Although only about 5% of the body weight is intravascular fluid, this is obviously a very important 5% in terms of haemodynamic variables. The fluid component of the blood is in equilibrium with the interstitial fluid. With changes in posture, the amount of fluid in the vasculature versus the interstitial space may be altered by gravity-induced effects. The most important implication of this relates to changes from supine to upright posture. Under normal circumstances, there is pooling of blood in the vasculature of the lower part of the body within a few seconds of assumption of upright posture. Normally, about 700 ml of blood moves to the lower abdomen, pelvis, and lower extremities on standing. Compensatory neurohumoral adjustments rapidly occur leading to increases in sympathetic nerve traffic, vasopressin, and the activation of the renin—angiotensin-aldosterone axis.
Much less appreciated but nevertheless very significant is the loss following the assumption of upright posture of 12—18% of plasma volume into the interstitial fluid compartment over the succeeding 20—30 minutes (Fig. 2) (Jacob et al 1996, 1998). This fluid shift translates into loss of about a unit of blood within 30 minutes of standing. This occurs in addition to the well-known intravascular pooling effect in the venous capacitance bed. The concurrence of these two effects render the period from 10 minutes after standing particularly vulnerable for maintenance of adequate blood £ow to the brain and other important organs. This is when most individuals who have a tendency to experience syncope due to low blood pressure will have this become manifest.
Perhaps more important than the average shift in plasma volume from the intravascular to the interstitial space is the substantial interindividual variability, which in a recent study of 25 individuals ranged from a change in plasma volume of 7% to a change of 28% (Fig. 3). The latter resulted in a shift in haematocrit from 42% to 34% within 30 minutes of lying down, even though no bleeding had actually occurred.
An important future direction for research is the understanding of interindividual variation in plasma volume shift with changes in posture and how pathophysiological conditions and drugs may alter this function. The determinants of and the effect of ageing on this shift, remain unknown.
Was this article helpful?