Sunday, 25 July 2010

Sodium and Water (3) - Sodium Balance

In the previous post, we discussed how the body regulates its free water content. Now we turn to sodium regulation.

We can lose a minimum of about 100 mmol per day. Therefore, this is the amount that we need to ingest on a daily basis. You lose some sodium in your sweat, and some sodium in your faeces, but these are largely unregulated losses. The place where the body does its sodium bookkeeping is in the kidneys.

Of the filtered load of sodium, about 98% is reabsorbed:
  1. Two thirds is reabsorbed in the proximal convoluted tubule.
  2. One quarter is reabsorbed in the thick ascending loop of Henle (via the Na+/K+/2Cl- cotransporter)
  3. About 5% is reabsorbed at the distal convoluted tubule (by the thiazide -sensitive Na+/Cl- cotransporter.

  4. These channels aren't particularly regulated from a sodium perspective. Rather it is at the last stage that the body finally turns its attention to the fate of sodium.

  5. The remaining sodium reabsorption occurs at the distal proximal tubule, and in the cortical and medullary collecting tubules. This stage is sensitive to hormonal manipulation.
And what are these sodium-controlling hormones? The most famous is aldosterone, which causes the principal cells in this area to reabsorb sodium (in exchange for potassium). Incidentally, it also performs as similar swap in the gut, although this is a less important phenomenon.

A slightly less well-known hormone is atrial natriuretic peptide, which is secreted in response to atrial stretch. It promotes sodium loss directly (by inhibiting distal tubular sodium reabsorption) and indirectly (by decreasing renin, one of the controlling hormones for aldosterone release).

In a similar class is brain natriuretic peptide, which is secreted in response to ventricular stretch and has similar properties to its atrial counterpart. (Note, it doesn't come from our brains, despite the name!)

Between them, these three hormones regulate the amount of sodium in our bodies. They don't really regulate sodium concentration though - this is the job of the body's water balance system described in the previous post. It's important not to get this mixed up. If your patient comes back with a low serum sodium concentration, he may still have a high total body sodium content.

If this last point seems a bit oblique, don't worry: we'll spend a bit of time on it next.