Lymph pools in the basement, sump pumps in the attic: the anuran dilemma for lymph movement

Amphibians are a vertebrate group transitional between aquatic and terrestrial environments. Consequently, both increases and decreases in blood volume are a natural biological stress associated with aquatic and terrestrial environments. In comparison with other vertebrate classes, anuran amphibians have the most rapid compensation and greatest capacity to compensate for changes in blood volume and survive dehydration. Unlike in mammals, a Starling transcapillary uptake mechanism does not account for this fluid mobilization because lymph flow is a substantial and important additional factor. The role of the lymphatic system in flux of fluids back into the circulation varies interspecifically in anurans and is an order of magnitude greater in anurans than in mammals. Current models of lymph movement in anurans are centered on the role of lymph hearts, but we suggest that these models are untenable. We present a new hypothesis for lymph movement involving (1) pressure differences created by compartmentalization of the hind limb lymph spaces into sacs of serially graded compliance to move lymph horizontally and (2) both negative and positive pressure differences created by contraction of skeletal muscles to move lymph vertically. The primary function of some of these skeletal muscles may be solely for lymph movement, but some may also be involved with other functions such as pulmonary ventilation.