A. Salt Hypothesis
The hypothesis accepted by the majority of researchers is that an inherited defect causes the kidney to retain excess sodium (salt) in the body. If you have such an inherited defect and your diet contains a large amount of sodium, your kidneys will retain more sodium and water. A normal kidney that has no defect in handling sodium will expel in the urine any excess that you may add in the diet. The sodium and water retained by the kidney get into the blood and also into the cells of the artery wall, thereby increasing the tone of the artery wall. This means that the artery becomes constricted or tightened and increases the resistance against which the heart must pump, that is, there is an increase in total vascular resistance and an increase in blood pressure (see Fig. 2).
The relationship between salt and hypertension is easier to understand after looking at the following example. If an individual is bleeding severely from a large cut or from the stomach or other body site, the blood pressure always falls, sometimes to very low levels such as 75/50. A transfusion of blood must be given quickly to increase the blood pressure, because at such a low level, not enough oxygen, glucose, and other nutrients will reach the brain, muscles of the heart, and other tissues of the body. How¬ever, blood is not usually available in emergency rooms for up to two hours. During this time the doctor rapidly gives sodium chloride (salt) diluted in water into a vein (intravenous saline), and this nearly always increases the blood pressure to safe levels until blood is available. During the bleeding described, the kidneys also immediately start to retain sodium and water and return it to the blood; that is, the kidney gives us an immediate transfusion of saline. Nature always finds a way to compensate.

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