Endocrine system

The endocrine system produces and regulates the body’s hormones. Hormones are chemical substances created and distributed by the body to act as “messengers” and coordinate functions of various body parts. Most hormones are proteins consisting of amino acid chains; some are steroids, fatty cholesterol-produced substances.

Although many different hormones circulate in the bloodstream, each affects only the cells that are genetically programmed to respond to it. Hormone levels can be influenced by stress, infection and changes in the balance of fluid and minerals in blood.

A gland is a group of cells that produces and secretes chemicals. Endocrine glands release more than 20 major hormones directly into the bloodstream, where they may be transported to cells in other parts of the body.

The major parts of the endocrine system include:

• Hypothalamus - The hypothalamus is a part of the brain near the optic chiasm. It secretes hormones that stimulate or suppress the release of hormones in the pituitary gland, in addition to controlling water balance, sleep, temperature, appetite and blood pressure.
• Pineal body - The pineal body is located below the corpus callosum, a part of the brain. It produces the hormone melatonin, a naturally-occurring antioxidant that helps regulate the body’s circadian rhythm.
• Pituitary  - The pituitary gland is located at the base of the brain. No larger than a pea, it gland controls many functions of the other endocrine glands.
• Thyroid and parathyroid glands - The thyroid and parathyroid glands are located in front of the neck, below the larynx (voice box). The thyroid plays an important role in the body's metabolism. Both the thyroid and parathyroid glands also play a role in the regulation of the body's calcium balance.
• Thymus - The thymus is located in the upper part of the chest and produces T-lymphocytes (white blood cells that fight infections and destroy abnormal cells).
• Adrenal glands - The two adrenal glands are located on top of both kidneys. They work hand-in-hand with the hypothalamus and pituitary gland.
• Kidney - The two kidneys are located near the middle of the back, just below the rib cage. They process the blood to sift out waste products and extra water. This waste and extra water becomes urine, which is stored in the bladder.
• Pancreas - The pancreas is located across the back of the abdomen, behind the stomach. The pancreas plays a role in digestion, as well as in hormone production.
• Ovary - A female's ovaries are located on both sides of the uterus, below the opening of the fallopian tubes (tubes that extend from the uterus to the ovaries). In addition to containing the egg cells necessary for reproduction, the ovaries also produce estrogen and progesterone.
• Testis - A male's testes are located in a pouch that hangs suspended outside his body. The testes produce testosterone and sperm.

What’s the difference between the endocrine and the exocrine systems?

The endocrine system secretes hormones directly into the bloodstream or to nearby cells; the exocrine system secretes hormones into ducts leading to the external environment (including sweat and salivary glands).