8.2: Introduction to the Endocrine System

Moon Face

The patient in Figure \(\PageIndex{1}\) has the characteristic moon face of a disorder named Cushing’s syndrome. Other signs and symptoms of this disorder include abnormal weight gain, acne, and excessive hairiness, among many other abnormalities. What can cause so many different problems in one patient? The answer is the overproduction of the endocrine system hormone cortisol.

Figure \(\PageIndex{1}\): a person with Cushing's syndrome

Overview of the Endocrine System

The endocrine system is a system of glands called endocrine glands that release chemical messenger molecules called hormones into the bloodstream. Other glands of the body, including sweat glands and salivary glands, also secrete substances but not into the bloodstream. Instead, they secrete them through ducts that carry them to nearby body surfaces. These other glands are called exocrine glands.

Endocrine hormones must travel through the bloodstream to the cells they affect, and this takes time. Because endocrine hormones are released into the bloodstream, they travel throughout the body wherever blood flows. As a result, endocrine hormones may affect many cells and have body-wide effects. Endocrine hormones may cause effects that last for days, weeks, or even months.

Comparison of the Nervous System and the Endocrine System

Communication is a process in which a sender transmits signals to one or more receivers to control and coordinate actions. In the human body, two major organ systems participate in relatively “long distance” communication: the nervous system and the endocrine system. Together, these two systems are primarily responsible for maintaining homeostasis in the body.

Neural and Endocrine Signaling

The nervous system uses two types of intercellular communication—electrical and chemical signaling. Electrical signaling occurs by the direct action of an electrical potential. Chemical signaling occurs through the action of chemical neurotransmitters such as serotonin or norepinephrine. Neurotransmitters act locally and rapidly. When an electrical signal in the form of an action potential arrives at the synaptic terminal, neurotransmitters diffuse across the synaptic cleft (the gap between a sending neuron and a receiving neuron or muscle cell). Once the neurotransmitters interact (bind) with receptors on the receiving (post-synaptic) cell, the receptor stimulation is transduced into a response such as continued electrical signaling or modification of cellular response. The target cell responds within milliseconds of receiving the chemical “message”; this response then ceases very quickly once the neural signaling ends. In this way, neural communication enables body functions that involve quick, brief actions, such as movement, sensation, and cognition.I

In contrast, the endocrine system uses just one method of communication: chemical signaling. These signals are sent by the endocrine organs, which secrete chemicals—the hormone—into the extracellular fluid. Hormones are transported primarily via the bloodstream throughout the body, where they bind to receptors on target cells, inducing a characteristic response. As a result, endocrine signaling requires more time than neural signaling to prompt a response in target cells, though the precise amount of time varies with different hormones. For example, the hormones released when you are confronted with a dangerous or frightening situation, called the fight-or-flight response, occur by the release of adrenal hormones—epinephrine and norepinephrine—within seconds. In contrast, it may take up to 48 hours for target cells to respond to certain reproductive hormones.

In addition, endocrine signaling is typically less specific than neural signaling. The same hormone may play a role in a variety of different physiological processes depending on the target cells involved. For example, the hormone oxytocin promotes uterine contractions in people in labor. It is also important in breastfeeding, and may be involved in the sexual response and in feelings of emotional attachment in humans.

In general, the nervous system involves quick responses to rapid changes in the external environment, and the endocrine system is usually slower acting—taking care of the internal environment of the body, maintaining homeostasis, and controlling reproduction (Table \(\PageIndex{1}\)). So how does the fight-or-flight response that was mentioned earlier happen so quickly if hormones are usually slower acting? It is because the two systems are connected. It is the fast action of the nervous system in response to the danger in the environment that stimulates the adrenal glands to secrete their hormones. As a result, the nervous system can cause rapid endocrine responses to keep up with sudden changes in both the external and internal environments when necessary.

Endocrine and Nervous Systems

	Endocrine system	Nervous system
Signaling mechanism(s)	Chemical	Chemical/electrical
Primary chemical signal	Hormones	Neurotransmitters
Distance traveled	Long or short	Always short
Response time	Fast or slow	Always fast
Environment targeted	Internal	Internal and external

Table \(\PageIndex{1}\)

Glands of the Endocrine System

Figure \(\PageIndex{2}\): Pineal gland, hypothalamus, Pituitary gland, Thyroid, Thymus, Pancreas, Adrenal gland, over and testes are part of the endocrine system. Note that the ovary and testis are the only endocrine glands that differ in males and females

The major glands of the endocrine system are shown in Figure \(\PageIndex{2}\). The glands in the figure are described briefly in the rest of this section. Refer to the figure as you read about the glands in the following text.

Pituitary Gland

The pituitary gland is located at the base of the brain. It is controlled by the nervous system via the brain structure called the hypothalamus, to which it is connected by a thin stalk. The pituitary gland consists of two lobes, called the anterior (front) lobe and posterior (back) lobe. The posterior lobe stores and secretes hormones synthesized by the hypothalamus. The anterior lobe synthesizes and secretes its own endocrine hormones, also under the influence of the hypothalamus. One endocrine hormone secreted by the pituitary gland is growth hormone, which stimulates cells throughout the body to synthesize proteins and divide. Most of the other endocrine hormones secreted by the pituitary gland control other endocrine glands. Generally, these hormones direct the other glands to secrete either more or less of their hormones. This is why the pituitary gland is often referred to as the “master gland” of the endocrine system.

Remaining Glands of the Endocrine System

Each of the other glands of the endocrine system is summarized below. Several of these endocrine glands are also discussed in greater detail in other concepts in the present chapter.

• The thyroid gland is a large gland in the neck. Thyroid hormones such as thyroxine increase the rate of metabolism in cells throughout the body. They control how quickly cells use energy and make proteins.
• The four parathyroid glands are located in the neck behind the thyroid gland. The parathyroid hormone helps keep the level of calcium in the blood within a narrow range. It stimulates bone cells to dissolve calcium and release it into the blood.
• The pineal gland is a tiny gland located near the center of the brain. It secretes the hormone melatonin, which controls the sleep-wake cycle and several other processes. The production of melatonin is stimulated by darkness and inhibited by light. Cells in the retina of the eye detect light and send signals to a structure in the brain named the suprachiasmatic nucleus (SCN). Nerve fibers carry the signals from the SCN to the pineal gland via the autonomic nervous system.
• The pancreas is located near the stomach. Its endocrine hormones include insulin and glucagon, which work together to control the level of glucose in the blood. The pancreas also has exocrine exocrine functions and secretes digestive enzymes into the small intestine.
• The two adrenal glands are located above the kidneys. Adrenal glands secrete several different endocrine hormones, including the hormone adrenaline, which is involved in the fight-or-flight response. Other endocrine hormones secreted by the adrenal glands have a variety of functions. For example, the hormone aldosterone helps to regulate the balance of minerals in the body. The hormone cortisol, which causes Cushing's syndrome when it is produced in excess, is also an adrenal gland hormone.
• The gonads include the ovaries in females and testes in males. They secrete sex hormones, such as testosterone (in people with testes) and estrogen (in people with ovaries). These hormones control sexual maturation during puberty and the production of gametes (sperm or egg cells) by the gonads after sexual maturation.
• The thymus gland is located in front of the heart. It is the site where immune system cells called T cells mature. T cells are critical to the adaptive immune system, in which the body adapts to specific pathogens.

Endocrine System Disorders

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

Hypersecretion

Figure \(\PageIndex{3}\): Martin Van Buren Bates

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 and goes untreated, it results in very long arms and legs and abnormally tall stature by adulthood. This condition is commonly known as gigantism. Martin Van Buren Bates is depicted in Figure \(\PageIndex{3}\) standing next to a man of average size. Bates was a Civil War-era American famed for his incredibly large size. He was at least 7 feet 9 inches tall and weighed close to 500 pounds. He was normal in size at birth but started to grow very rapidly by about age 6 years, presumably because of the hypersecretion of growth hormone.

Hyposecretion

Hyposecretion by an endocrine gland is often caused by the destruction of the hormone-secreting cells of the gland. As a result, not enough of the hormone is secreted. An example of this is type 1 diabetes, in which the body’s own immune system attacks and destroys cells of the pancreas that secrete insulin. This type of diabetes is generally treated with frequent injections of insulin.

Hormone Insensitivity

In some cases, an endocrine gland secretes a normal amount of hormone, but target cells do not respond normally to it. This may occur because target cells have become resistant to the hormone. An example of this type of endocrine disorder is Androgen Insensitivity Disorder. Generally, the typical sex chromosomes of males are one X and one Y chromosome whereas females typically have two x chromosomes and no Y chromosome. Individuals with Androgen Insensitivity Disorder disorder are born with an X and Y chromosome (typical of males) but they develop and are raised as females. This is due to a mutation in the Androgen Receptor (AR) gene which is located on the X chromosome. Testosterone is an androgen hormone that causes testes to descend and typical male characteristics to develop. People with this form of the condition have the external sex characteristics of females but do not have a uterus and therefore do not menstruate and are unable to conceive a child (infertile). They are typically raised as females and have a female gender identity. Affected individuals have male internal sex organs (testes) that are undescended, which means they are located in the pelvis or abdomen. Many people with this condition are not aware that they have it until later in life, if ever.

Review

1. What is the endocrine system? What is its general function?
2. Compare and contrast endocrine and exocrine glands.
3. How do endocrine system messages differ from those of the nervous system?
4. Describe the role of the pituitary gland in the endocrine system.
5. List three endocrine glands other than the pituitary gland, and identify their functions.
6. Which endocrine gland has an important function in the immune system? What is that function?
7. Define hypersecretion and hyposecretion.
8. Name an endocrine disorder in which too much of a hormone is produced.
9. What are two reasons people with diabetes might have signs and symptoms of inadequate insulin?
10. Choose one. Cushing’s syndrome is an example of (hyposecretion/hypersecretion).
11. True or False. The hypothalamus is the master gland of the endocrine system.
12. True or False. Mammary glands that produce milk for offspring are part of the endocrine system.
13. Melatonin is produced by the:
    1. A. Pituitary gland
    2. B. Hypothalamus
    3. C. Pineal gland
    4. D. Pancreas
14. Besides location, what is the main difference between the anterior lobe of the pituitary and the posterior lobe of the pituitary?
15. Which endocrine glands differ between males and females? Which hormones do they produce?

Explore More

https://bio.libretexts.org/link?16789#Explore_More

Most people want to live a long, healthy life. Geneticist Cynthia Kenyon’s research suggests that endocrine hormones may be a key to human longevity. Watch this fascinating TED talk to learn how.

Emily Quinn is an artist and activist. In this video, she talks about the hardship that she experienced while growing up as an individual with Androgen Insensitivity Syndrome.

Attributions

1. Cushing's face by Ozlem Celik, Mutlu Niyazoglu, Hikmet Soylu and Pinar Kadioglu CC BY 2.5 via Wikimedia Commons
2. Endocrine glands by Mariana Ruiz Villarreal CC BY-NC 3.0 via CK-12 Foundation
3. Martin Van Buren Bates by Magnus Manske; public domain via Wikimedia Commons
4. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0