Feedback Mechanisms

Homeostasis is a steady state

The statement on this slide is true: the concept of homeostasisThe maintenance of a stable internal environment in the body. is about maintaining a steady state.  If I asked you for some set points the body maintains, you’d probably mention things such as body temperature or blood pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange.. You might also mention elements in your blood, such as sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. and potassium(K⁺): Major ICF cation; essential for muscle and nerve function..  But, homeostasis is not only about maintaining an internal state.  It is about maintaining that state despite the factA statement based on direct observation that is repeatedly confirmed. that the environment is fluctuating.  For example, you get out of your air-conditioned car for class. Suddenly, the heat of a July day and a blacktop parking lot hits you. You feel as though you are going to burst into flames right there and then.  But you don’t burst into flames.  That’s because your body has mechanisms like sweating. These mechanisms regulate your body temperature. They keep it from reaching 104 degrees. This happens even though you went from a chilly 64 degrees to a murderous 104 degrees.  These mechanisms are called negative feedbackA control mechanism that reverses a change in the body to maintain homeostasis. mechanisms.  The word negative is not applied because these mechanisms are somehow bad.  They’re not.  They’re good.  The word negative is applied because these types of mechanisms reverse the stimulus.  For example, the stimulus in the parking lot is a rising body temperature.  But negative feedback mechanisms reverse that, bringing your body temperature down and/or keeping it at the correct set point. 

Negative Feedback Loop

Negative feedback can involve a few different organs to get the job done.  Usually it’s not one organ working on reversing the stimulus, but a few organs working together.  Let’s stick with our example of body temperature.  For all negative feedback mechanisms, there are at least three bodily structures. A receptorA structure that detects stimuli. senses the change in set-point. A control centerThe part of a feedback loop that processes information and initiates a response. decides what to do about that change. An effector carries out that change.  All three things must be either an organ, a tissue, or something located physically in the human body.  What I mean is that you can’t say “sweating,” you’d have to say “sweat glands.”  Sweating is a verb, you need nouns.  For this person down here that is really cold, you’d say that their muscles contract. You wouldn’t say “shivering” as the effector.  Again, shivering is a verb and you need nouns.  One more thing before we move on.  When you are hot, the blood vessels in your skinThe body’s largest organ, providing protection and regulation. dilate or get bigger in diameter.  This brings hot blood to the surface of your skin. The heat can then diffuse out of your cutaneous membrane via sweat.  The opposite is true when we are cold. Our skin blood vessels constrict. This keeps the blood centralized on your squishy organs.  If your vessel constrict long enough, frost bite can occur.  The face is the usual place for frost bite. 

Body Temperature Homeostasis

I like this diagram.  Probably because I made it!  Notice how the set point for body temperature is at the center of the diagram.  The top loop of the diagram shows you the events that occur if that temperature starts to creep up.  The bottom loop shows you what happens when your body temp starts to decrease.  Let’s do the top first.  We are going to start at the center of the diagram and move clockwise to tell a story.  So, first thing, body temperature increases.  That is our stimulus that we are going to reverse.  There are sensors in your skin that can sense both the outside and your internal temperature.  These sensors are the receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter here and they send a signal to the brain.  There’s this little part of the brain that we will get to know closely called the hypothalamusA small but vital brain region controlling hormones, temperature, and autonomic functions..  The hypothalamus is our control center.  It receives the signal from the receptors/sensors that the temperature has risen.  The hypothalamus can’t reverse the stimulus directly. However, it can communicate with other organs and tissues to make that happen.  And it does.  The hypothalamus tells the blood vessels in the skin to dilate bringing that heat to the surface of the skin.  The hypothalamus also tells the sweat glands in the skin to do their thing.  As the sweat secretes onto the surface of your skin, it dries, releasing the heat trapped in the waterThe universal solvent essential for life. moleculesGroups of atoms bonded together..  These two things help reverse the stimulus and we find ourselves back at the center of the diagram.  Please stop this recording right now. Write a few full sentences describing the bottom loop.  Start at the center of the diagram and move counter clockwise through the bottom loop where temperature decreases.  This picture tells that story – you tell it in words. 

Positive Feedback Mechanisms

Just as negative feedback is not a bad things, positive feedbackA control mechanism that amplifies a change instead of reversing it. is not necessarily a good thing.  Positive feedback is when the body works to enhance a stimulus instead of reverse it.  Let me explain with an example.  Blood clotting is a perfect example.  So, one of your little fragile capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues. around your eyes feels stressed. This happens because you were up all night studying.  The capillary bursts or rips.  It’s OK though because as blood flows through the capillary, there are platelets in the blood.  Some of those platelets attach and fill in the tear.  As blood continues to flow in the capillary, the attached platelets attract more platelets and more and more.  It’s like a platelet party in your capillary and everyone can invite two friends.  The attachment of that first platelet encourages the attachment of more and more.  This is positive feedback.  Other things such as labor, in which the contractions get worse and worse, are examples of positive feedback.  Anaphylaxis is an acute quick acting hypersensitivity or allergy. It intensifies quickly until someone uses an epi pen on you.  Kidding, they stab you in the side of the thigh.  This is your immune system trying to save you from eating whatever it is you are allergic to.  For me, that’s blue cheese and penicillin.  Positive feedback is not always good or bad. It simply enhances a stimulus. This process creates a graph like the one up top.  Negative feedback produces small fluctuations around a set point such as the graph on the bottom.