 So, next let's think about blood, which is also a connective tissue, and you've probably seen blood before if you've lived to be a college age student. Blood is a connective tissue that contains cells, and we're going to keep this really simple for this lesson, but later on you'll learn more about the different kinds of blood cells. The blood cells include red blood cells, which are also called erythrocytes, and white blood cells, and of course white blood cells are also called leukocytes, and then there are platelets, and platelets are called thrombocytes, and they're really little slivers of cells, they do not even have a nucleus. So the liquid portion of the blood is called the plasma, and there are all sorts of things that are suspended in the plasma, and we'll again keep it sort of simple for now. If we look at a micrograph of some blood, you can see the different things that we're talking about. When you look at this micrograph, you can see many of the cells are sort of pink-colored, and the pink ones are the red blood cells. When you look at ones flat on, like this piece of paper, if you look at it flat on, it would be this way. You see that those red blood cells are circular, but I want you to notice that in the middle of the red blood cells, they're much lighter. So you can see that on this one, for example, or on this one. So we're looking at this straight on, and we see that it's lighter in the middle. If we could look at a cross-section of one of these red blood cells, we would see that it has the shape of a biconcave disc, and so this biconcave disc shape is what makes it appear lighter in the middle. It's interesting, this shape maximizes the surface area to volume ratio for the cells, and later we'll learn that this is what allows red blood cells to be so efficient at exchanging oxygen and carbon dioxide with the tissues. So blood contains red blood cells, which we have looked at. It also contains white blood cells, and the white blood cells, interestingly enough, typically appear purple on the microscope slides. You can see three different kinds of white blood cells here. Later you'll learn about these, but these are white blood cells called neutrophils. This is a white blood cell called an eosinophil, and this is a white blood cell called a lymphocyte. You also see little specks throughout the tissue, and these little specks are the thrombocytes or the platelets. So we could go up and look at this on oil immersion, although I haven't put oil on the slide, but you can see some of these different cells that we were just looking at. In particular, it's interesting to find an eosinophil, which just by chance was there when I first put the microscope slide on the microscope, but eosinophils have red granules within their cytoplasm and a bilobed nucleus. So later we'll do a whole chapter on blood, but that's an introduction for now.