Effects of hypoproteinemia on fluid volumes and arterial pressure

The effects of both moderate and large decreases in plasma protein concentration on arterial pressure and fluid volumes were studied in 23 conscious dogs. In experiment 1, plasma protein concentration decreased 33% during a 5-day plasmapheresis period. During this time sodium space increased 11%, mean arterial pressure decreased slightly, and neither blood volume nor plasma volume decreased. Experiment 2 was performed to see if blockade of the alpha-sympathetic and angiotensin systems could prevent the blood volume homeostasis during moderate hypoproteinemia. Sodium space increased; however, blood volume was unchanged. During experiment 3 plasma protein concentration decreased 68% over a 12-day plasmapheresis period. By the last day of plasmapheresis, plasma protein concentration was 2.4 g/100 ml, mean arterial pressure had decreased 26 mmHg, sodium space had increased 12%, plasma renin activity had increased 11-fold, and blood volume and plasma volume were 63.9 +/- 4.0 and 66.9 +/- 2.5% of control, respectively. We conclude that the maintenance of a normal blood volume during moderate hypoproteinemia does not require active participation of the renin-angiotensin and alpha-sympathetic systems and large decreases in plasma protein concentration are accompanied by marked hypovolemia, hypotension, and hyperreninemia.