This book discusses and explains the importance of biochemistry knowledge in understanding what happens to patients during anesthesia and/or to those being in intensive care. It covers a wide range of topics, such as Cerebral Edema, Shock, Blood-Brain Barrier, The Pulmonary surfactant, The Acid – Base equilibrium, Local anaesthetics, Perineural adjuvants, Normobaric Oxygen Therapy, Theories of Narcosis. Hyperventilation effects and consequences are also presented. For instance, by hyperventilating a patient with a PaCO2 significantly below 25 mmHg, we risk blocking pyruvic acid carboxylation and transforming it into oxalacetic acid, which in turn knocks out the Krebs cycle, possibly leading to a complication, i.e. to metabolic acidosis and not to compensation for respiratory alkalosis.