Body water sits in compartments, and the exam mostly tests the extracellular consequences you can see and measure at the bedside. Get the two pictures straight and most fluid questions answer themselves.
FLUID VOLUME DEFICIT (hypovolaemia — losing water and often sodium through vomiting, diarrhoea, haemorrhage, burns, excessive diuresis or poor intake): the tank is running dry. Expect tachycardia, HYPOtension (especially postural/orthostatic drops), weak thready pulse, dry mucous membranes, poor skin turgor, sunken eyes, thirst, concentrated dark urine with LOW output (oliguria), weight LOSS, and a rising urine specific gravity, haematocrit and blood urea. Priority: restore volume with oral fluids or isotonic IV fluids (e.g. normal saline or Ringer's lactate), monitor intake/output and daily weight, and protect the patient from falls when they stand.
FLUID VOLUME OVERLOAD (hypervolaemia — too much water/sodium from heart failure, kidney failure, or over-infusion of IV fluids): the tank is overflowing. Expect bounding pulse, HYPERtension, raised jugular venous pressure, PERIPHERAL and pulmonary OEDEMA, crackles in the lungs, shortness of breath, weight GAIN, and dilute urine with LOW haematocrit. Priority: reduce fluid (restrict fluids and sodium, give the ordered diuretic), sit the patient upright to ease breathing, give oxygen if hypoxic, and monitor weight, output and breath sounds closely.
Two habits win marks throughout this topic: daily WEIGHT is the single most reliable measure of fluid status (about 1 kg equals roughly 1 litre of fluid), and accurate INTAKE/OUTPUT charting underpins nearly every intervention. A urine output persistently below about 30 mL/hour is a red flag for inadequate perfusion or kidney injury and should be reported.
