This is the process now that we need to discuss in terms of a central principle. The central
principle of physiology is basically homeostasis but we all need to be on the same page of what
homeostasis actually means. It is a regulated process by which a biological system maintains
a dynamic but relatively consistent internal condition during various stresses or pressures
incurred both from external and internal factors. That's a lot to think about so let's kind of deal
through each of those processes a little bit more so we will understand what this definition is.
First thing to think about is what variables of the body are important enough for you to
actually regulate them. If you think about this for a couple of seconds, one of them that comes
to my mind is glucose. So you need to regulate the amount of glucose in the blood so that an
optimal amount is delivered to each of the cells in the body. The same goes with oxygen.
Oxygen needs to be delivered to each cell in the body, it needs to be precisely regulated.
How do you make sure you get enough oxygen and glucose to the various spots in the body?
You need to have enough blood pressure or pressure to push the blood to those various spots.
So those are 3 really good examples. Other ones that you might not have thought of yet
include things like the regulation of body temperature, the regulation of pH balance are also
integral variables that we need to regulate. Now, the other thing about the definition that I
think we should discuss a little bit more that seems kinds of contradictory is that it's both a dynamic
and consistent and what we mean by that. It's not going to be one particular number but rather
a range of numbers that's going to be important in medicine. For example, a glucose level is not
going to be one number but rather a range. Blood pressure is not just one number that we are
after but rather a range of blood pressures and that is what is the dynamic component,
although we have to consistently keep it within a narrow range. What are some examples of
internal factors that can change homeostasis? This mainly involves changes in metabolism. So if
you undergo something like exercise you have an increase in metabolism while if you are
sleeping you have a decrease in metabolism. That's an internal factor. External factors are
almost too great to even name, these are anything that will be from the external environment
impacting the body. That can be something like heat generated from outside, from the sun or
from the environment and how that impacts our physiology. It could be cold, how cold impacts
our physiology or maybe in a stressful condition in which you are scared or you enacted the
fight or flight response. That's another example, that external factor that we need to now deal
with but maintain over regulated variables in a dynamic but consistent range. The other thing about
homeostasis that we need to have a firm grasp on is at what level are we talking about and this level
is a level of organization. So there are atoms and these form molecules which then form larger
molecules and then finally form various cellular components like mitochondria in this case,
it could be other local types of cellular organelles as well. Then finally we have the cell.
The cell is probably the first level of organization that we really need to maintain an
environment in and this is the homeostatic environment of whatever is going to be within
the cell. This is regulating what comes into the cell, it regulates what that cellular cytosol is
going to be composed of. Now each individual cell though is combined together to form a tissue.
At the level of tissue, we also have a homeostatic regulation, so a particular tissue will regulate
some of these parameters. Organ systems will also regulate a homeostatic norm. Organ
systems together regulate the body's various homeostatic mechanism such as the blood and
then finally we have the whole organism that needs to be regulated. So from the cell, tissue,
organ, organ system level and finally the organism as a whole, all of these need to have
homeostasis both at the individual level and the corporate level. So how is that done? Well one
thing to think about with regulation of homeostasis is that it's all based upon our genetics. So
you have DNA within each individual cell and so in fact a cell could become any tissue in the body.
It just so happens it is the cell and tissue that it is at the current time. So when we think about
tissues, organ, organ system and organism, we have to always realize that each individual cell
undergoes its own homeostasis and how that builds upon each other to regulate the homeostasis
of the whole entire organism but the basic genetic code is in each cell of this overall
hierarchy of organization. So let's bring it back together as all the organ systems and then
discuss how this process works in an integrated fashion. So remember you have something like
the nervous system at the top and the endocrine system at the bottom helping out with
the regulation of the entire organism. The musculoskeletal, respiratory, cardiovascular, renal
and GI systems are helping that regulation occur and some of the regulation is automatic in
nature meaning that it's induced through the autonomic nervous system and some of it is
behavioral. So for example if it's too hot out you can either do something like sit in the heat
and sweat to try to regulate your body temperature or you can simply get up and go to a
cooler environment. Those are the various choices between behavior and autonomic responses
that are available to try to maintain homeostasis. The last aspect of homeostasis that I think
will be most helpful to think about is what are the various components that allow you to
regulate to a different environment or a different stressor. That can be best enacted by
thinking about what an organ system might do. So in the example on the one side of your slide
here you have an increase in blood pressure. So if blood pressure goes up, you need to have
something to sense the blood pressure going up and then a signal coming back to the heart to
say "Hey, hey that's too much pressure." We're going to either have to slow down that
particular heart. This can be more complex to be not just one factor being involved with having
an increase in pressure and a slowing down of the heart but rather be multifactorial. So in this
next example we're going to show the same response with increase in mean arterial blood
blood pressure that's going to be sensed by a pressure receptor and these are baroreceptors
or pressure receptors. Then that is fed back to the brainstem, in this case the medulla. The
medulla then organizes the information and sends the signal out both to the heart as well as
the blood vessels. To the heart, it's going to send a signal that it should slow down, shouldn't
beat so fast, it also doesn't have to be disheart. Finally for blood vessels, it's going to say
"Hey you can relax a little bit, you're too tensed, there is too much blood pressure in the
system you need to dilate" and this case that will also reduce blood pressure. This induces two
things, a bradycardia and a vasodilation and hopefully those two in combination are enough
to lower mean arterial blood pressure down to a value in which we are trying to regulate.
Again, this is a dynamic process but we're looking at a consistent range that we're looking for.