Chapter 3 Basic chemistry of body fluids and electrolytes

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.

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