Fluids, electrolytes, and acid-base balance
Fluid and electrolyte balance underpins nearly every body function. Imbalances are common in hospitalised patients and can be dangerous. For study only, not clinical guidance: always follow your programme, current local protocols, and supervising clinicians.
Body fluids
Water makes up about 60% of adult body weight. About two-thirds is intracellular (inside cells) and one-third extracellular (plasma and the fluid between cells). Older adults and infants are more vulnerable to fluid imbalance: older adults have less body water and a weaker thirst response; infants lose fluid quickly.
Fluid volume deficit and excess
Fluid volume deficit (dehydration or hypovolaemia), from vomiting, diarrhoea, bleeding, or poor intake, causes thirst, dry mucous membranes, low urine output, rapid weak pulse, and low blood pressure. Fluid volume excess, as in heart or kidney failure, causes weight gain, oedema, crackles in the lungs, and high blood pressure. Daily weights are one of the best measures of fluid change.
IV fluids
Isotonic fluids such as 0.9% sodium chloride (normal saline) and lactated Ringer's expand the extracellular volume. Hypotonic fluids such as 0.45% saline move water into cells. Hypertonic fluids such as 3% saline draw water out of cells and are used only in specific situations with close monitoring.
Sodium
Commonly cited normal serum sodium is about 135-145 mmol/L. Hyponatraemia (low sodium) can cause confusion, headache, and seizures; it must be corrected slowly, because rapid correction can cause serious brain injury (osmotic demyelination). Hypernatraemia, often from water loss, causes thirst and confusion.
Potassium
Commonly cited normal serum potassium is about 3.5-5.0 mmol/L. Both low and high potassium affect the heart. Hypokalaemia, often from diuretics or vomiting, causes muscle weakness and arrhythmias. Hyperkalaemia, often from kidney failure, can cause life-threatening arrhythmias. IV potassium must never be given as a push; it is diluted and given slowly by pump.
Calcium and magnesium
Low calcium (hypocalcaemia) increases nerve and muscle excitability, causing tingling, muscle cramps, and the positive Chvostek's and Trousseau's signs. Low magnesium often accompanies low potassium and calcium and also causes arrhythmias. Replacing magnesium can be necessary before potassium levels will correct.
Acid-base balance
Normal arterial pH is about 7.35-7.45. The lungs regulate carbon dioxide (an acid) and the kidneys regulate bicarbonate (a base). Respiratory acidosis results from retained CO2 (hypoventilation, as in COPD or opioid overdose); respiratory alkalosis from hyperventilation; metabolic acidosis from conditions such as diabetic ketoacidosis or kidney failure; metabolic alkalosis from vomiting or excess antacids.
Worked example
An older patient with diarrhoea for three days has a heart rate of 112, blood pressure 92/58, dry mouth, and urine output of 15 mL per hour. These suggest fluid volume deficit. The nurse reports findings, anticipates isotonic IV fluids and electrolyte checks (diarrhoea can lower potassium), monitors intake, output, and weight, and assesses for dizziness and fall risk.
Key takeaways
Body water is about 60% of weight, two-thirds inside cells. Deficit causes thirst, low urine output, and tachycardia; excess causes weight gain, oedema, and crackles; daily weights track change. Isotonic, hypotonic, and hypertonic fluids shift water differently. Typical ranges: sodium 135-145, potassium 3.5-5.0 mmol/L. Correct sodium slowly; never push IV potassium. Normal pH is 7.35-7.45, regulated by lungs and kidneys.
Flashcard
About 60% of adult weight
Check your understanding
A patient on a loop diuretic develops muscle weakness and an irregular pulse. Which electrolyte problem is most likely?