BOURNE, G.B.: Hemodynamic properties of an open vascular bed

Open circulatory systems differ from closed ones primarily in the nature of the microvasculature. The dynamics of fluid passage through closed systems are fairly well understood. We have less knowledge of fluid dynamics in open vascular beds. In this study, pressure-flow relationships were used to further characterize functional attributes of the abdominal circulation of the lobster, Homarus americanus. In situ preparations were made by cannulating the dorsal abdominal artery via the ventricle and exsanguination was carried out. After allowing some time for the preparation to stabilize, repeated pressure-flow profiles were made using a modified version of the vertical-tube technique of Nichols et al. (Am. J. Physiol. 164: 330-344 [1951]). Experiments were carried out at 10 ^oC using artificial lobster saline as the perfusate. The results of this study compare favourably to that of Wilkens et al. (J. Exp. Biol. 200: 477-485 [1997]) who perfused their preparations using a different technique. Over the 90 min time course of an experiment, there was a small tendency for flow to decrease. Under conditions of unchanging output pressure, flow became relatively constant at higher input pressure. This maintenance of constant flow indicated that there are inherent mechanisms for adjusting hydraulic resistance in this region of the lobster circulatory system. Furthermore, an increase in resistance was found at low input pressure which was indicative of a “critical closing” pressure. The nature of the intrinsic factors which affect “critical closure” is unknown at this point. The dorsal abdominal artery possesses muscle but the muscle does not extend into the branches. Thus, there exists the possibility that flow maintenance in this vascular bed may occur at the level of the artery.