WATER

The two largest constituents of the body fluids are water and electrolytes. The four main functions of water in the body are:

Water is critical to maintaining a state of homeostasis because water is the medium in which most metabolic and chemical reactions in the body take place. Without sufficient water, cells cannot function and death results (deWit 2001).

MATTER

To assist in understanding normal body function and dysfunction, knowledge of certain chemical principles is important. An understanding of chemistry is the basis for the study of homeostasis, which is described later in the chapter.

All living and non-living materials are generally classified as matter. Matter can be defined as a substance that occupies space and has mass, or weight. All matter is composed of either the same or different kinds of atoms. A collection of atoms with the same atomic number represent a pure substance termed an element. Atoms, in turn, are composed of particles termed electrons, protons and neutrons. Protons and neutrons consist of smaller units such as the quark and are held together by electromagnetic forces. The human body, like all other matter, is composed of different elements and atoms. Some atoms form electrolytes or ions, while others combine to form molecules that form structures of the body’s cells and tissues.

ATOMS

An atom is composed of a central dense core of positively charged heavy particles termed protons and an equal number of lighter neutrons that bear little charge and are considered neutral. The nucleus is surrounded by a cloud of negatively charged electrons that are held in place by the positive electromagnetic force of the nucleus. The number of positively charged protons in each neutral atom normally equals the number of negatively charged electrons and therefore an atom is neither positive nor negative in its overall electrical charge under normal conditions. The number of neutrons may vary, but they do not affect the charge of the atom.

Atoms differ from each other in the number of particles they contain and, to aid in the identification of atoms, each one is assigned an atomic number. The atomic number of an atom is equal to the number of protons in its nucleus. For example, the hydrogen atom has one proton, so its atomic number is 1, the helium atom has two protons, so its atomic number is 2; thus, the larger the atomic number the larger and the heavier the atom is. Table 30.1 lists some common elements and their atomic numbers.

TABLE 30.1 SOME NATURALLY OCCURRING ELEMENTS

ISOTOPES

An isotope is an atom that has gained one or more extra neutrons in its nucleus. The atomic weight remains unchanged, as it is the sum of only the protons, but the mass of the atom increases by the mass of the neutron(s) added to the atom. For example, one isotope of carbon has six neutrons in the nucleus, another has seven, and another has eight. All have six protons, so all are carbon atoms. Isotopes are named according to the number of protons and neutrons in the nucleus: thus, the isotopes above are named carbon-12 (¹²C), carbon-13 (¹³C) and carbon-14 (¹⁴C). Many isotopes are radioactive and are used in medicine for diagnostic and therapeutic procedures, for example iodine-131 (¹³¹I).

ELEMENTS

Matter that is composed entirely of the same kind of atoms — that is, each with the same number of protons — is known as an element. There are 112 different elements that are known to exist, some of which do not exist in nature but have only been observed in physics laboratories. Each is classified by its own individual atomic number and particular chemical properties. Every element (and thus each kind of atom) is named and has been given a unique symbol that is used as a ‘shorthand’ for that element (see Table 30.1). When arranged into a table with a series of rows (or ‘periods’) based on increasing atomic weights, from lightest to heaviest, elements with similar properties, such as being a metal or an inert gas, become grouped in vertical columns (see the periodic table of the elements in any chemistry textbook). Elements can combine naturally with each other to form new substances; for example, one atom of sodium (Na) combines with one atom of chlorine (Cl) to form a salt, sodium chloride (NaCl); similarly one atom of Carbon (C) combines with two atoms of oxygen (O₂) to form carbon dioxide (CO₂).

Table 30.2 lists the common elements that make up the human body and their relative concentration within it. More than 95% of the body is made up of the elements oxygen, carbon, hydrogen and nitrogen, while the remaining 5% is comprised mainly of calcium and phosphorus with other elements in very small quantities.

TABLE 30.2 ELEMENTS IN THE BODY

MOLECULES

A molecule is the smallest unit of matter that can exist alone and exhibit the characteristic chemical properties of an element or a combination of elements. A molecule is composed of two or more atoms held together by electromagnetic forces (chemical bonds). A molecule can consist of atoms of the same kind (e.g. an oxygen molecule [O₂]) or of different kinds (e.g. water [H₂O]). A molecule can consist of as few as two atoms (e.g. carbon monoxide (CO) or many atoms (e.g. the 63,000,000,000-odd atoms in the molecules that contain our genetic material, deoxyribonucleic acid [DNA]). Adding or removing an atom or changing its location in a molecule alters the molecule and subsequently its chemical and physiological characteristics.

Molecules may be classified into two basic types: organic and inorganic. Organic molecules contain the element carbon. All known living things are carbon-based life forms. Inorganic molecules may or may not contain carbon and are smaller and simpler than organic molecules.

COMPOUNDS

A compound is a substance composed of two or more molecules of the same type, which cannot be separated by physical means. A compound has different properties from those of its individual elements.

MIXTURES

A mixture is made up of two or more different types of elements or compounds that have been mixed without forming a new compound. Therefore, the elements present in the mixture retain their individual properties, and the components of a mixture can be separated by physical means such as settling or filtering.

IONS AND ELECTROLYTES

An ion is an atom or molecule that has lost or gained one or more electrons. Elements or compounds that dissolve in a solvent, such as water, to form separate ions are known as electrolytes. By dissociating, an ion loses or gains an electron or electrons from the electron cloud and becomes electrically charged. If an electron is lost, the previously neutral atom becomes more positive, as the positively charged protons are no longer balanced by the same number of negatively charged electrons. Conversely, if an atom gains an electron, a previously neutral atom becomes more negatively charged. The number of electrons gained or lost is denoted by a number after the chemical symbol of the element or compound; for example, Ca⁺⁺ is a calcium ion that has lost two electrons, while OH⁻ is a hydroxide ion that has gained one electron. Movement of positively and negatively charged ions across a membrane produces an electric current or potential, for example, the electrical nerve signals in the brain. Positively charged ions are called cations, and negatively charged ions are called anions.

Electrolytes are chemical substances that, when dissolved or melted, dissociate into ions and can conduct an electric current. Electrolytes are a major constituent of all body fluids and affect the functioning of many physiological processes. Electrolytes are essential to the normal function of all cells and are involved in metabolic activities, fluid homeostasis and in creating charge differences on which the functioning of nerves and muscles depend.

The maintenance of electrolyte balance in the body depends on homeostatic mechanisms that regulate the absorption, distribution and excretion of water and the solutes dissolved in it. Many conditions can cause an electrolyte imbalance; for example, prolonged diarrhoea may cause a loss of many electrolytes.

THE MEANING OF pH

The pH scale is used to express the concentration of hydrogen ions (H⁺) in acids or bases. The ‘p’ stands for potential or power, and the ‘H’ stands for hydrogen. Therefore, pH represents the potential, or power, of hydrogen ions present. The pH of a solution is determined by measuring the amount of H⁺ present. The scale is numbered from 0–14; the lower the number on the scale, the more H⁺ is present, therefore the more acidic the solution is. A pH of 7 indicates a neutral solution, while a pH above 7 is termed basic, or alkaline, and a solution below 7 is acidic. Each integral step in the pH scale (i.e. 14, 13, 12 … to zero pH) represents a tenfold increase in H⁺ ion concentration; thus, the pH scale is an inverse logarithmic scale of the amount of hydrogen ions present.

ACID–BASE BALANCE

The pH of arterial blood is maintained between 7.35 and 7.45; however, the normal cellular metabolism of nutrients in the human body continuously produces acids, which are released into the capillaries. As acid enters the capillaries the blood becomes more acidic. The blood in venules is approximately 7.36, making venous blood more acidic than arterial blood.

The body’s capacity to form, excrete and ‘buffer’ acids and bases is known as the acid–base balance. The acid–base balance is critical to the survival of the human body. Variations outside the normal arterial range of a pH of 7.35–7.45 indicate serious dysfunction in the body. The normal metabolism of nutrients by cells produces large amounts of hydrogen ions (H⁺) that form acids. The concentration of hydrogen ions in the cells and tissues must, however, be kept low to prevent cellular damage. To maintain pH within normal limits, the body excretes acids at the same rate that they are produced and buffers any excess hydrogen ions.

BUFFER SYSTEMS

A buffer is a chemical substance that resists changes in the pH of solutions. Buffers react with a relatively strong acid or base and decrease the acidic or basic strength of the solution. Buffer systems provide the body fluids, especially the blood, with protection against changes in acidity or alkalinity.

For example, CO₂ is an end product of cellular metabolism and when combined with water, forms carbonic acid (CO₂ + H₂O → H₂CO₃), which drains back into the circulation. Excess carbonic acid in the blood is rapidly decomposed into carbon dioxide and water (H₂CO₃ ↔ CO₂ + H₂O) and the CO₂ is excreted by the lungs to maintain the pH at normal physiological levels. Conversely, if water is being lost from the blood, it is replaced by the ionisation of carbonic acid, and a rise in pH is prevented. Thus, carbonic acid acts as a buffer in the blood. An example of the buffering action of bicarbonate ion is its reaction with lactic acid. Lactic acid is produced from glucose during muscle contraction, and the dissociation of lactic acid tends to lower the pH. As lactic acid is a stronger acid than carbonic acid, its conjugate base (lactic ion) is weaker. The stronger base (bicarbonate ion) combines with the hydrogen from lactic acid forming dissociated carbonic acid.

ACIDOSIS AND ALKALOSIS

Acidosis is a condition in which the blood becomes acidic (pH < 7.35) due to an increase in hydrogen ion concentration as a result of an accumulation of an acid, or the loss of a base. Alkalosis is a condition characterised by an increased pH, above 7.45, due to a decrease in hydrogen ion concentration as a result of a loss of acids or the accumulation of a base.

An acid–base imbalance occurs when there is a deficit or excess of carbonic acid or base bicarbonate. In some situations in which a client cannot efficiently excrete enough CO₂, for example, in a client with pneumonia, the buffer system may be unable to keep the pH within the normal range. In this situation the excess CO₂ will make the blood more acidic, reducing the pH. If the pH falls below 7.35 the condition is termed acidosis; if the pH rises above 7.45 the condition is termed alkalosis.

Normally the body can soak up or neutralise acids as they are formed (buffering). This is done by:

CAUSES OF pH DISTURBANCES

COMPENSATION

A change in pH by one system of the body will tend to cause an opposite compensatory change in pH, known as respiratory or metabolic compensation. If respiratory acidosis exists, the body will compensate by producing a metabolic alkalosis. If metabolic alkalosis exists, the body will compensate by producing a respiratory acidosis.

DISTRIBUTION OF FLUID AND ELECTROLYTES

An average healthy adult body of 70 kg consists of about 40 L of water. Fluid contained in the body is divided into two compartments: the intracellular fluid compartment (ICF) comprises the fluid located inside cells (about 25 L) and makes up about 60% of total body fluid. The extracellular fluid compartment (ECF) comprises the fluid located outside the cells (about 15 L). The ECF is divided up further into sub-compartments:

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