Acids, Bases, and pH

Clinical Orientation

Mr. Okonkwo, 34, is admitted with diabetic ketoacidosis. His breathing is deep, rapid, and regular—Kussmaul respirations, the body's desperate attempt to blow off acid. His breath has a sweet, fruity odor (acetone). He is lethargic, responding only to painful stimuli. His ABG shows: pH 7.08, PaCO2 12 mmHg, bicarbonate 5 mEq/L. His body has accumulated massive amounts of acid, and his lungs are working furiously to compensate. But even with extreme hyperventilation, his pH is barely compatible with life. To understand what is happening to Mr. Okonkwo, you must understand acids, bases, and the three systems that defend pH: buffers, lungs, and kidneys.

Governing Question: What mechanism links acids, bases, and pH to its required bedside findings, tests, red flags, and nursing priorities?

What Is Normal?

Hydrogen Ion and pH: pH is the negative logarithm of the hydrogen ion concentration [H+]. A small change in pH represents a large change in [H+]. Normal arterial pH is approximately 7.35-7.45. When pH falls below 7.35, the condition is acidemia—too many hydrogen ions. When pH rises above 7.45, it is alkalemia—too few hydrogen ions. The body tolerates a very narrow pH range: below about 6.8 or above about 7.8 is incompatible with life because enzymes, ion channels, and cellular processes are exquisitely pH-sensitive. "Common adult reference range; verify the reporting laboratory."

Bicarbonate Buffer: The bicarbonate (HCO3-) buffer system is the most important extracellular buffer: CO2 + H2O ↔ H2CO3 (carbonic acid) ↔ H+ + HCO3-. This reaction is catalyzed by carbonic anhydrase. The beauty of this system is that both sides can be regulated independently: CO2 is controlled by the lungs (ventilation), and bicarbonate is controlled by the kidneys (reabsorption and regeneration). According to the Henderson-Hasselbalch equation, pH is proportional to the ratio of bicarbonate to CO2. If bicarbonate falls relative to CO2, pH falls (acidosis). If bicarbonate rises relative to CO2, pH rises (alkalosis).

Carbon Dioxide as Volatile Acid Component: CO2 is continually produced by cellular metabolism. In solution, CO2 combines with water to form carbonic acid, which dissociates into hydrogen ions and bicarbonate. CO2 is called a "volatile acid" because it can be eliminated as a gas through the lungs. The partial pressure of arterial CO2 (PaCO2) is normally about 35-45 mmHg. When PaCO2 rises, more hydrogen ions are produced → pH falls (respiratory acidosis). When PaCO2 falls, fewer hydrogen ions → pH rises (respiratory alkalosis).

Lung CO2 Removal: Alveolar ventilation determines CO2 elimination. When ventilation increases (deeper and/or faster breathing), more CO2 is blown off, PaCO2 falls. When ventilation decreases, CO2 accumulates, PaCO2 rises. The respiratory response is rapid—changes in ventilation affect PaCO2 within minutes. This is the "fast" arm of pH regulation.

Renal Bicarbonate and Acid Handling: The kidneys regulate pH by two mechanisms: (1) reabsorbing filtered bicarbonate (about 4000 mEq/day—nearly all filtered bicarbonate is reabsorbed, mostly in the proximal tubule), and (2) excreting the daily acid load (about 50-100 mEq of fixed, non-volatile acids from metabolism) by secreting hydrogen ions into the urine, where they are buffered by phosphate and ammonia. The kidneys also regenerate bicarbonate consumed by buffering. Renal compensation is slow—taking hours to days to reach full effect—but is powerful and sustained.

What Goes Wrong?

Acid Load/Removal or Base Gain/Loss Changes the Bicarbonate-to-CO2 Relationship: A primary change in bicarbonate (metabolic disorder) or in PaCO2 (respiratory disorder) disturbs the ratio, shifting pH. In metabolic acidosis, bicarbonate falls—either from acid gain (lactic acidosis, ketoacidosis, renal failure, toxins) or bicarbonate loss (diarrhea, renal tubular acidosis). In metabolic alkalosis, bicarbonate rises—from acid loss (vomiting, NG suction) or base gain (bicarbonate administration, contraction alkalosis from diuretics). In respiratory acidosis, PaCO2 rises—from hypoventilation (CNS depression, neuromuscular weakness, airway obstruction, COPD). In respiratory alkalosis, PaCO2 falls—from hyperventilation (anxiety, pain, hypoxemia, pulmonary embolism, early sepsis, salicylate toxicity).

Buffers Act First, Lungs Faster, Kidneys Slower: When a primary acid-base disturbance occurs, the defense goes in order of speed. Buffers (bicarbonate, proteins, hemoglobin, phosphate) act instantaneously to minimize pH change—but they are limited in capacity. The respiratory system responds within minutes—hyperventilating to blow off CO2 in metabolic acidosis, hypoventilating to retain CO2 in metabolic alkalosis. The kidneys respond over hours to days—excreting more acid and regenerating bicarbonate in acidosis, excreting more bicarbonate in alkalosis. Renal compensation is the most powerful but the slowest.

Compensation Never Fully Corrects pH: This is a key principle. Compensation moves pH back toward normal, but it never overshoots into the opposite disorder. If a patient has metabolic acidosis, the lungs hyperventilate to lower PaCO2—but the resulting pH will still be low (though closer to normal). If the pH appears normal with abnormal PaCO2 and bicarbonate, consider a mixed disorder or fully compensated chronic condition. A truly normal pH with abnormal gases does not mean "no problem."

Causes, Risk Factors, and Triggers

Cellular Metabolism: Normal metabolism produces CO2 (volatile acid, ~15,000 mmol/day, eliminated by lungs) and fixed acids (~50-100 mEq/day from sulfur-containing amino acids and phosphoric acid from phospholipids, eliminated by kidneys). Increased metabolism (fever, exercise, hyperthyroidism) increases CO2 production. Anaerobic metabolism (shock, hypoxia, severe exercise) produces lactic acid.

Ventilation Changes: Hypoventilation retains CO2, causing respiratory acidosis. Causes: CNS depression (drugs—opioids, benzodiazepines, anesthesia; stroke; head injury), neuromuscular weakness (Guillain-Barré, myasthenia gravis, spinal cord injury), chest wall restriction (obesity, kyphoscoliosis), airway obstruction (COPD, asthma, foreign body), and mechanical ventilation problems. Hyperventilation removes CO2, causing respiratory alkalosis. Causes: anxiety/panic, pain, hypoxemia (stimulates respiratory drive), pulmonary embolism, early sepsis, salicylate toxicity, hepatic failure, pregnancy (progesterone stimulates respiratory center), and mechanical hyperventilation.

Renal Dysfunction: Renal failure prevents acid excretion and bicarbonate regeneration → metabolic acidosis. Renal tubular acidosis (RTA) impairs bicarbonate reabsorption or acid secretion despite relatively preserved GFR → metabolic acidosis with normal anion gap. In CKD, reduced ammonia production limits acid excretion capacity.

GI Losses: Diarrhea loses bicarbonate-rich fluid → metabolic acidosis. Vomiting or NG suction loses HCl (hydrogen and chloride) → metabolic alkalosis. The stomach normally secretes HCl into the lumen; when this acid is lost, the body retains bicarbonate (the alkaline tide from gastric acid secretion is not neutralized by pancreatic bicarbonate secretion because gastric contents are not reaching the duodenum).

Exogenous Substances: Methanol, ethylene glycol, and salicylates cause metabolic acidosis with elevated anion gap. Excessive bicarbonate or citrate administration (massive transfusion) can cause metabolic alkalosis. Diuretics cause contraction alkalosis (volume depletion → RAAS activation → increased distal sodium delivery → increased hydrogen and potassium secretion).

Shock and Tissue Hypoxia: Inadequate oxygen delivery → anaerobic metabolism → lactic acid production → metabolic acidosis. This is an ominous sign—lactic acidosis indicates tissue hypoperfusion and is a marker of shock severity.

What Happens Inside the Body?

Causal Chain 1: The Bicarbonate-CO2 Equilibrium

CO2 + water ↔ carbonic acid ↔ hydrogen + bicarbonate; change in CO2 or bicarbonate shifts pH

This equilibrium is the center of acid-base physiology. An increase in CO2 (hypoventilation) drives the reaction to the right, producing more hydrogen ions → pH falls (respiratory acidosis, at least initially uncompensated). A decrease in CO2 (hyperventilation) drives the reaction left, consuming hydrogen ions → pH rises (respiratory alkalosis). A decrease in bicarbonate (metabolic acidosis from acid gain or base loss) means fewer hydrogen ions are buffered → pH falls. An increase in bicarbonate (metabolic alkalosis) pulls the reaction left, consuming hydrogen → pH rises. Every acid-base disorder can be understood as a primary disturbance on one side of this equilibrium, with the other side attempting to compensate. Key finding: The direction of pH, PaCO2, and bicarbonate together identify the primary disorder—this is the foundation of ABG interpretation (Chapter 11).

Causal Chain 2: Defense in Sequence

Primary disturbance → Immediate buffering → Respiratory response over minutes-hours or renal response over hours-days → Partial movement toward normal

When acid is added to the blood (e.g., lactic acid from shock), the excess hydrogen ions are immediately buffered by bicarbonate: H+ + HCO3- → H2CO3 → CO2 + H2O. Bicarbonate levels drop as it is consumed. The rising CO2 stimulates the respiratory center → hyperventilation → CO2 is blown off → pH partially corrects. Over hours to days, the kidneys increase acid excretion and regenerate bicarbonate → bicarbonate rises back toward normal → pH approaches normal. If the primary problem is respiratory (e.g., acute hypoventilation → CO2 rises → pH falls), the kidneys do not have time to respond acutely—the pH change is more severe in acute respiratory disorders than in chronic ones where renal compensation has occurred (bicarbonate retained). Key finding: The "expected" compensation for a given primary disorder follows predictable patterns. Compensation outside these patterns suggests a mixed disorder. Acute vs. chronic timing changes the expected degree of compensation.

What the Nurse May See

Breathing Pattern: This is the most immediately observable sign. Kussmaul respirations (deep, rapid, regular) suggest metabolic acidosis—the body is hyperventilating to compensate. Slow, shallow respirations suggest respiratory acidosis from hypoventilation. Rapid, shallow breathing may be hyperventilation from anxiety or early sepsis. Cheyne-Stokes respiration (crescendo-decrescendo pattern with apneic periods) may be seen in severe heart failure or CNS disorders.

Mental Status: Acidemia depresses the CNS: confusion, lethargy, stupor, coma. Alkalemia increases CNS excitability: anxiety, irritability, paresthesia, tetany, seizures. The mental status change often correlates with the severity of pH disturbance.

Weakness: Acidosis impairs muscle function. In chronic respiratory acidosis (COPD), respiratory muscles may be fatigued. In metabolic acidosis, the patient may be weak and lethargic.

Cardiac Rhythm and Hemodynamics: Severe acidemia depresses cardiac contractility and causes vasodilation → hypotension. It also predisposes to ventricular dysrhythmias. Severe alkalemia causes vasoconstriction and may trigger coronary artery spasm. Potassium shifts associated with pH changes also affect cardiac rhythm—acidosis shifts potassium out of cells (hyperkalemia); alkalosis shifts potassium into cells (hypokalemia).

Symptoms of Underlying Cause: A patient with DKA will have polyuria, polydipsia, weight loss, and a fruity breath odor. A patient with vomiting will have signs of volume depletion and metabolic alkalosis from acid loss. A patient with COPD will have chronic dyspnea, wheezing, and a barrel chest. Always look for the cause—the acid-base disorder is a manifestation of the underlying disease.

Tests, Labs, and Monitoring

ABG Components: pH (acidemia vs. alkalemia), PaCO2 (respiratory component—normal ~35-45 mmHg), and bicarbonate (metabolic component—normal ~22-26 mEq/L). These three numbers are sufficient to identify the primary disorder in most cases. PaO2 and SaO2 assess oxygenation separately.

Venous Gas Limitations: Venous blood gases (VBG) can approximate pH and bicarbonate reasonably well but are less reliable for PaCO2 and PaO2. A VBG showing normal pH and bicarbonate makes significant acid-base disturbance unlikely. For precise diagnosis and monitoring, ABG is superior. VBG is easier to obtain (less painful, less risk of arterial injury) and is often used for trending.

Electrolytes and Anion Gap: The anion gap = sodium - (chloride + bicarbonate). Normal is typically ~8-12 mEq/L (varies by laboratory). The anion gap represents unmeasured anions (proteins, phosphate, sulfate, organic acids). An elevated anion gap in metabolic acidosis means unmeasured acids (lactate, ketones, toxins, renal failure acids) are consuming bicarbonate. A normal anion gap (hyperchloremic) metabolic acidosis means bicarbonate is being replaced by chloride—from diarrhea, renal tubular acidosis, or saline administration.

Renal Function: BUN and creatinine determine whether the kidneys can compensate appropriately. In renal failure, metabolic acidosis is expected (impaired acid excretion and bicarbonate regeneration), and renal compensation for respiratory disorders is blunted.

Lactate or Ketones: When metabolic acidosis with elevated anion gap is present, lactate and ketones help identify the cause. Elevated lactate suggests tissue hypoperfusion, sepsis, or mitochondrial dysfunction. Elevated ketones (beta-hydroxybutyrate) suggest DKA, alcoholic ketoacidosis, or starvation ketosis. These are ordered by the provider.

Nursing Priorities

Identify Primary Problem from Direction of pH, PaCO2, and Bicarbonate: Before you interpret a gas, know the patient. Then: (1) Is pH low (acidemia) or high (alkalemia)? (2) Does PaCO2 or bicarbonate match the direction of pH change? If pH is low and PaCO2 is high → respiratory acidosis. If pH is low and bicarbonate is low → metabolic acidosis. If pH is high and PaCO2 is low → respiratory alkalosis. If pH is high and bicarbonate is high → metabolic alkalosis. This is the core pattern recognition.

Assess Patient Before Paper: The ABG is a tool, not the patient. A patient with mild metabolic acidosis who is alert with normal vitals is very different from a patient with the same ABG who is obtunded and hypotensive. Always ask: How does the patient look? What is their breathing pattern? What is their mental status? What are their vital signs? The ABG provides the mechanism; the patient provides the urgency.

Connect Results to Cause and Trend: A single ABG is a snapshot. Trends over time—serial gases, serial lactates, serial electrolytes—tell you whether the patient is improving or deteriorating. Rising lactate, falling pH, or worsening compensation are alarming trends.

Verify Specimen/Context: Is this truly an arterial sample? Was it collected properly (no air bubbles, on ice if delayed, proper heparin)? Was it drawn during mechanical ventilation with known settings? Could it be a venous sample mislabeled as arterial? When the gas does not match the patient, question the sample.

Complications and Red Flags

Patient and Family Teaching

One-Minute Mechanism: "Your body constantly produces acid just from living—from breaking down food and from the energy your cells use. Your body also needs to keep its pH almost exactly the same at all times—like pool water that must stay perfectly balanced. If the water gets too acidic or too basic, everything in it suffers. Your body uses three tools: instant-acting chemical buffers, fast-acting lungs (breathe faster to blow off acid or slower to keep it), and slow-acting kidneys that can fix the balance over hours. When one of these tools fails or is overwhelmed, your blood chemistry drifts into dangerous territory."

Key Points: If you have diabetes, check your blood glucose regularly and know the warning signs of DKA: nausea, vomiting, deep rapid breathing, fruity breath, confusion—these require emergency care. If you have COPD or asthma, follow your medication plan and seek help if you cannot catch your breath. If you have kidney disease, your body has less ability to manage acids—follow dietary recommendations and attend dialysis sessions as scheduled. Prolonged vomiting or diarrhea can affect your body's acid-base balance—seek care if it persists more than 24 hours.

Key takeaways and summary

Summary

Normal → Change → Consequence → Finding → Priority: The pH is maintained by the bicarbonate-CO2 buffer system, with lungs controlling CO2 (fast) and kidneys controlling bicarbonate (slow). A primary change in bicarbonate (metabolic) or PaCO2 (respiratory) shifts the equilibrium, changing pH. The unaffected system attempts to compensate but never fully corrects. Breathing pattern, mental status, and hemodynamics reflect the severity. The nursing priority is to identify the primary disorder from pH, PaCO2, and bicarbonate, assess the patient for clinical severity, and recognize when compensation is failing.

Causal Chain 1: CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-; change in CO2 or bicarbonate shifts pH. Causal Chain 2: Primary disturbance → immediate buffering → respiratory (minutes-hours) or renal (hours-days) response → partial movement toward normal.

If You Remember Nothing Else:

1. pH, PaCO2, and bicarbonate together identify the primary acid-base disorder.
2. Lungs compensate for metabolic disorders; kidneys compensate for respiratory disorders.
3. Compensation is partial—it never overcorrects or cures the cause.
4. Red flag: Extreme pH (<7.0 or >7.6) and respiratory fatigue in a compensating patient signal impending decompensation.
5. Test limitation: A normal pH with abnormal PaCO2 and bicarbonate may indicate a compensated or mixed disorder—not "no problem."

One-Minute Teach-Back: "Explain the three systems that protect pH—buffer, lungs, and kidneys—and why each one matters at a different speed."

Common Student Mistakes

Mistake: "A normal pH excludes an acid-base disorder." Wrong. A fully compensated chronic respiratory acidosis or a mixed metabolic acidosis-respiratory alkalosis can produce a normal pH. The presence of abnormal PaCO2 and bicarbonate with normal pH demands investigation—it is not reassuring. Look at all components, not just pH.

Mistake: "Compensation overcorrects and cures the primary cause." Wrong. Compensation is a physiological response that minimizes pH change, but it does not fix the underlying problem. It never overshoots—respiratory compensation for metabolic acidosis will not cause the pH to become alkalotic. If pH is on the "wrong" side given the primary disturbance, consider a mixed disorder.