If you increase or decrease the total body water independently of its sodium content, then it follows that the sodium concentration will be altered. Think about it: if I have 100 mmol of sodium in one litre, but I then add 200 mls of water, I've changed the sodium concentration from 100 mmol/L to 83 mmol/L [100/1.2]. But changing the sodium concentration hasn't meant that I've changed the sodium content (amount) - which has remained the same, at 100 mmol, no matter how much water I've added.
Therefore, as a general rule, changes in sodium concentration reflect disturbed water homeostasis. The problem will lie with one of the regulators of water balance, then: water intake or AVP.
Now look what happens if I take a human body and force it to retain sodium. You might think that the sodium concentration would again be affected, but remember that sodium is highly osmotically active: it more or less drags an equal amount of water with it. Therefore, reabsorbing (or failing to excrete) sodium will not change the body's sodium concentration. Rather, it will cause there to be a rise in the total amount (content) of sodium and water in the body.
Therefore, as a general rule, changes in sodium content cause hyper- or hypovolaemia. Clinically, this manifests as oedema or dehydration/shock respectively. The problem lies with one of the regulators of sodium balance, usually the renin-angiotensin-aldosterone system.
Before this gets too theoretical, let's look at a few clinical examples.
- Cardiac failure - in this state, the sodium concentration is often low, and the patient is oedematous. From this, we can infer that (1) water is being retained in excess of sodium, causing hyponatraemia, and (2) the body contains too much of both sodium and water, causing oedema. Sure enough, treatment involves water and salt restriction, and diuretics to promote water and salt loss.
- Diarrhoea - the sodium level here can be low, normal or high depending on whether sodium is lost in excess of water or vice versa. For the sake of argument, let's say that in this patient the sodium is low. Regardless of the sodium level, however, the patient is certainly dehydrated. Therefore, unlike in cardiac failure, the treatment of hyponatraemic diarrhoea will include giving (not restricting) sodium and water (e.g. via intravenous normal saline).
In the next post, we'll discuss an approach to hyponatraemia.