A diagrammatic representation of how substances in solutions move from areas of high concentration (right in the picture above) to areas of low concentration.
Why doesn't the blood itself leak out of the pores? That's because the pores are very, very tiny and the size of the blood cells are larger than the size of the pores. The pores are designed so that only the toxins and water can pass through them.
Now, it is possible that things from the dialysate also pass through these pores and enter the blood. It all depends on the size of the substance and the concentration difference on the two sides of the pores. Any substance can only move from one side to the other if the concentration (the amount of the substance per unit volume) is more on one side than the other. If the concentrations are the same, no movement will happen.
Now coming back to Sodium. The dialysate must be ideally designed such that the concentration of Sodium in it is around the same as that of a healthy human body. This is around 135 to 145 mEq/liter. The dialysate, therefore must also be maintained around this level.
The dialysate is prepared by mixing three liquids inside the machine - the Acid solution (Part A), the Bicarbonate solution (Part B) and RO water in a certain proportion.
The conductivity setting of the dialysis machine directly corresponds to the level of sodium in the dialysate. A higher conductivity means a higher sodium level in the dialysate and vice versa. By altering the conductivity desired, we can tell the machine what sodium level we would like the blood to be exposed to. The machine does this by altering the proportion in which it mixes these three liquids.
As in anything related to dialysis, as indeed, in medicine, every individual is different and there is no one single number for conductivity that is suited to all. Generally, a high conductivity causes excess sodium to be present in the dialysate and as a direct consequence, in our blood while a lower conductivity cause lower amounts of sodium to be present in the dialysate and in our blood. Excess sodium causes excess thirst and causes us to drink more water while low sodium causes low blood pressure and cramps as well.
So, if you come back with excess fluid weight gain, it is quite likely that your conductivity setting was higher than you need it to be. Blame the techs or this! Not yourself!
But I am sure most of us are lucky enough that our technician or nurse knows all this and sets the conductivity that is suitable for our body. I am sure most of them understand the problems related to excess sodium and low sodium and most of us don't have to ever worry about this at all. Right? :-)
For the minority among us whose technician or nurse do not know about this, start with a conductivity of anything between 13.8 to 14 and then if you are getting cramps or low blood pressure, go up a point or two (0.1 or 0.2) at a time. If, on the other hand, you are feeling too thirsty and feel you can drink the water in all the rivers in the country and still not tire, try going down a notch at a time (0.1 o 0.2) and see how it feels!
Remember, it YOUR health on the line. It is YOUR body that is being dialyzed. The least you can expect is that YOU be consulted on matters related to this. This will obviously make sense only when you are aware of these things.