In the respiratory chain equivalents of reduction from nutrients are transferred to O₂ building an electrochemical proton gradient across the internal mitochondrial membrane that is used to synthesize ATP. ATP yields the energy for biosynthetic processes and maintenance of homeostasis in cells. From this elementary panorama of cell energetics it emerges that an inadequacy of the O₂ supply, hypoxia, causes a derangement of cell ion gradients, a curtailment of the biosynthesis of cell constituents and eventual cell death.

Adult mammals possess mechanisms directed to secure an adequate O₂ supply to cells according to needs. Some mechanisms are general, as they impinge in the entire organism, conform regulatory loops characterized by a low affinity for O₂ (are activated at high PO₂), and are aimed to secure an adequate provision of O₂ to the entire organism. These regulatory loops are the carotid body chemoreceptor reflex, the hypoxic pulmonary vasoconstriction and the erythropoietin system. Additional local regulatory loops distribute O₂ to cells, tissues or organs adjusting supply to particular needs in an instant to instant fashion. However, in the postnatal periods of mammals some of the general O₂-regulated loops are immature and function with a high affinity for O₂, (are activated at low PO₂), bringing the offspring unassisted in situations of hypoxia. Yet, in these postnatal periods mammals have an alternative O₂-driven regulatory loop, namely the adrenal medulla and its secretion products that are capable of maintaining a high glycolytic rate and adequate ATP generation.

Grants: BFU2007-61848 (DGICYT), CIBER CB06/06/0050 (FISS-ICiii) and JCyL-GR242.