The pathway of venous return from the posterior regions of Bufo marinus
either the renal portal or hepatic portal systems before returning to the heart [1
]. Resting blood flow in the two renal portal vessels combined was quite similar to that of the single ventral abdominal vein (which drains into the hepatic portal vein), with a slightly higher value observed for the ventral abdominal vessel. Total flow in the two vessel systems was 12.85 ml min-1
, which comprises the total blood flow returning from the hind limb region of the toad. Ventral abdominal flow accounted for 57% of this venous return, with renal portal flow contributing the remaining 43% (Fig. ).
], using similar techniques to those used in this study, found cardiac output in Bufo marinus
to be 57.2 ml min-1
, with aortic blood flow being 26.8 ml min-1
]. Using values obtained in this study, venous return from the hind limbs of the toad would thus represent approximately 1/4 of cardiac output.
When blood flow was examined following an infused doubling in plasma volume, both renal portal and ventral abdominal blood flow increased significantly during the 10 minute post infusion period but flow in the vessels leading to the kidneys was greater. The site of injection, the existence of a vascular connection between the two flow pathways and the large volume of fluid injected would support an equal distribution of excess fluid in both pathways. Our results strongly suggest a preferential movement of fluid through the renal portal system.
A favored movement of fluid through the renal portal veins would support the hypothesis that, in a natural aquatic setting, the rapid elimination of incoming water could be made possible by the renal portal system [8
]. Carter [8
] postulates that this excess water may move from the peritubular capillaries into the tubular lumen due to osmotic concentration differences, and therefore be excreted as urine. In this case, fluid would move in a preferred direction such that its rapid elimination is maximized. It has also been suggested that the femoral vein may serve as a functional connection between the renal portal and ventral abdominal veins [3
]. This would seem to be supported if increased venous flow in the hind limbs is primarily directed through the renal portal veins. This connection could then serve as a means for delivery of increased volumes of blood to the kidneys, allowing for the elimination of excess fluid.
Interestingly, the pattern of venous blood flow observed in both of the vessels throughout the experiments was a regular, slow-wave type pulsation. Previous work has shown heart rate in Bufo marinus
to be 39.9 beats min-1
and lung ventilation rate to be 16.3 ventilations min-1
in normoxic conditions [9
]. The calculated resting rate of 19.5 ± 3.0 pulsations min-1
in the vessels appears closer to values for lung ventilation, however the rhythmic nature of the pulsation pattern seems consistent with changes due to heart contraction. Although we did not monitor lung ventilation, it is possible that ventilatory pressure changes within the toad result in regular oscillations in venous flow. Pulsations in flow due to negative pressure from the heart, however, seem unlikely, as both of the veins studied branch into portal systems before emptying into the vena cava.
Much of the speculation on pulsation patterns arose directly as a result of the observation of flow "spikes" in the renal portal tracings, which appeared as flow returned to normal after hypervolemia. The sharp extra pulsations observed were less rhythmic than the resting pulsations and contrasted with the flow tracings recorded simultaneously in the ventral abdominal vein. The pattern of sharp, irregular peaks corresponds remarkably with tracings observed when lymph flow is monitored directly from lymph heart efferent vessels using similar techniques [10
]. Williams et al.
] found that 50 min after a doubling in plasma volume, lymph heart rate was approximately 50 beats min-1
, consistent with the "spike" rate of 48.0 ± 6.3 pulsations min-1
calculated in this study during a similar time frame. It would appear that these tracings reflect contributions to renal portal blood flow by lymph, which is ejected from the nearby posterior lymph hearts.
Previous measurement of lymph heart function in response to systemic hypervolemia has shown lymph flow from a single posterior heart to be 20.7 ml kg-1
and stroke volume to be 0.0074 ml kg-1
at the 50 min post-injection mark [11
]. Our values, determined through measurement of flow peaks above baseline renal portal flow, are considerably lower, with lymph flow being 6.2 ml kg min-1
, and stroke volume 0.0018 ml kg-1
. These measurements of lymph flow may vary due to a more "downstream" location and the indirect method of lymph flow measurement used in this study. In addition lymph heart flow "spikes" in the renal portal veins were not visible under normal resting conditions. Williams et al.
] found that, 50 min following a 100% increase in plasma volume, lymph flow was increased, although this change was not significant. It is possible that these sharp pulsatile increases in flow are peaks of the lymph heart systolic output, which has been increased such that it becomes visible, superimposed upon the larger flow of the renal portal vein.
The contribution of lymph flow to renal portal flow can also be considered with respect to resting state. Jones et al.
] found lymph flow in Bufo marinus
under normal conditions to be 25.9 ml kg-1
from a single posterior lymph heart. Using renal portal flow values measured in this study, it is estimated that almost one-sixth of the renal portal circulation consists of fluid originating in the posterior lymph hearts.