Monday, January 9, 2017

Homeostasis

Homeostasis
The last thing that we'll cover today is Homeostasis.  I listed homeostasis as one of the functions of the body that we'll discuss, but truthfully, it will be covered with every module.  For this module, I'd like to talk about what it is and how it works, and give you a brief overview about what I mean when I talk about vitalistic A&P as a way of viewing how your body works.

What is homeostasis?  Homeo – means the same, and stasis – means unchanging.  So while the term could literally mean “the unchanging same” (boring!), the truth is far from that.  Homeostasis describes the stable internal environment of the body.  Every variable in the body can be seen to be maintained in a relatively narrow “margin” of what is considered normal.  Blood pressure, temperature, oxygen levels, carbon dioxide levels, heart rate... each has their own homeostatic normal range, and that range is further influenced by the internal and external environment in which your body operates. 

(As an interesting side note here, the term Homeostasis was coined by Walter Cannon, in a book published in 1932 entitled “The Wisdom of the Body” - sounds quite a bit like what we're talking about with vitalism, no?)

Another way of thinking of homeostasis is a dynamic state of balance or equilibrium.  If a variable moves to the edge of the margin of what is acceptable, or outside of it, the body responds in a way to bring it back into balance. 

Homeostasis, as you can probably guess, or you may already know, is incredibly important.  It is our body's prime directive – that order that overrides everything else.  Maintaining a stable internal environment is what every part of your body functions to do.  We live in an unstable world, and everything that we interact with affects our bodies.  Homeostasis is vital to life – loss of homeostasis results in disease, and ultimately death. 

            Feedback loops
Have you ever taken care of a swimming pool?  One of the things that you do regularly with a pool is test the water, right?  You test pH, chlorine levels, and other things to determine if things are within acceptable and safe limits, or if there are changes that need to be made.   I grew up in CA, and we had a pool – I always loved getting the pool water into the little tube things, adding different things, and watching colors appear.

The very first thing that needs to happen when your body is trying to maintain homeostasis around any given variable is detection of that variable, right?  A RECEPTOR that detects the changes of that variable must be present.  (In the pool, the receptor was the tube thing, and adding the different chemicals from the dropper made the changes visible to my eye).

The receptor is not intelligent in and of itself, it cannot do anything with the information it detects alone.  In the pool example, this would be like I just left the tubes by the pool, even after adding the different indicators, and didn't do anything further.  However, the receptor reports to a CONTROL CENTER.  When I was testing the pool water, the control center was the booklet that came along with the test kit.  If something turned blue, it told me chlorine was needed, etc..  I could know if chlorine or whatever was needed to the pool based on what was being detected in my tubes (the receptor).  In our bodies, the control center is typically within the central nervous system, and the control center takes in the information and dictates the proper response.

The EFFECTOR is what performs the response.  So in my pool example, I was the effector – I was the one that got to add what was needed or perform the necessary changes to the water.

There are two types of feedback systems – positive and negative feedback.

We'll cover negative first, because that is easier to understand: In a negative feedback loop, the effector action turns off the trigger to the loop.  If the system was triggered because of high blood sugar levels, for example, the effector action lowers blood sugar levels to stop the loop.  (Or, if it's low blood sugar levels that were the trigger, the effector action would raise blood sugar levels in response)

Because the effector action shuts off the loop, it's called negative feedback.

Positive feedback is the opposite.  The effector action increases the trigger, increasing the response.  Typically, positive feedback systems create a cascading effect that continues until something outside the system stops it. This sounds illogical and is kind of counter intuitive.  Let's look at times where positive feedback is used for examples.   Contractions during childbirth are an example of a positive feedback system:  Oxytocin is released from the pituitary gland, and causes contractions.  The contractions push the baby's head against the cervix.  Pressure on the cervix in turn cause a signal to go to the hypothalamus (where the hormone is created) to produce more oxytocin, which when released into the blood, creates stronger and more contractions, increasing pressure on the cervix and so on.  The process continues until the contractions completely push the baby (or babies) out, and the uterus is emptied.  The emptying of the uterus is what shuts off the loop, because the baby is no longer stretching the cervix.  Blood clotting and lactation both also work by positive feedback systems. 

Clear as mud, right?  It doesn't really matter if you completely understand the positive vs. negative feedback thing now.  Just be aware that our bodies have these two types of feedback, one turns off the initial stimulus, and the other turns in on higher and higher until something else happens to turn it off. 

I promised that we'd end this module with an example of what I'm talking about with vitalistic A&P.  So since we've started covering homeostasis, let's look at one of the basic homeostatic mechanisms of the body – temperature control.

Humans are fairly unique in the animal world because we are able to adapt, and even thrive, in quite a range of temperatures.  There are human populations in the cold arctic regions, as well as in hot desert climates, and everything in between.  As long as the external temperature is within our acceptable range, however, our bodies adapt and change so that the internal temperature remains acceptable as well.  Our internal temperature range of acceptable is quite a bit smaller than the external temperatures, and it's a testament to how amazing our bodies are that we are able to maintain ourselves within that small acceptable internal temperature range.

(And just to clarify, at the moment, I'm not talking about temperature increases that occur when your body is fighting something – we'll go over immune functions at a later date.  This is just looking at temperature for normal every day life.)

It's important to remember that if you know what needs to be accomplished, the things that the body does make perfect sense.  If we're too cold, our bodies work to get warm.  If we're too hot, our bodies work to cool off.  If you can step back and say “what are all the ways I can do X (generate heat, lose heat, raise blood sugar, etc), you'll generally come up with a mostly complete list of what your body will do in response to the stimulus.  So when we're looking at these different responses, we'll start by generating the list of things that your body does, then address them each specifically as far as how your body does that.

If your body starts getting too cold, what does it want to do – warm up, right?  So how can we do that?                                               Answer:  reduce heat loss and generate heat. 

When you start getting too cold, the first thing that happens is that the thermoreceptors, nerve receptors that measure temperature, both in your skin and internally, send information to the hypothalamus telling your brain that it's getting cold.  In response, the hypothalamus will trigger a series of responses that shift your body to help keep you warm.  The autonomic (or involuntary) responses triggered are: 
            Shivering – for heat generation from the muscles.  Although skeletal muscles are voluntary, shivering is typically not.  Your ANS takes over and says that heat production overrides the resting state of the muscles.  Unless you are very very cold, you can still move voluntarily, but if your muscles are resting, they will be shivering with quick contractions to generate heat.  
            Goose Bumps – we're relatively hairless (or most of us are!), so this doesn't make as much sense for us as it does for other mammals, but goose bumps help to retain heat by causing the hair to lift away from the skin and trap heat in between the hair and the skin.  They are caused when the arrector pili muscles, which attach to the shaft of hair in your skin, contract, pulling the hair shaft more upright.
            Shift in blood flow – heat is primarily lost through the skin, so if your body temperature is dropping, we want to avoid heat loss at the skin.  One way of doing this is for the arteries and vessels close to the skin to vasoconstrict (close) so that less blood (which carries heat) makes it to the surface of the skin.  Shifting blood flow away from the skin brings blood (and heat) internally, hopefully heating up the internal organs, and keeping the more internal thermoreceptors happy. 
            Warmth seeking behaviors – although we have control over these in respects to if we actually perform them (or are able to perform them), they are triggered by the information coming through your ANS.  So putting on a sweater, coming inside, standing by the fire, drinking something warm – are all driven by your subconscious mind making them happen.  Putting on a sweater, or a blanket, or coming in from outside, all of these decrease the information about cold temperatures, and help to restore normal temps in your body. Drinking something warm has the added benefit of also heating up the internal thermoreceptors, as well as holding something warm in your hands. 

In the next module, we'll cover how we maintain boundaries.  We'll talk about it on a cellular level, as well as from a systemic and whole organism level.  So we'll be focusing on the integumentary system, as well as on the nervous system, we'll mention a bit of circulatory physiology, and even touch on the respiratory, digestive, reproductive and endocrine systems. 




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