Quantitative approaches to the study of lymphatic contractile activity in vitro and in vivo: potential role of this dynamic 'lymph pump' in the re-expansion of the vascular space following hemorrhage.

Few investigators have considered a dynamic role for the lymphatic vessel in regulating the movement of fluid and protein from the interstitium back to the bloodstream. This view is based on the assumption that lymphatics are passive conduits and that hydrostatic pressure gradients and external compression forces acting on the vessels are primarily responsible for the movement of lymph. However, it is becoming increasingly evident that the intrinsic contractile capabilities of lymphatic vessels provide a major part of the propulsive force. Lymphatics have noradrenergic innervation and respond to a variety of humoral factors and inflammatory mediators suggesting that the pumping activity is centrally regulated and in addition, may respond to local factors. In this article, we will discuss what is known of the regulation of this 'lymph pump'. Techniques that permit analysis of contractile activity and fluid pumping in vitro and in vivo will be reviewed. Of particular interest is a sheep model that allows the quantitation of pumping activity in vivo without the complication of variable lymph inputs. While there is little information available at this time concerning the potential role of the 'lymph pump' in pathophysiological states, some preliminary experiments from our own laboratory suggest that an independently regulated 'lymph pump' may play an important role in hemorrhagic shock.