Homeostasis: Definition & Level of Organization

00:00 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.

00:41 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.

01:09 Oxygen needs to be delivered to each cell in the body, it needs to be precisely regulated.

01:14 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.

01:27 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.

04:00 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.

04:30 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.

05:31 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.

09:06 Again, this is a dynamic process but we're looking at a consistent range that we're looking for.