The Mechanism of Blood Circulation inAnodonta Anatina(L.) (Bivalvia, Unionidae)

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Abstract
1. The structure of the blood circulatory system of Anodonta anatina is described and the haemodynamics have been investigated by recording pressures in the ventricle, auricle, pericardial cavity and pedal haemocoele at rest and during burrowing. 2. Ventricular systolic pressure is usually 2-4 cm in the resting animal; during burrowing it increases to between 6 and 10 cm and this is sufficient to maintain the blood supply to the foot for most of the digging cycle. 3. Auricular and pericardial cavity pressures fall rapidly (by about 1·0 cm) during ventricular systole, confirming the operation of a volume-compensating mechanism for refilling the heart. 4. High peaks of pressure at spontaneous phasic adduction and during the adduction and retraction movements of the digging cycle are generated equally throughout all parts of the animal enclosed within the shell and do not create large gradients of pressure in the haemocoele; the longer duration of these pressure peaks in the pedal haemocoele produces small transient gradients of pressure which could result in the movement of blood out of the pedal haemocoele. 5. At spontaneous phasic adduction contraction of the pedal muscles may assist the flow of blood from the pedal haemocoele. There is some evidence that Keber's valve limits blood flow from the pedal haemocoele during active burrowing. 6. Although body movements may assist the movement of blood through parts of the peripheral circulation, they do not generate a high venous return pressure. The form of the circulatory system effectively isolates the heart from pressure changes in the pedal haemocoele.

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