What goes down must come up: regulation of lymph movement in anuran amphibians

Anuran amphibians have a unique body plan characterized by a high interstitial compliance as a consequence of numerous subcutaneous lymph sacs. Anurans produce lymph at very high rates owing to "leaky" capillaries and low capillary reflection coefficients. The copious amounts of formed lymph are stored in these lymph sacs but, owing to gravitational forces, lymph preferentially collects in the ventral lymph sacs. Lymph is returned to the circulation by dorsally located lymph hearts, which pump lymph into the venous side of the circulation. The major problem for anurans is moving the lymph from ventral lymph sacs, against gravity, to the dorsal lymph hearts. Lymph movement is accomplished by three distinct mechanisms: 1) horizontal movement of lymph along the hind limbs by differential lymph sac compliance; 2) vertical movement by skeletal muscles that insert on the urostyle, skin, and the margins of lymph sacs that change the compliance and pressure of lymph sacs; 3) lung ventilation and associated volume changes in the lungs are transmitted primarily to the subvertebral lymph sac overlying the lungs resulting in large negative pressures that aspirate lymph dorsally. Phylogenetic analyses reveal that lymph skeletal muscles have undergone bidirectional evolution with more terrestrial species showing greater elaboration of these muscles compared with aquatic species that have lost or reduced these muscles. More terrestrial species also have larger lung volumes and compliances than aquatic or semiaquatic anurans, which presumably enhance their ability to mobilize lymph movement in desiccating environments where maintenance of plasma volume is a greater challenge.