Endocrine System

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The Endocrine System

Endocrine System Glands
The principal glands of the female and male human endocrine systems. Encyclopaedia Britannica/UIG/Getty Images

What Is the Endocrine System?

The endocrine system regulates vital processes in the body including growth, metabolism, and sexual development. This system is comprised of several major endocrine glands. These glands secrete hormones into the blood. Once in the blood, the hormones travel along the cardiovascular system until they reach their target cells. Only cells with specific receptors for a certain hormone will be influenced by that hormone. Hormones control various cellular activities including growth; development; reproduction; energy use and storage; and water and electrolyte balance. Both the endocrine system and the nervous system are responsible for maintaining homeostasis in the body. These systems help to maintain a constant internal environment in response to environmental changes.

Endocrine Glands

The major glands of the endocrine system are the pineal gland, pituitary gland, thyroid and parathyroid glands, adrenal glands, pancreas, thymus, ovaries, and testes. There are also other organs in the body that have secondary endocrine functions. These organs include the heart, liver, and kidneys.

  • Pineal Gland

    The pineal gland is a pine cone shaped gland of the endocrine system. It is situated between the cerebral hemispheres of the brain. This gland produces several important hormones including melatonin. Melatonin influences sexual development and sleep-wake cycles. The pineal gland connects the endocrine system with the nervous system in that it converts nerve signals from the sympathetic system of the peripheral nervous system into hormone signals.
     
  • Pituitary Gland

    The pituitary gland is a small endocrine organ located in the middle of the base of the brain. It controls a multitude of important functions in the body. The pituitary gland is termed the "Master Gland" because it directs other organs and endocrine glands to suppress or induce hormone production. Hormones secreted by the pituitary gland include antidiuretic hormone, corticotropin, growth hormone, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), oxytocin, prolactin, and thyroid-stimulating hormone (TSH).
     
  • Thyroid and Parathyroid Glands

    The thyroid is a dual lobed gland located in the neck region. It secretes hormones that control metabolism, growth, heart rate, body temperature, and regulate calcium levels. Hormones secreted by the thyroid include thyroxin, triiodothyronine, and calcitonin. Parathyroid glands are found within thyroid tissue. They secrete parathyroid hormone which regulates calcium levels in the blood.
     
  • Thymus

    Although it is considered an endocrine gland, the thymus gland is the main organ of the lymphatic system. Its primary function is to promote the development of specific white blood cells called T-lymphocytes. The thymus produces several hormones including thymosin, which increases immune responses by promoting the production of antibodies. In addition to immune function, the thymus also stimulates the production of certain pituitary gland hormones that promote growth and sexual maturation.
     
  • Adrenal Glands

    There are two adrenal glands in the body. One located atop each kidney. The adrenal gland produces hormones in both the inner medulla region and the outer cortex region of the gland. Hormones produced within the adrenal cortex region are all steroid hormones. Adrenal cortex hormones include aldosterone, cortisol, and sex hormones. Aldosterone causes the kidneys to secrete potassium and retain water and sodium. This causes blood pressure to rise. Cortisol acts as an anti-inflammatory and helps maintain blood sugar levels and blood pressure. Hormones of the adrenal medulla include epinephrine and norepinephrine. These are secreted in response to stimulation from sympathetic nerves, typically in response to stress.
     
  • Pancreas

    The pancreas is a soft organ located near the stomach and small intestines. It is both an exocrine gland and an endocrine gland. The exocrine portion of the pancreas secretes digestive enzymes that are carried by a duct to the small intestines. The endocrine segment of the pancreas produces the hormones glucagon and insulin. Glucagon raises blood sugar levels, while insulin lowers blood sugar levels and stimulates the metabolism of glucose, protein, and fat.
     
  • Gonads (Ovaries and Testes)

    The endocrine system includes certain organs of the reproductive system. Male and female primary reproductive organs are called gonads. Gonads produce sex cells and also secrete reproductive hormones. The male gonads, the testes, produce hormones called androgens. Testosterone is the main androgen secreted by the testes. The female ovaries secrete the hormones estrogen and progesterone. Gonadal hormones are responsible for the development of male and female reproductive organs and sexual characteristics.

Hormone Regulation

Hormones can be regulated by other hormones, by glands and organs, and by a negative feedback mechanism. In negative feedback, the initial stimulus is reduced by the response it provokes. The response eliminates the initial stimulus and the pathway is halted. Negative feedback is demonstrated in the regulation of blood calcium. The parathyroid gland secretes parathyroid hormone in response to low blood calcium levels. As parathyroid hormone increases blood calcium levels, calcium levels eventually return to normal. When this happens, the parathyroid gland detects the change and stops secreting parathyroid hormone.

Sources:

  • SEER Training Modules, Introduction to the Endocrine System. U. S. National Institutes of Health, National Cancer Institute. Accessed 21 October 2013 (http://training.seer.cancer.gov/anatomy/endocrine/)
  • Hormones and the Endocrine System. The Ohio State University Wexner Medical Center. Accessed 21 October 2013 (http://medicalcenter.osu.edu/patientcare/healthcare_services/diabetes_endocrine/about_diabetes/endocrinology/hormones_and_endocrine_system/Pages/index.aspx)