The purpose of cortisol in the body is to increase the levels of blood glucose, according to Dummies.com. It lowers the muscles' uptake of protein, increases blood pressure, and suppresses the immune system. It also floods the body with chemicals that repair tissues in case of injury.
Cortisol is a hormone released in response to stress, says Dummies.com. It prepares the body to fight or flee.
Cortisol also suppresses the processes of digestion, growth and reproduction, says Mayo Clinic. These systems are not necessary for immediate survival, even though cortisol is a hormone responsible for supporting the development of a fetus and triggering childbirth, according to the Society for Endocrinology. Cortisol also sends glucose to the brain and communicates with the areas in the brain responsible for motivation, mood and fear. When the immediate threat has passed, the level of cortisol drops, and the systems of the body are allowed to return to normal.
Cortisol levels respond to a person's diurnal rhythm, says the Society for Endocrinology. They are usually high first thing in the morning, then decrease as the day goes on. Cortisol is controlled through the interaction of the adrenal gland, which sits on top of the kidney; through the hypothalamus, which is found in the brain; and through the pituitary gland, which is also found in the brain.