The bulbus arteriosus of the teleost heart possesses a static inflation curve that is r-shaped over the in vivo pressure range. To examine the possible significance of this in living animals, we recorded arterial blood pressure from anaesthetized yellowfin tuna and utilized a video dimensional analyser to simultaneously record changes in bulbar diameter. By plotting the changes in pressure against the changes in diameter, it was possible to create dynamic pressure-diameter (P-D) loops as well as calculate the instantaneous volume changes within the bulbus. The dynamic P-D loops showed the same features exhibited by static inflation. When nearly empty, a small stroke volume caused a large increase in blood pressure, while around systolic pressure large changes in volume resulted in small changes in pressure. We conclude that these features allow the bulbus to maintain ventral aortic flows and pressures over a large range of volumes
