Physiology — Body Fluids, Sodium and Potassium, NMC MBBS licence examination syllabus (Nepal Medical Council).
A sodium result is not a salt measurement. It is a ratio, and most of the time the thing that has changed is the water.
The first two physiology chapters covered the circulation, the kidney's filter, acid-base and gas transport. This one covers the compartments those systems move fluid between, and the two ions whose disturbance a candidate will be asked about repeatedly.
Most body water is inside cells. The remainder is outside them, and only a fraction of that sits inside blood vessels. That distribution explains something that otherwise seems wasteful: filling an under-filled circulation takes considerably more fluid than the apparent deficit, because much of what is given redistributes.
The two main ions are held apart deliberately. Sodium is kept outside cells and potassium inside, by a pump that spends energy continuously to maintain the gradient. Almost everything in this chapter follows from that arrangement.
The practical consequence: a salt-containing fluid stays largely in the extracellular space, which is why it is used to refill a circulation, while water without salt distributes through every compartment, including into cells. Choosing between them is choosing which compartment to fill. NMCPH06 covers the specific fluids.The most useful idea in this chapter is that sodium concentration is a ratio — sodium relative to water — not a measure of how much sodium the body contains. It falls when water is retained, and it can fall in a patient whose total body sodium is normal or even increased.
That reframing changes the approach. The question is not "how much salt should I give?" but "what is this patient's fluid state?" — and that is answered by examination rather than by the laboratory.
Assessing the patient as depleted, normal, or overloaded splits the causes into three manageable groups: losses that have not been replaced, retention of water without an excess of salt, and states where both salt and water are retained but the water more so — heart failure, liver failure, kidney failure.
The gradient across the cell membrane sets how excitable that membrane is. Because of that, both high and low potassium threaten the heart — from opposite directions but with the same consequence. Muscle weakness accompanies both, which is a good reason to check potassium in any patient complaining of weakness.
Get an ECG. It shows the risk rather than the number, and two patients with identical results are not necessarily in the same danger. In a patient with a high potassium, the ECG is what determines urgency.A depleted patient has an under-filled circulation: a fast pulse, a low or postural blood pressure, dry mucous membranes, reduced urine output and thirst. In a child, reduced skin turgor and sunken eyes matter more — NMCPD04 covers that assessment.
An overloaded patient has too much fluid in the extracellular space: peripheral oedema, a raised jugular venous pressure, and crackles at the lung bases.
For tracking a patient over days, daily weight is more informative than any single sign. Fluid has mass, and a change of a kilogram overnight is fluid rather than tissue.
An elderly woman on diuretics is confused, with a low sodium. The first step is assessing whether she is dry, which the diuretic makes likely. The confusion reflects the sodium, and correction must be gradual — she has probably been like this for some time.
A young man with severe diarrhoea has a fast pulse and reduced urine output. He is depleted, and the fluid he needs is one that stays in the extracellular space. Potassium is worth checking, since it is lost in diarrhoea.
A well outpatient's routine blood test shows a high potassium. If the patient is well, the ECG is normal, and the sample was difficult to obtain, repeat it before acting. A haemolysed sample is a common explanation.
A patient with liver failure has marked oedema and a low blood pressure. Overloaded tissues and an under-filled circulation together. A diuretic without careful assessment may worsen the perfusion.
Low sodium and salt deficiency. Usually a water excess rather than a salt deficit. Giving salt without thinking about water misses the mechanism in most cases.
Oedema and fluid overload of the circulation. Oedema is fluid in the tissues. The circulating volume can be low at the same time.
The potassium number and the patient's risk. The ECG, not the result, tells you how urgent the situation is.
Correcting quickly and correcting effectively. With sodium, speed is the danger rather than the goal.
Most body water is inside cells. Salt-containing fluid stays extracellular; water without salt goes everywhere.
Sodium concentration is a ratio. Low sodium usually means too much water, so assess the fluid state first.
Correct sodium slowly — rapid correction causes irreversible brain injury, particularly when the disturbance is long-standing.
Both high and low potassium can stop the heart. Get an ECG; it shows the risk.
An unexpected high potassium in a well patient — consider haemolysis and repeat, unless the patient is unwell or the ECG is abnormal.
Oedema plus an under-filled circulation is a real combination in liver and heart failure.
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