Haemodynamics in Tresus Nuttallii and Certain other Bivalves

1. Blood pressures were measured in the hearts and aortas of several bivalves.

2. Systolic pressures in the ventricles were commonly 0·5-2·0 mm. Hg, while diastolic pressure was zero.

3. The aortic bulb produced pressures similar to those produced by the ventricle, but beat less frequently and not in synchrony with the ventricle, except in the isolated heart.

4. Pericardial and auricular pressures were normally undetectable, except that small pressure pulses were recorded in the auricle during experimentally produced increases in venous return.

5. Aortic pressure sometimes exceeded ventricular pressure, although unusually strong contractions of the body muscles raised aortic pressures far above that in the ventricle. The heart does not provide pressure for extension of siphons or foot.

6. Sea water pumped by isolated Tresus ventricles demonstrated that neither pericardium nor auricles are needed for ventricular filling, as had been hypothesized earlier.

7. The morphology of the heart walls supports the concept of bivalve hearts as being a site of filtration, with the ventricle thought to play a larger role than the auricles.

8. Valves in the heart and aortas which control direction of blood flow are described.