okay so some topics for review so going way back to like our the first chapter that we talked about um we talked about homeostasis and I know that's a word that you've all heard in your other um science classes um but particularly we talked about two kinds of feedback mechanisms a negative feedback mechanism and a positive feedback mechanism in a negative feedback mechanism or Loop some change is corrected or or put back the way it was so um for example if the body gets too hot that triggers a response that cools the body back down so we call it a negative feedback because it negates or reverses whatever the stimuli is so if it's too hot the body cools down back to to zero or back to uh neutral if the body is too cold it heats up so it's opposite the direction of change is opposite uh the the initial change or the inciting change positive feedback mechanisms are a little bit weirder because a small change is Amplified so we call it positive because it's adding to the change that already took place so in blood clotting for example once Blood starts to clot it clots even more and even more and even more so it's like an amplification so those are those two primary feedback mechanisms um the example we gave for the negative feedback loop was the air conditioner you know when the room gets too warm the air conditioner turns on cools the room back down and then the air conditioner turns off most of the feedback mechanisms in the body are negative are the negative version because the body has a particular way it wants things to be so anything that departs from that and the body's going to do something to correct it to bring it back into that normal range all right and then we talked about uh anatomical position and directions so anatomical position is standing uh Palms forward you know it's not exactly the normal way we stand you know usually we stand with our our palms turned in but in anatomical position it's upright and Palms forward and uh we talked about the directional terms you're going to use these not only in this course but in the rest of of your you know medical career too to describe things so just as a review we have anterior and posterior anterior is towards the front posterior is towards the back now because the human being stands upright instead of walking on all fours like a lot of animals do anterior and posterior have other words too so anterior is also ventral or towards the belly and post or uh posterior is also dorsal or towards the back so if you remember that a shark fin you know the Jaws fin that's the dorsal fin of the fish and that will help you to remember that dorsal is in the back on the back of the animal all right um a lateral and medial lateral is away from the midline of the body medial is towards the midline and since the human being is uh symmetric across the midline you know we have a left half and a right half that are pretty much the same at least on the outside um we use these medial and lateral terms a lot to describe you know going further from the midline or going towards the middle proximal and distal um we use when we're talking about um uh the limbs in particular so proximal is towards the origin of the limb distal is toward the end um we also use these words when we talk about vessels um and nerves so um arteries and ve we'll talk about you know things a vessel is distal to another vessel or proximal to another vessel so proximal and distal um and then the one uh the set that is used least commonly is that cranial coddle one so cranial is towards the head coddle is towards the tail or towards the end of the spine now in um in uh other animals we use cranial and coddle a lot more because they are on all four so we can describe it easier that way but for the human only rarely and usually um we talk about cranial and coddle when we're talking about the central nervous system um because you know this the brain is at the top and the end of the spinal cord is at the bottom so they these words make sense in uh that context all right and then the um the planes the bodies of threedimensional thing and sometimes the best way to think about whatever Anatomy you're looking at is to think of it in one of uh these three planes so the transverse plane is the one that you actually see in clinical medicine the most often um and this is cutting a person in top and bottom you know so they're top half and bottom half and the reason that we see these most commonly is um the computed tomography the CT scan and the MRI because of how they work the pictures that they generate are usually in the transverse plane so um anybody who's going into uh you know Hospital based medicine or ICU you know U nursing or OT transverse plane is something you'll get familiar with but not in this class um so that's top and bottom if we cut the person in left and right that's A sagittal plane or A sagittal section the um uh down the middle um into left and right so we're going to divide the person in half we call that the mid sagittal plane because it's the middle sagittal plane um so we get pictures that look like this and then if we cut the person into front and back then that's the frontal plane now most of the pictures in anatomy books are usually rendered in the frontal plane you know you're usually looking at the person um uh head on you just have to remember that the anatomy is more complicated than that you know we're not South Park characters with no Third Dimension you know we do have a depth and organs are not you know uh even distributed from front to back so the three um planes sagittal frontal and uh all right let's see why is my mouse acting up there we go okay so the planes all right so that was chapter one at least in quick review um also in chapter one we talked about words and word roots you know how Anatomy words are like Legos you know you can you take three or four different words and you can put them all together to make a compound word hopefully you're starting to see that um and you'll see it more as we get into the organ systems and we start talking about things so like the um the epidermis you know we've talked about that Epi is above dermis is skin so this is the top part of the skin so just an example of leg work all right then we talked about chemicals we did a little chemistry review with a uh particular focus on how chemistry matters to the human being um we talked about water um water has a number of important properties that made it the the medium of life so to speak or you know all all living things that we've ever discovered um have water involved in their physiology some of the reasons for that water is known as the universal solvent now it can't actually dissolve everything but it can dissolve lots of things so because water can dissolve so many different things um it uh it allows uh complex Solutions or um mixtures of substances dissolved um in water so it's a very it has a high solvent potential water has a high heat capacity in other words it takes a lot of heat to to increase the temperature of water um and that matters because by being water-based it means that our temperature has a tendency to stay put um if we were uh you know if we were uh mostly alcohol for example alcohol's heat capacity is much lower so when it got cold outside we'd get colder faster and we would get hotter faster um so high heat capacity and then water is polar now this is a way to describe a chemical but it means it has a positive end and a negative end and the the reason that matters is polar substances um can dissolve in other polar substances so because water is polar it means that it can hold lots of different ions and um substances that have one end or another the other version is nonpolar and in the biologic molecules it's only the lipids that are non-polar all the others are uh are polar all right so water we talked about carbohydrates um carbohydrates uh their primary role in the body is as nutrients um so the most important carbohydrate to know is glucose glucose is um a monosaccharide it is a simple six carbon um ring um it's a simp it's the simplest sugar um and it is one of the primary fuel sources for the body the mitochondria they love ATP and or they love glucose and they make lots of ATP from it so glucose is a primary energy molecule if we string a bunch of glucose molecules together in kind of a branching pattern we create a starch called glycogen so a starch is um a long chain of simple sugars okay and our starch the starch that the body makes is called glycogen we make it out of glucose and it's a big branching uh chain all right so carbohydrates fuel molecules lipids um two uh well three primary functions in in terms of lipids one phospholipids create cell membranes um and we learned that there are multiple organel in the cell that are effectively made of membranes so these phospholipids are critical to life and uh you'll remember that the cell membrane is called a lipid by layer because what we get is we get the phospholipid heads all line up on the outside and and the Tails all Point towards the center and we make a three-dimensional structure out of this a sphere and that's the cell membrane so each one of these so one of these guys is what we call phospholipid so it has a polar head and it has a nonpolar tail and because of that it orients like this almost by itself um so it creates the cell membr all right so that's one important lipid another is triglycerides so most of by mass or by weight most of the lipid in the body is in the form of triglycerides triglycerides are the primary component in adapost tissue so in fat tissue and what a triglyceride is is it's um so this is a carbon backbone so these Each corner is a carbon molecule um and a triglyceride is this four carbon chain called glycerol and then attached to it are these long chains of carbon and hydrogen that we call fatty acids so a triglyceride is made up of three fatty acids and a glycerol back bone and it's triglycerides that are body makes for energy storage you know when you take in more calories than you spend the extra gets SA up as triglyceride um and then also because triglyceride is a good insulator um uh it also helps to keep us warm and it helps to cushion and protect things all right so phospholipids triglyceride and then the last one is cholesterol and the steroids which are chemically related cholesterol um and I'm not going to draw it accurately because I can never remember how but it looks something like this so cholesterol is um a a joint ring so it's multiple rings of carbons that are all sort of joined together cholesterol's primary role in the body is in the cell membrane um it lives in the non-polar region of the cell membrane so in between the heads where the tails are pointing together cholesterol lives in there and it helps to keep the cell membrane very flexible um and then cholesterol is made into uh certain kinds of hormones called steroid hormones which um directly affect the the U the nucleus so all that to say cholesterol and its uh sister the steroid hormones are the third important lipid that we see in the body all right so phospholipids triglycerides cholesterol then we talked about proteins proteins are long chains of amino acids and an amino acid is just a kind of uh molecule that has a nitrogen on one side and a carbon on the other but what we end up with is an unbranched long chain of amino acids so this is amino acid one this is amino acid 2 amino acid 3 and so on all right the bond between two amino acids we call this a peptide bond and it's called that because it's a nitrogen Bond peptide is always associated with nitrogen so because these amino acids have a nitrogen that connects them we they get this special name called a peptide bond so this long chain of amino acids is not going to do anything in this form you know a long chain isn't very interesting you know it's not going to do anything exciting so proteins have a shape that they develop as the long chain kind of twists and bends and loops around itself to make a kind of blob you know if you imagine a long string that you take and youve kind of bald up that's sort of what a protein looks like you know the long string gets turned into a shape and then that shape has effects so we talked about three or four um levels of structure for the protein so its primary structure is the order of amino acid AIDS and that order of amino acids is uh is what is stored in DNA you know DNA is a recipe for making a protein for putting a string of amino acids together in just the right order so that's the primary structure that comes from the DNA from the RNA the secondary structure is if we take this long string and we do something simple to it you know let's say we twist it into a helix or we line it up in kind of a flat sheet you know like you would do for weaving that's a secondary structure and then the tertiary structure is a single protein's final shape okay so we go from the order to a simple pattern and then we take that simple pattern and we make it complicated and that's the tertiary structure without the tertiary structure the protein doesn't work it doesn't do what it's supposed to do and then the last one we talked about was quatron structure there are a l lot of proteins that are made up of more than one part so like hemoglobin for example is has four separate protein parts that are joined together so the quadrin structure it is um multiple proteins that are joined together into one functional unit and there are a lot of important proteins that are like that all right so a little review of proteins um ions are um atoms or molecules that carry a charge um and they're very common in the body and they're very important in how the body uh works so a cat ion is a positive ion just and an anion is negative so cat ion and the common pneumonic there is the cat ion has a positive in the middle right there's a t in the middle and then anion a negative ion is an anion just little pneumonics and there are examples of both um and we talked a little bit about the the common ones found in the body so sodium potassium chloride um bicarbonate we talked about all all four of those calcium is a major player in terms of ions um but for now just know what an ion is it's a it's a molecule to charge all right pH is our measure of acidity and what PH actually measures is hydrogen ion concentration so you know the simplest atom in all the periodic table is the hydrogen ion the H+ because quite literally it's just a proton it's a single subatomic particle without an electron without a neutron it's it's simple and boring but because it's simple like that it's reactive so the more hydrogen ions you have in a solution the more reactive that solution is going to be that's acidity you know if you take something very acidic and you put like metal in it the metal will bubble and and eventually dissolve because those hydrogen ions are literally taking the metal apart um so the pH is a measure of that hydrogen ion concentration now pH uh matters a lot in the body because of that the body tries to keep it in a very narrow range the reason for that is proteins um change their shape as the pH changes and if the protein changes its shape the function of that protein changes so the body defends a pH of 7.4 so 7.4 is normal body pH and um it it departs from that very little and we briefly talked about buffers buffers are substances that when present reduce the change in PH so if they if you have a beaker with water in it that has a buffer in it and you add acid the pH doesn't change until you add enough acid to use up all the buffer and we're going to talk more about this next semester but just know that pH hydrogen ion concentration and that buffers help to keep the pH from changing okay um we talked uh and then our last sort of biological chemical we talked about was um DNA and protein synthesis which I know that you've heard in other classes um in your past so you know it shouldn't be a total review or a total new thing rather um but DNA stores the recipe for making proteins but proteins are made out in the cytoplasm DNA is in the nucleus right so we have a problem the recipe is in one place but the machine is in another the kitchen is somewhere else so that's where RNA comes in RNA copies the recipe takes it out into the cytool and then attaches to a ribosome and then that ribosome creates a protein matching that RNA recipe which matches that DNA recipe so a couple of important words that sound similar but are different transcription is the the creation of RNA from a DNA template okay we call it transcription because we're transcribing RNA language to DNA language which are not very different translation is turning that RNA template into an actual protein so translation is another name for protein synthesis and transcription is another name for RNA synthesis um and you'll hear those words come up so transcription is DNA to RNA translation is RNA to uh to protein all right and then we started talking about the cell um cells don't exist in the body um all by themselves they're always connected and associated with other cells and the connections between those cells can be critical to how a cell works so three I want to remind you about the Gap Junction is a connection between cells that allows stuff to pass through all right so we have cell so here's one end of cell one here's another of cell two so a gap Junction is like a little straw between the tube so there's a hole here so it creates a Channel or a path so that uh molecules that are small enough can actually go from one cell through this tube to the other cell without having to leave and come back inside so we see Gap Junctions in tissues where uh the cells have to work together or they have to do something all at the same time probably the most famous example is cardiac muscle cardiac muscle has these Gap Junctions so that the whole heart will contract at once so that the signal from this cell can pass directly to the next cell without having to go out and come back in we also see Gap Junctions in um smooth muscle like we see in the intestine or the uterus or the bladder same reason all those smooth muscle cells have to do the same thing at the same time or the whole system doesn't work all right so that's a gap Junction like the straw between the two um a desmosome is like a rivet or a nut and bolt so on one side we have a kind of protein plate on and then on the other we have the same thing so just underneath the membrane there's like a dis of protein on each side and then those discs are connected together by a very strong protein so desmosomes keep cells from being pulled apart you know if you grab this cell and this cell and you tried to pull them apart this those two plates are going to resist that you know with the uh The Binding in between so desmosomes gives uh cells strength against shearing or being torn apart so like our skin cells that we talked about they have lots of these you know because the skin cells have to hold on to each other otherwise if you rubbed your skin well the cells would just all rub off so that's a desmosome and then the last one that I wanted to talk about is called an including Junction okay so again we have cell one and cell two and including Junction is a little bit like um a a stitch in clothing it's sort of weaves in and out like this and what it does is uh because these cells are are sort of stitched together stuff cannot get between the cells and into the body or into the tissue so we call it an including Junction because it blocks the path between the cells and an including Junction looks a little bit like a a stitching pattern all right so those are three I wanted to remind you about okay um organel so we talked about you know just like the body cells have structures that have particular function the nucleus runs things and it does so by controlling what gen gen get activated or what genes uh a gene is an area of DNA that um is the recipe for a protein and because the nucleus has control of all the recipes it has control over what gets made in the cell so the reason we think of the nucleus as being in charge is it determines what the cell does by turning genes on and off essentially so um creating a protein or not creating a protein is going to alter what that cell does all right the endoplasmic reticulum is a site of manufacturing so the rough ER is where proteins are made the Smoothie R is where everything else is made so things like phospholipids um and U uh uh amino acids things like that are made in the smooth so endoplasmic reticulum manufacturing mitochondria ATP production um specifically mitochondria so if this is a mitochondria they look a little bit like that if we put small carbon chains in so um we put small carbon chains in and we put oxygen in and we get ATP out we get Co2 out and we get water out so essentially the mitochondria is is a factory that burns carbon chains into CO2 using oxygen and then forms ATP and water in that process so the reason that we breathe is because of the mitochondria and what they do we have to bring in oxygen and we have to get rid of the CO2 that has been formed by burning the food that we've eaten or the nutrients that we've eaten so mitochondria responsible for that and then ribosomes are protein factories um so they use a a messenger RNA template to create a chain of amino acids which then goes on to become the functional protein after it reaches its um tertiary or quinary structure all right and then we finally got started talking about the human being so up until that point we had been talking about you know General chemistry General biology and then we started talking about tissue types so remember all of the tissues in the body are one of those four types so it's either an epithelium connective tissue muscle or nervous tissue there's only four types now inside each of those groups there are different tissue uh types inside there but those are the four big categories um we talked a lot about epithelium um epithelium one of the things to uh remember is it's considered polar in other words an epithelium always has a top and a bottom the top we call apical um and that's going to point uh either towards the Lumen okay and then the bottom we call uh basil or basil so top and bottom um epithelium always sits on a membrane so the aular side sits on What's called the basement membrane that sort of gives the whole epithelium a structure um epithelium is either lines something in other words covers the surface of a thing or we find epithelium that form glands that produce a substance like sweat for example um another place we see epithelium is in um uh uh senses so like you know we have a sense of touch thanks to what are called neuroepithelium or epithelial cells that um respond to stimuli somehow so most of the uh our senses actually arise from epithelial cells that are specialized for that so like your eyes your ears um your sense of taste your sense of smell your sense of touch all of those are because of epithelium um okay so we talked a lot about epithelium a connective tissue the big characterist istics there are um there's a lot more stuff than there are cells you know and that stuff we call the Matrix um which is made up of protein fibers and then ground substance which is sort of the fluid part of connective tissue and uh we describe connective tissue as uh loose or dense and then regular or irregular um and you should have looked at some of these in lab too um the different tissue types now there won't be any questions on our lecture test about identifying specific tissues that's for lab but in terms of characteristics like which kind of tissue has a top and a bottom or an apical and Basler side well that's epithelial tissue none of those other tissues have that so do know the characteristics but you won't have to identify a tissue from a description for example all right so tissue types and then we actually got started talking about uh an organ system particularly the skin um the skin has two primary layers right there's the epidermis on the top um and then underneath the epidermis is the dermis and we divide the dermis into two layers the capillary layer sits just underneath the epidermis and this is where the blood vessels that supply the cells of the epidermis um uh run the papillary layer gets its name Capilla means Hill and the papillary layer has all these Hills and the epidermis has a matching Hills so to speak and that helps to keep the epidermis and dermis attached to it to each other um because there again if this were flat and smooth when you pushed on the skin it would have a tendency to separate from the underlying Derm so instead we have this kind of egg crate pattern that holds those epidermal and dermal layers very tightly together so capillary layer and then the larger layer underneath of the dermis is the reticular layer and this is where you're going to find the fibers that make the dermis strong and elastic so we have collagen fibers that give the dermis its strength and then we have elastic fibers that give it its stretchiness you know that the skin will stretch and then snap back like when you smile and then frown the skin can do that because of the elasticity of the dermis so capillary layer and reticular layer underneath the dermis is the hypodermal space which is not technically part of the skin it's under the skin um but this is areolar tissue like we've all seen in land um and it allows the skin to be uh to move separately from what's underneath it but okay so layers of the skin and then if we zoom in to just the epidermis so we're going to look at just the top part we will see back we'll see the layers of the skin or of the epidermis rather there are two kinds of skin there's thin skin and thick skin most of the body is covered with thin skin um the only places that are covered with thick skin are the walking surfaces so the the um Palms of the hand and the soles of the feet are the only place that we have thick skin and we have um in thick skin we have five layers one two three four five in thin skin we don't have this one so we only have four layers um and just to review the stratum let's use my pointer all right the stratum Bale which is right here this is uh the stem cell layer so this is where the mother cells are these mother cells divide and and one cell becomes a daughter and the other stays a mother so it can uh continue to replace the cells above it so that's the stratum basal the stratum spinosum is the largest layer of living cells um so we have kind of layer after layer of keratinocytes remember these are kot ten aites that's what we find in the skin um so that's the stratum spinosum the stratum granulosum is a thinner layer that lies uh um here at the top and you can see it in this picture it's kind of blue and we call it granulosum because it has granules in it so when you look zoom in under the microscope what gives this uh area a darker color is the cells have a bunch of granules or or little grains in them and those grains contain keratin which is then going to create this top layer that we'll see in a minute um in thick skin we have this layer called the stratum lucidum Lucid is clear so this is the clear layer and you can sort of see it here how there's a clearing not so much here but you can see it here and here and here so that's the one that's different so Straton lucidum only found in fixed skin okay and then above that we have this um uh the stratum corium and as you've seen under the microscope in lab this layer can vary in size a lot you know this is thick skin obviously because it has a lucum so um The Palms and Soles have just layer after layer after layer of these dead keratinized um keratinocytes so just huge numbers of layers if you look at the scalp or you know the skin from the hand you'll see many many fewer layers of stratum corn so like it might even only be like as thick as this you know right here this little part so huge variability the stratum Corum is made up of dead keratinocytes and the Keratin that they ended up making so all the while these cells are uh moving up because they start down here and they end up here as they're moving up they're creating more and more cartin and eventually they die in the um granulosum layer and then what remains is the Keratin that forms a kind of flexible waterproof shell um for the human being now do those dead um those dead layers that are just keratin those still have um melanin or is the melanin underneath no they still have melanin yeah they have the um the dead ones have melanin too yeah and these guys are sloughing off all the time so like up here you know we all know we lose skin cells we don't lose live skin cells we lose dead skin cells from the Strat of corneum all the time because new ones are being made you know the Skin's always replacing itself all right some accessory structures of the skin um we talked about hair a little we talked about Nails a little um but there are three glands that are important to be able to discriminate in the skin so we have two kinds of sweat glands um one is very common the other is found only in in U specific areas of the body so when you think sweat gland what you're usually thinking of are the mirrin sweat glands these are the sweat glands that produce a very watery um solution that has a little sodium chloride in it a little salt in it and um their purpose is to cool the body down by putting a little water on the skin it uh and that water evaporates it takes heat away with it so it's a kind of evaporative cooling mechanism um the mirran sweat glands also are a little bit antibacterial um so it makes our skin a little less of a nice place to live for uh potential bacteria all right the other kind of sweat gland is found in only certain areas of the body so axilla groin and nipples and these produce a very viscus secretion um so not watery at all it's it's thick it's kind of slimy it's uh very oily and it's has a lot of complex chemicals in it now the thinking is is that these apocrine sweat glands are probably therone glands um so old factory communication sending signals from one organism to the next um because they're very influenced by hormones they don't develop until puberty um so there's some thought that these are are pherone glands for the human being um these are the ones that are responsible for body odor um mirran sweat glands don't produce anything that bacteria can uh use for nutrients the aine sweat glands do so the bacteria take the secretion and uh in their metabolism they end up releasing G gases that have a smell so body odor is associated with the aine sweat glands not the mirin sweat glands all right and then the last gland is the sebaceous glands sebaceous glands are oil glands um that basically help to keep the skin nice and supple and nice and soft you always find them associated with hair so every hair has a sebaceous gland related to it and that's because without that oil hair gets very dry and brittle so it breaks but the sebaceous gland helps to keep that hair flexible um and soft so that it can do its job of of protection um but we also have sebaceous glands scattered around our skin as well so they're not all associated with hair but all hair cells have an associated sebaceous are we gonna need to know the difference between a gland and a fle no okay a follicle is just a place where a hair is okay okay I think that's let's see um in terms of obus tissue the stuff we just covered um you know there's compact bone and their spongy bone compact bone is arranged in osteons um osteons being sort of concentric cylinders uh compact bone is very strong and hard and heavy um and then spongy bone is arranged in truli or sort of a complex strut Network um it's less strong but it's better at transmitting forces in multiple directions um the main cell types osteoblasts make bone eventually the osteoblast surrounds itself with bone and becomes an Osteo site that lives in a laun or a little Gap in the bone where for the cell to live um osteoclasts dissolve bone um and then um I guess that's the three osteoclast disolve bone bone is always remodeling so it's always being dissolved and remade um to keep bones uh as heavy and strong as we need but not heavier or stronger than we need um and I think that's kind of all I have on there because we just talked about it so those things to know like um the flat bones yes the different kinds of Bones yep yep that classification system um all right okay so I spent most of our time here but let's just do a uh couple of questions here all right so get your socket thing out and I have everybody's name there we go okay so what is a tissue no it's not reliable enough that's why I have a sheet up here that I do it on our old clicker system you could do attendance like that but not new one all right all right I think that's everybody okay good a tissue is a group of cells with similar structure and function important to note that an organ is always made up of more than one tissue okay so all four tissue types epithelial connective muscle and nerve are found in all organs so don't confuse organ with tissue tissue is part of an organ but it's a set of cells that are all doing the same thing all right what's in organel Oh I thought I asked this question we got a diverse set of answers all right jump in there last couple of people all right there's my 24 all right so an organel is a a cell organ so it's a uh it's a I can see why there was a spread a small structure within a cell is not a very specific defition but you can say that's an organel um because a structure composed of several tissue types that's going to be an organ basic structural unit of all living organisms that's a cell right right um and group of organs with a common set of functions that's an organ system and we already talked about e as a tissue so kind of a weak definition for organel but once you've eliminated the others it would have to be a all right not the greatest we're going to skip that one okay this is a good one which of the following is most similar to a negative feedback mechanism this one you have to think of a little bout which is uh part of why it's in here all right five seconds jump in there gotta commit to an answer good unlike classes in the past most of you got this one this is C would be the best answer all right so why a negative feedback mechanism works to put a thing back the way it was you know so um before you flush the toilet it was full after you flush it the tank empty so a negative feedback is well we want to put it back the way it was we want to fill that tank back up um a car runs out of gas and stops there is no feedback there you know that's just running out of a thing so now if you stop to get gas because the car was out of gas you have participated in a negative feedback loop because you have put the car back the way you want it with a full tank right um uh the same is kind of true with b and e there isn't a feedback there b would actually be a neutral feedback mechanism because all the teacher is doing is identifying the the situation it's not responding at all um D would be an example of a positive feedback mechanism where the response to A Change Is to make something happen different happen you know you normally the door is closed when you approach it the door opens so that's a positive response you've made something happen uh not put something back all right we'll do actually let's call it right there because we only got a minute left anyway um so I'll post the review questions obviously that we didn't get to um online I'll open up all the clicker questions that we've had so far homework you'll be able to review after midnight when it's due um and then I'll see you all on Friday for our first test together do study but don't panic and you said that you were going to change that homework yeah yeah I'm going to look at that so it'll be out of nine instead of out of 10 yes yeah which screws up the percentages but it doesn't really matter