Chapter 30 MEETING FLUID AND ELECTROLYTE NEEDS
This chapter gives a brief overview of the physics and chemistry involved in homeostasis of the body. It also focuses on the fluid and electrolyte needs of clients over the lifespan and how a client’s wellbeing can be affected by alteration in fluid input or output. Nurses are required to assess, maintain and educate clients to maintain their fluid and electrolyte balance according to their specific needs. Fluid requirements vary according to age, height, sex, metabolism and the presence of any underlying conditions. Nurses must be able to accommodate for all these different factors when they plan, care and educate clients.
A basic knowledge of two sciences, physics and chemistry, is helpful in many aspects of nursing. This chapter therefore addresses some aspects of physics and chemistry that help to provide a valuable framework in the study of physiology and understanding of the rationale behind many nursing and medical practices. Although the boundary between the two sciences is often indistinct, physics may be defined as the study of the laws and properties of matter relating particularly to motion and force, while chemistry may be defined as the science dealing with the elements, their compounds, and the chemical structure and interactions of matter.
The concepts of atoms, atomic structure and bonding provide the link between physics and chemistry. Both physics and chemistry are interrelated and interdependent, and one action rarely happens in isolation. Therefore, this chapter outlines some of the physical and chemical principles that are commonly applied in nursing practice. This chapter also discusses the acid–base and the fluid and electrolyte balances that are essential components of the body’s homeostatic processes.
My whole family was so worried when my grandfather was admitted to hospital for a bad case of gastroenteritis. Grandad was only 70 years old and the day that he was admitted to hospital, he became very confused. He didn’t know who Grandma was, and she began to cry. He was wandering around, not really recognising anyone. We thought that maybe he had dementia, but we knew it didn’t come on that quickly. When we told the nurses that Grandad was not normally confused, they allayed our fears by telling us that because he had lost so much fluid through diarrhoea and vomiting that this had led to an imbalance with all his body fluids, and would soon be corrected after they had put an IV drip in and replaced all his fluids. They were right! Within a couple of days, Grandad was back to his normal self!
In health the body maintains a precise osmolarity and fluid balance within body compartments; the volume and constituents of body fluids varying only slightly in order to maintain a stable internal physical and chemical environment. When there is a disturbance in fluid and electrolyte balance, the body attempts to compensate by various adaptive mechanisms. If the imbalance is too great or prolonged, the body’s compensatory mechanisms may deplete the ability to maintain homeostasis and health.
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).
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.
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.
|The first 20 elements, by increasing atomic weight|
|Sodium||Na (Latin: natrium)||11|
|Potassium||K (Latin: kalium)||19|
|Some other well-known elements|
|Iron||Fe (Latin: ferrum)||26|
|Copper||Cu (Latin: cuprum)||29|
|Silver||Ag (Latin: argentum)||47|
|Tin||Sn (Latin: stannum)||50|
|Gold||Au (Latin: aureum)||79|
|Mercury||Hg (Latin: hydragyrum)||80|
|Lead||Pb (Latin: plumbum)||82|
|Some heavier naturally radioactive elements|
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).
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.
|Element||Atomic symbol||Percentage of body mass (approximate)|
Trace = less than 0.01%
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.
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.
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.
An acid is any substance that releases hydrogen ions (H+) when dissolved in water. Acids have chemical properties essentially opposite to those of bases. Examples of acids found in the body include hydrochloric, lactic, pyruvic, carbonic, citric, folic and fatty acids. A base is any substance that accepts hydrogen ions in chemical reactions. Alkalis are bases that are soluble in water. Examples of bases found in the body include hydrogen bicarbonate and sodium hydroxide.
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.
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.
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 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.
Respiratory acidosis occurs with hypoventilation, when the body is unable to ventilate enough CO2 out of the lungs, as may occur in airway obstruction, emphysema, asthma or opiate overdose. (The kidneys compensate, e.g. by retaining bicarbonate.) Respiratory alkalosis occurs with hyperventilation, when rapid respirations blow off too much carbon dioxide, resulting in alkalosis of a respiratory nature, as may occur in panic attacks. (The kidneys compensate, e.g. by excreting more bicarbonate.)
Metabolic acidosis occurs when the body is unable to excrete enough acids because of a problem with malabsorption, such as diarrhoea; metabolism, for example, diabetes (ketone acids [see Clinical Interest Box 30.1]); or organ failure, such as kidney failure. (The lungs compensate, e.g. by increasing ventilation and increasing excretion of acidifying CO2.)
CLINICAL INTEREST BOX 30.1 Diabetic ketoacidosis
Clients with diabetes who have not been administered enough insulin or are unwell, or those with undiagnosed diabetes, are at risk of developing diabetic ketoacidosis. Without adequate insulin the body uses its muscle and fat for metabolism, producing ketone acids. To rid the body of acids, emesis and later hyperemesis occurs, resulting in a loss of not only acid but fluid and other electrolytes as well. The ketone acids can severely affect the pH of the blood, causing a metabolic acidosis. The body tries to compensate by excreting acid by the lungs (compensatory respiratory alkalosis), kidneys and skin.
Hyperglycaemia causes an osmotic-induced polyuria and a subsequent fluid and electrolyte imbalance and may result in severe dehydration. If insulin is not administered and the correct rehydration therapy instigated, in severe cases the condition can result in cerebral oedema, coma and death.
Metabolic alkalosis occurs when too much acid is lost, for example by vomiting, nasogastric drainage, or use of some diuretics. (The lungs compensate, e.g. by decreasing ventilation and decreasing excretion of acidifying CO2.)
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.
Chemical reactions, whereby one substance is changed into another substance or substances, are accomplished by a transfer of energy. Chemical reactions that take place in the body cells result in the release of energy. The major source of energy for the body is the food that is consumed. The energy that the cell extracts from the metabolic breakdown of food is stored in adenosine triphosphate (ATP) molecules until needed. ATP is an organic compound produced in all cells and which can readily liberate its energy. When energy is required for cellular activity, one of the phosphate bonds of the ATP molecule is broken, releasing energy, and forming adenosine diphosphate, which is then recycled to form ATP for further energy storage. ATP provides the energy necessary for most physiological processes to take place efficiently. Energy is required by all cells to:
Most of the chemical reactions in the body require the assistance of catalysts to influence the rates at which chemical reactions proceed. The body’s catalysts are a group of proteins known as enzymes.
Homeostasis is the term that refers to the processes by which the internal environment of the body is maintained within narrow physiological parameters. Homeostasis can also be defined as the tendency of the body to maintain the stability of the internal environment. Homeostasis is dynamic and active, as the body constantly and actively pursues the maintenance of a stable internal environment.
Homeostatic mechanisms are the mechanisms by which the body is able to control the state of the internal environment. They are the processes and means by which the body is able to adapt to stresses (anything that threatens or upsets homeostasis) and yet maintain its inner balance. Any stress situation that arises activates protective homeostatic mechanisms that endeavour to compensate for that stress. Without homeostatic mechanisms to maintain the internal environment, the body cannot survive. When the ability of the body to maintain homeostasis is overwhelmed, illness, and sometimes death, occurs.
Much of nursing practice is aimed at maintaining or restoring the client’s homeostasis. Many of the topics discussed in this and other chapters relate to the state of homeostasis; for example, acid–base balance in body fluids, energy production, fluid and electrolyte balance, and body temperature regulation. Homeostatic regulation of the body is achieved by the cooperative action of most organs and tissues, including the lungs, kidneys and cardiovascular system, and the pituitary, suprarenal and parathyroid glands.
For these mechanisms to maintain homeostasis, the body must be able to detect changes and to react appropriately to those changes. The ability of the body to detect changes is through the process of feedback. Two types of feedback exist. Negative feedback brings the body’s internal environment back to its optimal state. Negative feedback is a decrease in function in response to a stimulus; for example, if the blood glucose level rises above normal, action is instituted by several control systems (such as the islets of Langerhans in the pancreas) to restore the blood glucose level to normal.
Positive feedback directs the body’s internal environment away from its optimal state. Positive feedback is an increase in function in response to a stimulus; for example, during childbirth one uterine contraction induces further contractions, which continue to increase in intensity and frequency until the baby is born. Many positive feedback situations are undesirable and can, for example, result in the over-production of a normal body chemical, thus compounding the problem.
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: