Normal kidneys maintain the balance between body water and the substances dissolved in it within the narrow limits necessary for life. Kidneys also excrete the waste products of protein metabolism. Dialysis partially substitutes for these two important functions when the normal kidneys fail. 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 system
A solid review of the basic system of measurement is necessary because the metric system is used in chemical and physical measurements that relate to body physiology. Length is expressed by the basic unit of the meter. The basic unit of mass is the gram, and the liter is the basic unit of volume. Table 3-1 lists common metric terms and their interrelationship.
Table 3-1 Commonly Used Metric Units
The metric system is entirely decimal. Prefixes indicate smaller or larger units (Table 3-2).
Table 3-2 Metric Decimal Prefixes
|
Multiplication factors |
Prefix |
Symbol |
|
1 |
||
|
0.1 = 10–1 |
deci |
d |
|
0.01 = 10–2 |
centi |
c |
|
0.001 = 10–3 |
milli |
m |
|
0.000001 = 10–6 |
micro |
μ |
|
0.000000001 = 10–9 |
nano |
n |
|
0.000000000001 = 10–12 |
pico |
p |
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-pound person weighs 70 kg.
The following are commonly used conversion factors to change metric units to the English 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)
Temperature is expressed in degrees centigrade. Zero degrees centigrade is the freezing point of water and 100°C is its boiling point. The following is a comparison of some centigrade temperatures with the Fahrenheit scale:
|
°F |
°C |
|
|
Boiling point of water |
212 |
100 |
|
Normal body temperature |
98.6 |
37 |
|
Freezing point of water |
32 |
0 |
For conversion of Fahrenheit values to centigrade and vice versa, use the formula:
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 changing its chemical properties. There are 108 known elements. They 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 boiling point. Many elements do not exist in an uncombined state but only as compounds. Oxygen as it exists in air is not monatomic oxygen (O) but a compound of two oxygen atoms, O2. Almost all hydrogen (H) exists in compounds, such as in water (H2O).
What is an atom?
An atom is the smallest particle of an element that retains the properties of that element. An atom is composed of a central nucleus that contains protons with neutrons with electrons moving in an orbital fashion around the nucleus. An atom resembles a miniature solar system. The sun represents the nucleus; the paths of the planets represent the orbiting electrons. Several electrons may occupy the same orbital path.
Protons are a part of all nuclei and have a positive charge equivalent to that of an electron. An electron is a particle of infinitesimal mass with a negative charge. A neutron is equal to a proton in mass but is electrically neutral.
What is a compound?
A compound is a chemical combination of elements. The proportion of elements is fixed for each compound. For example, water (H2O) always exists in a 2:1 ratio of hydrogen and oxygen.
What is a molecule?
A molecule is the smallest unit of a substance that retains its chemical properties. Most chemicals in the body are in the form of molecules. A molecule of oxygen (O2) has different chemical properties than an atom of oxygen (O).
What is atomic weight?
Atomic weights relate to one another based on an arbitrary scale that assigns a weight of 12 atomic mass units (amu) to the carbon isotope 12 (12C). On this scale, protons and neutrons each weigh 1 amu; electrons have negligible mass. The hydrogen atom (1H) is 1 amu, and the oxygen atom (16O) is 16 amu. One atomic mass unit is sometimes called a dalton (Da), after John Dalton, an early developer of the atomic concept.
What does atomic number indicate?
The atomic number is the number of protons in the nucleus of an atom. This is a unique number that characterizes each element. This number of protons in a nucleus determines the chemical nature of the atom; by contrast, the number of neutrons affects only the weight of an atom. When two atoms with the same number of protons have different numbers of neutrons, and hence have different weights, they are called isotopes.
How does atomic weight relate to molecular weight?
To calculate molecular weight, add the weights of each atom that makes up that molecule. For instance, water (H2O) consists of two hydrogen atoms and one oxygen atom. From this, you can calculate that the molecular weight of water is 18 Da (1 + 1 + 16). If the amu weight of an element is expressed in grams, it is called a gram atomic weight; for compounds, the term is gram molecular weight. This does not make calculating more complex, however. For instance, 16 g of oxygen contain the same number of particles as does 1 g of hydrogen because oxygen weighs 16 times as much as hydrogen.
What determines the physical state of a molecule?
Every molecule possesses kinetic energy, the energy of movement. The speed of a molecule depends on the temperature. An increase in temperature increases molecular speed; cooling reduces molecular speed. The rapid movement of each molecule acts to keep all particles separate from one another.
There are also powerful attractive forces between particles. These attractive forces tend to aggregate molecules. In a crystal of ice, the attractive forces are greater than the separating forces, and the molecules remain trapped in the crystal structure. When heat is added, kinetic energy increases until the separative forces are greater than the attractive forces. This is the process of melting. Further addition of heat increases the kinetic energy until some molecules acquire sufficient energy to escape the liquid state entirely by boiling and producing steam.
Conversely, as cooling occurs, steam condenses into water and then crystallizes into ice. These steps occur as kinetic energy and molecular speed are reduced and the attractive forces become more important.
What is a solution?
A solution is a homogeneous mixture of dissolved particles (solute) and a liquid (solvent). In physiologic solutions, the solvent is usually water. Physiologic saline contains 0.85 g of NaCl in 100 mL of water.
How is the concentration of a solution measured?
Nonionized particles have been measured in terms of weight of solute per volume of solvent. Blood glucose and urea commonly have been measured as milligrams per 100 mL (mg/dL). For ionized particles it is important to know the relative number of particles 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?
SI units is the abbreviation for le Systeme Internationale d’Unites. This is an extension of the metric system that provides uniformity of units of measurement and easy conversion. Since 1987 SI has been used to report data by most clinical laboratories in the United States. In this system the amount of a substance is written as moles per liter rather than as mass, such as g/L or 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 an electric current. It is illustrated by an electrolytic cell. Fig. 3-1 shows such a cell: a container of solution with two electrodes. The electrodes are connected by wires through a battery and an ammeter; the ammeter measures flow of current through the circuit. If the only communication between electrodes is through very pure water, little or no current flows; there is no way for electrons to pass through the water.
Figure 3-1 Electrolytic cell.
If sodium chloride is added to the water, current will flow. Sodium ions are attracted to the negative electrode (cathode), where each ion accepts an electron. At the same time, chloride 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 kinds of electrolytes that are present and their concentration. Conductivity monitors are vital components of dialysis fluid delivery systems that must produce solutions of constant, precise solute content.
Why is conductivity an important measurement in dialysis?
Dialysate is produced by mixing a concentrated solution of electrolytes with very pure water. The correct proportion of 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 the conductivity of dialysate is dependent on the amount of sodium in the solution. A dialysate solution containing too little sodium may cause water to shift into the patient’s blood cells. This may cause hypotension, cramping, and hemolysis. Too much sodium in the dialysate may cause high blood levels of sodium. When the sodium levels in the blood become too high, fluid may leave the cells, causing the blood cells to shrivel. This is known as crenation and may cause such symptoms as hypertension, profound thirst, and headache.
What is osmosis?
Osmosis is the movement of fluid from an area of low concentration of solutes to an area of high concentration of solutes (Fig. 3-2). A strong electrolyte solution has a reduced water concentration because some of the water has been replaced by solute. If two solutions of different concentrations are separated by a membrane permeable only to water, water flows from the area of greatest water concentration to the area of least water concentration. This is the same as saying that water flows from the area of least solute concentration to the area of greatest solute concentration. Only the water moves, not the solute.
Figure 3-2 Osmosis is the process of water movement 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 7, St. Louis, 2007, Mosby.)
What is diffusion?
Diffusion is the movement of solutes from an area of higher concentration of solutes to an area of lower concentration of solutes, so that both sides are equal (Fig. 3-3). Only the solutes move, not the water.
Figure 3-3 Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. A 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 7, St. Louis, 2007, Mosby.)
What is ph?
The measure of either acidity or alkalinity of a substance is expressed as pH (Fig. 3-4). The term pH stands for potential, or power, of hydrogen and the pH value demonstrates the concentration of hydrogen ions in a solution. Normal H+ ion concentration in human extracellular fluid is 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 an alkali. If a substance has a pH value of 7, it is considered neutral. pH measures only the free H+ in solution. If some H+ is bound and not ionized, it does not affect the pH. The pH is maintained by the action of buffers.
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.)
What is a buffer?
Buffers are substances that, in solution, maintain a constant hydrogen ion concentration despite the addition of either acid or base. Buffers minimize pH changes when acid or 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 this precise range of H+ concentration. If the H+ concentration exceeds that of pure water, the solution is acidic. If the concentration is less, it is basic, or alkaline. If the concentration becomes too great or too small, massive derangements of metabolism occur. The H+ concentration compatible with life lies between 16 and 160 nmol/L (pH 7.8 to 6.8) (Fig. 3-5). Two body organs are involved in H+ 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 so doing regulate the partial pressure of carbon dioxide in 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.
Figure 3-5 The normal range of plasma pH is 7.35 to 7.45. A 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 7, St. Louis, 2007, 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 the proton without the orbiting electron.
Body water
How much water does the body contain?
Water is the major constituent of the body and varies with age, sex, and body fat. It comprises 45% to 75% of the total body weight of an adult. The proportion 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 less body water. Infants and very young children have the highest proportion of body water (Fig. 3-6).
Figure 3-6 Percentage of 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 tissue is made up of living cells. Complex chemical processes within these cells produce energy in the form of heat, motion, and regeneration. Oxygen and nutrients are metabolized; 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. It is the vehicle for transportation of nutrients from—and wastes to—the outside environment.
Many conditions can disrupt the mechanisms that control fluid balance in the human body, and therefore disorders of body fluids are one of 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 the extracellular fluid (ECF). Approximately two thirds (or 40% of body weight) of the total amount of body water are contained in the ICF compartment and one third (or 20% of body weight) of the total amount of body water is contained in the ECF compartment. The ECF can further be separated into interstitial (spaces between the cells and outside the blood vessels), intravascular (fluid in the blood plasma), and transcellular (fluids outside normal compartments), which includes synovial, pericardial, intraocular, peritoneal, and other body fluids that do not interchange readily (Fig. 3-7).
Figure 3-7 Body fluid compartments.
(From Hansen JT, Koeppen BM: Netter’s atlas of human physiology, Philadelphia, 2002, Saunders. Netter illustration from www.netterimages.com. © Elsevier Inc. All rights reserved.)
What are the constituents of intracellular fluid?
ICF provides fluid to the cells to function. The composition of ICF varies with the specific tissue. Muscle values are commonly used in calculations. Potassium, the major intracellular cation, is 155 mEq/L; magnesium is 40 mEq/L; and sodium is only 10 mEq/L. Organic phosphates and protein are the important anions; chloride and bicarbonate total only 10 mEq/L.
What is the composition of extracellular fluid?
Plasma water and interstitial fluid are nearly the same. Sodium is the major cation (145 mEq/L). Chloride and bicarbonate are the major anions. About 7% of plasma volume is 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 plasma than in interstitial fluid. Clinical calculations of electrolytes usually ignore these small differences and assume that plasma electrolytes are representative of the total ECF.
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. It is known as the Starling effect.
How are the electrolyte concentrations kept different inside and outside the cell?
The cell membrane is impermeable to protein and the organic phosphate complexes, confining them inside the cell. There are metabolically active (energy-consuming) “pumps” in the cell wall 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 (Fig. 3-8).
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.)
Are there nonelectrolytes in body fluids?
Yes. These include glucose, amino acids, and other nutrients and metabolic wastes, such as urea. Their concentration is 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. Some 300 to 500 mL of water is produced each day by metabolism of food and from tissue breakdown. Other fluid taken in, such as coffee, tea, juice, or other beverages, obviously represents water intake and averages 1500 to 2000 mL/day.
Between 700 and 1000 mL of water is lost each day through evaporation from the lungs and by insensible perspiration (Box 3-1). Vigorous activity or a rise in temperature causes additional loss (measurable in liters if the environmental temperature increase 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.
Box 3-1 Normal Fluid Balance in the Adult
|
Intake |
|
|
Fluids |
1200 mL |
|
Solid food |
1000 mL |
|
Water from oxidation |
300 mL |
|
2500 mL |
|
|
Output |
|
|
Insensible loss (skin and lungs) |
900 mL |
|
In feces |
100 mL |
|
Urine |
1500 mL |
|
2500 mL |
The electrolyte composition, pH, osmolality, and so on are precisely maintained in the body’s internal fluid environment. The kidneys keep this balance, called homeostasis. The kidney conserves fluid or excretes excess as needed. When kidney failure occurs, meticulous attention to the balance of fluid intake and losses becomes a necessity.