Module 2: Fluids, Electrolytes, and Acid/Base Balance

What percentage of total body water is intracellular fluid (ICF)?

About two-thirds of total body water is intracellular.

What percentage of total body water is extracellular fluid (ECF)?

About one-third of total body water is extracellular.

What are the three types of extracellular fluid (ECF)?

Interstitial fluid, intravascular fluid (plasma), and transcellular fluid (CSF, pleural, synovial, GI fluids).

What is hydrostatic pressure?

The pressure that pushes water out of capillaries into tissues.

What is oncotic pressure?

The pressure (mostly from albumin) that pulls water back into capillaries from tissues.

What is the main cause of edema?

An imbalance between hydrostatic and oncotic pressures, or lymphatic obstruction.

What health conditions can cause edema?

Heart failure, kidney disease, cirrhosis, burns, lymphatic blockage.

How does edema happen physiologically?

Increased capillary pressure, decreased plasma proteins, or impaired lymph drainage cause excess fluid to accumulate in the interstitial space.

What hormone increases water reabsorption by the kidneys?

Antidiuretic hormone (ADH or vasopressin).

What hormone increases sodium and water retention?

Aldosterone.

What hormones reduce blood volume and pressure?

Natriuretic peptides (ANP, BNP, urodilatin).

What is the main function of sodium (Na⁺)?

Regulates water balance, blood pressure, and supports nerve/muscle activity.

What is the normal sodium range?

135–145 mEq/L.

What health conditions can cause hyponatremia?

SIADH, diuretics, heart failure, kidney disease, psychogenic polydipsia, vomiting, diarrhea.

How does hyponatremia happen physiologically?

Low Na⁺ lowers extracellular osmolality, causing water to enter cells and swell — especially brain cells.

What health conditions can cause hypernatremia?

Dehydration, diabetes insipidus, high fever, profuse sweating, diarrhea without adequate water intake.

How does hypernatremia happen physiologically?

High extracellular sodium draws water out of cells, causing cellular dehydration and CNS symptoms.

What is the main function of potassium (K⁺)?

Maintains intracellular electrical neutrality and supports nerve and muscle activity, especially in the heart.

What is the normal potassium range?

3.5–5.0 mEq/L.

What health conditions can cause hypokalemia?

Vomiting, diarrhea, diuretics, alkalosis, insulin therapy.

How does hypokalemia happen physiologically?

K⁺ is lost or shifted into cells, lowering serum levels and hyperpolarizing cells → decreased excitability.

What ECG changes occur with hypokalemia?

Flattened T waves, U waves, and increased risk of arrhythmias.

What health conditions can cause hyperkalemia?

Renal failure, tissue trauma, acidosis, potassium-sparing diuretics, Addison’s disease.

How does hyperkalemia happen physiologically?

Increased extracellular K⁺ depolarizes cell membranes, disrupting repolarization and conduction — especially in the heart.

What ECG changes occur with hyperkalemia?

Peaked T waves, widened QRS, bradycardia, and risk of cardiac arrest.

What is the main role of calcium (Ca²⁺)?

Supports bone structure, nerve transmission, muscle contraction, and blood clotting.

What is the normal calcium range?

8.8–10.5 mg/dL (total).

What health conditions can cause hypocalcemia?

Hypoparathyroidism, vitamin D deficiency, pancreatitis, massive transfusion (citrate), renal failure.

How does hypocalcemia happen physiologically?

Low serum Ca²⁺ increases neuronal membrane excitability → spontaneous depolarization and tetany.

What signs are associated with hypocalcemia?

Chvostek sign, Trousseau sign, muscle twitching, spasms, seizures.

What health conditions can cause hypercalcemia?

Hyperparathyroidism, bone cancer, prolonged immobilization, vitamin D toxicity.

How does hypercalcemia happen physiologically?

Elevated Ca²⁺ increases the threshold for depolarization, decreasing neuromuscular excitability.

What symptoms are seen in hypercalcemia?

Constipation, muscle weakness, lethargy, kidney stones, shortened QT interval.

What is the main role of magnesium (Mg²⁺)?

Stabilizes nerve and muscle cells, regulates cardiac conduction, and serves as an enzymatic cofactor.

What is the normal magnesium range?

1.8–3.0 mg/dL.

What health conditions can cause hypomagnesemia?

Alcoholism, diarrhea, malnutrition, diuretics, uncontrolled diabetes.

How does hypomagnesemia happen physiologically?

Low Mg²⁺ removes inhibitory control on calcium channels, increasing neuromuscular excitability.

What symptoms are seen in hypomagnesemia?

Tremors, seizures, hyperreflexia, torsades de pointes.

What health conditions can cause hypermagnesemia?

Renal failure, excessive magnesium intake (antacids/laxatives), tumor lysis syndrome.

How does hypermagnesemia happen physiologically?

High Mg²⁺ suppresses acetylcholine release and neuromuscular activity → weakness, bradycardia, respiratory depression.

What is phosphate’s primary role in the body?

Energy production (ATP), bone mineralization, and buffering of acids and bases.

What is the normal phosphate range?

2.5–5.0 mg/dL.

What health conditions can cause hypophosphatemia?

Alcohol abuse, refeeding syndrome, DKA recovery, malabsorption, respiratory alkalosis.

How does hypophosphatemia happen physiologically?

Low phosphate reduces ATP production, impairing muscle and organ function, especially in diaphragm and heart.

What health conditions can cause hyperphosphatemia?

Chronic kidney disease, tumor lysis syndrome, hypoparathyroidism, rhabdomyolysis.

How does hyperphosphatemia happen physiologically?

Excess PO₄³⁻ binds to Ca²⁺, forming insoluble salts, reducing free calcium and promoting soft tissue calcification.

What is the normal arterial blood pH?

7.35–7.45.

What is the normal PaCO₂ range?

35–45 mmHg.

What is the normal bicarbonate (HCO₃⁻) range?

21–28 mEq/L.

What causes metabolic acidosis?

DKA, renal failure, diarrhea, lactic acidosis, toxin ingestion.

How does metabolic acidosis happen physiologically?

Acid accumulates or bicarbonate is lost → blood pH drops; lungs try to compensate with hyperventilation.

What causes metabolic alkalosis?

Vomiting, NG suction, diuretic use, excess bicarbonate intake.

How does metabolic alkalosis happen physiologically?

Loss of H⁺ or gain of HCO₃⁻ raises pH; respiratory compensation causes hypoventilation to retain CO₂.

What causes respiratory acidosis?

COPD, hypoventilation due to CNS depression, sedatives, airway obstruction.

How does respiratory acidosis happen physiologically?

CO₂ retention increases carbonic acid, lowering pH; kidneys compensate over time by retaining HCO₃⁻.

What causes respiratory alkalosis?

Hyperventilation due to anxiety, pain, fever, or sepsis.

How does respiratory alkalosis happen physiologically?

Excess CO₂ loss raises pH; causes cerebral vasoconstriction and electrolyte shifts (e.g., hypocalcemia).

What is Kussmaul breathing and when is it seen?

Deep, labored breathing seen in metabolic acidosis, especially diabetic ketoacidosis.

What is the treatment for severe hyperkalemia?

IV calcium gluconate, insulin with glucose, sodium bicarbonate, or dialysis.

Which IV fluid is isotonic and expands plasma volume?

0.9% Normal Saline (NS).

Which fluid is hypotonic and moves water into cells?

0.45% NS or D5W (after glucose is metabolized).

Which fluid is hypertonic and pulls water from cells into vessels?

3% NaCl, D5NS, D5LR.

What are colloid fluids used for?

To increase oncotic pressure and pull water into the vasculature — used in hypovolemia, burns, or low albumin.

What is third spacing?

Fluid shifts into areas where it cannot be used (e.g., ascites, pleural effusion), reducing circulating volume.

A patient with Na⁺ = 129 mEq/L is confused and has nausea. What condition is most likely present?

Hyponatremia — low sodium has led to cerebral edema.

A dehydrated patient has Na⁺ = 151 mEq/L and intense thirst. What is the likely problem?

Hypernatremia — high sodium is pulling water out of cells, causing cellular dehydration.

A patient on furosemide has muscle cramps and a K⁺ of 3.0 mEq/L. What electrolyte imbalance is suspected?

Hypokalemia — due to renal potassium loss, leading to muscle weakness and arrhythmias.

A patient with chronic kidney disease has a K⁺ of 6.2 mEq/L and ECG shows peaked T waves. What is the concern?

Hyperkalemia — excess potassium is disrupting cardiac conduction.

A patient has numbness, twitching, and a positive Chvostek sign. Calcium is 7.9 mg/dL. What is the likely diagnosis?

Hypocalcemia — increased neuromuscular excitability is causing classic signs.

A cancer patient with bone metastases reports fatigue and constipation. Calcium is 11.5 mg/dL. What is the likely issue?

Hypercalcemia — calcium is leaching from bone into the blood, slowing muscle and GI function.

An alcoholic presents with tremors and seizures. Mg²⁺ is 1.2 mg/dL. What condition do you suspect?

Hypomagnesemia — magnesium deficiency is causing neuromuscular hyperexcitability.

A dialysis patient is receiving magnesium-containing antacids and becomes lethargic with diminished reflexes. What is likely happening?

Hypermagnesemia — excess magnesium is depressing neuromuscular and CNS activity.

A malnourished patient develops weakness after refeeding. Phosphate level is 1.5 mg/dL. What is this condition?

Hypophosphatemia — refeeding syndrome causes intracellular phosphate shifts and ATP deficiency.

A patient with chronic renal failure has phosphate = 6.5 mg/dL and calcium = 7.5 mg/dL. What complication is occurring?

Hyperphosphatemia with secondary hypocalcemia — phosphate is binding free calcium.

A diabetic patient in DKA has rapid, deep breathing and low blood pressure. What is this compensatory pattern called?

Kussmaul respirations — compensating for metabolic acidosis by blowing off CO₂.

A patient receiving loop diuretics for heart failure becomes dizzy, has poor skin turgor, and low BP. What is likely occurring?

Dehydration and possible hypovolemia — isotonic fluid loss is reducing plasma volume.

A post-op patient has new swelling in their legs, distended neck veins, and weight gain. What fluid issue should be considered?

Fluid volume excess (possibly isotonic) — likely from IV fluids or fluid retention.

A patient has normal Na⁺ but is receiving hypotonic fluids and becomes confused. What could be happening?

Dilutional hyponatremia and cerebral edema — even with normal sodium, excess free water is lowering osmolarity.

A burn patient has significant edema around wounds and low serum albumin. What fluid shift is likely?

Oncotic pressure loss → third spacing — proteins are low, so fluid leaks into interstitial and “third” spaces.

What is the imbalance? pH = 7.30, PaCO₂ = 50, HCO₃⁻ = 24

Respiratory Acidosis: Uncompensated

What is the imbalance? pH = 7.48, PaCO₂ = 30, HCO₃⁻ = 24

Respiratory Alkalosis: Uncompensated

What is the imbalance? pH = 7.28, PaCO₂ = 40, HCO₃⁻ = 17

Metabolic Acidosis: Uncompensated

What is the imbalance? pH = 7.50, PaCO₂ = 40, HCO₃⁻ = 32

Metabolic Alkalosis: Uncompensated

What is the imbalance? pH = 7.32, PaCO₂ = 55, HCO₃⁻ = 28

Respiratory Acidosis: Partially Compensated

What is the imbalance? pH = 7.49, PaCO₂ = 48, HCO₃⁻ = 30

Metabolic Alkalosis: Partially Compensated

What is the imbalance? pH = 7.35, PaCO₂ = 55, HCO₃⁻ = 30

Respiratory Acidosis: Fully Compensated

What is the imbalance? pH = 7.45, PaCO₂ = 30, HCO₃⁻ = 20

Respiratory Alkalosis: Fully Compensated

What is the imbalance? pH = 7.37, PaCO₂ = 32, HCO₃⁻ = 18

Metabolic Acidosis: Fully Compensated

What is the imbalance? pH = 7.41, PaCO₂ = 48, HCO₃⁻ = 30

Metabolic Alkalosis: Fully Compensated

What is the imbalance? pH = 7.26, PaCO₂ = 60, HCO₃⁻ = 26

Respiratory Acidosis: Uncompensated

What is the imbalance? pH = 7.25, PaCO₂ = 36, HCO₃⁻ = 14

Metabolic Acidosis: Uncompensated

What is the imbalance? pH = 7.51, PaCO₂ = 28, HCO₃⁻ = 23

Respiratory Alkalosis: Uncompensated

What is the imbalance? pH = 7.50, PaCO₂ = 50, HCO₃⁻ = 30

Metabolic Alkalosis: Partially Compensated

What is the imbalance? pH = 7.20, PaCO₂ = 60, HCO₃⁻ = 24

Respiratory Acidosis: Uncompensated (Severe)

What is the imbalance? pH = 7.55, PaCO₂ = 38, HCO₃⁻ = 34

Metabolic Alkalosis: Uncompensated

What is the imbalance? pH = 7.38, PaCO₂ = 48, HCO₃⁻ = 28

Respiratory Acidosis: Fully Compensated

What is the imbalance? pH = 7.36, PaCO₂ = 33, HCO₃⁻ = 18

Metabolic Acidosis: Fully Compensated

What is the imbalance? pH = 7.33, PaCO₂ = 29, HCO₃⁻ = 16

Metabolic Acidosis: Partially Compensated

What is the imbalance? pH = 7.34, PaCO₂ = 58, HCO₃⁻ = 31

Respiratory Acidosis: Partially Compensated

What is the imbalance? pH = 7.27, PaCO₂ = 27, HCO₃⁻ = 12

Mixed Acidosis (Metabolic + Respiratory): No Compensation