Hans Selye was a very important endocrinologist working in the field of biological stress. He developed the General Adaptation Syndrome (GAS) which is a model describing an animal's response to stress. GAS is comprised of three stages; the alarm, the resistance and the exhaustion stage.
During the resistance stage, the body's glands produce and secrete several hormones which protect the individual from the perceived stressor. Some of the hormones produced at this stage are the glucocorticoids.
Four chambered heart keeps oxygenated and deoxygenated blood separate and has double circulation while three chambered heart has a single circulation. Two chambered heart only has single atrium and single ventricle