The endocrine system involves chemical signalling via the secretion of molecules called hormones into extracellular fluid. They bind to chemical receptors in order to cause specific changes in target cells, these lead to changes in the body's internal environment called homeostasis. It includes the thyroid, parathyroid, pituitary, pineal and adrenal glands along with other regions. The bone, adipose tissue, heart, pancreas and liver are a few of the regions of the body which show endocrine function.
During homeostasis the body maintains a constant internal balance in pH, temperature, blood pressure etc. Cells in a multicellular organism become specialized for particular tasks and communicate with one another in order to maintain homeostasis. Within the human body these are known as hormone cascades, where several complex steps occur- the tissues signal to one another with the use of hormones released by the endocrine system. The regulation (increase and decrease) of these secretions is achieved by negative feedback loops, where the release of certain substances during a cascade in turn halts the secretion of hormones at earlier stages.
For instance, in the body calcium homeostasis, which is generally defined as the maintenance of specific internal conditions, is mediated by the endocrine system through hormonal control. This regulates the calcium flux between the bloodstream and the bone; and the hormones responsible, change the ratio of osteoclast activity to osteoblast activity -osteoblasts build bone while osteoclasts breakdown or degrade bone.
Hormones responsible include calcitonin (from special parafollicular cells in the thyroid gland), calcitriol (activated vitamin D) and the parathyroid hormone (from the parathyroid glands). These affect the absorption of calcium from the gut, or kidney reabsorption; both parathyroid hormone and calcitriol act by causing the increase of calcium ions into the bloodstream as the bone is degraded by osteoclasts.
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The endocrine system plays an important role in homeostasis because hormones regulate the activity of body cells. The release of hormones into the blood is controlled by a stimulus. For example, the stimulus either causes an increase or a decrease in the amount of hormone secreted. Then, the response to a stimulus changes the internal conditions and may itself become a new stimulus. This self-adjusting mechanism is called feedback regulation.