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UT Arlington NURS 5315 - Electrolyte Homeostasis and Fluid Balance/Imbalances

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1 N5315 Advanced Pathophysiology Electrolyte Homeostasis and Fluid Balance/Imbalances Introduction To maintain a healthy cellular environment, the body must maintain fluid and electrolyte homeostasis. This balance is maintained by renal, neural and hormonal functions. In this unit we will review sodium and potassium homeostasis, fluid balance and imbalance disorders. Fluid Compartments & Mechanisms of Fluid Balance Total body water is approximately 60% of the body weight and is spread between the three main fluid compartments. The three compartments are the intracellular space, extracellular space (interstium), and the intravascular space. The intracellular space contains the largest portion of the total body water (40%). The remaining two spaces each contain 20% of the total body water. The kidneys are responsible for eliminating the largest amount of water. Fluid is also lost, albeit in small amounts through stool, the skin and lungs which is known as insensible water loss. Total body water varies with age. Infants have a total body water of 70-80% because they have less adipose tissue. During the immediate postnatal time frame, an infant loses about 5% of body weight secondary to a physiologic loss of water. Infants are vulnerable to changes in TBW due to their higher metabolic rate and the potential for fluid loss through their skin due to their greater body surface. Diarrhea can result in a significant loss of TBW in infants. Renal controls of fluid and electrolytes are not mature enough to counter act the effects of the loss of TBW, therefore infants are very susceptible to dehydration and it can happen quickly. In childhood TBW decreases to 60-65% and during adolescence TBW approaches adult proportion. Adult males have a greater portion of TBW than females due to a difference in muscle mass. Females have more body fat and less muscle secondary to the effects of estrogen and therefore they have less TBW. TBW decreases as we age. This is secondary to an increase in body fat, decreased muscle mass and a decrease in the ability to regulate sodium and water balance. Renal function declines as we age and therefore they do not function as well to maintain TBW. Insensible water loss increases with age. The elderly are very susceptible to dehydration and it may be life threatening. Fluid balance between the spaces is maintained by a number of dynamics. These include osmolality (tonicity), osmosis, osmotic pressure, hydrostatic pressure, and oncotic pressure. Osmolality is the measure of solute concentration in a solution. In humans, it is basically the concentration of the plasma. The higher the number of solutes (particles) in a compartment (osmolality), the greater the concentration is and therefore the less water is in that compartment. Plasma osmolality is 280-295 mOsm/kg. The low side of normal may even go as low as 275. Sodium is largely responsible for maintaining the osmotic balance in the ECF and potassium is responsible for maintaining the ICF osmolality. Tonicity is the effective osmolality. Effective osmolality is the concentration of those solutes that do not readily cross the cell membranes and in effect contribute to the osmolality. Even though urea is a solute in the plasma it freely diffuses2 through the cell membranes and has no effect on osmolality, whereas sodium and glucose are limited to the extracellular space and greatly contribute to the effective osmolality. Tonicity and osmolality may be used interchangeably. Osmotic Pressure is the amount of pressure or force that is exerted by solute molecules of a given compartment. The higher the osmolality, the higher the osmotic pressure. This is a pulling force and will pull water into a compartment. This is the force that must be overcome by hydrostatic pressure to oppose osmosis. Osmosis is the movement of water between compartments from an area of low concentration of solutes to one that has a high concentration of solutes. It is a passive force because it does not require energy. Hydrostatic Pressure is the force within a fluid compartment - the mechanical force of fluid against the walls of the compartment, i.e. blood pressure. This pressure is a pushing force and pushes fluid outside the compartment. Oncotic pressure (colloid osmotic pressure) is the force which helps to keep water/fluid within a compartment. It contributes to osmotic pressure and is exerted by plasma proteins. The main plasma protein is albumin. Hydrostatic pressure is greater than oncotic pressure at the arterial end of the capillary bed. This causes fluid to move out of the capillary bed into the interstium. This loss of water then decreases hydrostatic pressure at the venous end of the capillary bed, where oncotic pressure is then greater. Fluids are then pulled back into the capillary bed thus resulting in a balance of fluids. Fluid homeostasis is dependent upon all of the mechanisms we just discussed. An alteration in one or more of these mechanisms will result in a fluid imbalance. IV solutions often alter osmolality and contribute to fluid shifts. The basis of understanding how fluid shifts is directly related to the principle of osmosis. Remember, osmosis is the passive movement of water from areas of low solute concentration to high solute concentration. A hypertonic IV solution, when administered, will increase the solute concentration in the intravascular space. The intravascular space will become more concentrated and will experience an increased osmolality. Because water flows via osmosis, water will then flow into the intravascular space from the extracellular space. This leaves the extracellular space more concentrated (higher concentration of solutes), and water will then move from the intracellular space to the extracellular space, which will cause the cell to shrink. Water will continue to flow into the intravascular space from the extracellular space if the intravascular space still remains more concentrated than the extracellular space. Administration of a hypotonic intravenous solution will cause the intravascular space to become more dilute (less solute concentration and a lower osmolality). Based on the concept of osmosis, water will move from the intravascular space to the extracellular space. This will cause a dilution of the extracellular space causing water to then shift into the intracellular space, consequently causing the cell to swell. One last concept to be familiar with


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