An individual or animal needs extra energy to give immediate reaction when they experience an external threat. In such situation automatic nervous system becomes active to satisfy the need of extra energy.
In 1920, Walter Canon described that the cells of the body become active to cope up with the threat of the situation. The neurons, the nervous system and the endocrine become very active. Canon calls this reactionary process as "fight or flight.
"Hypothalamus in the brain is the origin of psychological reactions related to stress. It is known as the centre of stress. In such situation hypothalamus reacts along two pathway glands (i) through automatic nervous system (ii) through activity of the pituitary gland. Automatic nervous system has two parts: (1) sympathetic system (2) parasympathetic system. When sympathetic system becomes active, the Tęspiratory process becomes fast and deep, heart beat increases and the blood pressure also increases as arteries get contracted. There is more flow of air to the lungs due to changes in the muscles of throat and nose. Changes in facial expressions also occur depending upon different emotions. The digestive system is arrested a little. The adrenal glands release a large amount of catecholamine.
The secretions from the pituitary gland stimulates the thyroid gland and the adrenal gland. The secretion of these two glands produce several changes like release of more blood-cells in the bone-hollow, production of more sugar in the pancreas, thickening of blood etc. which are very necessary to make the body system more active.