Osmotic Fluid Transfer from Tissue to Blood during Hemorrhagic Hypotension

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Abstract
Jarhult, J. Osmotic fluid transfer from tissue to blood during hemorrhagic hypotension. Acta physiol. scand. 1973. 89. 213–226.Arterial and venous plasma osmolality and glucose concentration, and net transcapillary fluid movements were studied in the cat lower leg muscles during hemorrhagic hypotension (50 mm Hg). Arterial plasma osmolality increased rapidly after bleeding and exceeded after 20 min the control value by about 20 mOsm/kg H2O, a level maintained during the remainder of the 90 min observation period. Popliteal venous osmolality rose much more slowly and did not reach the arterial level until some 50 min after the bleeding. Studies of plasma glucose concentration showed that the arterial and regional venous hyperosmolality almost entirely could be ascribed to a concomitant hyperglycemia, in particular during the first 30 min of hemorrhage. A clearcut arterio‐venous osmolar difference in the muscle (reflecting a transcapillary osmolar gradient) was thus present in the early (≅50 min) stages of bleeding. In the sympathectomized muscle region an absorption of extravascular fluid to the blood stream occurred as long as this arterio‐venous osmolar difference existed, strongly indicating that the fluid transfer was caused by osmosis. Comparative studies on the innervated and the sympathectomized muscle region showed that the extravascular fluid absorption due to reflex decrease of hydrostatic capillary pressure was most prominent in the early (< 20 min) stages of bleeding, whereas the osmotic fluid absorption was maintained for a more prolonged period of time. Both mechanisms no doubt are important for plasma volume restoration after hemorrhage.