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22.2: The Endocrine System

Difficulty Level: At Grade Created by: CK-12
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Lesson Objectives

  • List the glands of the endocrine system and their effects.
  • Explain how hormones work by binding to receptors of target cells.
  • Describe feedback mechanisms that regulate hormone secretion.
  • Identify three endocrine system disorders.


  • adrenal glands
  • endocrine system
  • gonads
  • hypothalamus
  • pancreas
  • parathyroid glands
  • pineal gland
  • pituitary gland
  • target cell
  • thyroid gland


The nervous system isn’t the only message-relaying system of the human body. The endocrine system also carries messages. The endocrine system is a system of glands that release chemical messenger molecules into the bloodstream. The messenger molecules are hormones. Hormones act slowly compared with the rapid transmission of electrical messages by the nervous system. They must travel through the bloodstream to the cells they affect, and this takes time. On the other hand, because endocrine hormones are released into the bloodstream, they travel throughout the body. As a result, endocrine hormones can affect many cells and have body-wide effects.

Glands of the Endocrine System

The major glands of the endocrine system are shown in Figure below. You can access a similar, animated endocrine system chart at the link below. http://www.abpischools.org.uk/page/modules/hormones/horm2.cfm

The glands of the endocrine system are the same in males and females except for the testes, which are found only in males, and ovaries, which are found only in females.


The hypothalamus is actually part of the brain (see Figure below), but it also secretes hormones. Some of its hormones that “tell” the pituitary gland to either secrete or stop secreting its hormones. In this way, the hypothalamus provides a link between the nervous and endocrine systems. The hypothalamus also produces hormones that directly regulate body processes. These hormones travel to the pituitary gland, which stores them until they are needed. The hormones include antidiuretic hormone and oxytocin.

  • Antidiuretic hormone stimulates the kidneys to conserve water by producing more concentrated urine.
  • Oxytocin stimulates the contractions of childbirth, among other functions.

The hypothalamus and pituitary gland are located close together at the base of the brain.

Pituitary Gland

The pea-sized pituitary gland is attached to the hypothalamus by a thin stalk (see Figure above). It consists of two bulb-like lobes. The posterior (back) lobe stores hormones from the hypothalamus. The anterior (front) lobe secretes pituitary hormones. Several pituitary hormones and their effects are listed in Table below. Most pituitary hormones control other endocrine glands. That’s why the pituitary is often called the “master gland” of the endocrine system.

Hormone Target Effect(s)
Adrenocorticotropic hormone (ACTH) Adrenal glands Stimulates the cortex of each adrenal gland to secrete its hormones
Thyroid-stimulating hormone (TSH) Thyroid gland Stimulates the thyroid gland to secrete thyroid hormone
Growth hormone (GH) Body cells Stimulates body cells to synthesize proteins and grow
Follicle-stimulating hormone (FSH) Ovaries, testes Stimulates the ovaries to develop mature eggs; stimulates the testes to produce sperm
Luteinizing hormone (LH) Ovaries, testes Stimulates the ovaries and testes to secrete sex hormones; stimulates the ovaries to release eggs
Prolactin (PRL) Mammary glands Stimulates the mammary glands to produce milk

Other Endocrine Glands

Other glands of the endocrine system are described below. You can refer to Figure above to see where they are located.

  • The thyroid gland is a large gland in the neck. Thyroid hormones increase the rate of metabolism in cells throughout the body. They control how quickly cells use energy and make proteins.
  • The two parathyroid glands are located behind the thyroid gland. Parathyroid hormone helps keep the level of calcium in the blood within a narrow range. It stimulates bone cells to dissolve calcium in bone matrix and release it into the blood.
  • The pineal gland is a tiny gland located at the base of the brain. It secretes the hormone melatonin. This hormone controls sleep-wake cycles and several other processes.
  • The pancreas is located near the stomach. Its hormones include insulin and glucagon. These two hormones work together to control the level of glucose in the blood. Insulin causes excess blood glucose to be taken up by the liver, which stores the glucose as glycogen. Glucagon stimulates the liver to break down glycogen into glucose and release it back into the blood. The pancreas also secretes digestive enzymes into the digestive tract.
  • The two adrenal glands are located above the kidneys. Each gland has an inner and outer part. The outer part, called the cortex, secretes hormones such as cortisol, which helps the body deal with stress, and aldosterone, which helps regulate the balance of minerals in the body. The inner part of each adrenal gland, called the medulla, secretes fight-or-flight hormones such as adrenaline, which prepare the body to respond to emergencies. For example, adrenaline increases the amount of oxygen and glucose going to the muscles. You can see an animation of this response at http://www.abpischools.org.uk/page/modules/hormones/horm8.cfm?coSiteNavigation_allTopic=1.
  • The gonads secrete sex hormones. The male gonads are called testes. They secrete the male sex hormone testosterone. The female gonads are called ovaries. They secrete the female sex hormone estrogen. Sex hormones are involved in the changes of puberty. They also control the production of gametes by the gonads.

How Hormones Work

Endocrine hormones travel throughout the body in the blood. However, each hormone affects only certain cells, called target cells. A target cell is the type of cell on which a hormone has an effect. A target cell is affected by a particular hormone because it has receptor proteins that are specific to that hormone. A hormone travels through the bloodstream until it finds a target cell with a matching receptor it can bind to. When the hormone binds to a receptor, it causes a change within the cell. Exactly how this works depends on whether the hormone is a steroid hormone or a non-steroid hormone. At the link below, you can watch an animation that shows how both types of hormones work. http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP13704

Hormones are discussed at http://www.youtube.com/watch?v=HrMi4GikWwQ (2:28).

Steroid Hormones

Steroid hormones are made of lipids, such as phospholipids and cholesterol. They are fat soluble, so they can diffuse across the plasma membrane of target cells and bind with receptors in the cytoplasm of the cell (see Figure below). The steroid hormone and receptor form a complex that moves into the nucleus and influences the expression of genes. Examples of steroid hormones include cortisol and sex hormones.

A steroid hormone crosses the plasma membrane of a target cell and binds with a receptor inside the cell.

Non-Steroid Hormones

Non-steroid hormones are made of amino acids. They are not fat soluble, so they cannot diffuse across the plasma membrane of target cells. Instead, a non-steroid hormone binds to a receptor on the cell membrane (see Figure below). The binding of the hormone triggers an enzyme inside the cell membrane. The enzyme activates another molecule, called the second messenger, which influences processes inside the cell. Most endocrine hormones are non-steroid hormones, including insulin and thyroid hormones.

A non-steroid hormone binds with a receptor on the plasma membrane of a target cell. Then, a secondary messenger affects cell processes.

Hormone Regulation: Feedback Mechanisms

Hormones control many cell activities, so they are very important for homeostasis. But what controls the hormones themselves? Most hormones are regulated by feedback mechanisms. A feedback mechanism is a loop in which a product feeds back to control its own production. Most hormone feedback mechanisms involve negative feedback loops. Negative feedback keeps the concentration of a hormone within a narrow range.

Negative Feedback

Negative feedback occurs when a product feeds back to decrease its own production. This type of feedback brings things back to normal whenever they start to become too extreme. The thyroid gland is a good example of this type of regulation. It is controlled by the negative feedback loop shown in Figure below. You can also watch an animation of this process at the link below. http://biologyinmotion.com/thyroid/

The thyroid gland is regulated by a negative feedback loop. The loop includes the hypothalamus and pituitary gland in addition to the thyroid.

Here’s how thyroid regulation works. The hypothalamus secretes thyrotropin-releasing hormone, or TRH. TRH stimulates the pituitary gland to produce thyroid-stimulating hormone, or TSH. TSH, in turn, stimulates the thyroid gland to secrete its hormones. When the level of thyroid hormones is high enough, the hormones feedback to stop the hypothalamus from secreting TRH and the pituitary from secreting TSH. Without the stimulation of TSH, the thyroid gland stops secreting its hormones. Soon, the level of thyroid hormone starts to fall too low. What do you think happens next? This process is discussed at http://www.youtube.com/watch?v=Vae5CcaPN_8 (1:35).

Negative feedback also controls insulin secretion by the pancreas. You can interact with a feedback loop of this process at the link below. http://www.abpischools.org.uk/page/modules/hormones/horm6.cfm?coSiteNavigation_allTopic=1

Positive feedback

Positive feedback occurs when a product feeds back to increase its own production. This causes conditions to become increasingly extreme. An example of positive feedback is milk production by a mother for her baby. As the baby suckles, nerve messages from the nipple cause the pituitary gland to secrete prolactin. Prolactin, in turn, stimulates the mammary glands to produce milk, so the baby suckles more. This causes more prolactin to be secreted and more milk to be produced. This example is one of the few positive feedback mechanisms in the human body. What do you think would happen if milk production by the mammary glands was controlled by negative feedback instead?

Endocrine System Disorders

Diseases of the endocrine system are relatively common. An endocrine disease usually involves the secretion of too much or not enough hormone. When too much hormone is secreted, it is called hypersecretion. When not enough hormone is secreted, it is called hyposecretion.


Hypersecretion by an endocrine gland is often caused by a tumor. For example, a tumor of the pituitary gland can cause hypersecretion of growth hormone. If this occurs in childhood, it results in very long arms and legs and abnormally tall stature by adulthood. The condition is commonly known as gigantism (see Figure below).

Hypersecretion of growth hormone leads to abnormal growth, often called gigantism.


Destruction of hormone-secreting cells of a gland may result in not enough of a hormone being secreted. This occurs in Type 1 diabetes. In this case, the body’s own immune system attacks and destroys cells of the pancreas that secrete insulin. A person with type 1 diabetes must frequently monitor the level of glucose in the blood (see Figure below). If the level of blood glucose is too high, insulin is injected to bring it under control. If it is too low, a small amount of sugar is consumed.

To measure the level of glucose in the blood, a drop of blood is placed on a test strip, which is read by a meter.

Hormone Resistance

In some cases, an endocrine gland secretes a normal amount of hormone, but target cells do not respond to the hormone. Often, this is because target cells have because resistant to the hormone. Type 2 diabetes is an example of this type of endocrine disorder. In Type 2 diabetes, body cells do not respond to normal amounts of insulin. As a result, cells do not take up glucose and the amount of glucose in the blood becomes too high. This type of diabetes is not generally treated with insulin injections. Instead, it is usually treated with medication and diet.

Lesson Summary

  • The endocrine system consists of glands that secrete hormones into the bloodstream. It is regulated by a part of the brain called the hypothalamus, which also secretes hormones. The hypothalamus controls the pituitary gland, which is called the “master gland” of the endocrine system because its hormones regulate other endocrine glands. Other endocrine glands include the thyroid gland and pancreas.
  • Hormones work by binding to protein receptors either inside target cells or on their plasma membranes. The binding of a steroid hormone forms a hormone-receptor complex that affects gene expression in the nucleus of the target cell. The binding of a non-steroid hormone activates a second messenger that affects processes within the target cell.
  • Most hormones are controlled by negative feedback in which the hormone feeds back to decrease its own production. This type of feedback brings things back to normal whenever they start to become too extreme. Positive feedback is much less common because it causes conditions to become increasingly extreme.
  • Endocrine system disorders usually involve the secretion of too much or not enough hormone. For example, a tumor of the adrenal gland may lead to excessive secretion of growth hormone, which causes gigantism. In Type 1 diabetes, the pancreas does not secrete enough insulin, which causes high levels of glucose in the blood.

Lesson Review Questions


1. Define hormone.

2. List the major glands of the endocrine system.

3. Name three pituitary hormones, and state how they affect their targets.

4. Define hypersecretion. Give an example of an endocrine disorder that involves hypersecretion.

Apply Concepts

5. Tasha had a thyroid test. Her doctor gave her an injection of TSH and 15 minutes later measured the level of thyroid hormone in her blood. What is TSH? Why do you think Tasha’s doctor gave her an injection of TSH? How would this affect the level of thyroid hormone in her blood if her thyroid is normal?

6. After the thyroid test, Tasha’s doctor said she has an underactive thyroid. What symptoms would you expect Tasha to have? Why?

Think Critically

7. Explain how the nervous system is linked with the endocrine system.

8. Compare and contrast how steroid and non-steroid hormones affect target cells.

9. Why are negative feedback mechanisms more common than positive feedback mechanisms in the human body? What might happen if an endocrine hormone such as thyroid hormone was controlled by positive instead of negative feedback?

10. Explain why a person with type 2 diabetes cannot be helped by insulin injections.

Points to Consider

In this lesson you learned that endocrine hormones can affect cells throughout the body because they travel in the blood through the circulatory system.

  • Do you know what organs make up the circulatory system?
  • Can you explain what causes blood to move through the system?

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