The tritium cycle of a fusion reactor is here-after investigated by a synthetic model of the tritium circulation between the blanket, the tritium recovery units from the breeder, the coolant, the plasma exhaust and the storage unit. Analytical expressions of the minimum required breeding capability and of the initial tritium supply are derived to analyse the sensitivity of these crucial parameters to the fractional burn up, to the tritium losses (radioactive and others) and to the processing time associated with the various units. As confirmed by the parametric study of a few typical situations, the necessary breeding capability and the initial tritium supply are essentially functions of the total equilibrium inventory. In addition, the distribution of this total inventory among the various units and the possible disproportion of the time scales required by different recovery processes, strongly influence the initial tritium requirement and the doubling time associated with given breeding performances.