Although we rarely think about the endocrine system, it influences almost every cell, organ, and function of our bodies. The endocrine system plays a role in regulating mood, growth and development, tissue function, metabolism, and sexual function and reproductive processes.
In general, the endocrine system is in charge of body processes that happen slowly, such as cell growth. Faster processes like breathing and body movement are controlled by the nervous system. But even though the nervous system and endocrine system are separate systems, they often work together to help the body function properly.
The foundations of the endocrine system are the hormones and glands. As the body’s chemical messengers, hormones transfer information and instructions from one set of cells to another. Many different hormones move through the bloodstream, but each type of hormone is designed to affect only certain cells.
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 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 include the:
- pituitary gland
- adrenal glands
- pineal body
- reproductive glands (which include the ovaries and testes)
The hypothalamus (pronounced: hi-po-THAL-uh-mus), a collection of specialized cells that is located in the lower central part of the brain, is the main 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 changes in the seasons. To accomplish this, the hypothalamus provides 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. The anterior lobe produces hormones such as:
- 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: thy-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-fins), chemicals that act on the nervous system and reduce feelings of pain. In addition, the pituitary secretes hormones that signal the reproductive organs 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 the balance of water in the body. The posterior lobe also produces oxytocin (pronounced: ahk-see-TOE-sin), which triggers the contractions of the uterus in a woman having a baby.
The thyroid (pronounced: THY-royd), located in the front part of the lower neck, is shaped like a bow tie or butterfly and produces the thyroid hormones thyroxine (pronounced: thy-RAHK-sin) and triiodothyronine (pronounced: try-eye-oh-doe-THY-ruh-neen). These hormones control the rate at which cells burn fuels from food to produce energy.
The production and release of thyroid hormones is controlled by thyrotropin (pronounced: thy-ruh-TRO-pin), which is secreted by the pituitary gland. The more thyroid hormone there is in a person’s bloodstream, the faster chemical reactions occur in the body.
Why are thyroid hormones so important? There are several reasons — for example, they help kids’ and teens’ bones grow and develop, and they also play a role in the development of the brain and nervous system in kids.
Attached to the thyroid are four tiny glands that function together called the parathyroids(pronounced: par-uh-THY-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 also 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.
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 when you sleep at night and when you wake in the morning.
The gonads are the main source of sex hormones. Most people don’t realize it, but both guys and girls have gonads.
In guys the male gonads, or testes (pronounced: TES-teez), are located in the scrotum. They secrete hormones called androgens (pronounced: AN-druh-junz), the most important of which istestosterone (pronounced: tess-TOSS-tuh-rone). These hormones tell a guy’s body when it’s time to make the changes associated with puberty, like penis and height growth, deepening voice, and growth in facial and pubic hair. Working with hormones from the pituitary gland, testosterone also tells a guy’s body when it’s time to produce sperm in the testes.
A girl’s gonads, the ovaries (pronounced: OH-vuh-reez), are located in her pelvis. They produce eggs and secrete the female hormones estrogen (pronounced: ESS-truh-jen) and progesterone(pronounced: pro-JESS-tuh-rone). Estrogen is involved when a girl begins to go through puberty. During puberty, a girl will experience breast growth, will begin to accumulate body fat around the hips and thighs, and will have a growth spurt. Estrogen and progesterone are also involved in the regulation of a girl’s menstrual cycle. These hormones also play a role in pregnancy.
Although the endocrine glands are the body’s main hormone producers, some other organs not in the endocrine system — such as the brain, heart, lungs, kidneys, liver, and skin — also produce and release hormones.
The pancreas (pronounced: PAN-kree-us) is also part of the body’s hormone-secreting system, even though it is also associated with the digestive system because it produces and secretes digestive enzymes.
The pancreas produces (in addition to others) two important hormones, insulin (pronounced: IN-suh-lin) and glucagon (pronounced: GLOO-kuh-gawn). They work together to maintain a steady level of glucose, or sugar, in the blood and to keep the body supplied with fuel to produce and maintain stores of energy.
What Does the Endocrine System Do?
Once a hormone is secreted, it travels from the endocrine gland that produced it through the bloodstream to the cells designed to receive its message. These cells are called target cells. Along the way to the target cells, special proteins bind to some of the hormones. These proteins act as carriers that control the amount of hormone that is available for the cells to use.
The target cells have receptors that latch onto only specific hormones, and each hormone has its own receptor, so that each hormone will communicate only with specific target cells that have receptors for that hormone. When the hormone reaches its target cell, it locks onto the cell’s specific receptors and these hormone-receptor combinations transmit chemical instructions to the inner workings of the cell.
When hormone levels reach a certain normal amount, the endocrine system helps the body to keep that level of hormone in the blood. For example, if the thyroid gland has secreted the right amount of thyroid hormones into the blood, the pituitary gland senses the normal levels of thyroid hormone in the bloodstream. Then the pituitary gland adjusts its release of thyrotropin, the hormone that stimulates the thyroid gland to produce thyroid hormones.
Another example of this process is parathyroid hormone, which increases the level of calcium in the blood. When the blood calcium level rises, the parathyroid glands sense the change and reduce their secretion of parathyroid hormone. This turnoff process is called a negative feedback system.