Secondary Lympoid Organs: The Spleen - Video Tutorials & Practice Problems
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concept
Function of the Spleen
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7m
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In this video, we're going to continue to talk about secondary lymphoid organs as we introduce the spleen and talk about functions of the spleen. Now, the spleen is actually the largest lymphoid organ. In fact, the spleen is about the size of a fist and it's located on the left side of the abdomen. So if we take a look at the image down below, notice that within this dotted box here, the purple structure here on the left side of the abdomen is the spleen. And notice that the spleen is tightly packed in and in direct contact with a bunch of different body structures and organs. Now, over here on the right, we have a zoom in of the spleen. Now, it turns out that the spleen is made up of really soft tissue. And so the shape of the spleen is impacted by the contact points that it has with other body structures and organs that are around it. And so the spleen tends to have a lot of different indentations in it from those contact points with other body structures. Now, in many ways, the spleen can be thought of as a really large lymph node. And that's because functionally the spleen is quite similar to lymph nodes except for the very important fact that the spleen filters blood, not lymph. Whereas recall from previous lesson videos that lymph nodes filtered lymph. Now, as the spleen is filtering the blood, it actually removes any abnormal old or defective erythrocytes or red blood cells and platelets or thrombocytes from circulation in the blood. But the spleen can also remove any cellular debris, foreign pathogens and foreign antigens from the blood as well. Now, in addition to removing these substances, the spleen can also recycle components from these abnormal erythrocytes such as for example, recycling the iron that's found in the hemoglobin of these abnormal erythrocytes. So that later down the line that iron can be reused to make fresh hemoglobin molecules. Now, very similarly to lymph nodes, the spleen is going to provide a site for lymphocyte proliferation and activation to generate immune responses either right then and there within the spleen or the activated lymphocytes can migrate through the bloodstream to the site of infection and carry out the immune response at that distant site of infection. Now, the spleen can actually serve as a reservoir for blood where it stores many blood components. And so it can actually store or stores erythrocytes or red blood cells, platelets or thrombocytes and many white blood cells such as monocytes and these stored blood components can actually be released back into the bloodstream in emergency situations. In fact, it's estimated that about 30% of all of the erythrocytes or platelets are stored in the spleen at any given time. And so this reservoir function of the spleen where it stores these blood components is somewhat analogous to a sponge. And that's because like a sponge, the spleen can absorb significant amounts of fluids uh of blood and upon being squeezed or compressed, it can actually eject those fluids back out into circulation. And so notice over here in the image down below, we're showing you a sponge for that reason. And so it turns out that the spleen actually has some smooth muscle in it and that smooth muscle can contract again during emergency situations such as lots of blood loss, for example. And uh the stored blood components within the spleen upon being the spleen compressing, uh those stored blood components can be ejected back out into circulation just like a squeezed sponge here would eject fluids out as well. Now, last but not least the last function that we have here of the spleen is that under certain conditions such as during fetal development or in adults that have anemia or low blood cell count, the spleen can actually serve as a secondary site for hematopoiesis or the formation of blood cells. But recall from previous lesson videos that the primary site for hematopoiesis is the red bone marrow. And so it's important to keep in mind that the spleen will only serve as a secondary site for hematopoiesis only under certain conditions. Now, down below, in this image, we also have some text that reveals that blood flow through the spleen is actually relatively slow, especially in comparison to blood flow through other tissues in the body. And the reason that this slow blood flow is so important is because the slow blood flow helps to optimize immune interactions. And we know that the spleen again serves as a site to generate immune responses. And also the slow blood flow through the spleen helps the spleen serve as a reservoir for blood storing these components because they move really slowly through the spleen. Now, the last note that I'll leave you all off with here is that it is possible to survive without the spleen. In fact, the term splenectomy refers to the surgical removal of the spleen. And again, because the spleen is made up of really soft tissue, it's really susceptible to damage whenever there's trauma to the abdomen region. And so, uh again, whenever there is some uh trauma, it's it's possible for the spleen to regenerate on its own. But in some cases, a splenectomy may be required the removal of the spleen and under those conditions, other organs such as the liver and bone marrow can actually help take over most of the functions of the spleen since a lot of the functions of the spleen overlap with these other structures. However, it is important to note that the absence of the spleen does decrease overall immune function and increase the overall risk of blood infections. And again, this is because we know that the spleen is involved in immunity and that it does filter pathogens from the blood. So this year concludes our brief introduction to the spleen and the discussion on the functions of the spleen. And as we move forward in our course, we'll be able to apply these concepts and talk more about the structure of the spleen. So I'll see you all in our next video.
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example
Secondary Lympoid Organs: The Spleen Example 1
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1m
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So here we have an example problem that asks during which of the following emergency situations, would the spleen be able to assist the body? And we've got these four potential answer options down below. Now, option A says a sudden increase in blood glucose concentration in a diabetic person. However, the spleen does not have a direct role in regulating blood glucose levels. And so the spleen wouldn't really be able to assist directly in this scenario. So we can eliminate answer option A and for a similar reason, we can also eliminate answer option C which says a sudden decrease in blood glucose concentration in a diabetic person. So now we're between either answer option B or answer option D and notice that option D says an inability to secrete functional digestive enzymes. However, the spleen is not directly involved in secreting digestive enzymes. So for that reason, we can eliminate answer option D and of course, this leaves answer option B as the only option and it is the correct answer which says a sudden decrease in the number of erythrocytes and platelets and circulation. And so, recall that the spleen can act as a reservoir for specific blood components such as erythrocytes, platelets and macrophages, for example. And so whenever there is a sudden decrease in these blood components, the spleen is able to release these blood components into circulation to help compensate for that decrease. And so option B is the correct answer to this example problem that concludes this example and I'll see you all in our next video.
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Problem
Problem
What is a common outcome among people who’ve had a splenectomy (spleen removal)?
A
Increased susceptibility to infection.
B
Increased risk of blood disorders.
C
Impaired hemostasis.
D
All of the above.
4
concept
Structure of the Spleen
Video duration:
10m
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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 trabecula. 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 hill, 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 hill 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 spleen 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 trabecular 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 two 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 and T and B lymphocytes or T and B cells. Whereas the red pulp does so mainly with innate immunity and macrophages. And 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 it 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 um 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 our 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 venus 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 venus sinusoids are review from previous videos, there's really nothing new here. These are just the specialized capillaries that are found in the spleen. And so these Venus 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 Venus sinusoids. And again, it's going to be quite permeable. However, only normal red blood cells are able to make their way into the Venus 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 venus sinusoids because they lack the normal flexibility that uh most normal uh blood cells have that they need to get into the venus 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 uh 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 um you know, the, the blood to circulate through the red pole. Whereas in other tissues, it only takes a few seconds for blood to circulate through those tissues. So, again, it's flowing very slow. 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 artery 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 Venus sinusoid from the Venus 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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example
Secondary Lympoid Organs: The Spleen Example 2
Video duration:
1m
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So here we have an example problem that wants us to choose one of the four potential answer options down below that best fills in the blank in this sentence which reads that the spleen's white pulp is composed primarily of blank. And the answer options are macrophages, platelets, splenic sinusoids or lymphocytes. And so we need to recall from our last lesson video that the white pulp of the spleen is named because it has lots of white blood cells, which means that we can eliminate answer option B platelets. Since platelets are not white blood cells recall platelets also sometimes referred to as thrombocytes are cell fragments involved in the blood clotting process. And we can also eliminate answer option C which says splenic sinusoids uh which are also sometimes known as venus sinusoids. And these are specialized capillaries found in the red pulp of the spleen. So now we're between either option, a macrophages, which are a type of white blood cell and option D lymphocytes, which again are also a type of white blood cell. And so we need to recall from our previous lesson video that the white pulp is composed primarily of lymphocytes specifically the T and B lymphocytes or the T cells and the B cells and the macrophages are not going to make up the primary component of the white pulp. And so we can indicate that answer. Option. D lymphocytes is the correct answer to this example. That concludes this example. And I'll see you all in our next video.
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Problem
Problem
What is the primary function of red pulp in the spleen?
A
Filtering & removing old/defective erythrocytes.
B
Activating the adaptive immune response when a pathogen is identified.
C
Producing new leukocytes.
D
Gas exchange.
7
Problem
Problem
Which of the following statements about the spleen is true?
A
It is a primary producer of red blood cells.
B
It plays a role in both blood filtration & immune response.
C
It only plays a major role in the immune system, but not blood filtration.
D
It plays a role in the respiratory system.
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