Ageing and water homeostasis

David Robertson, Jens Jordan*, Giris Jacobf, Terry Ketch, John R. Shannon and Italo Biaggioni

Autonomic Dysfunction Center, Departments of Medicine, Pharmacology, and Neurology Vanderbilt University, Nashville TN 37232-2195, USA, *Clinical Research Center, Franz Volhard Institut, Berlin, Germany and {Recanati Autonomic Dysfunction Center, Rambam Medical Center, Haifa, Israel

Abstract. This review outlines current knowledge concerning fluid intake and volume homeostasis in ageing. The physiology of vasopressin is summarized. Studies have been carried out to determine orthostatic changes in plasma volume and to assess the effect of water ingestion in normal subjects, elderly subjects, and patients with dysautonomias. About 14% of plasma volume shifts out of the vasculature within 30 minutes of upright posture. Oral ingestion of 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 raises 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 uncommon. Ingestion of water is a major determinant of blood pressure in the elderly population. Volume homeostasis is importantly affected by posture and large changes in plasma volume may occur within 30 minutes when upright posture is assumed.

2002 Endocrine facets of ageing. Wiley, Chichester (Novartis Foundation Symposium 242) p265-278

Water is the major constituent of living beings. It engenders and facilitates homeostasis by virtue of its effects as a solvent, its ionizing potential, its high thermal conductivity, its high specific heat content, and its high latent heat of evaporation. Its plasma proteins (via oncotic pressure) and its electrolytes (via osmotic pressure) maintain body fluid homeostasis. (Greenleaf & Morimoto 1996.)

Total body water accounts for 50-70% of body weight (Fig. 1). The cell membranes demarcate an intracellular compartment comprising about 50% of body weight and the extracellular compartment comprising about 20% of body weight. Extracellular fluid is further subdivided by the capillary endothelium into interstitial fluid (* 15%) and plasma volume (* 5%).

The crucial cation in the extracellular fluid is Na+ (* 150 mEq/l) and its associated anions are Cl7 (*110mEq/l) and bicarbonate (*28mEq/l). The

Intravascular 5%

IMonwat

Intravascular 5%

IMonwat

intracellular 50%

interstitial 15%

intracellular 50%

interstitial 15%

FIG. 1. The distribution of body water.

osmotic environment of the extracellular fluid is largely defined by these ions, and its osmolality is usually about double the Na+ concentration.

Thirst is a subjective quality defined as 'a desire to drink potable liquids'. Thirst appears to have several different causes, and its fundamental mechanisms remain uncertain (Gordon et al 1997). Cellular, extracellular and volume factors all seem to be involved. Osmotic stimuli account for about 70% and volume factors for about 25% of dehydration-induced drinking, but under circumstances where volume loss is large, for example with haemorrhage or phlebotomy, the contribution ofvolume factors increases considerably.

Greenleaf et al (1966) developed a linear regression equation for predicting actual water intake in 87 young military trainees in field conditions which reflects (r = 0.79; P < 0.01) the complexity of this functional expression of thirst:

Water intake (ml/day) = —11502+45.8 (serum osmolality)+1.2 (mean daily urine output) —18.9 (mean daily urinary potassium)+4.4 (mean daily urinary chloride) — 18.7 (lying heart rate) + 1.8 (daily sweat rate).

Ageing has a significant effect on body water compartments. With increasing age beyond 30 years, there is a gradual fall in the fraction of body weight that consists of water. This is accompanied by a less dramatic increase in the fraction of water in the fat-free body. These changes are primarily due to loss of skeletal muscle tissue, a relatively water-rich bodily component, with ageing.

FIG. 2. On assumption of upright posture after 60 minutes of supine posture, there is a rapid loss of plasma volume from the vasculature into the interstitial tissue. This is reflected in the increased concentration of total protein. In this figure, the per cent change in plasma volume in the lower part of the figure is calculated on change in haematocrit.

FIG. 2. On assumption of upright posture after 60 minutes of supine posture, there is a rapid loss of plasma volume from the vasculature into the interstitial tissue. This is reflected in the increased concentration of total protein. In this figure, the per cent change in plasma volume in the lower part of the figure is calculated on change in haematocrit.

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