Essentials of Anatomy and Physiology

Metabolic alkalosisis not common, but may be
caused by the overuse of antacid medications or the
vomiting of stomach contents only. As the pH of body
fluids begins to rise, breathing slows and decreases the
amount of CO 2 exhaled. The CO 2 retained within the
body increases the formation of Hions, which will
help lower the pH back toward the normal range.
The respiratory system responds quickly to prevent
drastic changes in pH, usually within 1 to 3 minutes.
For an ongoing metabolic pH imbalance, however, the
respiratory mechanism does not have the capacity to
fully compensate. In such cases, respiratory compen-
sation is only 50% to 75% effective.

RENAL MECHANISMS

As just discussed in Chapter 18, the kidneys help reg-
ulate the pH of extracellular fluid by excreting or con-
serving Hions and by reabsorbing (or not) Naions
and HCO 3 ions. One mechanism was depicted in
Fig. 18–6, and another, involving the phosphate buffer
system, is shown in Fig. 19–4.
The kidneys have the greatest capacity to buffer an
ongoing pH change. Although the renal mechanisms
do not become fully functional for several hours to
days, once they do they continue to be effective far
longer than respiratory mechanisms. Let us use as an
example a patient with untreated diabetes mellitus
who is in ketoacidosis, a metabolic acidosis. As acidic
ketones accumulate in the blood, the capacity of the
extracellular fluid (ECF) buffer systems is quickly
exhausted. Breathing rate then increases, and more
CO 2 is exhaled to decrease Hion formation and raise
the pH of ECF. There is, however, a limit to how
much the respiratory rate can increase, but the renal

buffering mechanisms will then become effective. At

this time it is the kidneys that are keeping the patient

alive by preventing acidosis from reaching a fatal level.

Even the kidneys have limits, however, and the cause

of the acidosis must be corrected to prevent death.

EFFECTS OF pH CHANGES

A state of acidosisis most detrimental to the central

nervous system, causing depression of impulse trans-

mission at synapses. A person in acidosis becomes con-

fused and disoriented, then lapses into a coma.

Alkalosishas the opposite effect and affects both

the central and peripheral nervous systems. Increased

synaptic transmission, even without stimuli, is first

indicated by irritability and muscle twitches. Progres-

sive alkalosis is characterized by severe muscle spasms

and convulsions.

The types of pH changes are summarized in Table

19–3.

AGING AND FLUID

AND pH REGULATION

Changes in fluid balance or pH in elderly people are

often the result of disease or damage to particular

organs. A weak heart (congestive heart failure) that

cannot pump efficiently allows blood to back up in cir-

culation. In turn, this may cause edema, an abnormal

collection of fluid. Edema may be systemic (often

apparent in the lower legs) if the right ventricle is

weak, or pulmonary if the left ventricle is failing.

The sense of thirst may not be as acute in elderly

450 Fluid–Electrolyte and Acid–Base Balance

Table 19–3 pH CHANGES

Change Possible Causes Compensation

Metabolic acidosis

Metabolic alkalosis

Respiratory acidosis

Respiratory alkalosis

Kidney disease, ketosis, diarrhea, or vomiting

Overingestion of bicarbonate medications, gastric

suctioning

Decreased rate or efficiency of respiration:

emphysema, asthma, pneumonia, paralysis

of respiratory muscles

Increased rate of respiration: anxiety, high

altitude

• Increased respirations to exhale CO 2


• Decreased respirations to retain CO 2


• Kidneys excrete Hions and reabsorb
  Naions and HCO 3 ions


• Kidneys retain Hions and excrete
  Naions and HCO 3 ions