Cortisol is a 'glucocorticoid' (a cortical hormone that increases blood glucose levels). It acts on a wide range of tissues and organs eliciting a wide range of effects. The main ones are:
It has several poorly understood effects such as increased appetite, increased acid production by the stomach and effects in the mental state (e.g. euphoria or mania).
Diagram illustrating control of cortisol release - click to enlarge
Cortisol production and release is controlled by the hypothalamus and pituitary, forming the hypothalamopituitary-adrenal axis. Cortisol is released in response to stress, which can be emotional (e.g. anxiety) or physiological (e.g. fluid deprivation or injury). Stressors cause a release of Corticotrophin Releasing Hormone (CRH). This travels in the portal system of the hypothalamus to the anterior pituitary (the adenohypophysis). There, it stimulates the cleavage of Pro-opiomelanocortin (POMC) into several molecules including melanocyte stimulating hormone (MSH) and Adrenocorticotrophic Hormone (ACTH). ACTH travels in the bloodstream to the adrenal cortex stimulating the production and release of cortisol. Cortisol then travels to the tissues where it exerts its effects. Cortisol inhibits the release of CRH and ACTH from the hypothalamus and pituitary gland respectively, preventing further cortisol release. Cortisol is inactivated in the liver to inactive cortisone.
A daily pattern (circadian rhythm) is also seen, with cortisol being at it's lowest concentration at midnight, rising to a peak between 6am and 8am, falling throughout the rest of the day.
Graph showing circadian rhythm of cortisol - click to enlarge