The direction and part of the rate of fluid filtration between synovial capillary (subscript capill) plasma and joint (subscript j) cavity fluid depends on the hydrostatic (P(capill)-P(j)) and colloid osmotic (COP(plasma)-COP(j)) pressure differences and on the osmotic efficiency (coefficient sigma) of 'colloids' across the barrier between plasma and synovial fluid. The rate of filtration also depends on capillary endothelial hydraulic conductance (L(p)) and surface area (A). Synovial fluid protein is removed by lymphatics, or split to be resorbed or used up by cells. Physiologists put together as equations the above, as well as other terms, all of these influenced by numerous factors. The equations offer a frame for a multifactorial approach to diseases associated with joint swelling and suggest, among others, that in the presence of weak generalized exsudation, synovial joints are loci minores resistentiae to swelling. The influence of joint cartilage energy demands on synovial blood flow may have been underrated. There are many accepted clinical, laboratory and morphological findings that fit this physiological frame that merits further clinical research.