An analysis of the mechanisms of adaptation of the newborn infant, immediately after birth, is put forth. Emphasis is placed upon cardiorespiratory and metabolic homeostasis, with a description of the internal medium variations in neonates seeking spontaneous equilibrium. In Mexico City this takes 4 hours. Hypoxia is stressed as one of the most important complications of the neonatal period, requiring rapid and efficient management of the infant to avoid severe complications or even death. Intra and extracardaic hemodynamic changes, stabilization of central arterial pressure as well as sufficient cardiac output, due to proper left ventricle function are described in reference to the cardiovascular sphere. A brief mention is made of the work of both ventricles during the first days of life. The diverse hemodynamic behaviour in different vascular areas of the newborn infant might be due to the concentrations of circulating gases, pH, enzyme factors and circulating hormones.