20. The Lymphatic System
Secondary Lymphoid Organs: The Spleen
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In this video, we're going to talk about the structure of the spleen. And so like lymph nodes, the spleen is externally covered by a capsule made of dense irregular connective tissue, which provides structural integrity to the spleen, and it also has inward extensions of that capsule called trabeculae. Now the spleen also has a single splenic artery and a single splenic vein, which are going to carry blood, respectively, into and out of the spleen at the hilum, which recall is the term for the indentation. And so if we take a look at the image down below in the bottom left, you can see the structure of the spleen. And again, it has this hillum or this indentation, which is where the single splenic artery is going to come in to carry blood into the spleen, and the single splenic vein will come out carrying blood out of the spleen. Now, in this image, we're taking a cross section of the and that's exactly what we can see over here in this image. And in this image, you can actually see that the capsule is being labeled this exterior layer of dense irregular connective tissue that goes all the way around the spleen, and you can also see that the trabecula are also being labeled these inward extensions of the capsule. Now, down below, you can actually see that the splenic artery is being labeled right here, which, again, is going to carry blood into the spleen, and the splenic vein is being labeled down below right here, which is going to be carrying blood out of the spleen. Now, internally, the spleen is going to have 2 different types of tissue, and those are going to be the white pulp and the red pulp. Now, both the white pulp and the red pulp play critical immune functions in filtering pathogens out of the blood. However, the white pulp does so mainly with adaptive immunity in T and B lymphocytes or T and B cells, whereas the red pulp does so mainly with innate immunity and macrophages. And the red pulp also has an additional function of removing old, defective, and abnormal erythrocytes, or red blood cells and platelets, from circulation in the blood. Now, the white pulp and the red pulp are both also named based on their appearance under fresh splenic tissue rather than their appearance under the microscope after it's stained. Now the white pulp, as its name implies, is going to have lots and lots of white blood cells or lots and lots of leukocytes, including both b and t lymphocytes. And also, the white pulp has lymphoid follicles or lymphoid nodules, which recall from previous lesson videos contain large populations of B cells or B lymphocytes. And these white blood cells and lymphoid follicles are going to be filtering pathogens from the blood. Now the white pulp tends to be clustered around these smaller branches of the splenic artery called central arteries. And so if we take a look at the image down below, notice that these smaller branches of the splenic artery, highlighted here, is being labeled as the central artery, and that's exactly where we can find the white pulp clustered around those central arteries. And so you can see the white pulp is labeled right there. Now over here on this side, you can also see some white pulp central, cuffed around or centered around a central artery as well. Now, the white pulp being clustered around the central artery is going to get an early jump on pathogens and foreign substances being brought into the spleen before the blood actually makes its way to the red pulp. And as you'll notice, there's a lot less white pulp in the spleen than there is red pulp, and so the white pulp kind of looks like islands in a sea of red pulp, if you will. Now, the red pulp, on the other hand, is going to contain all of the normal components of circulating blood, which includes really high numbers of erythrocytes, or red blood cells. And so, yes, the red pulp does have lots of red blood cells, but again it has all of the other normal components of blood as well. Now, the red pulp is also going to have a very high number of macrophages, which recall are cells that can perform phagocytosis or cellular eating. And the red pulp has a reticular fiber framework that's commonly referred to as splenic cords. So the splenic cords are essentially a reticular fiber framework with a really high number of macrophages. So these splenic cords are called that because the reticular fibers kind of look like cords within the spleen. And so, if we take a look at the image down below, notice that the red pulp is being labeled as all of this red tissue that you can see all throughout the spleen, so it makes up the vast majority of the splenic tissue. And over here in this dotted box, you can see that we are zooming into that region over here on the right, and what you'll notice is that these smaller branches of the splenic artery continuously get smaller and smaller and smaller until they get to the size of capillaries, and eventually, these capillaries are going to be open ended and end in the red pulp. And so when the blood gets to the red pulp, it's kind of leaving the traditional vasculature of the cardiovascular system as it enters into the red pulp of the spleen. And the blood will make its way through the red pulp of the spleen, And, again, in the red pulp, there will be high numbers of macrophages. So you can see that the macrophages are being labeled right here. Now, also, in the red pulp of the spleen are what are known as venous sinusoids, which recall from way back in some of our previous lesson videos that sinusoids are a special type of capillary. And so, really, these venous sinusoids are review from previous videos, so there's really nothing new here. These are just the specialized capillaries that are found in the spleen. And so these venous sinusoids are going to be just like the sinusoids we talked about in previous lesson videos, so they are going to be enlarged, very discontinuous, and they're going to be very permeable capillaries. And again, they're going to be found specifically in the red pulp of the spleen. So you can see over here in this image, we are labeling the venous sinusoids, And, again, it's going to be quite permeable. However, only normal red blood cells are able to make their way into the venous sinusoids. However, old, worn out, defective, and abnormal red blood cells, like these right here, are not going to be able to make their way into the venous sinusoids because they lack the normal flexibility that, most normal, blood cells have that they need to get into the venous sinusoids. And so these abnormal red blood cells will not be able to make their way in, and they get filtered out, and the macrophages are able to, digest them and break them down, and again, recycle their parts so that they can be reused later. Now, down below in this part of the image, what we're showing you is the very slow blood flow through the spleen, and what structures it's going to flow through and flow by. And again, the slow blood flow is incredibly important because it helps to optimize the immune functions, and it also helps the spleen act as a reservoir, since as the blood flows through the red pulp, it's flowing really, really slowly, so the blood can spend quite a bit of time in the red pulp, and it can take several minutes for, you know, the the blood to circulate through the red pulp. Whereas in other tissues, it only takes a few seconds for blood to circulate through those tissues. So again, it's flowing very slowly. And so again, recall that the blood is going to arrive at the spleen via the splenic artery. Now, from the splenic artery, it's going to enter into smaller branches of the splenic arteries, such as central arteries, and around the central arteries is where the white pulp is going to be clustered around. Now, the central artery will continuously branch into smaller and smaller vessels until eventually it gets the capillaries, and those capillaries can end in the red pulp, where, again, the blood is leaving the traditional vasculature of the red pulp of the cardiovascular system temporarily. And then, again, from the red pulp, the blood can enter into venous sinusoids, but only the normal blood cells are able to enter into the venous sinusoids. The abnormal ones will not be able to enter, and so the abnormal ones will get broken down and their parts will be recycled. And so then the venous sinusoid from the venous sinusoids, the blood will enter into the splenic vein and make its way out of the spleen. And so this here concludes our lesson on the structure of the spleen, and we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.
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