Fluid handling in ageing is altered in many ways. There is a reduced fraction of the body consisting of water in ageing. Postural plasma volume shifts occur although the precise role of ageing in interindividual variation in this variable remains unstudied. Oral water raises blood pressure in individuals with impaired autonomic reflexes and is an important source of noise in blood pressure trials in the elderly. On the average, oral ingestion of 16 ounces (473 ml) of water will raise blood pressure 11 mmHg in elderly normal subjects. In patients with autonomic impairment such as multiple system atrophy, strikingly exaggerated pressor effects of water have been seen with blood pressure elevations greater than 75 mmHg not at all uncommonly. These responses tend to be maximal within 30 minutes of water ingestion and to be largely dissipated within 90 minutes.

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