Basic Chemistry of Body Fluids and Electrolytes

18

Normal kidneys maintain a balance between body water and the substances dis-solved in it within the narrow limits necessary for life. The kidneys also excrete

the waste products of protein metabolism. Dialysis partially substitutes for these two important functions when the normal kidneys fail. Other functions of the kidney will be discussed in Chapter 4. Fundamental to understanding the processes used by the kidneys, whether natural or artificial, is a basic knowledge of the chemistry involved and the measurements used.

Metric SystemA solid review of the basic system of measurement is necessary because the metric sys-tem is used for the chemical and physical measurements related to body physiology. Length is expressed by the basic unit of meter. The basic unit of mass is gram, and liter is the basic unit of volume. Table 3-1 lists the common metric terms and their interrelationships.

The metric system is entirely decimal. Prefixes indicate smaller or larger units (Table 3-2). To relate the metric system to more familiar uses, the following approximations may be helpful: • A man who is 6 feet 4 inches tall is about 1.95 m in height. • A dime is about 1 mm thick. • A 154-lb person weighs 70 kg.

The following are commonly used conversion factors to change metric units to theEnglish system of pounds, inches, and quarts. • 1 meter (m) = 39.37 inches (in) • 1 inch (in) = 2.54 centimeters (cm) • 1 liter (L) = 1.057 quarts (U.S.) (qt) • 1 gallon (gal) = 3.785 liters (L) • 1 kilogram (kg) = 2.2 pounds (lb) • 1 ounce (oz) = 28.35 grams (g) • 1 fluid ounce (fl oz) = 29.57 milliliters (mL)

C H A P T E R 3Basic Chemistry of Body

Fluids and ElectrolytesJudith Z. Kallenbach

Chapter 3 Basic Chemistry of Body Fluids and Electrolytes 19

Temperature is expressed in degrees centigrade. Zero degrees centigrade is the freez-ing point of water and 100° C is its boiling point. The following table shows a comparison of some centigrade temperatures with the Fahrenheit scale:

° F ° CBoiling point of water 212 100Normal body temperature 98.6 37Freezing point of water 32 0

To convert Fahrenheit values to centigrade and centigrade values to Fahrenheit, use the following formulas:Fahrenheit temperature = 9/5 (centigrade temperature) + 32Centigrade temperature = 5/9 (Fahrenheit temperature) − 32

TABLE 3-1 Commonly Used Metric UnitsQuantity Unit Abbreviation Relationship of Units

Length MillimeterCentimeterMeterKilometer

mmcmmkm

1 mm = 0.001 m1 cm = 0.01 m1 m1 km = 1000 m

Area Square centimeterSquare meterSquare kilometer

cm2

m2

km2

1 cm2 = 0.0001 m2

1 m2

1 km2 = 1,000,000 m2

Volume MilliliterDeciliterLiterCubic meter

mLdLLm3

1 mL = 0.001 L1 dL = 0.01 L1 L1 m3 = 1000 L

Mass MilligramGramKilogram

mggkg

1 mg = 0.001 g1 g1 kg = 1000 g

TABLE 3-2 Metric Decimal PrefixesMultiplication Factors Prefix Abbreviation

10.1 = 10−1 deci- d0.01 = 10−2 centi- c0.001 = 10−3 milli- m0.000001 = 10−6 micro- μ0.000000001 = 10−9 nano- n0.000000000001 = 10−12 pico- p

20 Review of Hemodialysis for Nurses and Dialysis Personnel

An optional method to convert Fahrenheit values to centigrade and centigrade values to Fahrenheit is as follows:To convert Fahrenheit to centigrade, subtract 32 and divide by 1.8To convert centigrade to Fahrenheit, multiply by 1.8 and add 32

All physical things are composed of a finite number of kinds of matter. Matter is anything that possesses weight and occupies space or has mass. The basic kinds of matter are called elements. An element cannot be further divided without chang-ing its chemical properties. There are 108 known elements. These may exist alone, in mixtures, or in chemical combinations (compounds). Some elements exist alone in their natural form as a solid, liquid, or gas. For instance, gold nuggets are pure, crystalline gold (Au). Metallic mercury (Hg) is a liquid under ordinary conditions. Helium (He) is a monatomic gas. The physical state depends on the melting or boil-ing point. Many elements do not exist in an uncombined state but exist only as com-pounds. Oxygen, as it exists in air, is not monatomic (O) but rather a compound of two oxygen atoms (O2). Almost all hydrogen (H) exists in compounds, such as in water (H2O).

What Is a Solution?A solution is a homogeneous mixture of dissolved particles (solute) and a liquid (sol-vent). In physiologic solutions, the solvent is usually water. Physiologic saline contains 0.9 g of sodium chloride in 100 mL of water.

How Is the Concentration of a Solution Measured?Nonionized particles have been measured in terms of the weight of the solute per volume of the solvent. Blood glucose and urea are commonly measured as milligrams per 100 mL (mg/dL). For ionized particles it is important to know the relative number of par-ticles present and the contribution of their charge. These are measured more precisely by the use of molarity and normality and are usually expressed as mEq/L (milliequivalents per liter).

What Are SI Units?The term SI units is the abbreviation of le Systeme Internationale d’Unites. This is an extension of the metric system that provides uniformity for the units of measurement and easy conversion. Since 1987, SI has been used to report data by most clinical labo-ratories in the United States. In this system the amount of a substance is written as moles per liter rather than as mass, such as grams/liter (g/L) or milligrams/deciliter (mg/dL).

What Is an Electrolyte?An electrolyte is a substance that dissolves in water to form ionized particles.


What Is an Ion?An ion is a particle that has an electric charge. It may be a charged atom, such as a sodium ion, or a charged compound, such as a lactate ion.

What Is Conductivity?Conductivity is the ability of a solution to conduct electric current. This is illustrated by an electrolytic cell. Figure 3-1 shows such a cell: a container of solution with two elec-trodes. The electrodes are connected by wires through a battery and an ammeter; the ammeter measures the flow of current through the circuit. If the only communication between the electrodes is through very pure water, little or no current will flow, because there is no way for electrons to pass through water.

If sodium chloride is added to water, current will flow. Sodium ions are attracted to the negative electrode (cathode), where each ion accepts an electron. At the same time chlo-ride ions are attracted to the positive electrode (anode), where each gives up an electron.

The ease with which electrons flow in a solution depends on the kind of electro-lytes present and their concentration. Conductivity monitors are vital parts of dialysis fluid delivery systems that must produce solutions with a constant and precise solute content.

Why Is Conductivity an Important Measurement in Dialysis?A dialysate is produced by mixing a concentrated solution of electrolytes with very pure water. The correct proportion of the concentration of electrolytes and water is measured by the electrical conductivity of the solution. The proportion of electrolytes to water must be within certain limits to ensure patient safety. The conductivity of pure water is zero, whereas that of the dialysate depends on the amount of sodium in the solution. A dialy-sate solution containing too little sodium may cause water to shift into the patient’s blood cells, resulting in hypotension, cramping, and hemolysis. Too much sodium in the dialy-sate may cause a high blood level of sodium that may make fluid leave the cells, which causes the blood cells to shrivel. This is known as crenation and may cause symptoms such as hypertension, profound thirst, and headache.

Amperes(�) (�)

Directcurrentsource

Ammeter

Solution

Figure 3-1 An electrolytic cell.


What Is Osmosis?Osmosis is the movement of fluid from an area of low solute concentration to an area of high solute concentration (Figure 3-2). A strong electrolyte solution has reduced water concentration because some of the water has been replaced by the solute. If two solutions of different concentrations are separated by a membrane permeable only to water, water flows from the area of highest water concentration to the area of lowest water concentra-tion. In other words, water flows from the area of lowest solute concentration to the area of highest solute concentration. Only water moves, not the solute.

What Is Diffusion?Diffusion is the movement of solutes from an area of higher solute concentration to an area of lower solute concentration so that both sides are equal (Figure 3-3). Only the solutes move, not water.

Semipermeable membrane

Before osmosis After osmosis

Figure 3-2 Osmosis is the process of movement of water through a semipermeable membrane from an area of low solute concentration to an area of high solute concentration. (From Lewis SM, Heitkemper MM, Dirksen SR: Medical-surgical nursing, ed. 9, St. Louis, 2013, Mosby.)

Membrane

Before diffusion After diffusion

Figure 3-3 Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. Normal pH is maintained by a ratio of 1 part carbonic acid to 20 parts bicarbonate. (From Lewis SM, Heitkemper MM, Dirksen SR: Medical-surgical nursing, ed. 9, St. Louis, 2013, Mosby.)


What Is pH?The measure of either the acidity or alkalinity of a substance is expressed as pH (Figure 3-4). The term pH stands for the potential, or power, of hydrogen, and the pH value demonstrates the concentration of hydrogen ions in a solution. The normal hydrogen ion concentration in human extracellular fluid (ECF) ranges from 7.35 to 7.45. If a substance has a pH value less than 7, it is an acid. A substance that has a pH value greater than 7 is

1

2

3

4

5

6

7

8

9

10

11

12

13

14

0

– Gastric fluid

– Lemon juice

– Wine, orange juice

– Tomatoes

– Coffee

– Milk, urine

– Distilled water– Blood

– Baking soda– Egg white

– Borax

– Milk of magnesia

– Household ammonia– Lime water

– Lye, caustic soda

BASIC(alkaline) OH�

NEUTRAL

ACIDICH�

Figure 3-4 The pH range on a logarithmic scale of 1 to 14. The actual concentration of hydrogen ions changes tenfold with each pH unit on the scale. (From Thibodeau GA, Patton KT: Structure and function of the body, ed. 13, St. Louis, 2008, Mosby.)


an alkali. If a substance has a pH value of 7, it is considered neutral. pH measures only the free hydrogen ions in solution. If some hydrogen ions are bound and not ionized, it does not affect the pH. pH is maintained by the action of buffers.

What Is a Buffer?Buffers are substances that, in solution, maintain a constant hydrogen ion concentration despite the addition of either an acid or a base. Buffers minimize pH changes when an acid or a base is added to a solution. Bicarbonates, phosphates, amino acids, and proteins all act as buffers. Bicarbonate is the major plasma buffer.

Why Is the Hydrogen Ion Concentration Important?All metabolic processes of the body require a precise range of hydrogen ion con-centration. If the hydrogen ion concentration exceeds that of pure water, the solu-tion is acidic. If the hydrogen ion concentration is lower, the solution is basic, or alkaline. If the hydrogen ion concentration becomes too high or too low, massive metabolic derangement occurs. The hydrogen ion concentration compatible with life lies between 20 and 160 nmol/L (pH 7.8 to 6.8) (Figure 3-5). Two body organs are involved in hydrogen ion regulation: the lungs and the kidneys. The lungs eliminate carbon dioxide (the major end product of metabolism) as rapidly as it is produced, and in doing so, regulate the partial pressure of carbon dioxide in the blood. The kidneys regulate blood pH by reabsorbing or excreting acids or bases. Kidney failure causes retention of hydrogen ions; this is called metabolic acidosis. See Chapter 5 for further discussion.

Base bicarbonate

20

Carbonic acid

1

Death Acidosis Normal Alkalosis Death

pH6.8

pH7.35

pH7.45

pH7.8

Figure 3-5 The normal range of plasma pH is 7.35 to 7.45. Normal pH is maintained by a ratio of 1 part carbonic acid to 20 parts bicarbonate. (From Lewis SM, Heitkemper MM, Dirksen SR: Medical-surgical nursing, ed. 9, St. Louis, 2013, Mosby.)


What Is an Acid and What Is a Base?An acid is a substance that can donate a hydrogen ion, and a base is a substance that can accept a hydrogen ion. An acid may be called a proton donor, and a base may be called a proton receptor. Remember that the hydrogen atom consists of a positively charged nucleus, or proton, and a single negatively charged orbiting electron. The hydrogen ion (H+) is a proton without an orbiting electron.

Body WaterHow Much Water Does the Body Contain?Water is the major constituent of the body, and its volume varies with age, sex, and body fat. Water comprises 45% to 75% of the total body weight of an adult. The pro-portion varies inversely with the amount of body fat. A 70-kg man (154 lb) has about 42 L of total body water (60% of weight). Women have a lower proportion of body water. Infants and very young children have the highest proportion of body water (Figure 3-6).

100

50

0

Adultmale(60%)

Adultfemale(50%)

Newborninfant(80%)

Per

cen

tag

e o

f to

tal b

od

y w

eig

ht

Figure 3-6 Percentage of the total body weight composed of water. (From Thibodeau GA, Patton KT: Structure and function of the body, ed. 13, St. Louis, 2008, Mosby [Rolin Graphics].)


What Purpose Does This Fluid Serve?Body tissues are composed of living cells. Complex chemical processes within these cells produce energy in the form of heat, motion, and regeneration. Oxygen and nutrients are metabolized, and carbon dioxide and other wastes are produced. Water within the cell is the medium for these chemical processes.

Water also surrounds and bathes all cells, protecting them from the hazards of the external world. Water is the vehicle that transports nutrients from and wastes to the out-side environment.

Many conditions can disrupt the mechanisms that control fluid balance in the human body; therefore body fluid disorders are among the most commonly seen problems in patients seeking medical care.

How Is Water Distributed in the Body?Total body water is the sum of all fluids within all compartments of the body. The total body water is distributed between two major compartments: the intracellular fluid (ICF) and ECF. Approximately two thirds (or 40% of the body weight) of the total amount of body water is contained in the ICF compartment and one third (or 20% of the body weight) of the total amount of body water is contained in the ECF compartment. ECF can further be separated into interstitial (in the spaces between the cells and outside the blood vessels), intravascular (in blood plasma), and transcellular fluids (outside normal compartments), which include synovial, pericardial, intraocular, peritoneal, and other body fluids that do not interchange readily (Figure 3-7).

What Are the Constituents of the Intracellular Fluid?ICF provides a medium for cells to function. The composition of ICF varies with the specific tissue. Muscle values are commonly used in these calculations. The concentra-tion of potassium, the major intracellular cation, is 155 mEq/L, that of magnesium is 40 mEq/L, and that of sodium is only 10 mEq/L. Organic phosphates and protein are important anions; the total concentration of chloride and bicarbonate is only 10 mEq/L.

What Is the Composition of the Extracellular Fluid?Plasma water and interstitial fluid are nearly the same. Sodium is the major cation found in ECF (135 to 145 mEq/L). Chloride and bicarbonate are the major anions. About 7% of the plasma volume comprises protein and lipid material that does not cross the capillary wall. The protein molecules are anionic; to maintain electrical neutrality, there are slightly fewer sodium and chloride ions in the plasma than in the interstitial fluid. Clinical calcu-lations of electrolytes usually ignore these small differences and assume that the plasma electrolytes are representative of the total ECF.


Intravascular plasma3 L

Interstitial fluid11 L

INTAKE

Intracellular fluid28 L

Capillary membraneLymphatics

Extracellularfluid (14 L)

Cell membrane

OUTPUT•Kidneys•Lungs•Feces•Sweat•Skin

Figure 3-7 Body fluid compartments. (From Hall JE: Guyton and Hall’s textbook of medical physiology, ed. 12, Philadelphia, 2011, Saunders.)

What Determines the Distribution of Water Between Plasma and the Interstitial Compartment?The distribution of water between plasma and the interstitial compartment depends on the balance among colloid (protein and lipid) osmotic pressure, intracapillary blood pressure, and tissue turgor pressure. This is known as the Starling effect.

How Are Electrolyte Concentrations Kept Different Inside and Outside the Cell?The cell membrane is impermeable to proteins and organic phosphate complexes, confin-ing them inside the cell. The cell wall contains metabolically active (energy-consuming) “pumps” that transport sodium ions from within the cell to the outside while moving potassium ions from the exterior to the cell’s interior.


Does Water Pass Across the Cell Membrane?Yes, water moves quickly in either direction across the cell membrane to maintain total osmolar equality on both sides of the membrane (Figure 3-8).

Are There Nonelectrolytes in Body Fluids?Yes. These include glucose, amino acids, and other nutrients and metabolic wastes such as urea. Their concentrations are relatively low compared with the electrolytes.

Are Urea and Creatinine Electrolytes?No. Both urea and creatinine are soluble in water, but they do not form charged particles.

What Is Meant by Fluid Balance?A normal diet contains 500 to 1000 mL of water in the food itself. Approximately 300 to 500 mL of water is produced each day by food metabolism and tissue breakdown. Other fluids taken in, such as coffee, tea, juice, or other beverages, obviously represent water intake and average 1500 to 2000 mL/day.

300Urea

Otherions

Proteins Proteins,other

nitrogen-containing

solutes

Plasma Interstitialfluid

Intracellularfluid

(skeletal muscle)

GlucoseGlucoseUrea

Otherions

Otherions

Organicphosphates

mO

sm/k

g H

2O

Cl� Cl�

Na�

Na�

K�

Mg��

Na�

Cl�

200

100

0

HCO�3 HCO�

3

HCO�3

Figure 3-8 Osmotic composition of major body fluids. (Redrawn from Laiken ND, Fanestil DF: Best and Taylor’s physiological basis of medical practice, ed. 12, Baltimore, 1991, Williams & Wilkins.)


Between 700 and 1000 mL of water is lost daily through evaporation from the lungs and insensible perspiration (Box 3-1). Vigorous activity or an increase in temperature causes additional losses (measurable in liters if the increase in environmental tempera-ture is severe). A minimum of 400 mL of fluid or more must be excreted as urine each day to prevent the accumulation of metabolic wastes.

The electrolyte composition, pH, osmolality, etc. are precisely maintained in the body’s internal fluid environment. The kidneys maintain this balance, called homeostasis. The kidneys conserve fluid or excrete excess as needed. When kidney failure occurs, meticu-lous attention to the balance of fluid intake and losses becomes a necessity.

IntakeFluids 1200 mLSolid food 1000 mLWater from oxidation 300 mL

2500 mL

OutputInsensible loss (skin and lungs) 900 mLIn feces 100 mLUrine 1500 mL

2500 mL

BOX 3-1 Normal Fluid Balance in an Adult

Basic Chemistry of Body Fluids and Electrolytes

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