Fluid and Electrolyte Balance (Structure and Function) (Nursing) Part 2

Water

Water is vital for life and makes up the greatest portion of body weight; the human body consists of 45% to 77% water (see Table 17-1). It provides an efficient medium for delivery of nutrients to and export of waste products from body cells, and helps regulate many body processes and pressures. It protects and lubricates body surfaces. It is especially important in regulation of blood pressure and fluid and electrolyte balance (see Box 17-2).

BOX 17-2.

Functions of Water

♦    Primary solvent within the body

♦    Primary compound in all body fluids

♦    Suspension agent

♦    Helps regulate body temperature, body pH, and fluid pressures inside and outside cells

♦    Assists or participates in chemical reactions

♦    May be end product of chemical reactions

Water in Solution with Other Substances

♦    Transports nutrients and oxygen to cells

♦    Transports waste products away from cells

♦    Acts as a “bumper” to protect cells and organs

♦    Lubricates to prevent outer walls (parietal) from rubbing against inner walls (visceral) of organs

♦    Participates in maintenance of blood pressure

♦    Helps regulate acid-base and fluid-electrolyte balance


♦    Facilitates the use of water-soluble vitamins (C and B-complex)

Key Concept Water (H2O) is vital to human life.The body cannot carry on most of its activities without it.

Age, sex, and individual body composition cause variations in total percentage of water, as related to total body weight. Children can be composed of more than 75% body water; adult men about 60%. Fat cells contain the least water of any cells, thus adult women normally have the lowest water content (about 50%), because of the presence of greater amounts of subcutaneous fat. Table 17-1 summarizes the breakdown of body water by individual and by water compartment.

NCLEX Alert The NCLEX includes terminology found in this topic including concepts such as homeostasis, ECF, body fluid balance, and fluid volume excess or deficit. These concepts are basic to the understanding of your role in providing nursing interventions, the use of medications, and the prevention of complications.

TABLE 17-1. Water as a Percentage of Body Weight

WATER

COMPARTMENT

INFANT

(%)

ADULT (%)

ELDERLY

PERSON

(%)

MAN

WOMAN

Extracellular

Intravascular

4

4

5

5

Interstitial

25

11

10

15

Intracellular

48

45

35

25

Total body water

77

60

50

45

Special Characteristics of Water

Specific properties of water make it important in body chemistry. This will aid in your understanding of the rationales underlying nursing interventions.

•    A great temperature difference is needed to cause a physical change (to solid or gas) in water—which is normally liquid.Water boils at 212°F (100°C) and freezes at 32°F (0°C). This temperature span is sufficient to cause physical changes that do not affect the chemical composition of the water. However, this change is difficult and relatively slow to occur. Heat is needed to change water from liquid to gas, but water can absorb much heat before increasing its temperature. (The evaporation process [change from liquid to gas] removes heat from the body.)

•    Water directly and indirectly participates in all chemical reactions in the body. Chemically, each water molecule (H2O) is made up of two elements, hydrogen (2 atoms) and oxygen (1 atom). During metabolism, numerous chemical changes in the body separate water molecules into their component elements for use elsewhere. Hydrogen is the main component of the pH system of the body (described later). Oxidation is the process through which the body uses oxygen to form needed new substances. Indirectly, water acts as the solution in which other chemicals ionize (dissociate). When substances change into ions, also discussed later, they become available to participate in other chemical reactions.

•    Water is a good solvent. A solvent is a liquid that dissolves substances; a solute is the substance dissolved. Many compounds, such as salts and sugars, dissolve easily in water. This is known as a solution. (A solution does not involve a chemical change in the composition of either the solvent or the solute.) Nutrients and wastes are transported as solutes in water. Body water contains two main types of solutes: nonelectrolytes and electrolytes. Nonelectrolytes include proteins, glucose, carbon dioxide, oxygen, and organic acids. Electrolytes are solutes that generate an electrical charge when dissolved in water. They are discussed in the next section.

•    Water functions as a suspension agent. Many larger molecules, such as lipids and proteins, are easily suspended in water. (A suspension is not the same as a solution.) Suspensions must be kept in motion, or larger molecules will settle to the bottom. For example, red blood cells settle out of suspension unless kept in motion in the watery medium of blood. An individual’s blood pressure partially depends on intravascular water as a suspension agent, with specific amounts of proteins, electrolytes, and minerals as solutes.

•    Water exerts pressure against the walls or vessels that contain it—hydrostatic pressure. This occurs because water has weight and volume. The amount of hydrostatic pressure depends on the depth of the liquid. Regardless of the amount, water in a tall, thin container exerts more hydrostatic pressure than water in a shallow container.

•    Osmotic pressure is the pressure that develops when a semipermeable membrane separates two solutions containing different concentrations of solutes. In other words, the amount of solutes in water affects the pressure that can be exerted against surrounding membranes. The greater the amount of solutes, the greater the amount of pressure. Osmotic pressure in the body is normally maintained within very narrow limits. For example, body cells have an osmotic pressure nearly equal to that of the circulating fluid of the blood. Solutions exerting equal pressures on opposite sides of a membrane are said to be isotonic (of equal tension). Stronger solutions, compared with those on the opposing side of a membrane, are said to be hypertonic (increased tension). Immersion in a hypertonic solution will result in shrinkage of blood cells because osmosis (discussed later) will draw fluid out of the cells. Weaker solutions, compared with an opposing solution, are called hypotonic (reduced tension). Immersion in a hypotonic solution will result in swelling of blood cells (see Fig. 17-3).

Special Considerations :LIFESPAN

Risk for Fluid Volume Imbalances in Older Adults

Loss of thirst sensation in older adults often leads to decreased consumption of fluids and therefore increased risk for fluid volume deficit. Cardiovascular and renal problems, along with nutritional habits, may cause sodium and water retention, leading to overload states.

Electrolytes

An electrolyte is a substance that will dissociate into ions (see discussion below) when dissolved in water. An ion is able to conduct a weak electric current. Electrolytes are found in the form of inorganic salts, acids, and bases. Electrolytes are found in all body fluids. Specific electrolytes and their concentrations vary. Because electrolytes are active chemicals, their concentrations are measured according to their chemical activity and expressed as milliequivalents (mEq).

Laboratory tests are often done to determine blood levels of electrolytes. Electrolyte concentration affects fluid balance and specific electrolytes affect various body functions. An excess or deficiency of key electrolytes can cause serious physical disorders.

Ions

Each chemical element has an electrical charge, either positive (+) or negative (-). An element usually is able to connect or “bond” to another element. The bonding ability or attraction between chemicals is determined by the electrical charge and specific characteristics of each element.

Many elements are able to gain or lose electrons that circle around them. An atom that has gained or lost one or more electrons is called an ion and it has acquired an electrical charge and bonding ability. Ions are atoms or groups of atoms in search of a bonding partner. Some ions are able to bond with only one other ion; others can bond with two or more. This ability is expressed in terms of a positive (+) or negative (-) value and is called the valence. Thus, the valence represents the combining power of an element in a chemical compound, the result of a chemical change. This is also known as the “oxidation number.” For example, sodium has a valence of +1 (Na+); chlorine has a valence of -1 (Cl); sulfate (a combination of sulfur and oxygen) has a valence of -2 (SO4 ). The number of plus or minus signs in the valence indicates the number of ions with which a particular ion is able to bind. Therefore, sodium (Na+) can bind with only one negatively charged ion that has a valence of -1, such as chlorine (Cl). This compound would then be NaCl, which is sodium chloride or common table salt. If hydrogen (H+) combines with sulfate, two hydrogen ions would be required to fill the two minus bonds of the sulfate, yielding sulfuric acid (H2SO4). Water is composed of two hydrogen (H+) ions and one oxygen ion (O ) and is expressed as H2O.

Osmosis and red blood cells. Water moving through a red blood cell membrane in solutions with three different concentrations of solute. All these actions have the goal of equalizing the solute concentration on both sides of the cell membrane. Left: Isotonic (normal) solution has the same concentration as the cell, and the water moves into and out of the cell at the same rate. Center: Hypotonic (diluted) solution causes the cell to swell and eventually hemolyze (burst) because of the large amount of water moving into the cell. Right: Hypertonic (concentrated) solution draws water out of the cell, causing it to shrink.

FIGURE 17-3 · Osmosis and red blood cells. Water moving through a red blood cell membrane in solutions with three different concentrations of solute. All these actions have the goal of equalizing the solute concentration on both sides of the cell membrane. Left: Isotonic (normal) solution has the same concentration as the cell, and the water moves into and out of the cell at the same rate. Center: Hypotonic (diluted) solution causes the cell to swell and eventually hemolyze (burst) because of the large amount of water moving into the cell. Right: Hypertonic (concentrated) solution draws water out of the cell, causing it to shrink.

Key Concept A salt is any compound composed of a base and an acid. Common table salt is the compound yielded when sodium and chlorine chemically combine. Some elements have more than one valence value and, thus, can combine with ions that have various valence values.

A positively charged ion is known as a cation. A negatively charged ion is known as an anion. Examples of cations include sodium (Na+), potassium (K+), calcium (Ca++), magnesium (Mg++), iron (Fe++), and hydrogen (H+). Examples of anions include chlorine (Cl), bicarbonate (HCO3), sulfate (SO4    ), oxygen (O ), and phosphate (HPO4). (Use this memory helper for cation: [ca+ion]. This will help you remember the positive charge, thereby recalling which substances are cations and which are anions.) The cation, because it has a positive charge, is attracted by a negatively charged ion. The anion, because it is negatively charged, is drawn toward the positive charge. In other words, opposites attract. The normal situation in which the charges are equal is known as electroneutrality.

Ionization

The process of ionization (osmolarity) involves dissociation of compounds into their respective free-standing ions. This separation of chemical compounds into ions releases them for use in other chemical reactions. Ionization of the water molecule (H2O, HOH) releases hydrogen ions (H+) and hydroxyl ions (OH). Each of these ions is now free to combine with another substance to form an acid, base, or salt. The ionization of table salt, sodium chloride (NaCl), releases a sodium ion (Na+) and a chlorine ion (Cl). Each of these ions can recombine with other ions into new substances, such as the combination of hydrogen and chlorine, hydrochloric acid (HCl).

Important Electrolytes

The major intracellular (inside cells) electrolytes are potassium (K+), magnesium (Mg++), sulfate (SO4++), and phosphate (HPO4). The major extracellular electrolytes are sodium (Na+), chlorine (Cl), calcium (Ca++), and bicarbonate (HCO3). Sodium is the most important extracellular cation (positive charge). Chlorine is the most important extracellular anion (negative charge). As stated previously, these ions combine to form sodium chloride or NaCl (ordinary table salt), which is one of the most common compounds in the body. Normal (isotonic) saline (NS) is a salt solution (0.9% NaCl); it is commonly administered IV (intravenously) to augment body fluids. NS is called isotonic because it has the same NaCl concentration as normal body fluids.

Key Concept Remember that NaCl (salt)is a compound; a chemical change has taken place to form it. Saline solution (salt and water) is a mixture because it can be separated without a chemical reaction. A compound combines elements in exact proportions, which are the same each time. A mixture can mix components in different proportions. Therefore, a saline solution can be isotonic (0.9%), or it can be in any other proportion and still be a saline solution, such as half-normal saline (0.45% NaCl), also a common IV solution.

A gain or loss of sodium is the cause of most common electrolyte disorders. This is based on factors such as food content, urine excretion, vomiting, perspiration, and specific disorders

The most dominant cation intracellularly (inside the cells) is potassium (K+). The most dominant anion is phosphate (HpO4 ). ICF also contains sodium, but in much smaller amounts than outside the cell. The balance between intracellular potassium and extracellular sodium is an extremely important aspect of energy production.

TABLE 17-2. Major Functions and Food Sources of Electrolytes (Dissolved in Body Fluids)

ELECTROLYTE

MAJOR FUNCTIONS

FOOD SOURCES

Cations

Sodium (Na+)

(Major ion in extracellular fluid [ECF])

Maintenance of osmotic pressure; thus, maintains body fluid balance Assists with normal functioning of neurons and muscle cells Essential for buffer system (acid-base balance)

Table salt, meat, dairy foods, eggs; many processed and preserved foods including bacon, pickles, and ketchup

Potassium (K+) (Major ion in intracellular fluid [ICF])

Maintenance of osmotic pressure; thus, maintains body fluid balance Normal functioning of neurons and muscle cells, including the heart Essential for buffer system (acid-base balance)

Dry fruits, nuts, many vegetables, meat

Calcium (Ca++)

Assists with normal functioning of neurons and muscle cells,

including the heart

Essential for neurotransmitter release

Maintenance of bones; bone formation

Essential for blood clotting

Milk and other dairy products, broccoli and other green leafy vegetables, sardines

Magnesium (Mg++) (Mainly in ICF)

Assists with normal functioning of neurons and muscle cells, including the heart; required for ATP use; enzyme production Maintenance and formation of bones

Green leafy vegetables, legumes, chocolate, peanut butter, whole grains

Anions

Chloride (Cl_) (Mostly in ECF, combined with Na+)

Maintenance of osmotic pressure; thus, maintains body fluid balance Essential for buffer system (acid-base balance)

Maintains acidity of gastric juice (stomach acid-HCl)

Cheese, milk, fish

An excess of chloride ions is called acidosis. (NaCl = table salt)

Bicarbonate (HCO3_) (Most important in ICF)

Maintenance of osmotic pressure; thus, maintains body fluid balance Essential for buffer system (acid-base balance)

Does not need to be specifically included in the diet.

Excess bicarbonate ions can result from overuse of antacids, such as sodium bicarbonate (NaHCO3, baking soda). The body also can lose acids as a result of illness. An excess of bicarbonate ions is called alkalosis.

Phosphate (HPO4_) (mostly occurs in ICF)

Maintenance of bones and teeth

Assists with normal functioning of nerves and muscle cells

Assists with formation of ATP (adenosine triphosphate); energy storage

Assists with metabolism of nutrients

Whole grains, milk and other dairy foods, meat, fish, poultry

Sulfate (SO4 ) Proteins

Important in protein metabolism; amino acids Maintenance of osmotic pressure; organic acids

Protein-rich foods

Meat, fish, legumes, eggs, nuts, dairy products

The body needs all these electrolytes and more for normal functioning of nerves and muscles, developing body cells, blood clotting, and coordinating all body activities. Table 17-2 summarizes major electrolyte functions and their dietary sources. Organs involved in homeostatic mechanisms to maintain electrolyte balance include the kidneys and the adrenal, parathyroid, and thyroid glands. In the clinical setting, electrolyte balance refers to the maintenance of normal serum concentrations of electrolytes. Measuring electrolyte concentration in the ICF is difficult; therefore, serum concentrations (ECF: intravascular) are used to assess and manage clients with imbalances. Table 17-3 provides normal serum electrolyte ranges for adults.

FLUID AND ELECTROLYTE TRANSPORT

Total electrolyte concentration affects the body’s fluid balance. Nutrients and oxygen must enter body cells, whereas waste products exit the body. During this exchange, substances pass through various fluid compartments and cellular membranes (plasma membranes). Each ion or molecule has a specific way or ways in which it can be transported across these membranes. The cell membrane separates the intracellular environment from the extracellular environment. The composition of these two environments is very different. These differences must be maintained for the cell (and thus the organism) to survive. Cell membranes allow some molecules to pass through, while resisting or preventing others from entering (or leaving) the cell.

TABLE 17-3. Normal Serum Electrolyte Values

ELECTROLYTE

SERUM VALUE

Cations

Sodium (Na+)

135-145 mEq/L

Potassium (K+)

3.5-5.0 mEq/L

Calcium (Ca++)

4.3-5.3 mEq/L (8.9-10.1 mg/dL)

Magnesium (Mg++)

I.5-I.9 mEq/L (1.8-2.3 mg/dL)

Anions

Chloride (CP)

95-108 mEq/L

Bicarbonate (HCO3~)

22-26 mEq/L

Phosphate (HPO4~, H2PO4~)

1.7-2.6 mEq/L (2.5-4.5 mg/dL)

Normal value ranges may vary slightly from laboratory to laboratory

The ability of a membrane to allow molecules to pass through is known as permeability. Factors that affect permeability include:

•    The size of pores in the membrane (which can be altered in response to pressures or hormones)

•    The external and internal pressures exerted on the molecules (osmotic pressure)

•    The pressure of fluid against the membrane (hydrostatic pressure)

•    The electrical charges of the molecule, the plasma membrane, or the body fluid

•    The solubility of the molecules

•    The size of the molecules

Permeability of Membranes

Freely permeable membranes allow almost any food or waste substance to pass through. Freely permeable walls allow easy transfer of fluid and substances from intravascu-lar fluid to interstitial fluid. After substances arrive in the interstitial fluid around the cells, they must still penetrate the cellular membrane to reach the ICF, where the majority of the body’s work occurs.

The cellular membrane is selectively permeable, meaning that each cell’s membrane allows only certain specific substances to pass through. Movement across the cellular membrane occurs in one of four ways:

•    Molecules move through the cell membrane, including oxygen, carbon dioxide, and steroids.

•    Substances pass through membrane channels. These channels are of various sizes and allow only a certain size range and electrical charge to traverse the membrane.

•    Carrier molecules in the membrane assist substances across the barrier.

•    A vesicle (membrane-bound sac) transports large molecules or whole cells across the plasma membrane.

Some molecules move passively through the membrane; thus, they do not require any energy output by the body. Passive transport mechanisms include diffusion, osmosis, and filtration. Another form of transport uses energy and requires assistance. It is called active transport. This type of transport is used when molecules are too large or too specialized to pass through membranes without assistance.

Passive Transport Diffusion

Diffusion, or the process of “being widely spread,” is the random movement of molecules from an area of higher concentration to an area of lower concentration. Molecules constantly move and bombard each other at random, with the goal of equalization—the molecular equivalent of seeking homeostasis. If molecules are highly concentrated, they collide often and attempt to move to a place with fewer collisions. Following total diffusion, equilibrium is reached, with no further exchange of molecules. The molecules continue to move, but the number in each area or on each side of a membrane stays the same. (Heat speeds up diffusion because heat makes molecules move faster.) Diffusion commonly occurs in liquids and gases. For example, when liquid cream or powdered creamer is added to coffee it spreads out (diffuses). Smoke or perfume diffuses in a room.

Diffusion is the most important mechanism by which nutrients and wastes pass across the cell membrane. In the body, oxygen and carbon dioxide diffuse across the cell membranes of the alveoli in the lungs, as shown in Figure 17-4A. When a person takes in a breath, more oxygen (O2) molecules are drawn into the alveoli of the lungs. The oxygen molecules are pushed passively toward the pulmonary (lung) capillaries where the O2 level is lower. This difference in pressures forces oxygen to cross the cell membrane out of the lungs (diffuse) into the pulmonary capillaries. The oxygenated (oxygen-rich) blood then passes into the pulmonary vein and from there is transported to various parts of the body. Carbon dioxide (CO2) gas is exchanged in the same manner, except in the reverse direction because of the pressure. The designation “Pa” or “P” indicates the “pressure” or “potential” of oxygen or carbon dioxide.

Key Concept Dialysis is an example of diffusion. An isotonic solution on one side of a membrane causes wastes from the body to flow into it (due to higher pressure), in an effort to equalize pressures.

Osmosis

The homeostatic mechanism of osmosis equalizes the concentrations of nondiffusible solutes within the body. Thus, osmosis is the diffusion of a pure solvent, such as water, across a semipermeable membrane in response to a concentration gradient, in situations where the molecules of a higher concentration are nondiffusible. In other words, water molecules move passively from an area where the water molecules are higher in number (more dilute solution, with fewer nondiffusible solutes), to an area where they are lower in number (more concentrated, with more nondiffusible solutes (see Fig 17-4B). Water moves from a hypotonic solution (a dilute solution with fewer solutes) to a hypertonic solution (a concentrated solution with more solutes). As stated before and as shown in Figure 17-3, if water from a less-concentrated solution is moving into a red blood cell (RBC), the RBC becomes swollen and may rupture (hemolysis) because of this influx of water. An RBC will shrink (crenation) if it is losing water to a surrounding hypertonic solution (a concentrated solution with a solute level higher than that found in the RBC).

Although there are many possible solutes, a common one is salt (NaCl). Therefore, when thinking about osmosis, think of salt as the solute and think about the phrase “water follows salt.” This will help you to remember which direction the water will move. So, osmosis can be thought of as “pulling pressure,” pulling the water in to equalize or dilute the concentrated solution.

Key Concept The term osmotic pressure refers to pressure exerted to stop the flow of water across a membrane.

A. The process of osmosis, showing the flow of water; in an attempt to equalize concentrations of a solute on both sides of the semipermeable membrane. (The water flows from the dilute solution into a more concentrated solution.) The level of the fluid column is maintained by osmotic pressure. B. The process of diffusion, showing gas exchange in the alveoli. Oxygen is transported across the alveolar-capillary membrane from the alveoli of the lungs to the capillaries, to add oxygen to the unoxygenated blood returning from the body (systemic circulation). The carbon dioxide in the capillaries (area of greater concentration) moves into the alveoli (area of lesser concentration). This simple diffusion is an example of passive transport.

FIGURE 17-4 · A. The process of osmosis, showing the flow of water; in an attempt to equalize concentrations of a solute on both sides of the semipermeable membrane. (The water flows from the dilute solution into a more concentrated solution.) The level of the fluid column is maintained by osmotic pressure. B. The process of diffusion, showing gas exchange in the alveoli. Oxygen is transported across the alveolar-capillary membrane from the alveoli of the lungs to the capillaries, to add oxygen to the unoxygenated blood returning from the body (systemic circulation). The carbon dioxide in the capillaries (area of greater concentration) moves into the alveoli (area of lesser concentration). This simple diffusion is an example of passive transport.

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