The endocrine system regulates the functioning of every cell, tissue, and organ in the body. It acts to maintain a stable internal body environment, regardless of changes occurring within or outside of the body. Endocrine cells have the ability to sense and respond to changes via the excretion of specific chemicals known as hormones. The endocrine system is one of the body’s two major communication systems, the nervous system being the other. Communication within the nervous system is rapid, while the signals sent by the endocrine system may have much longer delays and last for much greater lengths of time. The endocrine system consists of all those glands termed endocrine glands, which secrete hormones. Hormones are chemical messengers that enter the blood, which carries them from endocrine glands to the cells upon which they act. The cells influenced by a particular hormone are the target cells for that hormone. The endocrine system differs from most of the other organ systems of the body in that the various glands are not anatomically connected; however they do form a system in a functional sense. Endocrine glands are ductless glands that secrete their products which are biologically active molecules called hormones directly into the blood. The blood carries the hormones to target organs that respond in a specific fashion to them. Many endocrine glands are discrete organs whose primary functions are the production and secretion of hormones. The main endocrine glands include the pituitary gland, pancreas, ovaries, testes, thyroid gland, and adrenal glands. The hypothalamus is a neuroendocrine organ. Other organs which are not so well known for their endocrine activity include the stomach, which produces such hormones as ghrelin. Most regulation of hormone levels in the body is conducted by negative feedback: if a particular hormone is needed, production of that hormone will be stimulated; if there is enough of a particular hormone present, production of that hormone will be inhibited. In a few very specific instances, hormonal output is controlled by positive feedback mechanisms. One such instance is the output of the posterior pituitary hormone oxytocin. This hormone causes the muscle layer of the uterus, the myometrium, to contract during childbirth. Contraction of the myometrium causes additional oxytocin to be released to aid in the contraction, regardless of the amount of hormone already present.
Secretions can be classified by several properties
1. Classification by site of action.
Secretions can be categorized by the site of action relative to the site of secretion.
Autocrine secretion - substance released by cell that affects the secreting cell itself (e.g. norepinephrine is released by a neurosecretory cell in the adrenal medulla, and norepinephrine itself inhibits further release by that cell)
Paracrine secretion - substance released by cell that affects neighboring cells. Not released into bloodstream (e.g. histamine released at site of injury to constrict blood vessel walls and stop bleeding)
Endocrine secretion - substance released by cell into bloodstream that affects distant cells.(e.g. testosterone is secreted by Leydig cells in testis, makes hair grow on your back)
Exocrine secretion - substance released by cell into a duct that leads to epithelial surface (onto skin or into gut). Action doesn’t depend on receptors in target tissue.(e.g. sweat, saliva, spider silk)
Endocrine and exocrine secretions are glandular secretions; they come from specialized secretory cells that are clumped together to form a gland. Endocrine glands are sometimes called ductless glands. A secretion may have several sites of action simultaneously.
2. Classification by origin. Another set of terms, related to those just discussed, is commonly used to classify secretions, based both on origin and site of action
Neurohormones - endocrine, source = nerve...