Why there are two problems, not one
It is tempting to think the body just keeps track of “fluid.” But it actually watches two things that can move in opposite directions. The first is concentration: how many particles (mostly sodium and its tag-along ions) are dissolved per litre of body water. The second is volume: how much fluid is in the tank, especially the part that fills the blood vessels. You can be concentrated but low on volume (badly dehydrated), or dilute but overloaded (heart failure with swollen ankles). Two problems, so the body needs two control systems.
Here is the cleanest way to keep them apart. Water follows particles. If you change how concentrated the blood is, you fix it by moving *water*. If you change how much volume there is, you fix it by moving *salt* (and water follows along). So roughly: the body controls concentration by adjusting water through vasopressin (ADH) and thirst, and it controls volume by adjusting sodium through the renin-angiotensin-aldosterone system. That single sentence is the spine of this whole track.
Osmolality: the number the body defends
Concentration has a name in the lab: osmolality, the number of dissolved particles per kilogram of water. Healthy blood sits in a narrow band, roughly 285-295 milliosmoles per kilogram, and the body guards it fiercely because every cell sits in this fluid. Too concentrated and cells shrink as water leaves them; too dilute and cells swell as water rushes in. Brain cells hate both, which is why the osmoreceptors that sense osmolality live right next to the centres for thirst and water hormones.
You can estimate osmolality from a basic blood panel, which is useful because it shows that sodium dominates the picture. The classic bedside formula doubles the sodium (to account for the chloride and bicarbonate that ride with it) and adds the small contributions of glucose and urea.
Estimated osmolality = 2 x [Na] + glucose/18 + BUN/2.8 (US units, mg/dL) or = 2 x [Na] + glucose + urea (SI units, mmol/L) Example (SI): Na 140, glucose 5, urea 5 = 2 x 140 + 5 + 5 = 280 + 10 = 290 mOsm/kg -> right in the normal band Notice: doubling sodium gives 280 of the 290. Sodium concentration basically IS your osmolality.
Volume: what the heart and kidneys feel
Volume is harder to measure than concentration, so the body senses it indirectly through pressure and stretch. Sensors in the big arteries, in the heart's own walls, and in the kidney all report on how full the circulation feels. When volume drops, pressure sags, and a cascade switches on to hold onto salt and water and tighten the vessels. When volume is high, the stretched heart releases hormones that tell the kidney to dump salt. We will meet both of these systems in detail; for now just register that volume control is mostly about sodium, kept in line through the kidney by aldosterone and friends.
It helps to see that this whole apparatus is just homeostasis applied to fluid: sensors compare the current state to a set point, and effectors push it back. The cleverness is that two set points (concentration and volume) are defended at once by partly overlapping machinery, which is exactly why fluid disorders can look so tangled until you separate the two threads.