title: Testing the constant-volume hypothesis by magnetic resonance imaging of Mytilus galloprovincialis heart.
abstract: 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 seawater at 23°C (immersed condition), the end-diastolic volume (EDV), end-systolic volume (ESV) and stroke volume (SV) were 50%, 21% and 29% of the heart volume, respectively, and the auricle volume (VA) was maximized at end-systole. Assuming a constant volume of the heart, venous return to the auricles (IV) was constant, and out-flow from the pericardium to the kidney (IPK) was 2/3 of SV. During aerial exposure (emersed condition), EDV, ESV and SV decreased to 33%, 22% and 11%, respectively. VA was maximized at end-diastole and associated with the decrease of systolic IV to 1/2 of diastolic IV, while IPK 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 pressure in the pericardium--we modified two postulates: (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.
authors: Department of Marine Ecosystem Dynamics, Division of Marine Life Science, Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa 277-8564, Japan.
authors: Eriko
authors: E
authors: Seo
authors: Kazue
authors: K
authors: Ohishi
authors: Tadashi
authors: T
authors: Maruyama
authors: Yoshie
authors: Y
authors: Imaizumi-Ohashi
authors: Masataka
authors: M
authors: Murakami
authors: Yoshiteru
authors: Y
authors: Seo
mesh_terms: D000388:Air; D000818:Animals; D002306:Cardiac Volume; D006321:Heart / Q000502:physiology; D008279:Magnetic Resonance Imaging; D049878:Mytilus / Q000502:physiology*; D012623:Seawater; D013318:Stroke Volume
keywords: Bivalve; Cardiac cycle; Ejection fraction; Haemolymph flow; Heart rate; Renocardiovascular system; Stroke volume