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Fundamentals · chapter 20 · Fluid, Electrolyte, and Acid-Base Balance

Acid-Base Balances

The 13 things this section of the textbook says that you are most likely to be asked about — quoted word for word, not retold. Then 6 practice questions with the reasoning.

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Key points

  1. Normal PaCO2 and breathing

    The normal PaCO 2 level is 35 to 45 mm Hg. The act of breathing involves the exhalation of carbon dioxide (CO 2).

  2. Bicarbonate regulation

    The kidneys regulate the bicarbonate level by either excreting bicarbonate in the urine or reabsorbing it back into the bloodstream. The normal serum bicarbonate range is approximately 22 to 29 mmol/L (Hopkins et al., 2022); however, reference ranges may vary slightly by laboratory.

  3. Acidosis and alkalosis thresholds

    A pH below 7.35 indicates acidosis, signifying an abundance of hydrogen ions in the blood. Conversely, a pH exceeding 7.45 suggests alkalosis, indicating a shortage of hydrogen ions in the blood (Hopkins et al., 2022).

  4. Severe acidosis effect

    However, extended or severe acidosis causes potassium ions to shift out of cells to buffer the net influx of positive hydrogen ions, resulting in systemic hyperkalemia (Mount, 2024).

  5. Blood gas interpretation

    Although it is also possible to monitor pH via a capillary blood gas or a venous blood gas, both of those tests tend to report more acidotic results. The type of blood gas sample must be considered when interpreting results (Castro Keenaghan, 2024).

  6. Imbalance categories and compensation

    Acid-bases imbalances fall into two general categories: metabolic imbalances and respiratory imbalances. If an imbalance occurs in one system, the other system will compensate in order to bring the blood pH back into normal range.

  7. Metabolic acidosis seriousness

    Metabolic acidosis is never a benign process and signifies an underlying medical problem that needs to be addressed (Burger Schaller, 2023).

  8. Metabolic acidosis causes

    The root cause of metabolic acidosis is divided into four categories: an increase in acid production, a decrease in acid excretion, acid ingestion, and renal or gastrointestinal bicarbonate loss.

  9. Anion gap significance

    Therefore, having a gap greater than 12 mmol/L indicates that the patient has a large number of additional anions in the plasma, indicating an acid has been added to the system.

  10. Metabolic alkalosis complications

    Metabolic alkalosis is a common diagnosis among hospitalized patients. If not corrected, it can lead to decreased myocardial contractility, arrhythmias, decreased cerebral blood flow, confusion, and even death.

  11. Ventilation disruption

    However, if ventilation is disrupted for some reason, the lungs are not able to exhale excess CO 2 and the patient develops respiratory acidosis (Patel Sharma, 2023).

  12. Mechanical ventilation monitoring

    Patients who are on mechanical ventilation need to have their carbon dioxide levels monitored regularly to ensure that the ventilator settings are appropriate. If the respiratory rate on the ventilator is too high or if the tidal volume is excessive, the patient may develop respiratory alkalosis (Singh Gill, 2019).

  13. Fatal pH extremes

    A pH greater than 7.8 or less than 6.8 is often associated with death.

Terms to know

Acid
An acid is a molecule that can donate a hydrogen ion (H +) in chemical reactions.
Base
A base is a molecule that donates hydroxide ions (OH –) in chemical reactions.
pH
The pH, or potential of hydrogen, serves as the indicator for the acid-base balance in a solution.
Acidosis
Acidosis, pH less than 7.35, is any process that increases the concentration of hydrogen ions in a solution.
metabolic acidosis
An increase in the hydrogen ion concentration as a result of an abnormally low serum bicarbonate level (HCO 3 –), or a serum bicarbonate level less than 22 mEq/L is known as metabolic acidosis.
metabolic alkalosis
A net increase in bicarbonate ions due to loss of hydrogen ions or retention of bicarbonate ions by either the renal or gastrointestinal systems is called metabolic alkalosis.
respiratory acidosis
Having a blood pH less than 7.35 with a concurrent increase in carbon dioxide (CO 2) is called respiratory acidosis.
respiratory alkalosis
A systemic acid-base disorder that is caused by a reduction in carbon dioxide, which produces an elevation in pH above 7.45, is called respiratory alkalosis.

Practice questions

Stuck on select-all-that-apply? How to take them one option at a time.

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  1. A nurse reviews an arterial blood gas result: pH 7.31 and PaCO2 52 mm Hg. Which acid-base imbalance should the nurse identify?

    1. Respiratory acidosis
    2. Respiratory alkalosis
    3. Metabolic acidosis
    4. Metabolic alkalosis

    Answer: Respiratory acidosis

    The pH is below 7.35, indicating acidosis, and the PaCO2 is above the normal 35 to 45 mm Hg range. A low pH with increased carbon dioxide is respiratory acidosis, not a metabolic problem driven by bicarbonate.

  2. A nurse is reviewing a clinic schedule to identify patients who are at increased risk for acid-base imbalance and need closer assessment. Which chronic disease histories should the nurse flag? Select all that apply.

    1. Kidney disease
    2. Lung disease
    3. Endocrine disorders
    4. Isolated stomach acid secretion during eating
    5. Cardiac disease

    Answer: Kidney disease, Lung disease, Endocrine disorders, Cardiac disease

    The section identifies kidney, lung, cardiac, and endocrine disorders as chronic diseases that place patients at risk for acid-base imbalance. Stomach HCL secretion during eating is described as isolated to the gastrointestinal tract and not affecting whole-body acid-base balance.

  3. A capillary blood gas sample has been collected, but the unit’s analyzer is unavailable and the sample must be sent to the lab. What should the nurse do to protect the accuracy of the results?

    1. Discard the sample because capillary blood gas testing is never appropriate
    2. Apply excessive pressure to the puncture site to obtain another tube faster
    3. Send the sample on ice as a STAT lab
    4. Leave the sample at room air temperature until pickup

    Answer: Send the sample on ice as a STAT lab

    If the blood gas sample cannot be analyzed immediately, it should be stored on ice and sent as a STAT lab. Leaving it at room air temperature or using excessive pressure can lead to inaccurate results.

  4. A hospitalized patient has pH greater than 7.45 with a net increase in bicarbonate ions. Which assessment findings would support metabolic alkalosis as described in the section? Select all that apply.

    1. Large-volume vomiting or gastric suctioning
    2. Diarrhea with bicarbonate loss in stool
    3. Overuse of antacids
    4. Renal tubular acidosis with excessive bicarbonate excretion in urine
    5. Use of loop or thiazide diuretics causing secondary hyperaldosteronism

    Answer: Large-volume vomiting or gastric suctioning, Overuse of antacids, Use of loop or thiazide diuretics causing secondary hyperaldosteronism

    Metabolic alkalosis can result from gastrointestinal loss of hydrogen ions, renal loss of hydrogen ions, or retention/addition of bicarbonate. Diarrhea and renal tubular acidosis are described as common causes of nonanion gap metabolic acidosis, not metabolic alkalosis.

  5. A patient admitted for vomiting and diarrhea is tachypneic with clear breath sounds. The patient later reports taking an entire bottle of aspirin, and the provider notes a significant anion gap. Which explanation best connects the tachypnea with the acid-base problem?

    1. The respiratory system is compensating for metabolic acidosis by increasing the respiratory rate to blow off carbon dioxide
    2. The patient’s clear breath sounds rule out any acid-base imbalance
    3. The kidneys are compensating for respiratory alkalosis by excreting bicarbonate immediately
    4. The vomiting has caused respiratory acidosis by increasing carbon dioxide retention

    Answer: The respiratory system is compensating for metabolic acidosis by increasing the respiratory rate to blow off carbon dioxide

    The section’s case describes aspirin ingestion causing anion gap metabolic acidosis, with tachypnea as respiratory compensation. Clear breath sounds do not rule out an acid-base imbalance, and vomiting is discussed as a cause of metabolic alkalosis through hydrogen ion loss.

  6. A preceptor is teaching a student nurse how to interpret different blood gas sample types. Which statements should the preceptor include? Select all that apply.

    1. Venous blood has less oxygen and more carbon dioxide than arterial blood
    2. Capillary and venous blood gas samples tend to report more acidotic results
    3. An arterial blood gas is the most accurate way to measure blood pH
    4. Venous blood has more oxygen and less carbon dioxide than arterial blood
    5. The type of blood gas sample must be considered when interpreting results

    Answer: Venous blood has less oxygen and more carbon dioxide than arterial blood, Capillary and venous blood gas samples tend to report more acidotic results, An arterial blood gas is the most accurate way to measure blood pH, The type of blood gas sample must be considered when interpreting results

    The section explains that arterial blood gas sampling is the most accurate for blood pH, while venous blood contains less oxygen and more carbon dioxide. Capillary and venous samples may be useful, but they tend to look more acidotic, so the sample type must be considered.

Where every quote comes from

Section 20.2 Acid-Base Balances of Fundamentals of Nursing by Christy Bowen, Lindsay Draper, Heather Moore, OpenStax, 2024. Read the whole section free at openstax.org.

The textbook is licensed under CC BY-NC-SA 4.0. This page quotes it word for word and adds the headings, the order and the practice questions; references to the book's figures and stray spaces left by its formatting are removed. The page is shared under the same licence. Nothing here is sold.