The purpose of the Na/K pump is to create an electrochemical gradient. Water follows ions to keep osmolarity the same between different communicable compartments. If there are more osmoles in the vasculature then more water will go to the vasculature and thus create greater hydrostatic pressure, which is manifested with increased blood pressure.
When we sweat we typically lose hypotonic fluid; we lose some salt but proportionally more fluid. Thus more fluid may move from the ICF to the ECF to help keep the osmolarity the same. However, there is now decreased fluid altogether; thus the renin-angiotensin-aldosterone system can activate and anti-diuretic hormone can increase to lead us to reabsorb more Na from the kidneys and make less urine.
Drinking too much hypotonic fluid too quickly can cause hyponatremia as the Na gets diluted. This is how marathoners can die from drinking too much water. If you drink hypotonic fluid slowly and have working kidneys your kidneys will excrete extra free water to keep the Na concentration the same.
Na and K do not have a job of creating different osmotic pressure. We don't even use K when we calculate osmolality (2*Na + glucose/18 + BUN/2.8). The body likes to keep Na and K concentrations in a set range; this helps with many functions including setting off action potentials in nerve cells correctly. A consequence of keeping the concentrations normal is that free water moves around and generate different pressure, but that's not the primary role of Na & K.
On the bright side, none of this matters whatsoever for a normal person trying to exercise to be healthier. Just make sure you drink enough fluids to stay hydrated and if you do a ton of activity where you do sweat a lot (ie, play football in the summer or run long distance), try to replenish your electrolytes with an oral hydration solution of some sort during the activity.
|