The smoothness of flow and pressure in the ventral aorta of teleosts is due to the presence of a large central compliance

teleosts is due to the presence of a large central compliance that is the product of elastic and resistive elements downstream of the heart. While the resistance of the gills is only about 30–50% of the total peripheral resistance, the ventral aorta and branchial arteries are short, resulting in a small total compliance of the central arterial circulation. The bulbus arteriosus, the most anterior of the four chambers of the teleost heart, greatly increases central vascular compliance, largely subserving the Windkessel functions of the whole mammalian arterial tree (von Skramlick, 1935; cited in Mott, 1950; Satchell, 1971; Stevens et al., 1972; Licht and Harris, 1973; Jones et al., 1974, 1993; Priede, 1976; Farrell, 1979; Watson and Cobb, 1979; Benjamin et al., 1983, 1984; Santer, 1985; Bushnell et al., 1992; Jones, 1999). 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. Stage 2 is a result of the specialized material properties of the bulbus. The bulbar wall has a very high elastin:collagen ratio and is almost entirely composed of novel elastin (low hydrophobicity, high solubility) aligned in a novel manner (loose fibrils, no lamellae). These modifications produce very low wall stiffness and the ability to undergo large strain changes and result in the compliance of the plateau. At large extensions, stiff adventitial collagen is recruited to resist the expansion of the bulbus. Knowing the causes of the strange bulbar P-V loop is an important first step in understanding how the bulbus works. However, in order to make inferences based on the in vitro inflation curve, it is vital that the bulbus shows similar traits in vivo. To this end, in vivo changes in pressure and bulbar diameter during normal beating in anaesthetised yellowfin tuna were recorded using video dimensional analysis (VDA) and pressure recordings. 3327 The Journal of Experimental Biology 206, 3327-3335 © 2003 The Company of Biologists Ltd doi:10.1242/jeb.00576