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The Bulbus arteriosus of tuna : Form and function Braun, Marvin Herbert

Abstract

The bulbus arteriosus, the fourth chamber of the teleost heart, is very similar in structure and function to blood vessels. However, when the inflation properties of the bulbus are compared with those of an artery, a striking difference appears. The pressurevolume loops of arteries are J-shaped, indicating an increasing stiffness with increasing distension. The bulbus pressure-volume curve can best be described as an r-J curve, with an initial sharp rise followed by long plateau phase, and a final upturn following the plateau. This suggests that the bulbus has a high initial stiffness that decreases as volume increases before rising again at high distensions. To analyse the cause of this inflation behaviour, 1 utilised light microscopy, electron microscopy, mechanical testing of bulbar and arterial walls, and in vivo recordings of diameter and pressure changes of bulbi. The results show that the answer to the bulbus question is multifaceted. The steep initial rise in pressure is due to the geometry of the bulbus. When empty, the bulbus lumen is very small. According to the Law of Laplace, a small radius requires a large pressure increment in order for expansion to occur. As the lumen radius increases, the pressure increments decrease in size. The extremely compliant plateau phase of the bulbar inflation is due to the specialised material properties of the bulbar wall. The combination of a high elastin/collagen ratio, lack of elastic lamellae and specialised elastin result in a decreased elastic modulus (stiffness) within the bulbar wall. Once on the plateau phase of the bulbar inflation, this decreased stiffness allows the bulbus to expand a tremendous amount for small changes in pressure. The tensile tests show that the bulbar material exhibits J-shaped changes in stiffness when stretched and, despite the low modulus, at high extensions, the bulbar material can become quite stiff, resulting in the final rise in stiffness seen in inflation. The inflation behaviour of the bulbus allows to it function as an exceptional pressure and volume reservoir, mimicking the Windkessel effects o f a much longer arterial tree.

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