Evidence for the failure of the Laplace law as a sole explanation for wall thickening of arteries in hypertensive and aging normotensive kidneys.

Nephrosclerosis, the morbid condition of the kidney that accompanies essential hypertension, is characterized by thickening of the walls of cortical arteries. According to the law of Laplace, the tension in the wall of an artery should be proportional to the product of pressure (P) times diameter (D). Thus, if wall thickness (T) is governed by wall tension, then it should be proportional to the same product: T = kPD, where k is a proportionality constant. A comparison of immersion-fixed kidneys in normotensives aged 75 to 90 years with those aged 19 to 34 years showed the magnitude of k to increase with age. The increase was 41% in vessels that were relatively close to the heart (outer diameter, 150 to 300 microns) and 30% at the more remote level (outer diameter, 80 to 140 microns). In perfusion-fixed specimens, k also increased with age, being 77% and 35% greater, respectively, for the comparison between normotensives in the two age groups. Normotensives were defined to be those with mean blood pressure less than 115 mm Hg. A similar result was found by defining normotension to be systolic pressure less than 140 mm Hg. Wall thickness was not proportional to pressure in hypertensive compared with normotensive subjects, but rather to a quadratic function that combined age and blood pressure taken together. The law of Laplace did not fully encompass the data that relate the thickening of renal arterial walls to hypertension or to aging in the absence of hypertension. The thickening with age is structurally a metaplasia that exchanges fibrotic intima in the elderly for muscular media in the young. The result was reproduced in perfusion- as well as immersion-fixed specimens and is therefore not distorted by postmortem collapse. The results favor the conclusion that arterial wall thickening in hypertensive and aging normotensive kidneys is sclerosis and not adaptive hypertrophy.(ABSTRACT TRUNCATED AT 250 WORDS)