Editing Anatomy and Physiology (section)

== Fluid, Electrolyte, and Acid-Base Balance ==

Total body water is roughly 60% of adult body weight, distributed intracellularly (about two thirds) and extracellularly (about one third, split into interstitial and intravascular compartments). Infants have a higher proportion of body water and older adults a lower proportion, so both dehydrate faster.

Common adult reference ranges:
* Sodium 135 to 145 mEq/L
* Potassium 3.5 to 5.0 mEq/L
* Total calcium 9.0 to 10.5 mg/dL
* Magnesium 1.3 to 2.1 mEq/L
* Chloride 98 to 106 mEq/L
* Bicarbonate 22 to 26 mEq/L

'''Sodium''' is the main extracellular cation and determines water distribution, so sodium disorders present neurologically. Hyponatremia causes cerebral swelling with confusion, headache, and seizures; hypernatremia causes cellular dehydration with thirst, agitation, and neurologic changes.

'''Potassium''' is the main intracellular cation and governs cardiac and neuromuscular excitability. Both hypokalemia and hyperkalemia cause dysrhythmias. Potassium is '''never''' given by IV push and is always diluted and infused on a pump.

'''Calcium''' stabilizes nerve membranes. Low calcium means increased excitability (tetany, Chvostek and Trousseau signs, laryngospasm); high calcium means decreased excitability (lethargy, weakness, constipation, kidney stones).

'''Magnesium''' behaves much like calcium: low magnesium causes hyperexcitability and torsades de pointes; high magnesium causes depressed reflexes and respiratory depression. Deep tendon reflexes are the bedside monitor during magnesium sulfate therapy.

=== Acid-Base ===

The lungs correct acid-base disturbance in minutes by changing carbon dioxide elimination; the kidneys correct over hours to days by handling hydrogen ion and bicarbonate.

A practical interpretation approach:
# Look at pH: below 7.35 is acidosis, above 7.45 is alkalosis.
# Look at PaCO2 (respiratory) and bicarbonate (metabolic).
# Whichever one moves in the same direction as the pH abnormality is the cause; if PaCO2 moves opposite to pH, the problem is respiratory, and if bicarbonate moves in the same direction as pH, the problem is metabolic.
# Check whether the other value has shifted to compensate.

Common causes: respiratory acidosis from hypoventilation (opioid oversedation, chronic obstructive pulmonary disease, atelectasis); respiratory alkalosis from hyperventilation (anxiety, pain, early sepsis); metabolic acidosis from diabetic ketoacidosis, shock and lactic acid, diarrhea, and kidney failure; metabolic alkalosis from vomiting, nasogastric suction, and excessive antacid intake.