Two types of homeostasis are usually distinguished—physiological and developmental—although intermediate conditions exist. Physiological homeostatic reactions enable organisms to maintain certain physiological steady state in spite of environmental fluctuations. The regulation of the concentration of salt in blood by the kidneys, or the hypertrophy of muscle owing to strenuous use, are examples of physiological homeostasis. Developmental homeostasis refers to the regulation of the different paths that an organism may follow in the progression from fertilized egg to adult. The process can be influenced by the environment in various ways, but the characteristics of the adult individual, at least within a certain range, are largely predetermined in the fertilized egg.