**4. Hypothalamic-pituitary-adrenal axis**

Hypothalamic – pitutitary – adrenal system is the central stress response system linking neural regulation to neurohormonal and humoral control. In response to cortical signals e.g. fear, pain, deep emotions or immune derived factors like TNF α, Il-6 corticotropin realizing hormone, vasopressin, prolactin and growth hormone are released. Corticotropin releasing hormone stimulates sympathetic system and ACTH secretion. It reaches the adrenal cortex and stimulates cortisol production from cholesterol. Cortisol cooperates with sympathetic activation to prepare metabolism for stress response. These mechanism inhibit all growth and developmental functions, prepare metabolic substrates (glucose, fatty acids, amino acids), increase blood pressure and intravascular volume.

There is insufficiency of hypothalamic-pituitary-adrenal axis after pediatric cardiac surgery observed, best described as a critical illness–related corticosteroid insufficiency (CIRCI). Together with other axes derangement it is considered as one of the causes of low cardiac output syndrome in postoperative period. Many causes of this phenomenon were proposed: brain hypoperfusion, central hypothalamus and pituitary gland insufficiency, tissue resistance to adrenocorticotropic hormone (ACTH), adrenal dysfunction, cyanosis and tissues immaturities.
