 Now, one of the characteristics of life is that living systems exhibit this hierarchical organization. And this is something that you've seen in general biology and you've seen it in anatomy. And so this is totally review, but I want to give you a sort of physiological context. This hierarchical organization represents the structure of a living system or the structure of a human in this case that will ultimately determine the function of that human. So let's just briefly review this stuff. You know that the subatomic particles, because you've already had chemistry, rock stars, subatomic particles are the little pieces that make up an atom. Protons, neutrons, and my symbol for electrons. Protons are positively charged. The number of protons determines the element that you're looking at. Number of neutrons determines the element's stability and the number of electrons can actually change in an element and affect its charge. The atoms interact with each other by sharing electrons and that's how you actually can form chemical bonds. Subatomic particles make up the atoms. We're going to be talking about lots of different atoms. Oftentimes the atoms that we're talking about are actually ions and that just means that the atom is a charged particle. So a chlorine, chlorine atom that gains an electron, an electron from someone else and then has a negative charge becomes a chloride ion. Sodium is an ion with a positive charge. Calcium is an ion with a positive two charge. So we're going to be looking at all sorts of atoms and we can put those atoms together to make molecules. Water is an example of a molecule. Oxygen gas is an example of a molecule and carbon dioxide. I mean, we're going to mess with these things all the time. I want to throw one more atom in there. I want to throw hydrogen ions in there because those are pretty important in physio. If we take molecules and atoms and put them together, we can build biomolecules and remember that biomolecules are just big molecules that are in found in living systems and there are four categories. If they are, go ahead and holla, holla those out. You got your carbs, that's right. You got your proteins, uh-huh. You got your lipids, those are your fatty fats and you have your nucleic acids. Nucleic acids, don't forget, that's like DNA and RNA and very important biomolecules. If we're going to study nucleic acids, remember from your Bio 1 rock star days that nucleic acids include DNA and RNA and those provide the instructions for building proteins and protein, holy protein. We're going to be talking about a lot of proteins in this class. Proteins often are molecules that hold a certain structure and therefore carry out a certain function. If we put a bunch of biomolecules together, we can actually build organelles and organelles are things like mitochondria, ribosomes. We've got the Golgi apparatus, the smooth and the rough endoplasmic reticulum, the cell membrane, like all this of the nucleus, the nucleolus, all this stuff, all of these are organelles that we're going to look at in this class. Now, if we put our organelles together, you know you want to put your organelles together. We can build cells. Cells are the first place where we see life. In Physio, we'll look at cells and we'll look at how they do the things that they do, how they're talking to each other, how they're producing hormones, how they're responding to hormones. We definitely are functioning in a cellular level of organization. We're learning about how the body functions at a cellular level of organization. But what we know from anatomy is that we can put together cells, multiple different kinds of cells, and we can ask them to function together in a common function and that is a tissue. So cells, and let's just make a note that that was different kinds of cells that work together for a common function. And yes, I shorthanded that whole thing. But you know what I mean because you are histology superstars. I love histology. If we put together our cells, we can build tissues. If we put together tissues, different tissues working together for a common function, we can end up with organs. And our organs can be put together, like multiple organs can work together for a common function. Let's just include that in all of these definitions. So organs are tissues working together and organ systems are organs working together. And the organism is the whole shebang. It's the whole set of organ systems working together to maintain a common function. Now, in the next section, we're going to be looking at the organism level because we're going to talk about how organ systems maintain homeostasis. And in order to talk about homeostasis, we sort of have to define it. It's a novel concept. Let's do that. Let's dive into homeostasis.