The Endocrine System

A labelled diagram of the major organs of the human endocrine system.
Unit 7 figure 1 Endocrine System

The Endocrine System, together with the nervous system, controls everything that occurs in the body’s internal environment. Some endocrine organs act independently, without nervous system control. Other times, the nervous system controls the endocrine gland. When the nervous system causes a gland to secrete, that gland is part of the endocrine system, and the secreted substance is a hormone.

The endocrine system is comprised of primary endocrine glands (hormone secretion is their only job) and secondary endocrine glands/tissues (secreting hormones is their second job). 

Each hormone has specific cell types that it targets to produce a particular action. The table below provides a limited number of hormones to learn.
Hormone Origin Target Action
adrenaline | epinephrine adrenal medulla cells in many organs stimulates sympathetic nervous system (i.e. “fight-or-flight response” when the body is under stress
adrenocorticotropic hormone (ACTH) anterior pituitary adrenal cortex stimulates adrenal cortex
antidiuretic hormone (ADH) hypothalamus & posterior pituitary kidneys stimulates the kidneys to conserve water so that less urine is produced
calcitonin a subset of thyroid cells bones & kidneys helps to regulate calcium levels in the blood
cortisol adrenal cortex cells in many organs stimulates metabolism; inhibits inflammation; part of a class of hormones called glucocorticoids
estrogens ovaries uterine lining; many other cells stimulate development of female sexual characteristics & helps the female body prepare for pregnancy; a class of hormones, the most common of which is 17-β-estradiol
glucagon pancreas primarily liver cells stimulates the creation of glucose (gluconeogenesis) and breakdown of the storage form of glucose (glycogenolysis)
growth hormone anterior pituitary primarily bones and muscles promotes growth
insulin pancreas most cells in the body stimulates cells to take in glucose, which decreases blood sugar levels
oxytocin hypothalamus & posterior pituitary uterus, mammary glands, & brain cells stimulates the uterus to contract during childbirth; stimulates the mammary glands to release milk; promotes bonding to another person
parathyroid hormone (PTH) parathyroid glands bones & kidneys helps to regulate calcium levels in the blood
releasing hormones (many different kinds) hypothalamus anterior pituitary stimulate the cells of the anterior pituitary to make and release hormones
testosterones testes many cells in the body stimulate development of male sexual characteristics; stimulate development of sperm; a class of hormones, the most common of which is dihydrotestosterone
thyroid stimulating hormone (TSH) anterior pituitary thyroid gland stimulates the thyroid gland to produce T4 and T3
thyroxine (T4) & triiodothyronine (T3) thyroid gland cells throughout the body stimulate metabolism
Expanded view of anterior and posterior pituitary location in relation to hypothalamus and thalamus.
Unit 7 figure 2 Hypothalamus and Pituitary

The hypothalamus is a part of the brain that produces hormones which either target or are stored in the pituitary gland. The pituitary gland is located inferior to the hypothalamus and has two parts, the anterior (adenohypophysis) and posterior (neurohypophysis). The hypothalamus controls the anterior pituitary (adenohypophysis) through a “private” blood system called the hypophyseal portal system. Releasing hormones are dropped into this private bloodstream and carried to the anterior pituitary, where they encounter cells that release hormones such as adrenocorticotropic hormone, growth hormone, and thyroid stimulating hormone.

The posterior pituitary (neurohypophysis) works differently. Neurons in the hypothalamus make the hormones antidiuretic hormone and oxytocin. They ship the hormone down their nerve cell axons which end in the posterior pituitary, where the release of hormones into the bloodstream occurs.

Diagram and names of the hormones involved in calcium cycle.
Unit 7 figure 3 Calcium Homeostasis

Another job of the endocrine system is regulating the amount of calcium carried in the blood. A small subset of thyroid gland cells releases calcitonin, and the nearby parathyroid glands release parathyroid hormone (PTH). The kidneys release calcitriol (vitamin D), a closely related compound, calcidiol, is made by the skin. Calcitonin moves calcium from the blood to the bones, lowering blood calcium levels; parathyroid hormone moves calcium from the bones to the blood, increasing blood calcium levels. Calcitriol (vitamin D) helps in the absorption of calcium in the intestines, which also increases blood calcium levels.

Endocrine System (Female)
Unit 7 figure 4 Female Endocrine System

Hormones are also used to regulate sexual function. Genes on the X or Y chromosome will direct brain cells (through a complex, multi-step process) to begin producing hormones, which in turn promote the development of primary sexual characteristics (the gonads and external genitalia). Primary sexual characteristics are those a person is born with. After a pause of several years, another burst of hormones at puberty promotes the development of secondary sexual characteristics (for example, menstruation and breast development in the female). The beginning of menstruation is called menarche and the beginning of breast development is thelarche.

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