Transvascular fluid shifts accompanying short bodily activity in the alert rat and their relation to arterial pressure.

Abstract

Hematocrit and arterial blood pressure were measured continuously in alert rats with chronic vascular catheters implanted in the aorta and in various veins. Changes of arterial pressure and hematocrit in the various veins occurred either spontaneously or were induced by mild stimulation of the animal. Results: 1. Slow, spontaneous fluctuations of hematocrit without or with very little activity of the animal are directly related to blood pressure changes, such that a definite hematocrit corresponds to a definite blood pressure. 2. Fast Hct changes instantaneous to 2–20 sec of activity are negative in blood from the liver, the kidney and the gut and positive in blood from the lower extremities. Immediately after this first phase of Hct change, there is a rise of Hct in all veins and the aorta (second phase). This is related to the preceding arterial pressure rise. Intense continued activity is followed in a third phase by an ultimate drop of Hct. 3. Heart rate may either drop or rise on stimulation of the animal, but is invariably elevated during the second phase of Hct response. 4. Evidence is presented that the Hct changes (δHct), being the results of vascular reflexes, are due to appropriate fluid shifts (Δv) in the tissues, which can be calculated by considering time-integrated Hct and flow rates. Thus Δv is, for instance, 0.15 ml from the liver, 0.08 ml from the kidneys, 0.04 ml from the gut, and 0.05 ml into tissues of the lower extremities in a few seconds of activity. The slow spontaneous Hct fluctuations indicate a Δv of only 56μl per mm Hg of arterial pressure change. Blood-volume increase after 3–10 min of activity is by about 5%.