The two adrenal glands, one sitting on top of each kidney, are divided into an outer part, the adrenal cortex, and an inner part, the adrenal medulla. The adrenal cortex secretes aldosterone, which acts on the kidneys to reabsorb salt and water from the urine. The purpose is to restore blood volume after dehydration or haemorrhage. aldosterone is part of a complex control system. On sensing a fall in blood pressure due to loss of blood volume, the kidneys secrete an enzyme called renin. Renin reacts with a plasma protein from the liver to form angiotensin I. Converted to angiotensin II in the lungs, this stimulates the adrenal cortex to produce aldosterone. Thus initial action of the kidneys culminates in the restoration of blood volume and blood pressure.
The second group of hormones from the adrenal cortex called glucocorticoids. are concerned with normal metabolism and resistance to stress. They include cortisol and cortisone. By assisting the conversion of amino acids and fats to glucose they provide a quick source of energy to combat fight, temperature extremes, infection, and trauma. They are also anti-inflammatory agents. Their secretion is controlled by the hypothalamus, which directs the pituitary to send adrenocorticotrophic hormone (ACTH) to the adrenal cortex to summon their release.
The adrenal medulla secretes two hormones, adrenalin, and noradrenalin, which literally spurt from the gland when fear, anger, a severe injury, or some other cause of stress assaults the brain. The glandular reaction is part of the fight-or-flight response. Impulses from the hypothalamus traveling along fibers of the sympathetic division of the unconscious nervous system, jolt the adrenal medulla into discharging its hormones. They accelerate breathing and heart rate, diverting blood from the periphery to the skeletal muscles by constricting blood vessels of the skin, increase blood sugar level and bring about other changes that prepare the body for immediate action.