Central role of the kidney and abnormal fluid volume control in hypertension

In human essential hypertension, and in all forms of experimental hypertension studied thus far, volume regulation and the relationship between blood pressure (BP) and sodium excretion (pressure natriuresis) are abnormal. Considerable evidence indicates that resetting of pressure natriuresis plays a key role in causing hypertension, rather than merely occurring as a consequence of increased BP. In patients with essential hypertension, resetting of pressure natriuresis is characterized either by a parallel shift to higher BPs and salt-insensitive hypertension, or by a decreased slope of pressure natriuresis and salt-sensitive hypertension. This clearly indicates that essential hypertension cannot be ascribed to a single abnormality of kidney function. Multiple physiological studies have shown that salt-sensitive hypertension can be elicited by renal abnormalities that cause either loss of functional kidney mass or an inability to modulate the renin-angiotensin-aldosterone (RAA) system appropriately; these abnormalities include loss of functional nephrons, decreased glomerular capillary filtration coefficient, patchy venal ischemia, and increased distal and collecting tubular reabsorption. Renal abnormalities that cause salt-insensitive hypertension are characterized by normal functional kidney mass, and the ability to appropriately modulate the renin-angiotensin system during changes in sodium intake; important causes of salt-insensitive hypertension include widespread increases in preglomerular resistance and increased reabsorption in the proximal tubules and loops of Henle. By comparing the characteristics of pressure natriuresis in essential hypertensive subjects with those found in experimental hypertension of known origin, we can gain considerable insight into the etiology of human hypertension.