Influence of Plasma Hypertonicity on Blood Viscosity Studied in Vitro and in an Isolated Vascular Bed

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Abstract
The effects of plasma hypertonicity on blood viscosity have been examined in vitro as well as in a vascular bed. NaCl was added to ox‐blood to produce plasma osmolalities up to about 1000 mosm/kg. The isolated rabbit ear was perfused at 70 and 30 mm Hg. Apparent viscosities of blood relative to plasma were calculated from the corresponding flow ratios (‘in vivo’ viscosity). Relative viscosities in vitro of the perfusates were determined by means of a cone‐plate viscometer. Blood viscosity measured ‘in vivo’ as well as in vitro was significantly increased by increasing osmolality. Maximal increments (almost 100 per cent) were observed between 700 and 900 mosm/kg. Plasma viscosity and vascular resistance was not influenced by hypertonicity. Viscosities ‘in vivo’ were generally lower than in vitro–the discrepancy was more pronounced at 70 than at 30 mm Hg and was greater in isotonic than in hypertonic blood. The effect of hypertonicity on blood viscosity is explained mainly by a reduced deformability of the hypertonically crenated red cells. It seems likely to explain the inverse relationship between renal medullary blood flow and medullary hypertonicity (Thurau et al. 1960) as caused partly by passive mechanisms involving blood viscosity changes with varying osmolality.

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