Electrolyte disturbances
Why a sodium disorder is really a water disorder, how potassium and calcium set the excitability of nerves and heart muscle, and why the speed of an electrolyte change matters as much as its size.
By the end, you can
- Explain how sodium concentration reflects the balance of water and why hyponatraemia swells brain cells.
- Describe where the body’s potassium is held and how insulin, acid-base status and the kidneys move it.
- Relate high and low potassium to the resting membrane potential, muscle weakness and arrhythmias.
- Explain how ionised calcium affects nerve excitability and relate low calcium to tetany.
- Explain why rapid changes in sodium and potassium are more dangerous than slow ones.
- Recognise the ECG and laboratory patterns, classifications and key values of electrolyte disorders, and explain how treatments shift, remove or replace ions and water.
What it is
An electrolyte disturbance is a plasma concentration of a dissolved ion (sodium, potassium, calcium, magnesium, phosphate) outside its normal range. The concentration reflects the amount of the ion and the water it is dissolved in, its movement between cells and plasma, and its intake and excretion, so the same number can arise in several ways.
3 types to tell apart
- Sodium (hypo- and hypernatraemia)
- Mostly disorders of water: too much water for the sodium present, or too little.
- Potassium (hypo- and hyperkalaemia)
- Disorders of shifts between cells and plasma, or of kidney excretion; their danger is to heart rhythm and muscle.
- Calcium (hypo- and hypercalcaemia)
- Disorders of parathyroid hormone, vitamin D, bone and the kidneys; their effects come from ionised calcium on nerve and muscle membranes.
US spelling: litre → liter.
Normal function
How the kidneys keep ions steady
Sodium is the main ion outside cells and potassium the main ion inside them. A pump in every cell membrane, the sodium-potassium ATPase, moves three sodium ions out for two potassium ions in, keeping this difference and with it the cell’s resting electrical charge.
The kidneys match excretion to intake. The reabsorbs sodium without water; the fine-tunes sodium and calcium; and in the , aldosterone exchanges sodium for potassium while antidiuretic hormone (ADH, vasopressin) decides how much water is reabsorbed.
The release renin when blood pressure or sodium delivery falls, starting the hormone cascade that ends in aldosterone. Thirst and ADH respond to the concentration of the blood (its osmolality), keeping sodium within a few millimoles per litre.
Underlying change · Step 1 of 5
Sodium disorders are water disorders
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Underlying change · Step 2 of 5
Potassium: held in cells, excreted by the kidneys
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Disrupted function · Step 3 of 5
Potassium and the heart’s electrical stability
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Disrupted function · Step 4 of 5
Calcium and nerve excitability
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Consequences · Step 5 of 5
Why the speed of change matters
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Symptoms and signs
Symptoms, signs and findings
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Exam detail
Exam detail
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Treatment
How treatment works
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Recap
Recap
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Self-check
Check your understanding
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .