Video Transcript
Which of the following is true
about label Z? (A) It is a gland that secretes its
hormones through ducts. (B) It is a gland that consists of
a ductal glandular part and a ductless glandular part. (C) It is a gland called the master
gland in the human body. Or (D) it is a gland that secretes
its hormones directly into the bloodstream.
This question asks us to identify
the type of gland indicated by label Z in our diagram, which shows many organs of
the endocrine system. Let’s look at some of the main
points we need to know about the endocrine system so we can then answer the question
correctly.
The endocrine system is a series of
glands which produce and secrete hormones that the body uses for a wide range of
functions. The endocrine system works closely
with the nervous system to coordinate the body, enabling it to function
properly. They both respond to stimuli, which
are changes in the environment, to bring about a response. The nervous system brings about a
rapid short-lived response, while the endocrine system is slower to respond, but its
response lasts longer.
The hormones released by the
endocrine system are chemical messengers that travel throughout an organism’s body,
usually in the blood or another transport medium, to their target cells where they
bring about a response. The glands of the endocrine gland
are ductless glands as they secrete the hormones directly into the blood. Some glands do have ducts. These are the exocrine glands, and
they secrete substances directly to a body surface, such as the salivary glands
releasing saliva into the mouth or sweat glands secreting sweat directly onto the
surface of the skin.
Now that we understand the
difference between exocrine and endocrine glands, let’s review some of the organs in
the endocrine system to narrow down our options and answer our question
correctly.
Near the top of the diagram is the
pituitary gland. This is controlled by a part of the
brain called the hypothalamus, indicated by label X in our diagram. It sits just above the brainstem,
and its main role is to maintain homeostasis, a constant normal internal
environment. The hypothalamus secretes releasing
and inhibiting hormones, which stimulate or inhibit the release of hormones from the
pituitary gland. It therefore links the nervous and
endocrine systems.
The pituitary gland secretes seven
different hormones, which control the release of many other hormones from a variety
of endocrine glands, including the thyroid, ovaries, adrenal cortex, and testes. It is therefore often called the
master gland. We can therefore cross out option
(C) as it is the pituitary gland, not gland Z that is a gland called the master
gland in the human body.
Label Y indicates the pancreas. The pancreas is a special gland as
it has both exocrine and endocrine secreting cells. The exocrine cells release
digestive enzymes down the pancreatic duct into the duodenum. It also acts as an endocrine gland,
secreting hormones directly into the bloodstream. The hormones are released from an
area of the pancreas called the islets of Langerhans, and they are very important
for the regulation of blood glucose levels. Insulin is released when blood
glucose levels rise above the norm, and it enables liver and muscle cells to take up
glucose and store it as glycogen. If blood glucose levels fall too
much, glucagon is released. And this causes the liver to
release glucose into the blood to bring the levels back up.
Another example of an organ which
has both exocrine and endocrine roles is the liver. Most other endocrine glands
including gland Z only have hormonal roles, so only release their secretions into
the bloodstream. Therefore, we can also rule out
option (B).
So what is gland Z? It is a butterfly-shaped endocrine
gland in the neck and is the thyroid gland. This gland is made up of two types
of secretory cells: the follicular cells and parafollicular cells, shown in this
diagram. The follicular cells secrete the
thyroid hormones thyroxine and triiodothyronine, which regulate the basal metabolic
rate of the body. The parafollicular cells secrete
calcitonin, which works with the parathyroid gland to control the calcium levels of
the blood. If you look at the diagram, you can
see blood capillaries between the cells because all three of these hormones are
secreted directly into the blood.
After reviewing some of the glands
involved in our diagram, we can return to our question. We now know that the correct answer
is (D). The gland labeled Z is a gland that
secretes its hormones directly into the bloodstream.
