Developing a Comprehensive Physical Model of Lymph Flow in Amphibians

Portland State University, Portland OR

Investigators

Abstract

The objective of this work is to develop a comprehensive physical understanding of how lymph moves in anuran amphibians (frogs and toads) and investigate the anatomical and physiological control systems which lead to variations in the movement of lymph. Lymph is the fluid that filters from the capillaries of the circulatory system into the interstitial space as a consequence of the hydrostatic pressure generated by the heart. The principal role of the lymphatic system is to return this fluid back into the circulation. If this functional role of the lymphatic system is prevented, fluid is lost from the circulation much like a hemorrhage, and cardiovascular function declines eventually leading to circulatory shock. Anuran amphibians form lymph, at about ten times the rate of mammals. Hence the return of lymph to the circulatory system is an even more critical variable to their maintenance of blood volume. Amphibians, in relation to all vertebrate classes, have the most rapid compensatory ability to replace lost blood volume with lymph and this lymph mobilization capacity varies with a species' degree of terrestriality. Understanding how lymph moves is fundamental not only to our understanding of how amphibians function at an organ system level, but also aids our understanding of what adaptations are involved in the colonization of terrestrial environments. Consequently, amphibians represent an excellent evolutionary model system for studying comparative mechanisms involved in how lymph mobilization regulates blood volume. Current models of lymph movement are untenable from a physical perspective. The working hypothesis is that lymph moves in response to pressure differences created by two mechanisms, compartmentalization of lymph sacs creating sacs of different compliance and skeletal muscle contractions varying lymph sac compliance. Compliance is the relationship between lymph volume and pressure in a lymph sac. The primary function of many of these skeletal muscles may be solely tied to lymph movement and may represent the first vertebrate example of skeletal muscle fluid pumps. The pathways for lymph movement will be delineated using dye-labeled plasma, the pressures in these pathways will be monitored via implanted cannulae and skeletal muscle contraction monitored via EMGs recorded from implanted electrodes . How blood volume status influences the lymph sac pressures and skeletal muscle activities will also be investigated. Comparative anatomical data will then be collected to test what species' anatomical differences correlate with differences in lymph flow.