Toads have a little problem with water retention; their circulatory system is extremely leaky. Lymph is continuously lost from the blood as it filters out through capillaries, and the animals have to recycle the precious fluid to maintain their blood volume. Michael Hedrick, from California State University, USA, says, ‘Lung ventilation and specialised skeletal muscles move lymph from the ventral [lower] part of the animal to the dorsally located lymph hearts [in the back] that pump lymph back into the venous circulation’. However, it was not clear to Hedrick and Tobias Wang, from Aarhus University, Denmark, how the animals pumped fluid from cavities low down in the body up to the lymph hearts. ‘Knowing how the lymph sacs and sinuses are connected doesn't tell us how much lymph might move through these pathways toward the lymph hearts’, explains Hedrick. Teaming up with Kasper Hansen, Hedrick injected a solution that shows up in CT scans into the lymph sacs of cane toads and scanned the animals at Aarhus University Hospital to see how the fluid was pumped through the body prior to being returned to the animals' bloodstream (p. 2990).
‘The toads were extremely cooperative’, recalls Hedrick, explaining that although each scan took only 10 s, the toads had to remain still for several hours at a time as the dye moved slowly through their bodies. However, after scanning six toads and analysing the data with Hansen, Jesper Thygesen, Michael Pedersen and Henrik Lauridsen, Hedrick was pleased to identify several lymph return routes that had been predicted, including one where lymph flowed vertically from the interfemoral sac through the pubic sac up to the lymph hearts. In addition, they identified several novel routes, including one where dye injected into a lymph sac above the lungs moved down between the lungs before travelling back toward the hindlegs and the lymph hearts. ‘This was completely unexpected’, says Hedrick, concluding, ‘The lymphatic system in frogs and toads is a complex arrangement of lymph sacs and sinuses, and movement of lymph occurs through complicated and surprising pathways’.