Testing the constant-volume hypothesis by magnetic resonance imaging of Mytilus galloprovincialis heart

Summary The constant-volume (CV) hypothesis was tested using the Mytilus galloprovincialis heart under two conditions. The volume of the ventricle, auricles and pericardium, and the flow in the heart and adjacent vessels were measured by magnetic resonance imaging. In synthetic sea water at 23°C (immersed condition), the end-diastolic (EDV), end-systolic (ESV) and stroke (SV) volumes were 50%, 21% and 29% of the heart volume, respectively, and the auricle volume (V A ) was maximized at the end-systole. Assuming a constant volume in the heart, venous return to the auricles (I V ) was constant, and out-flow from the pericardium to the kidney (I PK ) was 2/3 of SV. During aerial exposure (emersed condition), EDV, ESV and SV decreased to 33%, 22% and 11%, respectively. The V A was maximized at the end-diastole and associated with the decrease of the systolic I V to 1/2 of the diastolic I V , while the I PK remained at 80% of the immersed condition. Based on these results, in addition to two postulates of the CV hypothesis: 1) the total volume of the heart is always the same, and 2) ventricle contraction causes a decrease in the pressure in the pericardium, we modified two postulate: 3) the low pericardial pressure maintains venous return from the anterior oblique vein to the auricle, and 4) the pressure difference between the auricle and the pericardium drives haemolymph filtration through the auricle walls. We also added a new postulate: 5) dilatation of the ventricle is associated with the haemolymph output to the kidney via the renopericardial canals.