Hydrostatic and oncotic determinants of microvascular fluid balance in normal canine joints

Water moves between plasma (p) and synovial fluid (SF) in response to gradients in the balance of opposing hydrostatic pressures (HP) and oncotic pressures (OP). At the vascular site where proximal filtration ceases and distal reabsorption begins, all forces are theoretically in balance. At this point, the transitional microvascular pressure (TMP) may be estimated from the equation TMP = HP_SF + OP_p − OP_SF. We measured these forces in the shoulders, wrists, and knees of 10 normal dogs, ages 2–10 years. The mean HP_SF in the knee was lower than that in the shoulders and significantly lower than that in the wrist. Conversely, the OP_SF in the wrist was significantly lower than that in the shoulder or the knee. These factors combined indicate that the microvascular bed in the normal knee has a remarkably low mean TMP (7.9 mm Hg). We also found a strong positive correlation between the age of each dog and the mean oncotic pressure of its SF.