There is strong evidence for couplings between the five oscillatory processes that characterize the human blood distribution system. In particular, mutual modulation and/or episodes of synchronization between the oscillations may be observed for a variety of different cardiovascular signals. Such phenomena can reveal information about the nature and strength of the couplings, which in turn reflect the state of the organism in sickness or in health. Earlier work on the inter-oscillator interactions is reviewed briefly, and some recent research on cardio-respiratory synchronization is considered in more detail. It is reported that, for rats, the directionality of the cardiorespiratory coupling reverses under anæsthesia. The interplay between synchronization and modulation is discussed and their coexistence in the cardiorespiratory interaction is illustrated.