Video Transcript
In this video, we will learn how to
describe the structure of the adrenal glands that are part of the human endocrine
system in each of our bodies. We will discover that the adrenal
glands are made of two main parts: the adrenal cortex, which releases hormones that
are essential to our survival, and the adrenal medulla, which releases other
hormones with important, albeit less vital, functions to our bodies. Furthermore, we will investigate
some examples of the main hormones that each of these sections of the adrenal gland
releases and their function.
Have you ever wondered why
performing on a stage in front of lots of people can be so frightening, or why some
people might be terrified at the sight of a snake or spider? These responses and many, many
others in our bodies are partly the result of hormones, specifically those released
by two little organs called the adrenal glands. Remember, hormones are chemical
messengers that travel throughout our bodies, usually via our bloodstream. Hormones are released by groups of
specialized cells which make up the human endocrine system, called endocrine
glands. The endocrine glands release
specific hormones into the bloodstream. These hormones are then transmitted
via the blood throughout the entire body.
When hormones reach specific cells
called their target cells, the hormones can bind to receptors either on the surface
of the target cell or within the cell itself, depending on the type of hormone. By activating these receptors,
hormones can trigger a specific effect to occur in their target cells. The adrenal glands, sometimes
called the suprarenal glands, make up part of the human endocrine system. The prefix ad- is a Latin word for
near to, and the prefix supra- is a Latin word for above. The suffix -renal in both words is
a Latin word referring to the kidney. This is because, as you can see in
the diagram, the adrenal glands sit just above each of our kidneys. As humans tend to have two kidneys,
we also tend to have two adrenal glands.
Let’s take a closer look at the
internal structure of the adrenal gland. The diagram on the right shows a
simple cross section through one of the adrenal glands. As you can see, each of the adrenal
glands is surrounded by a capsule, which is made of connective tissue forming a
protective layer around the gland. Moving inwards from the capsule is
a layer called the adrenal cortex. The adrenal cortex is responsible
for releasing steroid hormones that are essential to survival, such as those which
control our metabolism. The innermost region of the adrenal
gland is called the adrenal medulla. Though the hormones released by the
adrenal medulla are less vital than those released by the cortex, they are still
very important as they are responsible for functions such as helping our body react
appropriately to stressful situations.
As the adrenal medulla and adrenal
cortex are responsible for releasing very different types of hormones, let’s look at
some examples of the hormones released from each region in some more detail,
starting with the adrenal cortex. The adrenal cortex releases three
main groups of steroid hormones that are synthesized from lipids and trigger a
response by binding to receptors inside a target cell. These groups include
mineralocorticoids, glucocorticoids, and small quantities of sex hormones. Let’s see how these hormones differ
in their functions. We are going to look at an example
of a hormone that belongs to each group, their target tissues, and some of the
effects that they might have on the body.
Let’s look at mineralocorticoids
first. The name mineralocorticoids derives
from the fact that this group of hormones is involved in the retention of sodium
minerals or salts in the blood. “Corti” refers to the region of the
adrenal gland from which these hormones are released, the cortex. A key hormone in this group is
aldosterone, which helps to retain this sodium and to maintain a fine balance of
salt and water in the blood.
The release of aldosterone is
mediated by signals from the kidney. Once aldosterone has been secreted
from the adrenal cortex, it doesn’t have to travel very far to find its target
cells. They’re right below the adrenal
gland in the kidney. Aldosterone stimulates the kidneys
to absorb more sodium and water back into the blood. It also stimulates the kidneys to
excrete any excess potassium, both of which help regulate the blood pressure and the
acid–base balance in the blood.
Let’s look at glucocorticoids
next. You might notice that the name for
this group of steroid hormones contains the word gluco-. This alludes to one of their many
functions, that is, their involvement in the metabolism of carbohydrates like
glucose. Remember, “corti” refers to the
adrenal cortex from which glucocorticoids are also released. The secretion of glucocorticoids is
triggered by signals from the hypothalamus and pituitary gland in the brain. An example of a glucocorticoid is
cortisol, which helps regulate metabolism, blood pressure, and the stress and immune
responses, among many other functions. Cortisol has various roles and
various target tissues, including the liver, skeletal muscle cells, and fat
cells. Keep in mind that these drawings
are not to scale.
This red structure represents a
blood vessel, and the green hexagon represents a molecule of glucose, which is
circulating in the blood. Usually, a lot of this glucose will
be taken up by muscle cells to release energy through cellular respiration. In the presence of cortisol,
however, the muscle cells actually take up less glucose. And fat and muscle cells increase
the breakdown of fats and proteins, releasing them into the blood. The liver uses these broken-down
fats and proteins to produce more glucose, which is then released into the
bloodstream to increase the blood glucose concentration. This helps the body expend energy
wisely in response to stress, which is why cortisol is sometimes called the stress
hormone. Its release is usually triggered by
stimulation of the adrenal cortex by the sympathetic nervous system.
Let’s look at our final key group
of hormones released from the adrenal cortex, sex hormones. Though most sex hormones in the
human body are released from the ovaries in females and testes in males, small
quantities are also secreted by the adrenal glands when it’s triggered by signals
from the pituitary gland in the brain. Sex hormones are converted into
hormones called androgens. An example of an androgen that is
also produced in the male testes is testosterone. Androgens are also converted into
hormones such as estrogen in a female’s ovaries. Both of these hormones have various
target tissues in both males and females, including the reproductive organs in both
sexes, which you can see pictured here.
Androgens can also cause many
different effects, which include the development of these sexual organs in both
males and females and the development of secondary sexual characteristics when a
person reaches puberty in both males and females. For example, testosterone can lead
to increased growth and thickness of a male’s facial hair when he begins
puberty. It’s important to remember that
typically male and typically female sex hormones are actually released in both
sexes.
Let’s look at the hormones released
from the adrenal medulla, the innermost region of the adrenal glands, next. The adrenal medulla releases two
main hormones: adrenaline, sometimes called epinephrine, and noradrenaline,
sometimes called norepinephrine. Both of these hormones are amine
hormones. These differ from steroid hormones
released from the adrenal cortex mainly as amine hormones, which are synthesized
from amino acids, bind to receptors on the surface of target cells rather than
entering the cells first. These two hormones are released
when the adrenal medulla is stimulated by the sympathetic nervous system. The sympathetic nervous system
helps the body to prepare for action and respond to stressful situations through the
fight-or-flight response.
Humans experience the
fight-or-flight response in scary situations, such as standing on top of a very tall
building or more rarely escaping from a predator like this shark. Let’s see how adrenaline and
noradrenaline work together to allow your body to respond to these physically and
emotionally stressful situations. One of the target tissues of
adrenaline and noradrenaline is heart tissue. Adrenaline increases the heart rate
and the force of the heart’s contractions. The faster and stronger that your
heart is beating, the faster your blood is being oxygenated. Adrenaline and noradrenaline also
target liver cells, stimulating the breakdown of stored glycogen into glucose
molecules.
By increasing the blood oxygen and
blood glucose, the rate of cellular respiration in our cells can increase. These two hormones can also cause
changes to blood vessels to increase the blood flow to specific tissues. For example, our skeletal muscle
cells will require a larger blood flow as they require more energy to be released
via cellular respiration in fight-or-flight situations such as to swim away from
that shark. These are just some of the many
target cells and functions of the adrenal medulla hormones, which can also target
the circular muscles in the iris of the eye causing the pupil to dilate to lessen
more light, the air passages to the lungs allowing more oxygen to enter the
bloodstream, and many others.
The fight-or-flight response can
even cause psychological responses in the brain like increasing anxiety, the fear
you might feel in these sort of situations. The effect of adrenaline and
noradrenaline on the brain aims to warn you that a thing you’re planning on doing
could be dangerous and even potentially lethal. Though anxiety is usually a
negative feeling, as it is often highly energy consuming without being beneficial,
experiencing anxiety at the appropriate times can be useful to our survival.
Let’s see how much we’ve learned
about the adrenal glands and the hormones they release by having a go at a practice
question.
What is the primary function of the
adrenaline, epinephrine, and noradrenaline, norepinephrine, hormones? To balance the mineral and water
contents of the body. To regulate the metabolism of
carbohydrates in the body to maintain normal concentrations of blood glucose. To regulate the development of
sexual organs and characteristics. Or to respond to signals from the
sympathetic nervous system and induce the fight-or-flight response.
The question is asking us about the
function of adrenaline and of noradrenaline, two hormones that are released from the
adrenal glands. So that we can work out which
option is correct, let’s have a look at the main hormones released from the
different regions of the adrenal glands and their functions. The adrenal glands are small
hormone-releasing glands that sit above each of our kidneys. This image shows us a cross section
taking through one of the adrenal glands, showing us that it consists of two main
distinct regions, the adrenal cortex and the adrenal medulla.
These two regions are responsible
for releasing different groups of hormones with different structures and
functions. The adrenal cortex releases three
main groups of hormones. One of these groups are called
mineralocorticoids. The mineralocorticoids are
responsible for balancing the mineral or salt and water levels in the blood by
stimulating the selective reabsorption of sodium and water in the kidneys. Another group of hormones released
by the adrenal cortex are glucocorticoids. Glucocorticoids are partly
responsible for regulating carbohydrate metabolism in the body. For example, glucocorticoids help
to increase the production of glucose from fats and proteins when the human body is
under stress and requires more energy.
Androgens, sometimes known as sex
hormones, are mainly secreted from the ovaries in females and testes in males but
are also released in small quantities from the adrenal cortex. Sex hormones are responsible for
regulating the development of sexual organs. They also control how secondary
sexual characteristics develop when a person reaches puberty. For example, males often start to
grow facial hair during puberty. This is an example of a secondary
sexual characteristic.
The main hormones secreted by the
adrenal medulla are adrenaline, sometimes called epinephrine, and noradrenaline,
sometimes called norepinephrine. These hormones are released when
the adrenal medulla are stimulated by the sympathetic nervous system, which is
responsible for initiating the fight-or-flight response during dangerous or
stressful situations. For example, adrenaline and
noradrenaline can increase the rate and force of your heartbeat when you’re about to
run a race. These hormones can also increase
the rate at which your liver converts glycogen into glucose, increasing your blood
glucose concentration. This can also increase the rate at
which your muscle cells use glucose and cellular respiration to release the energy
required to escape or fight through these stressful situations.
Let’s review our answer options to
see which is correct. We know that the mineral and water
balance in the blood is maintained by mineralocorticoids. Therefore, this is not a function
of adrenaline and noradrenaline. We also know that carbohydrate
metabolism is a function of glucocorticoids, so this cannot be our answer
either. The development of sexual organs
and characteristics is controlled by androgens and the sex hormones that they are
converted into. So this is also not a function of
adrenaline and noradrenaline. Therefore, we can conclude that the
main function of adrenaline and noradrenaline is to respond to signals from the
sympathetic nervous system and induce the fight-or-flight response.
Let’s have a look at some of the
key points that we’ve addressed in this video. The adrenal glands are small glands
that are located on top of each of our kidneys. Each adrenal gland consists of an
inner medulla and an outer cortex surrounded by a protective capsule. The adrenal cortex releases
mineralocorticoids, which help to balance the mineral and water content of the
blood; glucocorticoids, which regulate the metabolism of carbohydrates in the body;
and small quantities of androgens, which regulate the development of sexual organs
and secondary sexual characteristics in both males and females. The adrenal medulla releases
adrenaline and noradrenaline to help the body respond to stressful situations by
initiating the fight-or-flight response. These responses include increasing
heart rate and breathing rate to increase the rate of cellular respiration.
