Page 190 - Cardiac Nursing

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166 PA R T II / Physiologic and Pathologic Responses

depend on the specific role of that electrolyte in normal cardio- carbonic acid is excreted as the two gases CO 2 and H 2 O, it some-
d
d
vascular function. times is called volatile acid.
People who do not have cardiovascular disease may acquire an Metabolic acids are produced primarily from the metabolism
electrolyte imbalance that subsequently causes cardiovascular im- of phosphate-containing compounds and amino acids that con-
pairment. In addition, people who have pre-existing cardiovascu- tain sulfur. These metabolic acids include sulfuric and phosphoric
lar disease have specific risk factors for electrolyte imbalances. If acids. Metabolic acids are handled differently by the body than is
imbalances develop in these individuals, then the cardiovascular carbonic acid. For this reason, they sometimes are called noncar-
effects of the electrolyte imbalances may cause severe disturbance bonic acids or nonvolatile acids.
to an already compromised cardiovascular system. Successful nurs- Cellular metabolism also produces small amounts of base (bi-
–
ing management of these individuals involves careful assessment of carbonate ions; HCO 3 ) as a result of oxidation of small organic
risk factors, elimination of those risk factors when possible, sur- anions such as citrate. Much more metabolic acid is produced
veillance for the manifestations of fluid and electrolyte imbalances, than base. In a standard adult, a net 50 to 100 mEq of hydrogen
and nursing interventions to protect and support function during ions is generated per day from metabolism. 3
the correction of fluid and electrolyte imbalances. 1
Acid Buffering
Buffers in the body act to minimize changes in pH because of gain
PRINCIPLES OF ACID–BASE of acid or base. They neutralize acids by taking up excess hydro-
BALANCE gen ions and neutralize bases by releasing hydrogen ions. Buffers
are located in all body fluids; however, the most important buffers
The degree of acidity of the body fluids plays an important role in are those in the extracellular fluid, intracellular fluid, bone, and
physiology. It influences the structure and function of many en- urine. Different body fluids contain different buffers, which meet
zymes and also modifies the affinity between oxygen and hemo- specific needs (Table 7-14).
globin. Deviations of acid–base balance from normal can affect The major extracellular buffer is the carbonic acid–bicarbon-
cellular function and tissue oxygenation. In the extreme, these im- ate–carbon dioxide buffer system (commonly termed the bicar-
balances can be fatal. bonate buffer system). Carbonic acid is a weak acid, which means
that it dissociates partially when in solution so that it is in equi-
librium with bicarbonate and hydrogen ions. The carbonic acid
Terminology Review
concentration can be altered by variations in alveolar ventilation

An acid is a substance that donates hydrogen ions (H ) in solu- (variations in CO 2 excretion). The chemical equation for the bi-
tion. A base is a substance that accepts hydrogen ions. The more carbonate buffer system is written as follows:
hydrogen ions a solution contains, the more acidic it is. The ac- CO 2
H 2 O Δ H 2 CO 3 Δ H

HCO 3

tual number of hydrogen ions in extracellular fluid is small and carbon dioxide water carbonic acid hydrogen ion bicarbonate ion
unwieldy to write (0.00004 mmol/L). 135 Therefore, the degree of
acidity of body fluids is reported as the pH. The pH is the nega- To maintain the pH of the blood within the normal range, there
tive logarithm of the hydrogen ion concentration. It ranges from must be 20 bicarbonate ions for every carbonic acid molecule.
1 (very acidic) to 14 (very alkaline). A pH of 7 is neutral. The The Henderson–Hasselbalch equation, a mathematical descrip-
blood is normally slightly alkaline. The normal pH range of the tion of the pH of a buffered solution, shows how this 20:1 ratio is
blood is 7.35 to 7.45. If the pH of the blood falls below the nor- necessary: [A ]

mal range (i.e., becomes more acidic), the person has acidemia. pH pKa
log (general equation)
3
The process that tends to decrease the pH is called acidosis. Sim- [HA]
ilarly, if the pH of the blood rises above the normal range (i.e., be- [HCO 3 ] (substituting values for

comes more alkaline), the person has alkalemia. The process that pH 6.1
log bicarbonate buffer system)
tends to increase the pH is called alkalosis. [H 2 CO 3 ]
20
pH 6.1
log
Processes Involved in Acid–Base 1
Balance pH 6.1
1.3
Normal cellular metabolism continually produces acids, which pH 7.4
can cause dangerous acidemia without the closely regulated
processes by which the body maintains pH within the normal
range. After acid production, the processes of acid buffering and
acid excretion work to maintain or re-establish a normal pH.
Table 7-14 ■ THE MAJOR BUFFERS
Acid Production Extracellular Fluid Intracellular Fluid Bone Urine
Cellular metabolism produces two types of acids: carbonic acid
and metabolic acids. Carbonic acid (H 2 CO 3 ) is produced as car- Bicarbonate Proteins Carbonates Inorganic
phosphates
bon dioxide (CO 2 ); the enzyme carbonic anhydrase combines the Inorganic phosphates Organic and inorganic Phosphates
CO 2 with water (H 2 O) to produce carbonic acid. In a standard phosphates
adult, approximately 15,000 mmol of carbonic acid are generated Plasma proteins Hemoglobin
3
per day from metabolism of carbohydrates and fats. Because (in erythrocytes)