1. A device for measuring hematocrit values using optical spectrophotometry comprising: a probe housing configured to be placed proximal to a tissue location which is being monitored; an occlusion device connected to said housing and configured to magnify the fractional change in the vascular blood volume to a value greater than said fractional change produced by normal arterial pulsations; light emission optics connected to said housing and configured to direct radiation at said tissue location; light detection optics connected to said housing and configured to receive radiation from said tissue location; a processing device connected to said housing and configured to process radiation from said light emission optics and said light detection optics to compute said hematocrit values.