So clearly cells have an internal environment
and an external environment.
And they need to maintain that internal environment
distinctly from the external environment in a consistent way.
So this is where we'll introduce the concept of homeostasis
which we'll revisit regularly throughout the course.
In homeostasis, let's take the example of
a house and your room temperature.
There's a stimulus. That stimulus would be heat.
The heat is detected by the thermostat or the sensor in your home.
And that sensor will integrate that with
what the set point should be for your house temperature.
And once the house temperature has been compared,
if it's too cool or too warm.
We're going to have an effect. So the air conditioner
or the heater are going to be the effectors
which will either heat or cool the environment.
And that change in environmental temperature
itself is going to feedback on the sensor,
and the sensor again will detect this in a cyclic manner.
And so there are two mechanisms for homeostasis. The predominant
one in biological systems is the negative feedback loop.
However on occasion, we'll see a positive feedback loop.
Let's first take a look at
a normal situation with negative feedback.
Negative feedback is what keeps most systems in biology in balance.
Let's now move on from the house example
into a body temperature example.
In much the same way, we will have a stimulus.
Perhaps, body temperature rises a little too much.
We need to keep that in pretty close check. So it's important
that we have a sensor to detect that increase in body temperature.
This occurs in the hypothalamus. And the sensory neurons that are
entering the hypothalamus are going to compare that information
with the set point, the 37 degree C
that our body should be maintained at.
Of course sometimes we increase or decrease
a little bit around that
but it's important that we keep it right around that set point
so all of the enzymes work properly.
So if we're a little bit warm,
the effect is that blood vessels will dilate.
We'll have vasodilation in the skin
in order to shed some heat to the environment.
In which case, our body temperature will then drop,
which is the response.
And that response then is detected by the sensor.
On the other end of the scale, perhaps we got a little bit
too cold, in which case we don't want blood at the skin.
We would like the blood then to be shunted
into the system, so we have vasoconstriction.
With the vasoconstriction,
we may even get so cold that we need to shiver.
The shivering increases muscle activity which generates heat.
And that increase in heat is the response.
And that response then feeds back to the sensor and
shuts down all of the mechanisms that act to warm our body.
So in essence now, you can see that on both ends of the spectrum,
there's a negative feedback loop
where the response itself feeds back negatively
to shut down production of whatever it is.
We could be managing blood sugar levels.
We could be managing pH in the internal cellular environment.
Negative feedback is going to be the mechanism
that helps all biological systems manage that
There are very few examples of positive feedback in the human body.
One of them is blood clotting. Another one we'll look at here
is the event of birth or parturation.
When a fetus is ready to be born,
it pushed up against the cervix.
And the cervix is stretched and the sensory muscle fibers
within the cervix will detect that stretching.
And that stimulus will be sent to the hypothalamus and integrated.
And the hypothalamus will then say "Well we need to have
a secretion of oxytocin in order to have contractions,
so that we can expel the baby." So that secretion of oxytocin
is then detected by the sensor, more contractions happen
which stimulates more oxytocin so on and so forth
until we have so much oxytocin present that the baby is expelled.
And it's the expulsion of the baby
that actually shuts down this positive feedback loop.
So you can see, a positive feedback loop
doesn't really maintain anything.
It escalates, escalates, escalates. While in negative
feedback loop, we'll regulate and keep things within homeostasis.
So hopefully this was a good introduction
to where we're going in the course.
You'll be able to tell your friends and family
what exactly it is that you're planning on learning in this course
In addition to that, you'll have an idea of the
relative scale of things that we're going to explore.
You know some of the differences between prokaryotes and eukaryotes,
we'll explore this in much more detail later.
And finally, you recognize that homeostasis is critical
to all living systems.
Thank you so much for listening to the introduction
and I'll look forward to seeing you in future lessons.