Lesson Explainer: The Adrenal Glands Biology

In this explainer, we will learn how to describe the structure of the adrenal glands in the human body and recall the function of hormones released from the medulla and cortex.

Have you ever wondered why public speaking can make people so nervous or why seeing a spider can make someone scream and run away in fear? These responses are in part caused by hormones released from the adrenal glands, which are sometimes called the suprarenal glands, and are designed to help us survive in potentially dangerous situations. The adrenal glands can also release numerous other hormones that regulate our metabolism of food and the water balance in our cells and even contribute to the development of our sexual characteristics!

The word renal is used to refer to anything concerning the kidneys. The “ad” in adrenal glands is a Latin word meaning “near to,” and the “renal” is the Latin word for “kidney.” This is because the adrenal glands are related to and sit above each of our kidneys, as you can see in Figure 1.

Key Term: Adrenal Glands (Suprarenal Glands)

The adrenal glands are two small glands located above the kidneys responsible for releasing various hormones.

The adrenal glands are endocrine glands, as they release hormones into the bloodstream. A hormone is a chemical messenger that travels through the blood to cause an effect in certain target cells, leading to a response. The adrenal glands release many different hormones, with many different functions.

Key Term: Endocrine Gland

An endocrine gland is a group of specialized cells that secrete hormones into the blood.

Key Term: Hormone

Hormones are chemical messengers that travel throughout an organism’s body, usually in the blood or another transport medium.

Before we look at the functions of the hormones released by the adrenal glands, let’s first take a look at the structure of the adrenal glands themselves.

As you can see in Figure 2, each adrenal gland is surrounded by a capsule made of connective tissue that forms a protective layer around the gland. Moving inward from this outer layer is the adrenal cortex. The inner region of the adrenal gland is called the adrenal medulla.

The adrenal cortex and adrenal medulla release different hormones. The adrenal cortex is responsible for releasing steroid hormones that are essential for life processes, while the adrenal medulla releases less vital, but still incredibly useful, hormones, such as those that help our body react to stressful situations.

Key Term: Adrenal Cortex

The adrenal cortex is the outer region of the adrenal glands responsible for releasing vital hormones, such as those that control metabolism.

Key Term: Adrenal Medulla

The adrenal medulla is the central region of the adrenal glands responsible for releasing less essential hormones to survival compared to those released from the adrenal cortex, such as those that help the body respond to stress.

Example 1: Identifying the Regions of the Adrenal Glands

What are the two distinct regions of the adrenal glands?

  1. Matrix and cristae
  2. Medulla and cortex
  3. Capsule and body
  4. Large and small
  5. Frontal lobe and parietal lobe

Answer

The adrenal glands are endocrine glands, as they release hormones into the bloodstream. A hormone is a chemical messenger that travels through the blood to cause an effect in certain target cells that can lead to a cellular response. The adrenal glands release many different hormones with many different functions.

As you can see in the figure below, each adrenal gland is surrounded by a capsule that forms a protective layer around the gland. Moving inward from this outer layer is the cortex of the adrenal gland. The inner region of the adrenal gland is called the adrenal medulla.

Therefore, the two distinct regions of the adrenal glands are the medulla and cortex.

One group of steroid hormones released from the cortex are mineralocorticoids. The main mineralocorticoid is called aldosterone. Aldosterone plays an important role in maintaining blood pressure by balancing the salt and water concentration in blood. Aldosterone targets kidney tissues, instructing them to absorb more sodium and water into the blood, increasing their retention in the body. Aldosterone also promotes the excretion of excess potassium, which helps to regulate the blood pressure and acid–base balance.

Key Term: Mineralocorticoids

Mineralocorticoids are hormones released from the adrenal cortex, such as aldosterone, which helps to regulate salt and water balance in the blood.

Key Term: Aldosterone

Aldosterone is a hormone released from the adrenal cortex that helps maintain a constant blood pressure and acid–base balance by balancing the salt and water concentration in the blood.

Example 2: Identifying the Adrenal Cortex Hormone from Its Function

State the adrenal hormone being described: This is the main hormone released by the adrenal cortex to regulate the balance of salt and water in the body by promoting the retention of salt and water by the kidneys.

Answer

The adrenal cortex and adrenal medulla release different hormones. The adrenal cortex is responsible for releasing hormones that are essential for life processes, while the adrenal medulla releases less vital, but still incredibly useful, hormones, such as those that help our body react to stressful situations.

The adrenal cortex releases a mineralocorticoid hormone called aldosterone. Aldosterone plays a vital role in maintaining blood pressure by balancing the salt and water concentration in blood. Aldosterone’s target tissue is in the kidneys, instructing them to absorb more salt and water into the blood, which also helps to control the blood pressure and acid–base balance.

The hormone being described is, therefore, aldosterone.

The cortex also releases a group of steroid hormones called glucocorticoids.

Examples of glucocorticoid hormones are cortisol and corticosterone. Cortisol mainly functions to regulate metabolism in our cells and can also help to maintain blood glucose levels. This can be helpful, especially so our bodies respond appropriately to stressful situations by releasing extra energy. Cortisol also helps to regulate blood pressure and control our sleep cycle and can even help regulate our immune response!

The hypothalamus and pituitary gland in the brain monitor the levels of glucocorticoids in the blood and respond by stimulating either the increase or decrease of glucocorticoid levels, depending on the need. When the human body encounters stress, for example, the hypothalamus releases the corticotropin-releasing hormone (CRH). CRH travels the short distance to the pituitary gland and stimulates it to release adrenocorticotropin hormone (ACTH). ACTH then travels through the blood to the adrenal glands, stimulating them to produce and secrete cortisol into the blood.

Key Term: Glucocorticoids

Glucocorticoids are hormones released from the adrenal cortex, such as cortisol, which helps regulate metabolism, blood pressure, and the stress and immune responses.

Example 3: Describing the Primary Function of Glucocorticoid Hormones

Which of the following best describes the primary function of hormones belonging to the glucocorticoids group?

  1. To regulate the development of sexual organs and characteristics
  2. To increase the activity of the immune system and initiate an inflammatory response
  3. To respond to signals from the sympathetic nervous system and decrease the heart rate
  4. To regulate the metabolism of glucose in the body to maintain normal concentrations of blood glucose

Answer

The cortex also releases a group of hormones called glucocorticoids.

One glucocorticoid is a hormone called cortisol. Cortisol mainly functions to regulate metabolism in our cells and can also help to maintain blood glucose levels. This can be helpful, especially so our bodies respond appropriately to stressful situations by releasing extra energy. Cortisol also helps to regulate blood pressure and control our sleep cycle and can even help regulate our immune response!

The hypothalamus and pituitary gland in the brain monitor the levels of glucocorticoids in the blood and respond by stimulating either the increase or decrease of glucocorticoid levels, depending on the need.

The primary function of glucocorticoid hormones is, therefore, to regulate the metabolism of glucose in the body to maintain normal concentrations of blood glucose.

The adrenal cortex also releases small volumes of sex hormones, which are precursors to hormones known as androgens. The word androgen includes the stem “andro,” which means “man,” as the hormones released from the adrenal cortex are actually small quantities of male sex hormones. In spite of this, they are released in both males and females as they are useful for both sexes and will be converted into other sex hormones, such as the hormone estrogen in the female ovaries and testosterone in the male testes. The ovaries and testes release far larger quantities of these hormones, but the sex hormones released by the adrenal cortex are still vital, especially in women after menopause.

Key Term: Androgens

Androgens are sex hormones released from the testes and ovaries and in small volumes from the adrenal glands to control development of male sex organs, secondary sexual characteristics, and arousal in both males and females.

The functions of androgens are plentiful. Some androgens are converted into estrogen and other hormones. All these hormones control the development of sex organs and secondary sexual characteristics, such as body hair growth in males and arousal in both males and females.

Conditions such as tumors can result in an unbalanced secretion of sex hormones from the adrenal cortex. High concentrations of testosterone in females can lead to development of characteristics such as voice deepening, excess body hair, and muscle growth. High concentrations of estrogen in males can lead to an enlargement of breast tissue. A severely unbalanced secretion of either hormone may also lead to infertility in both sexes.

We have outlined hormones released by the adrenal cortex, but what about the other region of the adrenal gland, the adrenal medulla?

The adrenal medulla releases amine hormones when it is stimulated by the sympathetic nervous system. The sympathetic nervous system helps the body prepare for action and respond to stressful situations through the fight-or-flight response.

The fight-or-flight response aims to help us either escape a dangerous situation or battle our way out of it. These responses likely originated long ago in our human ancestors as a method of escaping from predators. They are still in play today, however, as you might experience fight-or-flight responses in scary situations, like performing on stage, standing at the top of a very high building, or even seeing a spider! The sympathetic nervous system enhances the body’s response to dangerous and stressful situations. It does this by sending nerve impulses to many different organs and tissues, some of which you can see in Figure 3.

Key Term: Sympathetic Nervous System

The sympathetic nervous system is the part of the autonomic nervous system responsible for the body’s fight-or-flight response when the body is preparing for action.

Key Term: Fight or Flight

Fight or flight is an automatic physiological response triggered by the sympathetic nervous system that occurs when the body is preparing to escape from or fight through a stressful situation.

The hormones released from the adrenal medulla which aid the fight-or-flight response are adrenaline, also called epinephrine, and noradrenaline, also called norepinephrine.

Key Term: Adrenaline (Epinephrine)

Adrenaline is a hormone and a neurotransmitter that is secreted from the adrenal glands and prepares the body for activity by increasing breathing rate, blood circulation, and metabolism of carbohydrates.

Key Term: Noradrenaline (Norepinephrine)

Noradrenaline is a hormone and a neurotransmitter that is secreted from the adrenal glands and primarily functions to increase heart rate and the force of skeletal muscle contraction.

Let’s see how adrenaline and noradrenaline work together to allow your body to respond to physically and emotionally stressful situations.

One of the main functions of adrenaline is to increase heart rate and the force of the heart’s contractions. The faster and stronger your heart is beating and the faster your blood is being oxygenated, the higher the rate of respiration in cells. This means that your cells, especially your muscle cells, can release more energy to carry out emergency action.

Glycogen is a poorly soluble carbohydrate polymer; sugars can be stored as glycogen in animal cells. Adrenaline causes more glycogen storage molecules to be broken down into glucose in the liver and in muscles. This increases the concentration of glucose in blood so more glucose can be supplied to cells for cellular respiration.

Adrenaline and noradrenaline tend to have similar roles. The hormones can increase the blood flow to these muscles to ensure glucose and oxygen can get there faster. They can cause the opposite effect on blood vessels in nonessential organs to the fight-or-flight response. For example, they can reduce the blood flow to the stomach and intestines so it can be diverted more immediately to essential organs, slowing digestion.

Another target tissue of adrenaline and noradrenaline are the passages to the lungs, bronchi, and bronchioles. When adrenaline binds to these target cells, the smooth muscle in their walls relaxes. This allows ventilation of the lungs to increase so more oxygen is supplied to the blood vessels surrounding them.

Adrenaline and noradrenaline also bind to target cells in the circular muscles in the iris, causing them to relax. Relaxation of iris muscles dilates the pupils so more light can enter the eyes. This improves vision and allows the person to respond better to sudden changes in the environment.

Example 4: Describing the Primary Function of Adrenaline and Noradrenaline

What is the primary function of the adrenaline (epinephrine) and noradrenaline (norepinephrine) hormones?

  1. To respond to signals from the sympathetic nervous system and induce the fight-or-flight response
  2. To balance the mineral and water contents of the body
  3. To regulate the development of sexual organs and characteristics
  4. To regulate the metabolism of carbohydrates in the body to maintain normal concentrations of blood glucose

Answer

The adrenal medulla releases hormones when it is stimulated by the sympathetic nervous system. The sympathetic nervous system helps the body prepare for action and respond to stressful situations through the fight-or-flight response. The fight-or-flight response aims to help us either escape a dangerous situation or battle our way out of it.

Let’s summarize some useful responses that aid the fight-or-flight response brought about by these hormones. They work together to increase heart rate, increase the volume of air we can breathe in, and break down the storage molecule glycogen into glucose. All these responses increase cellular respiration, so we have more energy to run away. They also increase blood flow to skeletal muscles and decrease it to digestive organs. This is because if we need to run away from a predator, we need our legs muscles to have more energy than our intestines!

Mineralocorticoids released from the adrenal cortex balance water and mineral content in the bodily fluids. Androgens released from the adrenal cortex, ovaries, and testes cause development of sex organs and characteristics. Glucocorticoids released from the adrenal cortex regulate the metabolism of carbohydrates in the body to maintain normal concentrations of blood glucose.

Therefore, the primary function of adrenaline and noradrenaline is to respond to signals from the sympathetic nervous system and induce the fight-or-flight response.

You may have heard of an “adrenaline rush,” which refers to people aiming to encourage these stress responses of the human body to provide a sudden release of energy. By doing activities like extreme sports, or even riding roller coasters, the fight-or-flight response is activated in the body, which causes the release of adrenaline and noradrenaline and achieving this “rush.”

This results in the physical bodily responses we have already discussed, like increased heart rate and energy release, and even some psychological responses, like increasing anxiety. This anxiety is the fear you may feel when you are about to run a race, speak to a crowd of hundreds of people, or even jump off a high diving board into a swimming pool. The effect of adrenaline and noradrenaline on the brain aims to warn you that the thing you are planning on doing could be dangerous and even potentially lethal. So, though being afraid constantly is wasteful, anxiety in small doses and at appropriate times can be hugely beneficial to our survival!

Let’s recap some of the key points we have covered in this explainer.

Key Points

  • The adrenal glands are small glands located on top of each kidney.
  • Each adrenal gland consists of an inner medulla and an outer cortex surrounded by a protective capsule.
  • The adrenal cortex releases glucocorticoid, mineralocorticoid, and small volumes of androgen hormones.
  • The hormones released by the cortex function to regulate blood pressure and metabolism and influence sexual characteristics.
  • The adrenal medulla releases adrenaline and noradrenaline to help the body respond to stressful situations in the fight-or-flight response.
  • Responses in the body caused by the release of adrenaline and noradrenaline include increased heart rate and breathing rate to increase the rate of cellular respiration.

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