Form and function of the bulbus arteriosus in yellowfin tuna (Thunnus albacares), bigeye tuna (Thunnus obesus) and blue marlin(Makaira nigricans): static properties

Abstract

SUMMARY The juxtaposition of heart and gills in teleost fish means that the Windkessel function characteristic of the whole mammalian arterial tree has to be subserved by the extremely short ventral aorta and bulbus arteriosus. Over the functional pressure range, arteries from blue marlin (Makaira nigricans) and yellowfin tuna (Thunnus albacares) have J-shaped pressure-volume (P-V) loops, while bulbi from the same species have r-shaped P-V loops, with a steep initial rise followed by a compliant plateau phase. The steep initial rise in pressure is due to the geometry of the lumen. The interactions between radius, pressure and tension require a large initial pressure to open the bulbar lumen for flow. The plateau is due to the unique organization of the bulbar wall. The large elastin:collagen ratio, limited amount of collagen arranged cirumferentially, lack of elastin lamellae and low hydrophobicity of the elastin itself all combine to lower stiffness, increase extensibility and allow efficient recoil. Even though the modulus of bulbus material is much lower than that of an artery, at large volumes the overall stiffness of the bulbus increases rapidly. The morphological features that give rise to the special inflation characteristics of the bulbus help to extend flow and maintain pressure during diastole.

Form and function of the bulbus arteriosus in yellowfin tuna (Thunnus albacares): dynamic properties

“…However, by looking at beats covering the pressure range of 2.5-21.5·kPa, the features of the static bulbar inflation curve ( Fig.·3A) were recreated: the initial steep rise, the plateau and the final steep rise at large inflations and high pressures (Braun et al, 2003). At the low end of the pressure range, the bulbus was operating on the steep part of the inflation curve and small changes in volume resulted in large, rapid changes in pressure (Fig.·3B).…”

“…In a Windkessel, the arteries expand with each heartbeat and recoil elastically, causing the highly pulsatile inflow to become relatively smooth in the periphery. How a relatively short bulbus mimics these effects of a longer arterial tree has never been explained.Like an artery, the bulbus is composed of elastin, collagen and smooth muscle; however, it is highly modified, resulting in specialized inflation properties (Braun et al, 2003). Over the in vivo pressure range, an artery has a J-shaped P-V (pressure-volume) loop, while the bulbus has an r-shaped P-V loop.…”

“…The bulbar curve can be broken into distinctive stages: (1) a sharp initial rise in pressure for a relatively small volume change and (2) a plateau stage where the bulbus is largely unaffected by even large changes in volume. There is even some evidence to suggest that there is a third stage of the bulbar inflation (Braun et al, 2003); when greatly distended, the bulbar material rapidly increases in stiffness.Stage 1 is due to the relationship between the wall tension, pressure and volume of the bulbus, as described by the Law of Laplace. The bulbar lumen is very small at low pressure and therefore bulbar expansion requires a large initial pressure increment.…”

“…Like an artery, the bulbus is composed of elastin, collagen and smooth muscle; however, it is highly modified, resulting in specialized inflation properties (Braun et al, 2003). Over the in vivo pressure range, an artery has a J-shaped P-V (pressure-volume) loop, while the bulbus has an r-shaped P-V loop.…”

“…The bulbar curve can be broken into distinctive stages: (1) a sharp initial rise in pressure for a relatively small volume change and (2) a plateau stage where the bulbus is largely unaffected by even large changes in volume. There is even some evidence to suggest that there is a third stage of the bulbar inflation (Braun et al, 2003); when greatly distended, the bulbar material rapidly increases in stiffness.…”