The Science of Everything Podcast - Episode 25: Organs, Tissues and Systems

Episode Date: December 10, 2011

An overview of the various levels of biological organisation, from the atomic and molecular level through cells, tissues, organs, and up to populations and ecosystems. Also includes a more detailed examination of the structure and function of the four main types of human tissue, including epithelial, connective, muscle and nervous tissues.

You're listening to The Science of Everything podcast, episode 25, tissues, organs and systems. And I'm your host, James Fodor. In this episode, it's going to be a bit of a mishmash of a few different things. I'm going to go through, first of all, I'm going to discuss the different levels of biological organization, from the cellular and atomic level, right up to the population and ecosystem level, to give you an overview of how the biological world is categorized for study. And then I'm going to talk in more detail about tissue. that is sort of around the middle level, just between cells and organs,

which is a level that's sort of often neglected a little bit in biological causes, perhaps. And it's important to have a basic understanding of the different types of tissues before we go on to talk about the different organs and organ systems in the human body, which we'll do in subsequent episodes. So this will be a foundation for those future episodes. Okay, so let's get started. First of all, the levels of biological organization. So the purpose of this sort of categorization is to,

give some order to the biological world because biology is very complex and it can be studied at many different levels or many different degrees of fineness, if you will, just as you can study astronomy from the perspective of galaxies or clusters of galaxies or solar systems, planets, etc., as described in the Our Place in the Cosmos episode. You can also look at biology from different levels, different perspectives as well. So I'm going to go through those now in order from smallest up to largest. And there's three very basic levels of these categorizations. The cellular level, or the atomic cellular level, molecular level, that's everything from atoms right up through cells. That's the sort of the realm of biochemistry, molecular biology, cell biology, genetics and so on.

The next group is the organismal level. That's tissues, organs, systems, and organisms. So that's basically functional biology, botany, zoology, that sort of thing, or most of zoology, and also medicine. And then the last group is sort of the population level, where we look at populations, communities, ecosystems, and the biosphere. That last one, the population level, gets a little bit of less focus than the first two levels, if you do an intro biology course or other things like that. You'll spend much more time on the cellular and organismal level than the population level. But still it's important to have a perspective of all these different levels. Okay, so remember those three groups, cellular, organismal, and population,

and now we're going to go through each of the specific categories of levels of organization within those three basic groups. And by the way, these categories are fairly standardized. Some, if you look at websites or books or whatever, will be slightly different. but they're pretty much mostly the same. So this is not my idiosyncratic classification. This is a fearless standard way of doing it. So the lowest level is atoms, or the atomic level.

An atom is the smallest portion into which an element can be divided while still maintaining its essential properties. So we've talked about that before in previous episodes about atoms and chemistry and so on. A single atom or even a small group of atoms isn't really alive, so that's not life itself. But we still need to know about atoms because cells are made of molecules which are then made of atoms. So we need to understand how atoms interact and behave in order to understand higher levels of life.

So that's why we put that at the base level. So important examples of atoms include carbon, hydrogen, oxygen, phosphorus. They're common non-metallic elements that are found in, commonly found in life. The next level above atoms is molecules. A molecule, as we've discussed before, is a group of at least two atoms that are held together by chemical bonds, caused by the attraction between the positively charged nucleus and the negatively charged electrons and the electron shells. Examples of biomolecules or molecules that are found in biological organisms, well, water is one, of course, but others include monosaccharides like glucose, so these are small sugar molecules, ATP or adenosine triphosphate, which is used for energy transfer, nucleotides, which are the components of nucleic acids, and also amino acids, which are the individual components of proteins. So those are all molecules. Other ones include neurotransmitters, as I said, water, and also phospholipids, those that have found. in the membrane of cells and so on, made up of a number of atoms.

The next level above that is macromolecules. Now, this is sort of a subset of molecules because macromolecules are molecules too. And these macromolecules are what we talked about in the biochemistry, principles of biochemistry episode. They're basically just really big molecules. But the reason I included them as well is because macromolecules, at least in biology, in many other cases as well, are generally composed of, they're generally polymers, which are composed of multiple units of a monomer,

which is sort of a smaller molecule, which can exist by itself. So, for example, proteins, a class of macromolecules, are composed of amino acids, which themselves are molecules and can exist free in cells and in life forms and so on, but when joined together, form proteins. And DNA and RNA are other examples of macromolecules, those are nucleic acids which are formed out of nucleotides.

Polysaccharides are another example of macromolecules. Those are longer sugar molecules, like, for example, cellulose is a polysaccharide made up of many monosaccharide units bonded together. So macromolecules are very important in cell structure and understanding of so on, and that's the realm of biochemistry there. So atoms molecules, that's more chemistry, maybe organic chemistry, sort of separate to biology but relevant to it. Macromolecules is what you study in biochemistry predominantly.

The next level above that is organelles, which is within the realm of cell biology, maybe a bit biochemistry too. An organelle is a specialised subunit within a cell that has a specific function and is also usually enclosed within its own lipid bilayer, so within its sort of a membrane. And we talked about these in the episode on the cell. Examples of organelles include the mitochondria, used for energy production, chloroplasts, which conduct photosynthesis, flagella for movement, ribosomes, which build proteins, and centrioles which are used for cell division and also maintaining the structure of the cell.

So organelles are generally, they're sort of like mini organs within a cell, which is where they get their name, but they're generally composed of a number of different macromolecules. For example, ribosomes are composed of a number of different proteins connected together or joined together. Same with mitochondria and chloroplasts. They're both, they have some proteins and enzymes in there and also their numerous membranes. So that's organelles.

Next level is cells. A cell is the smallest independently functioning unit of life. Usually consisting of some genetic materials, usually DNA, sometimes RNA, surrounded by cytoplasin and enclosed by a membrane. There is also often organelles, but not all cells have organelles. For example, bacteria don't really have any organelles, just have the genetic material. Examples of different types of cells include archaea and bacteria,

yeast, which are eukaryotic, single-celled organisms, neurons, brain cells essentially found in higher forms of animals, and red blood cells. And there are many hundreds of thousands of different types of cells. But cells are effectively made up of a number of different organelles, plus membrane, plus cytoplasm and a few other things. Organelles are made up of a number of micro molecules, which in turn are made up of a number of molecules, which in turn are made up of a number of atoms.

So you can see how the structure builds up here. Each level is made up of a number of elements from the previous level, plus maybe some even lower than that. For example, a cell is made up of organelles, but it also has macromolecules floating around inside like proteins and so on, and even individual molecules, like water molecules, for example, making up the cytoplasm. Okay, so that's the cellular level. That's a really big chunk of biology that we've just sort of given an overview of there,

especially the sort of more modern fields of biology like biochemistry, genetics, cell biology and so on, that have expanded a lot in recent years, particularly with the discovery of DNA and increasing knowledge of that and so on. Next level above that, the organismal level. So the first level in that is, or the first subcategory within that, is tissues, which is what we'll talk about later on in this podcast. Tissues are an ensemble of cells, so a group of cells, not necessarily identical. the cells can be somewhat different, but they're all from the cells are from the same origin, and they generally reasonably similar, or at least they share, they act in concept to

carrying out a specific function. Examples of different types of tissues are epithelial tissues, connective tissues, muscle, and nervous tissue, and we'll talk more about what those are later. So basically tissues are just a group of cells that act together to form a common function. An example of that function might be to facilitate movement or facilitate information transfer, for example, a nerve cell, or to protect another organ or some body part, that might be an epithelial or connective cell. So the next level above that is organs. Organs are collections of tissues joined together in a single structural unit to serve a common function. So just as tissues are a group of cells that carry out a common function, an organ is just a group of tissues,

or a collection of tissues, that carries out a common function. Examples of organs include the liver and the brain, but also things that we don't normally think about as organs, like, for example, the skin, bones are also organs, stems, roots, and flowers are plants, they're all organs. So, organs is a bit more of a broader category than people generally think about. They generally think as an organ is a sort of a squishy internal thing that sits inside the body and does stuff like the stomach, the liver, the heart, whatever. Those are all organs, but the skin, muscles, as I said, stems and roots of plants, bones, they're all also organs. So most of the body is actually made up of organs. And we'll talk more about that when we get into the growth of the human systems.

But that leads us into the next level up, which is systems, or also called organ systems. And so, as you might guess, a system is a group of organs that work together to perform a general sort of task. Examples of systems include the skeletal system, the immune system, respiratory system, endocrine system, nervous system, etc. We'll go through the rest of those later on. So you can see they build up fairly consistently here. Tissues, just a group of cells, organs, a group of tissues, systems, a group of organs. They just keep building up. The highest level within the organismal level is organisms.

Now, interestingly, this is the level of biology that we are most familiar with, an organism, but it's also the hardest to define. So I'll give some examples. An ant is an organism, a human being is an organism, an elephant is an organism, a tree is an organism, a single bacterium is an organism, a whale, a fungus, etc., those are organisms. So it's a single living thing, basically. But if you think about it, it's hard to define what that is exactly like.

does it have to have multiple organ systems? Well, a bacteria doesn't have organ systems, in fact, that doesn't even have tissues, it's just a single cell. What about a tree? What does that have in common with an ant, that they can both be called organisms? And an ant's not even exactly another wasp and so on.

Some of those social insects are not even really, they can't really survive by themselves. They need to survive within the context of their colony. So is it necessarily meaningful to call them a single living thing? So it's actually a little bit more complicated than you might think. But for that one, it's hard to give a definition. a single continuous living thing, a living system is kind of the best I could find, but basically just go with your intuition in that one.

Sort of when you think of an organism, that's sort of a single living thing, that's what it is. So for more complex forms of life like plants, fungi, and animals, an organism will be made up of a number of organ systems, which in turn made up of a number of organs and so on. But for simple forms of life, like I said, bacteria, protozoan and stuff like that, they don't have to have, they don't even have to have tissues. they might just have numerous cells, or they might essentially just be one organ, or a couple of different organs, not necessarily have very meaningful systems and so on.

Depends how complicated they are. Okay, so that's the organismal level. Next, we have the population level. So the first level within this is populations. This is a fairly simple one to understand. A population is simply all of the organisms that belong to a single group or species. Generally, they belong to the same species when we define them as a population. So it's all the organisms belong to the same species that live in a particular

the geographical area. Generally, a population will be defined so that individuals within that group have some chance or some likelihood of interbreeding with each other. So, for example, a colony of bacteria on a given petri dish, a colony of the same bacteria on a petri dish 10 kilometers away would not be the same population. Although bacteria don't breed, they're not in the same location, they're not sharing the same resources and so on, so they're not the same population. A herd of bison or a herd of castle or something like that, which may be interbreeding with each other, living in the same place, sort of moving together, that's another population. Swarm of bees that live in the same nest, that would be another example of a population. In terms of

humans, it's a little tricky to define that level, because you can talk about the population of any given place, you can have the population of a street, the population of a city, the population and so on. For humans, it's probably most meaningful in the biological sense to talk about the world population, because there's a substantial degree of, obviously, travel between different parts of the world, into marriage and into breeding with different groups. So essentially the world's the world's human population makes up a single population because we're all interact together. But a few hundred years in the past, that probably wouldn't have been very meaningful. Like, for example, particularly before the discovery of the Americas by Europeans,

the Americas and maybe even different groups of the Americas, but certainly the Americas as a whole would have been a different population to, at least the old world, and maybe you could have further subdivided it, but certainly those two would have been separate because there was effectively no contact between the two, no substantial contact anyway, so they would have been different populations. Next level of populations is communities. This one's a little harder to define. But a community is a group of interacting populations of organisms sharing a single environment.

Remember, a community, sorry, a population essentially had to be a group of the same species, living in the same area. Community doesn't have to be of the same species. It's basically multiple populations of different species of organisms, but living in the same environment, sharing the same basic resources or physical environment. They'll generally have different niches within that environment, but they'll share the environment. So, for example, the fish community of this lake or that lake, or this pond or that pond, that would be a community. So there's different species of fish there. There's going to be bacteria, maybe some fungi, whatever. But they're all living in the same basic environment, sharing the same sort of set of resources, same water, sunlight, and so on.

So that's a community. A given area of a rainforest would be another example of a community. So you're going to have multiple different species of trees, shrubs perhaps. You know, maybe you'll have some monkeys living there, a lot of beetles, whatever else is living there, obviously bacteria as well. But they're going to be sharing the same sort of soil, new trees. the same rainfall and so they are a community. Often a community is useful because we can talk about food web interactions. So, for example, if you know you've got predators,

at the top of the food chain who eat herbivores lower down, which then in turn eat the primate producers, which in turn produce their own food, their autotrophes, they collect sunlight and someone, or maybe their bacteria, their chemotroats. We'll talk about what all those things are later on, but the point is that if you're defining a food web or a food chain, it's also called, food web is probably more accurate. The different species are going to be interacting, eating each other or eating something that eats something else that eats the first thing, whatever.

They're interacting, they're sharing the same resources, so in a sense we can define them as a community. Different species living in completely different areas are not interacting that don't share the same resources or that don't predate on each other. They're not going to be the same community. So it's a little bit of a messier definition than some of the earlier ones, but hopefully you can get the sort of idea. So it's basically a community, basically a collection of populations of different organisms which share the same basic environment. The next level above communities are ecosystems. This is basic... An ecosystem is sort of the same as a community.

So it's all the organisms living in a particular area, generally perhaps a bit of a wider area than a community, so it's a bit bigger, but it doesn't necessarily have to be. But the key difference is that it also includes all the non-living physical components of the environment with which the organisms interact. So that's the air, the soil, the water, the sunlight and so on. So generally we talk about...

I mean, you can talk about the ecosystem of that pond or whatever, so you can talk about localized ecosystems, But generally, they're more commonly referred to in sort of broader sense. So rather than the fish community of that lake or that area of the rainforest or this local area of forest or whatever, would talk about a desert as being an ecosystem, or even all deserts as being similar ecosystems, or tundra, rainforest, farms, coral reefs and so on. They're all different types of ecosystems. Biomes is also a fairly similar concept.

It's like different regions of the planet which support sort of different types. of organisms and different interactions of, particularly different types of vegetation because there's different levels of sunlight or rainfall or whatever, and therefore different types of food webs and different interactions between the organisms. Yeah, an ecosystems, basically it's a community plus the non-living stuff, but generally it's applied in a more broader context to a larger area. And finally, the final largest level of biological organization is the Biosphere. The Biosphere is the global sum of all ecosystems, or the total area of land, sea, and air that

is inhabited by living things. So it's basically everything, that everything and all places where life exists. The only biosphere we know of is Earth currently. However, there may be other biospheres in our own solar system. For example, there could be life on Mars. There could be life under the ice of Europa, which is a moon of Jupiter. There could also be life on Titan, which is a moon of Saturn, and also Enceladus, another moon of Saturn could be life there.

So, I mean, there could be life in other places as well, but those are the main contenders. But the point is, if life were to be found there, most probably bacterial life or something like it, but there could be more complicated forms of life. Almost certainly not intelligent life, by the way. But if life were to be found in any of these places, then they would be their own biospheres,

because they're separate to Earth, obviously, they're different planetary bodies or moon bodies, and so they would have their own ecosystems and their own populations and so on down through the different types of organisms. So anyway, that's the levels of biological organization. So just a quick recap, I'll just run through them all very quickly from small to largest. There's atoms, molecules, macromolecules,

organelles, cells, tissues, organs, systems, or organ systems, organisms, populations, communities, ecosystems, and finally the biosphere. So that's the levels of biological organisation. They're the levels at which we study biology. And if you get into a biology textbook or lecture course or anything like that, you'll see that basically each of these categorizations has a certain number of, at least one chapter on a, sometimes more than one chapter.

Organ systems, for example, generally have multiple chapters because there are lots of different organ systems in the human body. Now, we're going to move on to the second part of the podcast, in which we'll look at tissues in more detail. So remember, multicellian organisms consist of many cells, which collectively support the functions of life, but tissues are groups of cells that specialize in a particular function and also generally have a similar structure, a shared structure.

They sort of bind together and form a given unit. in certainly humans and more generally other animals, there are four main types of tissues. Plants and other things like that, of course, are going to have different types of tissues, but at least in certainly mammals and definitely humans, there are four main different types of tissues. Epithelial, connective, muscle, and nervous. And we'll look at each of those in turn. So we'll start with epithelial tissue. Epithelial tissue, essentially the simplest way of thinking about it is that it's covering tissue.

So epithelial tissue sort of covers things, it protects things, it lines inner body surfaces, So on, that's its general function, it's a general purpose. But let's unpack that concept and look at it in more detail. Most epithelial tissues are sort of sheets of cells that line and cover external body surfaces and also internal cavity surfaces. So basically all internal surfaces of the body are lined by epithelial tissues. So, for example, the gastrointestinal system is lined inside and out, essentially, by epithelial tissues. Skin is an epithelial tissue, so basically the entire outside of the body is lined by skin and so on.

inside of your mouth is lined by epithelial tissue. So any membranes you'll find, any cavities, they're pretty much always going to be lined or covered by epithelial tissues of various sorts. Inner linings of the liver and also blood vessels, they're all lined by epithelial cells as well. So whenever, rule of fun, whenever there's a hollow cavity inside the body, it's probably lined by epithelial tissue of some sort. So as I said, the purpose of the epithelial tissues is to protect and contain and provide some support to the tissues and structures and organs underneath.

and also they're often designed to help with the transportation of substances along the body. For example, the blood vessels are smooth to aid in the transportation of blood cells, gastrointestinal system, the inside of that's designed so that it can promote the transportation of food products through the body and so on. Another function of epithelial tissues is also to absorb necessary nutrients or, in the opposite, to secrete wastes or other products that are necessary. So glands, for example, as for example I found in the endocrine system, are specialised epithelial tissues, which produce and then secrete a product, often a hormone or something like that, which is used for signaling purposes or other regulatory functions inside the body.

Once again, to recap that, epithelial tissues, they don't just protect and support tissues or underlying tissues and organs. They also help to transport things and absorb or secrete necessary products. So, for example, the lining of the small intestine is aligned with epitulmonary. epithelial tissue, which is designed or which functions in order to absorb necessary nutrients. Okay, so epithelial cells within the tissue are very densely packed together, kind of like bricks in a wall, leaving very little intercellular space. So there's not very much space between these cells. Remember, each cell is surrounded by membrane, and there can be more or less space between the different membranes of the different cells. Epithelial tissues are generally, have very

little space between them. In fact, they're often sort of pushed up right against each other, and you have gap junctions between the different cell membranes, which is sort of like a protein that joins the two together. It's kind of like a little passageway connecting the two cells. We'll talk more about those when I go through cell membranes in more detail in a later podcast. But anyway, the reason for those sort of tight junctions to keep them together is so that nothing or very little can pass between the cells. And so that's obviously going to be beneficial for epithelial cells

because their point is to protect and to seal off different sections of the body or from each other or also the outside of the body from the inside of the body. so you need to prevent things like water or viruses or other cells from passing in between when you don't want them to. So, for example, the external layer of the skin, the epidermis has, or even below that, actually, the cells are very closely connected to each other so closely, in fact, that water can't pass through, that's what makes skin waterproof, so that's obviously very useful for protecting the human body against water and potential pathogens that might be in the water and so on.

Now, one interesting thing about epithelial cells, because they're so tightly bound together, there's no space for vascular tissue, that is, blood vessels like capillaries, to pass between, the cells, as is the case for most other tissues. And so it's harder to get nutrients and oxygen and so on and also remove the waste from the cells. The way that the body has overcome this problem is that the oxygen and nutrients must diffuse directly through the tissues, so through the cells themselves, from one cell to the other, through these gap junctions to transport the nutrients and waste as necessary. So you can do that. You can diffuse through the cells. The disadvantage is that it takes longer and it's more difficult, and so that puts a limit on the number of cells you can have

sort of next to each other without having an intervening blood vessel. So in turn, that means that epithelial tissues are limited in how thick they can be. They can't be very thick. They're generally less than a millimeter thick, I think, yeah, a fraction of a millimeter the diameter. The outer layer of the skin is like one-tenth of a millimeter thick or something like that. So epithelial tissues are generally very thin, as I've just said, that's because these cells are so closely packed together that you can't fit blood vessels between them, so the nutrients have to diffuse through the cells, and that's difficult, and therefore places a limit on how thick they can be. Okay, so that's a basic outline of epithelial tissues. I'm now going to go through

and look at some of the different types of epithelial tissues, because there are a number of important different types of them, and we'll talk about those in turn. So the way we classify epithelial tissues is generally determined by the shape of the cells. And there are a few main types. There are squamous, cuboidal, columnar, stratified, and pseudo-stratified. So five different types. Other classification systems will be a bit different, but they're some of the main ones. So these words are a little odd, but let's go through them one of the time. Squamous. The way I remember that is it sounds kind of like squashed, squamous squashed, and that's appropriate because squamous cells are very thin. They sort of look like flat plates. They're kind of squashed together.

And an example of squamous epithelia are the epidermis, the atomost layer of the skin. And the reason that they're sort of flat and squash together is they provide a smooth, a low friction surface of which fluids can easily pass through which sub, by which substances can move without very much trouble. Squamous epithelia also found, for example, on the lining of surfaces inside the lungs to help with the diffusion of air through the lungs. Also, the lining of blood vessels is covered by squamous epithelial cells, once again, because they're flat and smooth, which helps for the transportation of blood. They're also being flat, they don't take up as much space so the blood vessels can get smaller, thinner, and thin into more places. That's particularly important

for the capillaries, the smallest type of blood vessels, which need to be, well, very thin, and so they have the flat squamous cells lying their edges. So that squamous epithelial tissues or tissue cells. Next form are the cuboidal cells. By the way, just to avoid confusion, I'm sort of using the word cell and tissue interchangeable here. The reason for that is because we classify these types of epithelial tissue by their cell type. So a cuboidal epithelial tissue is essentially epithelial tissue comprised of cuboidal type cells. So if you were confused by that, that's why I'm using the terms in that way.

So, cuboidal, as you might expect, roughly cuboidal or cubic in shape. They're most commonly found in secretive or absorbative tissue, for example, in those glands that we were talking about. An example being the exocrine gland of the pancreas, or in the lining of the kidneys, which are involved in the absorption of water from the blood. The next level down is columnar. Or the next category is columnar epithelial cells. These are sort of elongated in column-shaped. So we're sort of moving away. First we had squamous, which is sort of flat, then cuboidal, sort of cube.

now a Columna, sort of long and tall. Because they're taller and thicker, they provide more protections, so they're often sort of found in areas that require more protection, like, for example, the lining of the stomach and the intestines. Fourth category of epithelial cells are the stratified, excuse me, stratified epithelial tissues. This is not actually a single classification. It's a bit more complicated because each of the, each of the three shapes of epithelial tissues that we just talked about, that is squamous, cuboidal, and column, can be further classified into one of two groups, they can be either simple or stratified. This just means that you either have one layer, which is simple, a single layer of the cells, or stratified, you have many layers.

Stratified tissues obviously provide more protection, but they're going to require more, they're going to be thicker and require more nutrients than someone to support. So not always would that be necessary to have that many cells. So, for example, a simple squamous layer of epithelial cells provides the lining for a capillary, a single small blood vessel. But the skin, for example, of stratified sclamis and laurent cuboidal epithelial cells, because more protection is necessary. The final type of epithelial cells are the pseudo-stranusified, which are basically just their simple column epithelial tissues, whose nuclei appear at different heights so that it looks like there's more than one layer. Don't worry too much about those. It's kind of complicated

to understand. You might want to look at a diagram for that, but I just include it there for completer. So basically, a pseudo-stratified. It's a single layer of columnna cells, but it looks like it stratified. So hence, pseudos. Okay, so that's the thing. the epithelial tissues. Now I'm going to move on to talk about the connective tissues. Connective tissues, as you might expect, connect different parts of the body together, and also particularly provides support of the organs, of body organs against gravity and against other forces. So they often work in consort with epithelial tissues. Key characteristic of connective tissues, because connective tissues are quite diverse,

as you'll see when I described them in more detail, but a key common characteristic is that unlike epithelial tissues, remember epithelial tissues, the cells are a bunched together, there's pretty much nothing outside or between the cells. Connective tissues are kind of the opposite. There's lots of stuff outside or between the cells, lots of space in between them. So the cells are kind of sparse. The stuff in between the cells, we call the matrix, nothing to do with the movie, the intercellular matrix, or extracellular matrix, and that's composed of non-living material.

So remember, a cell is the smallest living unit, the smallest entity that we describe as living. So the stuff in between the cells, it's part of an organism, but we don't call that living. So the extracellular matrix is non-living. living. And it's often mostly water, but there's other important elements in there as well, which we'll talk about later on. So the type of matrix, or the stuff between the cells is often a very important determining factor in what distinguishing the different types of connective tissues and also their function. Connective tissues are, as I said, very diverse, mostly distinguished by what their extracellium matrix is. Another way we can classify connective tissues is by the number

of collagen fibers that they have. Collagen is just a protein that's very strong. And so, and they're and stiff, but also sort of flexible, depending on how it's joined up and what type of collagen it is, which is used to support cells and tissues. So there are various different classifications of connective tissues, but most of them focus around the different types of collagen fibres, how many of them there are, how they're arranged. So for example, we've got loose connective tissue and dense connective tissue, dense connective tissue having more collagen fibers, therefore making it stronger and stiffer than the loose connective tissue. But I'm not going to go through all the details of that. So just remember that connective tissue is basically for support and

joining and providing structure. And so obviously to do that, you're going to want to have a decent amount of collagen, which is a protein that provides that support and structure, and a different amounts and arrangements of collagen define the different types of connective tissue. Now I want to focus more on special connective tissues because there's two very broad categories of connective tissues. There's sort of the more generic ones that just, you know, support the stomach or provide the support underneath the skin and so on. They're not as interesting. The specialized connective tissues are more interesting. These do more specialized tasks and at first glance at, they may not sound like tissues at all. They are tissues. So remember earlier I said that

some of the things we don't think about as being organs, like bones, or the skin are actually organs. So similarly, some of the things we maybe don't think about as being tissues. We think about them as being something different. They are actually tissues. And many of them specialized connective tissues. So the first one of these I want to talk about is cartilage. And cartilage is a flexible connective tissue found in areas of humans and also other animals that need strength but also need some degree of flexibility. So it's kind of like you need a compromise between tissue and bone and so that's what cartilage is for. Formed from a lot of dense connective tissue, as I said, that has lots of collagen, dense collagen fibres packed together. Regions where they're found include between the joints or at the joints of bones, at certain sections of the rib cage where you need flexibility especially for breathing, supporting the ear, the nose,

elbow, ankle, and also in the discs between the vertebrae in the spine. So if you just touch the tip of your nose and sort of pinch it and wiggle it from side to side, you'll find, well, hopefully, you'll find something sort of a bit hard but flexible there. That's a bit of collagen there that's protecting your nose and providing it with some structure, because otherwise it would just be sort of floppy like your ear. If it didn't have that, those collagen fibres protecting it there. Collagen tissue is composed of specialized cells called chondroblasts that produce a large amount of collagen, which provides the structure to support the tissue or whatever else is surrounding it. The next specialised connective tissue I want to talk about is bone. Now, we definitely

don't think about bone as being a tissue, but it is. So when people think about bones, they often think about them as sort of just being lumps of, well, perhaps you think calcium or just mineral. And that's kind of what we find when we dig up bones, you know, from a graveyard or wherever we're digging them up. But that's not all bone is. When bone is a living tissue. Now, the bone itself, that is the hard thing that we dig up out of the ground, or that survives, that's fossilized or whatever, is that is non-living material. That specifically, that's hard stuff that we find,

is mineral deposits of calcium and phosphate, which make up essentially the extracellular matrix. So remember, I said that the matrix is the non-living part that fits between the cells. So that, in the case of bone, the matrix is, the extracellular matrix, are these deposits, large deposits of calcium and phosphate, which are mineralized and therefore form a very brisk.

well not very brittle, but somewhat brittle, but very hard support structure. And remember from one of the previous episodes, one of the previous chemistry episodes, which I think it was on a chemical bonding, we've talked about ionic bonding, which is very strong but also kind of brutal because they form a lattice structure. That's sort of what's going on in these calcium phosphate mineral deposits. Now, the mineral deposits are formed by cells called osteoblasts, which sort of form the extracellular matrix. And this extracellular bone matrix is constantly being renewed

because there are osteo, as I said, osteoblasts, that produce, excrete the minerals that form the bone matrix, but then there are also osteoclasts which reabsorpe the bone, which are constantly reabsorbing the bone extracellular matrix mineral structure, and for then reforming by new osteoblasts. So essentially in bones, we've got osteocytes, which is the generic name for mature osteoblasts, basically bone cells, sitting in a matrix of this extracellium matrix of calcium and phosphate mineral.

and it's obviously more complicated in that because in the interior there's different types of bone you've got for example spongy bone which is the interior that's where blood vessel not blood vessels are produced the outside of bones is a sort of a layer of osteosites we'll look into that in more detail when we talk about the skeletal system but just i want to give you the idea that bone is an actual tissue and the non-living element the extracellium matrix is a mineral structure which is excreted and constantly being reabsorbed and changed around by the osteosites, which are the cells that actually form the living tissue. So bone is not just sort of a dead thing that's sitting in your body there. They're actually a dynamic living tissue.

So we talked about collagen and bone. The next specialized connective tissue I want to cover is blood. Now, once again, blood is definitely not something you'd think about as being a connective tissue or a tissue of any sort. But think about it. Particularly, well, blood is a liquid, but it's got cells in it, separated by, liquid or the extracellular matrix, which is mostly water, but also has electrolytes and some other various dissolved things in them. Now, the different cells, there are a number of different cells in blood. You're probably familiar with red blood cells, which are called erythrocytes. They carry

oxygen. There are also leukocytes, which are the white blood cells, part of the immune system. Thrombocytes, which are platelets. They're not actually cells, but they're kind of like cells. And those are the cells that form the, actually the living component of the tissue. The liquid with dissolved nutrients, glucose amino acids, fatty acids, carbon dioxide, lactic acid, other bits and pieces that's dissolved in the blood plasma. That's all part of the matrix, which is the extracellular material. And the purpose of blood is obviously to transport nutrients and oxygen to the cells and transport metabolic waste products away from those cells. So blood is an important tissue in the body. So if you ever give a blood donation, you can say I'm giving you some of my tissue.

I gave a tissue donation. Finally, the last category of special connective tissue is adipose tissue, basically fat. Adipose tissue is specialized tissue that stores fat. Unlike most connected tissues, it doesn't actually have that much extra cellular structure. Most of the cells are just comprise of sort of big storage vessels which store lipids or fat deposits basically. Interestingly, when we put on weight or when we gain weight, our fat cells expand because there's more lipids being stored in each of the fat cells, vice versa when we lose weight, they shrink. But the weight loss does not actually change the number of fat cells that we have. That's essentially constant. If you have liposuction, that actually sucks out or removes fat cells. So the trouble with that

is that if you put on weight in the future, or even a little bit of weight, since some of your fat cells have been removed, the energy has to go somewhere, so the lipids are stored somewhere else in remaining adipose tissue. And so what can happen is that you can put on weight in very strange locations, so you might have a relative of a flat stomach, but very chunky arms or feet or something like that, which can be a bit odd. That's just an interesting fact about adipose tissue. So that's also considered to be a connective tissue, obviously because the antipose tissue also functions to provide support and protection for body structures. So, for example, the Bartix obviously provides a lot of antipose tissue there, provides support for when you sit down and so on. There's also an antipose tissue beneath the skin in most places, provides protection for the muscle and for the other things, bones and so on.

It can also be used what is also, also functions as heat storage to maintain internal temperature. Okay, so that's connective tissues. The next type of tissues are muscle tissues. Now these are a little bit simpler. Muscle tissues are made up of muscle cells, which basically have the sole purpose of causing motion, moving things, particularly moving the body. Specifically, they produce force and cause motion.

Now, there are three different types of muscle cells, smooth muscles, skeletal muscles, and cardiac muscles. I'll actually talk about cardiac muscles first. They are only found in the heart, and they are an involuntary muscle in the sense that we can't really control, consciously control their contraction or their movement. They basically serve simply to keep blood-pulled.

pumping around the body by contracting the heart muscle. Now, cardiac muscle has many small interconnected cells, connected via gap junctions, and this is useful because it helps for the transmission of electrical signals, which produced spontaneously by the cardiac muscle itself. So the brain doesn't tell the heart to contract or to pump. It can send signals to moderate the rate at which the heart is pumping, but it doesn't need to tell it to.

The heart itself, cardiac muscle is special, and then it can spontaneously generate. electrical signals which then propagated through these gap junctions between the relatively small cells. And so that's why you can be brain dead, for example, and still have a functioning heart, because the heart can continue to contract, continue to pump blood by itself. Of course, if your brain's stem is also destroyed, then you might have trouble with actually getting your lungs to function, but anyway, and digestion, but that's another issue. So that's cardiac muscle. The next type of muscle is smooth muscle, which is so-called because

it sort of looks smooth. It is also cannot, is not, a conscious control and it's used, it's found within the walls of organs and other structures, like the esophagus, for example, it's used essentially to control the motion of various substances throughout the body. So for example, it lines the intestines and the stomach to control the movement of food and nutrients through there to push it through. It's also found in blood vessels, obviously, to help blood continue to move. The bladder and urethra has smooth muscles there, which can control the pushing out or of urine. when that time comes. Same with the uterus.

Muscles contract and push out the baby and so on. The smooth muscles are made up of long, thin cells, which look kind of smooth, hence the name for them. Skeletal muscles, the final type, are anchored by tenens to bones and are used to move the bones around. So hence are used for maintaining posture and locomotion and so on, useful walking and so on. They're the things we generally think about when we think of muscles.

Scleetal muscles, these are the ones that are under conscious control so we can decide when to move skeletal muscles. And so when people, you know, if someone is muscular, you say they're muscular, they work out a lot. What they're doing is they're building up, increasing the mass and size of their skeletal muscles. Because they're the only, because essentially muscles increase in mass or size with exercise or with use. But you can't voluntarily choose to use your smooth muscles. You can actually choose to use your cardiac muscle a lot by doing cardiovascular exercise, but you do that by moving skeletal muscles, which then has a sort of an indirect effect of requiring more oxygen,

which in turn requires the heart to pump more rapidly. but directly you can choose to move your skeletal muscles, and so that's what people would generally think about when we think of being muscular or think of muscles. It's differentiated from smooth muscle, not only in that it moves bones around and is under conscious control, but also it looks different.

It looks sort of like it's sort of got lots of strips in it. It doesn't look as smooth, it looks sort of divided up into segments, so that's why smooth muscle has that name. Generally a matter of many long thin muscles. In fact, a muscle fiber generally only has a single long, at least along its length has a single muscle cell, but which has many nuclei,

because the muscle cells fuse together to form one big long sort of fibre there. And an individual muscle is made up of numerous fibers. Now, I'll go into muscles more detail when we talk about the muscular system, but I'm just introducing the tissue now so that we can have that as background knowledge for later. Finally, the last type of tissue is nerve tissue,

or nervous tissue. This is perhaps the most specialized of all the different types of tissues because its only purpose is in generating and transmitting electrical impulses throughout the body. Neurve tissue is composed both of neurons, which actually do the transmission of energy, and also neuroglea cells, which sort of assist neurons. They're like the support cells. They provide them with nutrients, they provide neurons with support, protection, and also with

sort of insulation, which can increase the speed of electrical impulses and so on. There are a few different types of nerves tissues or nerve cells. For example, sensory neurons found in the retina, in the ear and so on, they collect information from the external world and use it to produce electrical signals and convey that to internal tissues and organs into the sensory and to the central nervous system, which is like the brain and the spinal cord. Motion neurons are the ones that... So sensory neurons get the information transmitted in.

Motor neurons do the opposite. They take information from the, say, the central nervous system or the brain, and transmit it via signals to the effector cells, like in the muscles, for example. So when we decide to move our skeletal muscles to pick up something, skeletal muscles are doing the actual moving, but the motor neurons are what's telling the skeletal muscles to contract, and therefore that motion to occur.

In between the sensory neurons and the motion neurons are the interneurons. They're the really interesting ones because they're the ones that form the brain and the central nervous system, and they're the ones that do the processing of the information. We'll look at those in more detail later on. As I said, glial cells provide support, protection, insulation and so on for the other cells. They're found, they're not just found in the proofery, they're also found in the human brain. In the human brain, there's roughly one glia forever in neuron. So glia cells, they're often sort of underappreciated when, in studying neuroscience, for example,

but they're also very important, and they're considered to be an aspect of nervous tissue. Okay, so that's all the different types of tissue that I wanted to cover. Now, I'm not going to have time to go through human systems. I think I'll do that in a separate podcast, which will probably be a bit of a short one, but I want to do that anyway. So look out for that soon. I hope you enjoyed this podcast. As usual, if you have any feedback or questions or anything you want to say,

send me an email at FODS12 at gmail.com. Please post a comment or a review of the podcast on iTunes or any other aggregator site you may be familiar with. I've got a few reviews so far. More always appreciated. Please tell your friends about the podcast. or even your enemies, really. I don't really care who you tell. Just invite more people to listen. Thanks for listening, and I'll talk to you next time.