Homeostasis, Hormones and Feedback Control

Homeostasis, Hormones and Feedback Control

February 24, 2020 3 By Jose Scott


Hi! This video will introduce you to hormones. We all have heard the term “hormone”,
but what are hormones, exactly? And are all chemical signals hormones? We will also see that different types of hormones
have different chemical properties and that these chemical properties determine
how they interact with their target cells. Finally, we will end with two examples of
how hormones help animals maintain a steady state, or homeostasis. Let’s start by making sure we all understand
what we mean by homeostasis. The word homeostasis is made up of two roots
– “homeo” meaning the same,
and “stasis”, meaning stable. And that is just what homeostasis refers to
– animals regulate their internal environment, such as body temperature, even when they find themselves
in changing external conditions. This allows your cellular machinery
to operate optimally despite changes in the environment around
you. When the temperature outside
becomes very hot or very cold, your body temperature may begin to equilibrate. But receptors in your body sense
the change in temperature and send information about
your core body temperature to the control center that decides
whether or not the controlled condition is within the correct range. If not, the control center sends a message
out to effectors that then correct the condition. So, for instance, if you get too cold,
you start to shiver to warm yourself up. Or if you get too hot,
you start to sweat to cool yourself off. There are two major systems that coordinate to control an organism’s
physiology and behavior to maintain or return the body
to these internal set points. The nervous system typically is involved
in high-speed responses – like pulling your hand away from a hot stove. The endocrine system typically is slower acting, but stimulates longer acting responses. We are not going to consider the nervous
system in any detail in this video. But I am going to introduce you to cells
that provide a link between the nervous system and the endocrine system that allow
organisms to respond to external conditions. So what are hormones, anyway? Hormones are chemicals that move around
the body through the bloodstream. Hormones are in contact
with all cells in the body, but only certain cells recognize
the message that the hormone is sending. These cells are called target cells. Cells that are not targets can
ignore the hormone message because they don’t have the machinery
necessary to respond to the message. That machinery is a receptor that binds
the hormone and causes the cell to respond. Not all chemical messengers
are classified as hormones. Again,
hormones are defined as chemical signals that travel through the bloodstream
from the cells that produce them, the cells of an endocrine organ, to the target cells that have a receptor
that binds to the hormone. So how does this work? Let’s start with the endocrine cell
– the green cell. In this example there is a hormone
stored in secretory vesicles. Some signal stimulates the endocrine cell
to release the hormone, which then travels through the bloodstream
and eventually encounters a target cell that binds the hormone
and responds to the message. Some chemical signals travel
only short distances between the signaling cell
and the responding cells. This type of signaling is called paracrine
signaling. The process is similar to endocrine signaling, but the signaling molecule
only travels between neighboring cells. The third type of chemical signaling
is autocrine signaling. In this case,
the cell releasing the signaling molecule is also the cell responding the message. Weird, huh? Paracrine and autocrine signaling
are cell signaling systems you are likely to learn much more about
if you go on to take cell biology. But we aren’t going to consider
them any further in this case study. There is one more type of signaling
that I need to introduce you to. This type of signaling will become important
in our next video and provides an important link between
the nervous system and the endocrine system, as you might guess from the name
– neuroendocrine cell signaling. These cells are
“a little bit country, a little bit rock and roll.” Neuroendocrine cells look and
behave a bit like nerve cells, in that they generate an action potential. But at the synapse end,
instead of releasing a neurotransmitter they release a hormone into the bloodstream. You will see these cells again in the next
video. But right now, let’s talk a bit more about our
standard hormones and endocrine cells. Hormones come in two flavors. We classify hormones based on their solubility. Some hormones are soluble in water. These hormones are composed
of amino acids and small peptides. The other class of hormones are lipid soluble. These hormones are often synthesized from
cholesterol and can dissolve in the lipid bilayers. The different properties
of these two types of hormones mean that they have different ways
of moving around the body and interacting with the target cells. The first difference between water
and lipid soluble hormones is how they are synthesized and stored. Peptide hormones are synthesized and
stored in secretory vesicles within the cell until the endocrine cell is stimulated
to release them. But think about lipid soluble hormones for
a minute. They can’t be stored in secretory vesicles,
can they? They would just diffuse across the vesicle
membrane. So when an endocrine cell is stimulated
to release steroid hormones, they have to synthesize them and then
the hormone can diffuse out of the cell. How hormones move through the bloodstream
is another difference. Water soluble hormones are of course soluble
in the aqueous plasma of the blood. So they just move along with the blood. Lipid soluble hormones, though,
are also insoluble in the aqueous plasma. So they would just drop out of
solution in the bloodstream. This problem is solved by the presence of
transport proteins in the bloodstream. These transport proteins bind
a particular steroid hormone and keeps it soluble in the bloodstream. So the steroid hormones
move out of the endocrine cell, into the blood stream
where they bind a transport protein, and then away they go! The ability to cross the plasma membrane also affects how hormones
interact with their target cells. Remember, a target cell must
contain a receptor for that hormone. Water soluble hormones can’t pass freely
through the plasma membrane and so their receptors are located
at the cell surface. The receptor is a trans-membrane protein that has a binding site outside the cell,
where the hormone is, and an active site inside the cell. When the hormone binds to the receptor, a signal is transmitted to the
cytoplasm and stimulates a response. Without a receptor, a water soluble hormone can’t get in
to the cell and there is no response. The same principle applies
to lipid soluble hormones. But in this case, the hormone can diffuse
through the plasma membrane into the cytoplasm
without the need for a receptor. So the receptor is actually inside the cell
rather than at the plasma membrane. Again, the binding of the hormone to
the receptor stimulates a response. If the cell is not a target for a
particular lipid soluble hormone, the hormone can still diffuse into the cell,
but without a receptor there is no response. Finally, this difference in
where receptors are located means that the two different types of
hormones have different effects on the cell. Water soluble hormones bind to their
receptor on the outside of the cell and then binding triggers a response
inside the cell, usually through a series of biochemical events
called a signal transduction pathway. Lipid soluble hormone receptors
typically interact directly with the DNA. The receptor binds the hormone
and this triggers this receptor-hormone complex to enter into the nucleus and bind to a special control region in the DNA called a hormone response element. The binding of the hormone and its receptor
to the DNA results in a change in gene expression. Now we have some understanding
of how hormones work, let’s think about how they maintain homeostasis. We are going to think about two types of control. The first is a simple negative feedback control
pathway. The second type of control
relies on two different hormones that have opposite effects to keep
the controlled condition at a steady state. As an example of how a simple,
negative feedback hormone loop can be used to maintain homeostasis, let’s consider the controlled condition
of pH in the duodenum, the first part of the small intestine
where it connects to the stomach. The set point is for a neutral pH,
around seven. But imagine when partially digested food come
in from the acidic stomach, causing the pH to
drop. The S cells detect this disturbance
to the controlled condition and effect a response by releasing the hormone
secretin from the stored secretory vesicles. The secretin travels to its target cells. Once receptors on target cells
bind the hormone, they respond by releasing bicarbonate ion
which raises the pH and the controlled condition is
returned to the set point. Some controlled conditions
require two hormones that have opposing responses to keep the
condition balanced and maintain homeostasis. An example of such an antagonistic hormone
pair is used to maintain blood calcium levels. Let’s imagine you have just
had a big slice of cheese. As you body digests the cheese,
your blood calcium levels will begin to rise. Your thyroid gland is the sensor and
integration center for rising blood calcium, and it responds by releasing the hormone
calcitonin into the bloodstream. There are several target cells for calcitonin; cells in bone respond by
increasing calcium deposition, while cells in the kidney decrease how much
calcium is reabsorbed from the filtrate. Together these responses bring
your blood calcium levels down. Now imagine it’s been a while
since your last meal. Your blood calcium levels will begin to drop. The sensor and integration center for
dropping blood calcium is the parathyroid gland, which responds by releasing parathyroid hormone. Target cells in the bone, kidney and intestine respond
to parathyroid hormone. Bone cells release stored calcium, kidneys increase reabsorption of
calcium from blood filtrate, and the intestine increases
calcium uptake from food. Together these responses restore blood calcium
to the set point. Okay, so now we’ve seen how hormones act, and how they are regulated. And now we’re going to look at how they work to
control physiological systems, and keep them at homeostasis.