B. Other Hypotheses
Other hypotheses for the causation of primary hyper¬tension are shown in Fig. 3. Hereditary factors and stress cause an increase in discharge from a center in the brain (sympathetic center) which triggers the secretion of adrenaline and noradrenaline. These compounds not only cause an increase in heart rate and cardiac output, but a marked constriction of arteries and increased total vas¬cular resistance too, thereby increasing blood pressure. This sympathetic stimulation activates enzymes in the kidney and adrenal glands (the renin-angiotensin-aldosterone system). Angiotensin is a powerful constrictor of arteries which elevates blood pressure. Aldosterone, a hormone secreted by the adrenal glands, causes the kidney to retain sodium and water and this further increases blood pressure. The renin-angiotensin-aldosterone system appears, how¬ever, to have only a small role, and this is still undefined in the causation of primary essential hypertension. A low intake of calcium has been associated with an increase in blood pressure in two studies, but the evidence is not sufficient to implicate a low-calcium intake in the cau¬sation of hypertension. Further studies are necessary to clarify the aforementioned theories of causation.

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