why is important to keep the sodium level of the plasma volume so much higher th

where's the rest of the question? 

sodium is a non-penetrating substrate, meaning that water will follow the sodium particles in order to maintain a homeostatic balance. Having more sodium particles in plasma draws water out of the cells keeping the blood volume high. However, this prolonged condition cause problems, one of them being hypertension. If sodium levels were not high, a low blood volume would result, thus causing high blood pressure and this is characteristic of hypertension. Therefore, to get rid of hypertension, the patient must maintain a relatively higher sodium content in the plasma in order to keep drawing water into the blood. Remember, pressure and volume are inversely proportional..

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