Dynamics of blood circulation during diving in the bottlenose dolphin ( Tursiops truncatus ). The role of the retia mirabilia.

The retia mirabilia are vascular nets composed by small vessels dispersed among numerous veins, allowing blood storage, regulation of flow, and pressure damping effects. Here we investigated their potential role during the diving phase of the bottlenose dolphin ( Tursiops truncatus ). To this effect, the whole vertebral retia mirabilia of a series of dolphins were removed during post-mortem analysis and examined to assess vessel diameters, estimate vascular volume, and flow rate. Here we formulate a new hemodynamic model to help clarify vascular dynamics throughout the diving phase, based on the total blood volume of a bottlenose dolphin, and using data available about the perfusion of the main organs and body systems. We computed the minimum blood perfusion necessary to the internal organs, and the stroke volume (sv) and cardiac output (co) during the surface state. We then simulated breath-holding conditions and perfusion of the internal organs under the diving-induced bradycardia and reduction of sv and co, using 10 bpm as limit for the heart rate for an extended dive of over 3 minutes. Within these simulated conditions, the retia mirabilia play a vital role as a reservoir of oxygenated blood that permit functional performances and survival of the heart and brain. Our theoretical model, based on the actual blood capacity of the retia mirabilia and available data on organ perfusion, considers the dynamic trend of vasoconstriction during diving phase and may represent a baseline for future studies on the diving physiology of dolphins and especially for the blood supply to their brain.