General Health

What Is the Endocrine System?

The foundations of the endocrine system are the hormones and glands. As the body's chemical messengers, hormones (pronounced: hor-moanz) transfer information and instructions from one set of cells to another. Although many different hormones circulate throughout the bloodstream, each one affects only the cells that are genetically programmed to receive and respond to its message. Hormone levels can be influenced by factors such as stress, infection, and changes in the balance of fluid and minerals in blood.

A gland is a group of cells that produces and secretes, or gives off, chemicals. A gland selects and removes materials from the blood, processes them, and secretes the finished chemical product for use somewhere in the body. Some types of glands release their secretions in specific areas. For instance, exocrine (pronounced: ek-suh-krin) glands, such as the sweat and salivary glands, release secretions in the skin or inside of the mouth. Endocrine glands, on the other hand, release more than 20 major hormones directly into the bloodstream where they can be transported to cells in other parts of the body.

The major glands that make up the human endocrine system are the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pineal body, and the reproductive glands, which include the ovaries and testes. The pancreas is also part of this hormone-secreting system, even though it is also associated with the digestive system because it also produces and secretes digestive enzymes. Although the endocrine glands are the body's main hormone producers, some non-endocrine organs - such as the brain, heart, lungs, kidneys, liver, thymus, skin, and placenta - also produce and release hormones.

The hypothalamus (pronounced: hi-po-tha-luh-mus), a collection of specialized cells that is located in the lower central part of the brain, is the primary link between the endocrine and nervous systems. Nerve cells in the hypothalamus control the pituitary gland by producing chemicals that either stimulate or suppress hormone secretions from the pituitary.

Although it is no bigger than a pea, the pituitary (pronounced: puh-too-uh-ter-ee) gland, located at the base of the brain just beneath the hypothalamus, is considered the most important part of the endocrine system. It's often called the "master gland" because it makes hormones that control several other endocrine glands. The production and secretion of pituitary hormones can be influenced by factors such as emotions and seasonal changes. To accomplish this, the hypothalamus relays information sensed by the brain (such as environmental temperature, light exposure patterns, and feelings) to the pituitary.

The tiny pituitary is divided into two parts: the anterior lobe and the posterior lobe. The anterior lobe regulates the activity of the thyroid, adrenals, and reproductive glands. Among the hormones it produces are:

• growth hormone, which stimulates the growth of bone and other body tissues and plays a role in the body's handling of nutrients and minerals
• prolactin (pronounced: pro-lak-tin), which activates milk production in women who are breastfeeding
• thyrotropin (pronounced: thigh-ruh-tro-pin), which stimulates the thyroid gland to produce thyroid hormones
• corticotropin (pronounced: kor-tih-ko-tro-pin), which stimulates the adrenal gland to produce certain hormones

The pituitary also secretes endorphins (pronounced: en-dor-finz), chemicals that act on the nervous system to reduce sensitivity to pain. In addition, the pituitary secretes hormones that signal the ovaries and testes to make sex hormones. The pituitary gland also controls ovulation and the menstrual cycle in women.

The posterior lobe of the pituitary releases antidiuretic (pronounced: an-ty-dy-uh-reh-tik) hormone, which helps control body water balance through its effect on the kidneys and urine output; and oxytocin (pronounced: ahk-see-toe-sin), which triggers the contractions of the uterus that occur during labor.

The thyroid (pronounced: thigh-royd), located in the front part of the lower neck, is shaped like a bowtie or butterfly and produces the thyroid hormones thyroxine (pronounced: thigh-rahk-seen) and triiodothyronine (pronounced: try-eye-uh-doe-thigh-ruh-neen). These hormones control the rate at which cells burn fuels from food to produce energy. As the level of thyroid hormones increases in the bloodstream, so does the speed at which chemical reactions occur in the body. Thyroid hormones also play a key role in bone growth and the development of the brain and nervous system in children. The production and release of thyroid hormones is controlled by thyrotropin (pronounced: thigh-ruh-tro-pin), which is secreted by the pituitary gland.

Attached to the thyroid are four tiny glands that function together called the parathyroids (pronounced: par-uh-thigh-roydz). They release parathyroid hormone, which regulates the level of calcium in the blood with the help of calcitonin (pronounced: kal-suh-toe-nin), which is produced in the thyroid.

The body has two triangular adrenal (pronounced: uh-dree-nul) glands, one on top of each kidney. The adrenal glands have two parts, each of which produces a set of hormones and has a different function. The outer part, the adrenal cortex, produces hormones called corticosteroids (pronounced: kor-tih-ko-ster-oydz) that influence or regulate salt and water balance in the body, the body's response to stress, metabolism, the immune system, and sexual development and function. The inner part, the adrenal medulla (pronounced: muh-duh-luh), produces catecholamines (pronounced: kah-tuh-ko-luh-meenz), such as epinephrine (pronounced: eh-puh-neh-frun). Also called adrenaline, epinephrine increases blood pressure and heart rate when the body experiences stress. (Epinephrine injections are often used to counteract a severe allergic reaction.)

The pineal (pronounced: pih-nee-ul) body, also called the pineal gland, is located in the middle of the brain. It secretes melatonin (pronounced: meh-luh-toe-nin), a hormone that may help regulate the wake-sleep cycle.