 The octopus is, however, able to control how much oxygen it pulls out of the water with each breath using receptors on its gills, allowing it to keep its oxygen uptake constant over a range of oxygen pressures in the surrounding water.The three hearts are also temperature and oxygen dependent and the beat rhythm of the three hearts are generally in phase with the two branchial hearts beating together followed by the systemic heart.The Frank–Starling law also contributes to overall heart function, through contractility and stroke volume, since the total volume of blood vessels must be maintained, and must be kept relatively constant within the system for the heart to function properly.