The Big Picture
The endocrine system is a group of glands that release chemical substances called hormones into the bloodstream. These hormones help control the activities of other organs and glands.
Hormones affect many body processes. They help control growth, reproduction, energy use, water balance, and other chemical activities in the body.
Different glands produce different hormones. A good way to study this topic is to connect each gland with the hormones it releases and what those hormones do.
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Start Here If You’re Overwhelmed
Focus on these core ideas:
- An endocrine gland releases a hormone into the bloodstream.
- The pituitary gland releases several hormones that affect growth and other glands.
- Thyroid hormones help regulate metabolism, growth, and development.
- The parathyroid glands and thyroid gland both produce hormones that affect calcium levels.
- The adrenal glands produce several hormones involved in energy, stress, inflammation, salt, and water balance.
- Insulin and glucagon have opposite actions that help maintain homeostasis.
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Course Notes: Key Ideas You Need to Know
The Endocrine System and Hormones
The endocrine system is made up of endocrine glands. These glands release hormones into the bloodstream.
A hormone is a chemical substance made by cells in specialized glands and released into the blood.
Hormones affect specific target tissues. They bind to receptor sites that fit the hormone, similar to a key fitting a lock.
Hormones influence:
- the use of fats, proteins, and carbohydrates
- water and electrolyte balance
- cell growth and development
- chemical substances in cells and body fluids
- growth
- reproduction
The Pituitary Gland
The pituitary gland is also called the master gland. It is about the size of a cherry and sits at the base of the brain behind the eyes.
It has two parts:
- anterior pituitary lobe
- posterior pituitary lobe
Anterior Pituitary Hormones
The anterior pituitary lobe releases several important hormones.
Growth hormone: This hormone supports the growth of muscles, bones, and other organs. It is also needed throughout life to maintain normal-size bones and muscles.
If a baby is born with too little growth hormone, growth is stunted . Too much growth hormone causes excessive growth.
Thyroid-stimulating hormone (TSH): This hormone is needed for thyroid gland function.
Adrenocorticotropic hormone (ACTH): This hormone stimulates the adrenal glands.
Reproductive hormones: These hormones regulate the growth, development, and function of the male and female reproductive systems.
The Posterior Pituitary and Hypothalamus
The posterior pituitary lobe releases:
- antidiuretic hormone (ADH)
- oxytocin
Antidiuretic hormone (ADH) prevents the kidneys from removing excessive amounts of water from the body.
Oxytocin causes the muscles of the uterus to contract during childbirth.
Both ADH and oxytocin are produced in the hypothalamus, a nearby part of the brain.
The hypothalamus also releases several other hormones. Because of this hormone activity, it is considered part of the endocrine system as well as part of the brain.
Hormones produced in the hypothalamus control or inhibit the release of other hormones.
The Thyroid Gland
The thyroid gland is shaped like a butterfly. It sits in the neck in front of the larynx.
The thyroid releases:
- thyroxine, also called T₄
- tri-iodothyronine, also called T₃
- calcitonin
T₃ and T₄
T₃ and T₄ are thyroid hormones that regulate metabolism.
Metabolism refers to chemical processes in body cells that create and use energy and produce cellular waste products. The thyroid hormones regulate the burning of nutrients by cells to produce heat and energy.
T₃ and T₄ also support:
- proper functioning of all other hormones
- maturation of the nervous system
- normal growth and development
Too little T₃ and T₄ causes:
- slowed body processes
- slower movements
- weight gain
Too much T₃ and T₄ causes:
- increased metabolism
- excess energy
- weight loss
If a baby is born with too little thyroid hormone, physical and mental growth will be stunted.
Calcitonin
Calcitonin is produced by C cells in the thyroid gland.
It helps regulate calcium by:
- lowering the amount of calcium in blood plasma
- increasing the amount of calcium in bone
The Parathyroid Glands
The parathyroid glands are four small collections of cells. Two sit on each side of the thyroid gland.
They release parathormone.
Parathormone regulates the body’s use of calcium by increasing the amount of calcium available in blood plasma.
Calcium is needed for:
- proper nerve function
- muscle contraction
- bone development
- blood clotting
Too little calcium causes tetany, which involves severe muscle contraction and spasm. Untreated tetany can result in death.
The Adrenal Glands
The body has two adrenal glands. One sits on top of each kidney.
Each adrenal gland has two parts:
- adrenal medulla
- adrenal cortex
Adrenal Medulla
The adrenal medulla releases:
- epinephrine
- norepinephrine
During emergencies, these hormones stimulate the body to produce energy quickly.
They increase:
- heart rate
- blood pressure
- muscle power
- energy
Adrenal Cortex
The adrenal cortex releases three groups of hormones that are essential for life.
Glucocorticoids: These hormones regulate carbohydrate metabolism and control the body’s response to stress and inflammation.
Mineralocorticoids: These hormones regulate how much salt and water the kidneys absorb and lose.
Sex hormones: The adrenal cortex also releases small amounts of male and female sex hormones.
The Pancreas
The pancreas releases two hormones that regulate glucose, or sugar, in the blood:
- insulin
- glucagon
These hormones regulate how much glucose in the blood is available for cells to use.
Insulin
Insulin is needed for sugar to enter cells.
If there is too little insulin, sugar cannot enter the cells. Excess sugar then builds up in the blood.
Insulin and Glucagon Work in Opposite Ways
Insulin and glucagon are antagonistic hormones. This means they cause opposite actions in the body and help balance one another.
Their actions are examples of a negative feedback mechanism that helps maintain homeostasis.
Homeostasis is a stable internal environment maintained when the body’s organ systems work together.
The Gonads
The gonads are the glands involved in human reproduction.
Male sex glands, called testes, release testosterone.
Female sex glands, called ovaries, release:
- estrogen
- progesterone
Fast Review
Use these points for a quick review before a quiz or exam:
- Endocrine glands release hormones into the bloodstream.
- Hormones affect target tissues by binding to matching receptor sites.
- The pituitary gland is called the master gland and has anterior and posterior lobes.
- Growth hormone supports the growth and maintenance of muscles and bones.
- TSH supports thyroid gland function, while ACTH stimulates the adrenal glands.
- ADH prevents the kidneys from removing excessive amounts of water.
- T₃ and T₄ regulate metabolism and support normal growth and development.
- Calcitonin lowers calcium in blood plasma, while parathormone increases calcium available in blood plasma.
- The adrenal medulla releases epinephrine and norepinephrine, which increase energy and several body activities during emergencies.
- The adrenal cortex releases glucocorticoids, mineralocorticoids, and small amounts of sex hormones.
- Insulin allows sugar to enter cells. Insulin and glucagon have opposite actions.
- Testes release testosterone. Ovaries release estrogen and progesterone.
Final Pep Talk
There are many gland and hormone names in this topic, so study them in pairs. Match each gland with its hormones and main functions, then review the pairs again until the connections feel familiar.