Reduction in plasma water due to ultrafiltration increases hematocrite and blood density, which can be monitored using changes in optical or ultrasonic properties during treatment, as is used in relative blood volume monitoring. If the rate of plasma water removal exceeds the compensatory refilling rate, then the slope in relative blood volume monitoring becomes steeper. As hematocrit differs between capillaries and the capacitance arteriovenous system, small capillaries constrict first in response to ultrafiltration, thus returning a relatively lower hematocrit blood to the central vessels. As such, the change in relative blood volume monitoring lags behind these compensatory changes in ultrafiltration. It is not known wheter repetitive intradialytic hypotension adversely affects cardiac and cerebrovascular disease, or residual renal function, or whether patients with preexisting vascular disease are more prone to intradialytic hypotension.