CONCEPTS OF FLUID AND ELECTROLYTE BALANCE
A. Electrolytes
1. Description: A substance that is dissolved in solution and ome of its molecules split or dissociate into electrically charged atoms or ions.
2. Measurement
a. The metric system is used to measure volumes of fluids—liters (L) or milliliters (mL).
b. The unit of measure that expresses the combining activity of an electrolyte is the milliequivalent (mEq).
c. One milliequivalent (1 mEq) of any cation will always react chemically with 1 mEq of an anion.
d. Milliequivalents provide information about the number of anions or cations available to combine with other anions or cations.
B. Body fluid compartments
1. Description
a. Fluid in each of the body compartments contains electrolytes.
b. Each compartment has a particular composition of electrolytes, which differs from that of other compartments.
c. To function normally, body cells must have fluids and electrolytes in the right compartments and in the right amounts.
d. Whenever an electrolyte moves out of a cell, another electrolyte moves in to take its place.
e. The numbers of cations and anions must be the same for homeostasis to exist.
f. Compartments are separated by semipermeable membranes.
2. Intravascular compartment: Refers to fluid inside a blood vessel
3. Intracellular compartment
a. The intracellular compartment refers to all fluid inside the cell.
b. Most bodily fluids are inside the cell.
4. The extracellular compartment is the fluid outside the cell.
a. The extracellular compartment includes the interstitial fluid, which is fluid between cells (sometimes called the third space), blood, lymph, bone, connective tissue, water, and transcellular fluid.
b. Transcellular fluid is the fluid in various parts of the body, such as peritoneal fluid, pleural fluid, cerebrospinal fluid, and synovial fluid.
C. Third-spacing
1. Third-spacing is the accumulation and sequestration of trapped extracellular fluid in an actual or potential body space as a result of disease or injury.
2. The trapped fluid represents a volume loss and is unavailable for normal physiological processes.
3. Fluid may be trapped in body spaces such as the pericardial, pleural, peritoneal, or joint cavities, the bowel, or the aabdomen, or within soft tissues after trauma or burns.
4. Assessing the intravascular fluid loss caused by third-spacing is difficult. The loss may not be reflected
in weight changes or intake and output records and may not become apparent until after organ malfunction occurs.
D. Edema
1. Edema is an excess accumulation of fluid in the interstitial space.
2. Localized edema occurs as a result of traumatic injury from accidents or surgery, local inflammatory processes, or burns.
3. Generalized edema, also called anasarca, is an excessive accumulation of fluid in the interstitial space throughout the body and occurs as a result of conditions such as cardiac, renal, or liver failure.
E. Body fluid
1. Description
a. Body fluids transport nutrients to the cells and carry waste products from the cells.
b. Total body fluid (intracellular and extracellular) amounts to about 60% of body weight in the adult,
55% in the older adult, and 80% in the infant.
c. Thus, infants and the older adult are at ahigher risk for fluid-related problems than younger adults; children have a greater proportion of body water than adults and the older adult has the least proportion of body water.
2. Constituents of body fluids
a. Body fluids consist of water and dissolved substances.
b. The largest single fluid constituent of the body is water.
c. Some substances, such as glucose, urea, and creatinine, do not dissociate in solution; that is, they do not separate from their complex forms into simpler substances when they are in solution.
d. Other substances do dissociate; for example, when sodium chloride is in a solution, it dissociates or
separates into two parts or elements.
F. Body fluid transport
1. Diffusion
a. Diffusion is the process whereby a solute (substance that is dissolved) may spread through a solution or solvent (solution in which the solute is dissolved).
b. Diffusion of a solute will spread the molecules from an area of higher concentration to an area of lower concentration.
c. A permeable membrane will allow substances to pass through it without restriction.
d. A selectively permeable membrane will allow some solutes to pass through without restriction but will prevent other solutes from passing freely.
e. Diffusion occurs within fluid compartments and from one compartment to another if the barrier
between the compartments is permeable to the diffusing substances.
2. Osmosis
a. Osmotic pressure is the force that draws the solvent from a less concentrated solute through a selectively permeable membrane into a more concentrated solute, thus tending to equalize the concentration of the solvent.
b. If a membrane is permeable to water but not to all the solutes present, the membrane is a selective or semipermeable membrane.
c. Osmosis is the movement of solvent molecules across a membrane in response to a concentration gradient, usually from a solution of lower to one of higher solute concentration.
d. When a more concentrated solution is on one side of a selectively permeable membrane and a less concentrated solution is on the other side, a pull called osmotic pressure draws the water through
the membrane to the more concentrated side or the side with more solute.
3. Filtration
a. Filtration is the movement of solutes and solvents by hydrostatic pressure.
b. The movement is from an area of higher pressure to an area of lower pressure.
4. Hydrostatic pressure
a. Hydrostatic pressure is the force exerted by the weight of a solution.
b. When a difference exists in the hydrostatic pressure on two sides of a membrane, water and diffusible solutes move out of the solution that has the higher hydrostatic pressure by the process of filtration.
c. At the arterial end of the capillary, the hydrostatic pressure is higher than the osmotic pressure; therefore, fluids and diffusible solutes move out of the capillary.
d. At the venous end, the osmotic pressure or pull is higher than the hydrostatic pressure, and fluids and some solutes move into the capillary.
e. The excess fluid and solutes remaining in the interstitial spaces are returned to the intravascular compartment by the lymph channels.
5. Osmolality
a. Osmolality refers to the number of osmotically active particles/kilogram of water; it is the concentration of a solution.
b. In the body, osmotic pressure is measured in milliosmoles (mOsm).
c. The normal osmolality of plasma is 270 to 300 milliosmoles/kilogram (mOsm/kg) water.
G. Movement of body fluid
1. Description
a. Cell membranes separate the interstitial fluid from the intravascular fluid.
b. Cell membranes are selectively permeable; that is, the cell membrane and the capillary wall will allow water and some solutes free passage through them.
c. Several forces affect the movement of water and solutes through the walls of cells and capillaries.
d. The greater the number of particles within the cell, the more pressure exists to force the water through
the cell membrane.
e. If the body loses more electrolytes than fluids, as can happen in diarrhea, then the extracellular fluid will contain fewer electrolytes or less solute than the intracellular fluid.
f. Fluids and electrolytes must be kept in balance for health; when they remain out of balance, death can
occur.
2. Isotonic solutions
a. When the solutions on both sides of a selectively permeable membrane have established equilibrium or are equal in concentration, they are isotonic.
b. An example of an isotonic solution is 0.9% sodium chloride, which is referred to as isotonic saline solution or normal saline solution.
c. Isotonic solutions are isotonic to human cells, and thus very little osmosis occurs; isotonic solutions have the same osmolality as body fluids.
d. Other solutions that are isotonic are 5% dextrose in water, 5% dextrose in 0.225% saline, and Ringer's
lactate solution.
3. Hypotonic solutions
a. When a solution contains a lower concentration of salt or solute than another more concentrated solution, it is considered hypotonic.
b. A hypotonic solution has less salt or more water than an isotonic solution; these solutions have lower osmolality than body fluids.
c. 0.45% sodium chloride, 0.225% sodium chloride, and 0.33% sodium chloride are examples of hypotonic solutions.
d. Hypotonic solutions are hypotonic to the cells; therefore, osmosis would continue in an attempt to bring about balance or equality.
4. Hypertonic solutions
a. A solution that has a higher concentration of solutes than another less concentrated solution is hypertonic; these solutions have a higher osmolality than body fluids.
b. Hypertonic solutions include 3% sodium chloride, 5% sodium chloride, 10% dextrose in water, 5% dextrose in 0.9% sodium chloride, 5% dextrose in 0.45% sodium chloride, and 5% dextrose in Ringer's lactate solution.
c. Refer to Table 14-1 (Chap. 14) for a list of isotonic, hypotonic, and hypertonic solutions.
5. Osmotic pressure
a. The amount of osmotic pressure is determined by the concentration of solutes in solution.
b. When the solutions on each side of a selectively permeable membrane are equal in concentration, they are isotonic.
c. A hypotonic solution has less solute than an isotonic solution, whereas a hypertonic solution contains more solute.
d. A solvent will move from the less concentrated solute side to the more concentrated solute side to equalize concentration.
6. Active transport
a. If an ion is to move through a membrane from an area of lower concentration to an area of higher concentration, an active transport system is necessary.
b. An active transport system moves molecules or ions against concentration and osmotic pressure.
c. Metabolic processes in the cell supply the energy for active transport.
d. Substances that are transported actively through the cell membrane include ions of sodium, potassium, calcium, iron, and hydrogen, some of the sugars, and the amino acids.
H. Body fluid excretion
1. Description
a. Fluids leave the body by several routes, including the skin, lungs, gastrointestinal tract, and kidneys.
b. The kidneys excrete the largest quantity of fluid.
c. As long as all organs are functioning normally, the body is able to maintain balance in its fluid content.
2. Skin
a. Water is lost through the skin in the amount of about 400 mL/day.
b. The amount of water lost by perspiration varies according to the temperature of the environment and of the body, but the average amount of loss by perspiration alone is 100 mL/day.
c. Water lost through the skin is called insensible loss (the individual is unaware of losing that water).
3. Lungs
a. Water is lost from the lungs through expired air that is saturated with water vapor.
b. The amount of water lost from the lungs varies with the rate and the depth of respiration.
c. The average amount of water lost from the lungs is about 350 mL/day.
d. Water lost from the lungs is called insensible loss.
4. Gastrointestinal tract
a. Large quantities of water are secreted into the gastrointestinal tract, but almost all this fluid is reabsorbed.
b. A large volume of electrolyte-containing liquids moves into the gastrointestinal tract and then returns again into the extracellular fluid.
c. The average amount of water lost in the feces is 150 mL/day, equal to the amount of water gained through the oxidation of foods.
d. Severe diarrhea results in the loss of large quantities of fluids and electrolytes.
5. Kidneys
a. The kidneys play a major role in regulating fluid and electrolyte balance.
b. Normal kidneys can adjust the amount of water and electrolytes leaving the body.
c. The quantity of fluid excreted by the kidneys is determined by the amount of water ingested and the amount of waste and solutes excreted.
d. The usual urine output is about 1500 mL/day; however, this varies greatly depending on fluid intake, amount of perspiration, and other factors.
I. Body fluid replacement
1. Description: Water enters the body through three sources—orally ingested liquids, water in foods, and water formed by oxidation of foods.
2. Amounts
a. The average total amount of water taken into the body by all three sources is 2500 mL/day.
b. About 10 mL of water is released by the metabolism of each 100 calories of fat, carbohydrates, or proteins.
3. Electrolytes
a. Electrolytes are present in foods and liquids.
b. With a normal diet, an excess of essential electrolytes is
taken in and the unused electrolytes are excreted.
J. Maintaining fluid and electrolyte balance
1. Description
a. Homeostasis is a term that indicates the relative stability of the internal environment.
b. Concentration and composition of body fluids must be nearly constant.
c. In a client, when one of the substances is deficient, either fluids or electrolytes, the substance must be replaced normally by the intake of food and water or by therapy such as intravenous solutions and medications.
d. When the client has an excess of fluid or electrolytes, therapy is directed toward assisting the body to eliminate the excess.
2. The kidneys play a major role in controlling all types of balance in fluid and electrolytes.
3. The adrenal glands, through the secretion of aldosterone, also aid in controlling extracellular fluid volume by regulating the amount of sodium reabsorbed by the kidneys.
4. Antidiuretic hormone from the pituitary gland regulates the osmotic pressure of extracellular fluid by regulating the amount of water reabsorbed by the kidney.
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