The endocrine system is a vital system which consists of various endocrine glands secreting chemical substances commonly known as hormones for coordinating and controlling the body functions and processes. Endocrine system along with the nervous system helps in the proper functioning of the body. Although endocrine system is a much slower process in comparison to the transmission of the nervous system but its response on target organs last for a longer period of time than the nervous system. The endocrine glands secrete a very small amount of hormone directly in the blood or lymph (extracellular tissue) and blood transport the hormone to the target organ to respond to the particular stimuli or for the proper functioning of the organ. The nervous system controls the functioning of the endocrine system but the endocrine system doesn't have much of a control over the nervous system. The study of the endocrine system along with the formation of hormone and their function is known as endocrinology.
Types of endocrine glands
Endocrine glands are isolated ductless glands and they directly secrete the hormones in the extracellular tissues which transport it the target organs. They are also known as 'Glands of internal secretion'. There are mainly three types of endocrine glands,
Permanent glands: These glands are present in the human being throughout their life and they secrete some of the most important hormones which play a vital role in the coordination of the human body. Some examples of permanent glands are adrenal gland, hypothalamus, pituitary, thyroids etc.
Temporary glands: Although these glands are present throughout lifetime in the human being but they are only functional for a certain period of time and then they become totally non-functional for the rest of the life. One of the most common examples of temporary glands is thymus gland.
Recurrent glands: These glands are periodic in natures. They appear and secrete hormones and then they disappear and after certain period of time they again appear. Most of the recurrent glands are found in the female body. Some o the examples of the recurrent glands are placenta, corpus luteum etc.
Target organs and tissue types
Hormones which are secreted by the endocrine glands act on a particular organ or tissue. There are usually three types of target organs on which hormone acts on,
Primary targets: These targeting organs are considered to the first and foremost target of the hormone. For example, gonadotrophin releasing hormone (GnRH) secreted from the hypothalamus first acts on the pituitary gland to secrete gonadotrophin hormone. The pituitary gland is the primary target of the hormone.
Secondary target: The hormones of the primary target organs act on theses organs. For examples, under the action of the TSH-RH (thyroid stimulating hormone releasing hormone) from the hypothalamus gland the pituitary gland (primary target) secretes thyroid stimulating hormones (TRH) into the blood. This thyroid stimulating hormone reaches the thyroid gland and acts upon it. The thyroid gland is the secondary target organ.
Final target: Theses organs receive information from the secondary target organs. For examples, the thyroxine hormone which will be secretes by the thyroid gland under the stimulation of the TRH from the pituitary, will act over the cells of our body to control the metabolic activities.
Hormones, functional unit of the endocrine system
Hormones are chemical substances which are non-nutrient in nature and are produced in very small quantities by the endocrine cells of the glands. Their act as chemical messengers, who controls, activates and regulates various functions and physiological processes of our body. There are more than 45 hormones secreted in our body from various endocrine glands. Starling and Bayliss were the first biologist to discover hormones in the year 1902 and Starling was the one who use the term hormone for the chemical substances. Most of the hormones that secreted are glandular hormones but the hormone which is secreted by the hypothalamus is neuro-hormone.
Hormone types based on their chemical nature
Hormones are divided into seven types on the basis of their chemical nature.
Functions and properties of hormone
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