NURSING 3PA2 Study Guide - Final Guide: Intracranial Pressure, Circulatory Collapse, Molality

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2 May 2016
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NURSING 2PF3 Module 9: Fluid and Electrolyte Balance
MODULE 9: FLUID AND ELECTROLYTE BALANCE
Learning Outcomes
Upon conclusion of this unit the student will:
1. Describe the mechanisms of fluid balance and imbalance at the capillary level.
2. Discuss Na+ and H2O balance and imbalances in terms of the physiologic mechanisms, possible
causes, manifestations, and possible diagnostic measures and treatment.
3. Describe disorders of K+ balance.
4. Briefly discuss disorders of calcium, phosphate and magnesium balance.
Some Key Terms:
Osmotic pressure: the pressure which needs to be applied to prevent the inward flow of water across a
semipermeable membrane
Oncotic pressure or colloid osmotic pressure: is a form of osmotic pressure exerted by proteins in a blood
vessel’s plasma that usually tends to pull water into the circulatory system. It is the opposing force of
hydrostatic pressure
Osmolarity: the osmolar concentration in 1L of solution (mOsm/L). Usually used when referring to fluids
outside of the body
Osmolality: the osmolar concentration in 1kg of water (mOsm/kg of H2O). Usually used when describing
fluid inside the body. Because 1 L of water weighs 1 kg, the terms osmolarity and osmolaity are often
used interchangeably
Fluid Balance at the Capillary Level
Fluid Balance:
Fluid balance is determined by the push and pull of fluids across the semi-permeable capillary
membrane. It is important to remember that the normal movement of fluid depends on the integrity
of the capillary membrane
Fluid balance at the level of the capillary relies on a balance between opposing forces:
-Pushing force of hydrostatic pressure
-Pulling force of oncotic pressure
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NURSING 2PF3 Module 9: Fluid and Electrolyte Balance
Hydrostatic pressure: “Push”
-Hydrostatic pressure is the pushing force exerted
by a fluid.
-Inside the capillaries, the hydrostatic pressure is the
same as the capillary filtration pressure, about 30 mmHg at the arterial end and 10 mmHg at the
venous end
-At the arterial end of an capillary, hydrostatic pressure is higher than oncotic pressure in the
capillary so fluid moves into or is “pushed out” into the interstitial space
-Current research shows interstitial hydrostatic pressure has a small negative value (-3 mmHg)
which contributes slightly to the movement of the fluid from the capillary to the tissue
Colloid osmotic pressure: “Pull”
-The colloid osmotic pressure is the pulling force created by
the presence of evenly dispersed particles, such as the
plasma proteins, that cannot pass through the pores of the
capillary membrane
-The capillary colloid osmotic pressure is normally
about 28 mm Hg throughout the length of the capillary bed. In contrast, the interstitial colloid
osmotic pressure represents a pulling pressure exerted by the small amounts of plasma proteins
that leak through the pores of the capillary wall into the interstitial space. This pressure is only 8
mm Hg.
-At the venous end of the capillary, much of the fluid has moved into the interstitial space leaving
the solutes behind—this creates higher capillary osmotic pressure which effectively pulls fluid
from the interstitial space back into the vessel
-Some particles like glucose and electrolytes move from the vessel into the interstitial space and
create interstitial colloidal osmotic pressure, pulling a small amount of fluid into the interstitial
space
Capillary fluid balance:
-The lymphatic system represents an accessory system by which fluid can be returned to the
circulatory system.
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NURSING 2PF3 Module 9: Fluid and Electrolyte Balance
-Normally, the forces moving fluid out of the
capillary into the interstitium are greater than
those returning fluid to the capillary. Any excess
fluids and osmotically active plasma proteins that
may have leaked into the interstitum are picked
up by vessels of the lymphatic system and
returned to the circulation.
-Without the function of the lymphatic system,
excessive amounts of fluid would accumulate in
the interstitial space.
Fluid Imbalance:
Increased capillary hydrostatic pressure:
-If hydrostatic pressure continues to be high at the venous end of the capillary, net fluid
movement will be out of the capillary
-Can be caused by:
oIncreased fluid pressure (e.g. Hypertension)
oIncreased fluid volume (e.g. Sodium and water retention)
oBack-up of blood flow (e.g. Deep vein thrombosis (DVT) obstructing venous blood flow
resulting in higher than normal pressure at the venous end of the capillary)
Decreased capillary colloid osmotic pressure:
-When there is insufficient capillary oncotic pressure, there is insufficient “pull” to bring fluid
back into the intervascular space at the venous end of the capillary. Thus the net fluid movement
will be out of the capillary
-Because albumin is the most prevalent colloid (or solid) in the plasma, any clinical situation that
results in decreased serum albumin can result in a decrease of capillary oncotic pressure.
Common medical diagnoses associated with low serum albumin include burns, liver disease,
malnutrition, and excess wound drainage
Increased interstitial colloid osmotic pressure:
-When solutes or particles escape from the vessel into the interstitial fluid, they will take fluid
with them and hold that fluid in the interstitial space. Thus the net fluid movement will be out of
the capillary and into the interstitial space
-The capillary membrane should allow only some solutes to escape. Capillary permeability
increases in response to the chemical mediators of the inflammatory process = ”leaky capillaries”
Increased tissue hydrostatic pressure:
-Can occur due to obstruction of the lymphatics, causing an obstruction of lymph flow and
inability to remove excess fluid (i.e. lymphedema)
-Normally, osmotically active plasma proteins and other large particles that cannot be reabsorbed
through the pores in the capillary membrane rely on the lymphatic system for movement back
into the circulatory system. This increases tissue hydrostatic pressure.
-Additionally, there is increased interstitial colloid osmotic pressure and increased tissue oncotic
pressure which continues to pull fluid from the vessel
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