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154 PA R T I I / Physiologic and Pathologic Responses
is distributed between the extracellular and intracellular com- Similarly, if the extracellular fluid becomes more dilute (decreased
partments. Different processes regulate these two types of fluid osmolality), then the osmotic pressure of the extracellular fluid be-
distribution. comes lower than the osmotic pressure of the intracellular fluid.
Fluid distribution between the vascular and interstitial spaces is Water moves by osmosis into the intracellular compartment until
regulated by filtration. Filtration is the net result of four opposing the intracellular fluid becomes as dilute as the extracellular fluid.
forces. Two of these forces tend to move fluid out of the capillar- This process increases the amount of water that is distributed into
ies, whereas the other two tend to move fluid into the capillaries. the intracellular compartment.
Which direction the fluid moves in any one location depends on In summary, fluid distribution between the vascular and inter-
which forces are stronger. The two forces that tend to move fluid stitial compartments depends on filtration, the net result of four
out of capillaries are the blood hydrostatic pressure (outward force forces that act on fluid at the capillary level. Fluid distribution be-
against the capillary walls) and the interstitial fluid osmotic pres- tween the extracellular and intracellular compartments depends
sure (inward pulling force caused by particles in interstitial fluid). on osmosis, the movement of water across cell membranes to
The two forces that tend to move fluid into capillaries are the equilibrate particle concentrations.
blood osmotic pressure (inward pulling force caused by particles in
blood) and the interstitial fluid hydrostatic pressure. Fluid Excretion
Usually, the blood hydrostatic pressure is highest at the arterial Normal routes of fluid excretion are respiratory tract, urine, feces,
end of a capillary, and there is filtration from the capillary into the and skin (insensible perspiration and sweat). In a standard adult,
interstitial fluid. This flow of fluid out of the capillaries is useful in approximately 400 mL of water is excreted daily through the res-
carrying oxygen, glucose, amino acids, and other nutrients to the piratory tract, even if the person is fluid-depleted. This amount
cells that are surrounded by interstitial fluid. Most proteins are too increases during fever. The urine volume of a healthy adult varies
large to cross into the interstitial fluid and remain in the capillary. according to the fluid intake, the needs of the body, and the hor-
At the venous end of a capillary, the blood hydrostatic pressure is monal status. It averages 1,500 mL. Major hormones that regulate
usually lower and the blood osmotic pressure higher because fluid urinary excretion of fluid are summarized in Table 7-1. Diuretics,
has left the capillary but the proteins have remained. These changes ethanol, and caffeine increase urine volume. Fecal excretion of wa-
cause a net flow of fluid from the interstitial space back into the ve- ter averages 200 mL per day in healthy adults who have a normal
nous end of a capillary. The flow of fluid back into the capillaries fluid balance and a fully functioning bowel. Diarrhea causes a
is physiologically useful in carrying carbon dioxide, metabolic dramatic increase in fecal excretion of water.
acids, and other waste products into the blood for further metab- Insensible perspiration is fluid excretion through the skin that
olism or excretion. is not visible. It averages 500 mL per day in a healthy adult. In-
Changes in any of the four forces that determine the direction sensible perspiration occurs even if the person is fluid-depleted. It
of filtration at the capillaries can cause abnormal distribution be- increases during fever. Sweat is visible fluid excretion through the
tween the vascular and interstitial compartments. The most com- skin. The volume of sweat varies greatly depending primarily on
mon abnormal distribution is edema, which is expansion of the thermoregulatory needs.
interstitial space. Edema can be caused by increased blood hy-
drostatic pressure (e.g., venous congestion), increased microvas- Fluid Loss by Abnormal Routes
cular permeability that allows proteins to leak into interstitial Examples of abnormal routes of fluid loss are emesis, drains, suc-
fluid, increased interstitial fluid osmotic pressure (e.g., inflam- tion, paracentesis, and hemorrhage. Third-spacing (e.g., ascites)
mation), decreased blood osmotic pressure (e.g., hypoalbumine- can be considered abnormal fluid loss, even though the fluid re-
mia), or blockage of the lymphatic system, which normally mains in the body, because the fluid is not freely available to the
removes excess fluid from the interstitial space and returns it to normal fluid compartments.
the vascular compartment.
The second type of fluid distribution occurs between the ex- Summary of Fluid Balance
tracellular and intracellular compartments. This process is regu- In summary, the processes of fluid intake, fluid distribution, fluid
lated by osmosis. Cell membranes are freely permeable to water, excretion, and fluid loss by abnormal routes act together to deter-
but the passage of ions and other particles depends on membrane mine fluid balance or imbalances. A change in one of these
transport processes. Osmotic pressure is an inward-pulling force processes must be matched by a change in another to maintain
caused by particles in a fluid. Both the extracellular and intracel- fluid balance. For example, if an increased urine output is
lular fluids exert osmotic pressure. Because the osmolality of the matched by an increased fluid intake, then fluid balance can be
two compartments normally is the same, the osmotic pressures are maintained. If changes in one or more of these processes are not
the same. Therefore, the force pulling water into the cells balances matched by changes in the others, however, then a fluid imbalance
the force pulling water into the interstitial space, maintaining the occurs. Fluid imbalances may be characterized by altered volume
normal fluid distribution. If the osmolality of the extracellular of fluid (ECV imbalances), altered concentration of fluid (osmolal-
fluid changes, however, then osmosis occurs, altering the fluid dis- ity imbalances), or a combination of both.
tribution until the osmolality in the extracellular and intracellular
compartments again is the same. For example, if the extracellular
fluid becomes more concentrated (increased osmolality), then the EXTRACELLULAR FLUID VOLUME
osmotic pressure of the extracellular fluid becomes higher than the BALANCE
osmotic pressure of the intracellular fluid. Water leaves the intracel-
lular compartment until the intracellular fluid becomes as concen- The ECV is the net result of fluid intake, fluid distribution, fluid
trated as the extracellular fluid. This process decreases the amount excretion, and fluid loss by abnormal routes. A normal ECV is
of water that is distributed into the intracellular compartment. maintained when fluid excretion and any fluid loss are balanced