Secondary Lympoid Organs: The Spleen - Online Tutor, Practice Problems & Exam Prep

The spleen, the largest lymphoid organ, is crucial for filtering blood, removing old erythrocytes, and recycling components like iron. It consists of white pulp, which supports adaptive immunity through T and B lymphocytes, and red pulp, which aids innate immunity via macrophages. The spleen also acts as a blood reservoir, releasing stored components during emergencies. Its structure includes a dense connective tissue capsule and trabeculae, with blood flow optimized for immune interactions. While splenectomy is possible, it can reduce immune function and increase infection risk.

Function of the Spleen

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, the purple structure 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. 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 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.

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 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, 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 it turns out that the spleen actually has some smooth muscle in it that can contract, again, during emergency situations such as lots of blood loss, for example, and the stored blood components within the spleen, upon the spleen compressing, those stored blood components can be ejected back out into circulation, just like a squeezed sponge here would eject fluids out as well.

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.

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 is trauma to the abdomen region. And so, again, whenever there is some trauma, 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 here 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.

Secondary Lympoid Organs: The Spleen Example 1

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 4 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 in 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.

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.

Structure of the Spleen

In this video, we're going to talk about the structure of the spleen. And, 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 hilum or this indentation, 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 trabeculae 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 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, are 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 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 is 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 can make their way into the venous sinusoids. However, old, worn out, defective, and abnormal red blood cells, like these right here, will not 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 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 to 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 can 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.

Secondary Lympoid Organs: The Spleen Example 2

So here we have an example problem that wants us to choose one of the 4 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, which are also sometimes known as venous 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.

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.

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.

Here’s what students ask on this topic:

What are the main functions of the spleen in the human body?

The spleen has several critical functions: it filters blood, removing old or defective erythrocytes and platelets, and recycles components like iron from hemoglobin. It also filters pathogens and foreign antigens, supporting immune responses. The spleen serves as a reservoir for blood, storing erythrocytes, platelets, and white blood cells, which can be released during emergencies. Additionally, under certain conditions, such as fetal development or anemia, the spleen can act as a secondary site for hematopoiesis, the formation of blood cells.

How does the spleen's structure support its function?

The spleen is covered by a dense irregular connective tissue capsule with inward extensions called trabeculae, providing structural integrity. It has a single splenic artery and vein for blood flow. Internally, the spleen contains white pulp and red pulp. White pulp, rich in T and B lymphocytes, supports adaptive immunity, while red pulp, containing macrophages, aids innate immunity and removes old erythrocytes. The slow blood flow through the spleen optimizes immune interactions and allows it to act as a blood reservoir.

What is the difference between white pulp and red pulp in the spleen?

White pulp and red pulp are two distinct tissues in the spleen. White pulp is involved in adaptive immunity, containing T and B lymphocytes clustered around central arteries. It filters pathogens from the blood early. Red pulp supports innate immunity with high numbers of macrophages and removes old, defective erythrocytes and platelets. It also contains venous sinusoids, specialized capillaries that allow only normal blood cells to pass, while abnormal cells are filtered out and broken down.

Can a person live without a spleen, and what are the consequences?

Yes, a person can live without a spleen, a condition known as splenectomy. Other organs, such as the liver and bone marrow, can take over many of the spleen's functions. However, the absence of the spleen reduces overall immune function and increases the risk of blood infections. This is because the spleen plays a crucial role in filtering pathogens from the blood and supporting immune responses.

What role does the spleen play in hematopoiesis?

Under normal conditions, the primary site for hematopoiesis, the formation of blood cells, is the red bone marrow. However, the spleen can serve as a secondary site for hematopoiesis under certain conditions, such as during fetal development or in adults with anemia or low blood cell count. This secondary role allows the spleen to produce blood cells when the body's demand increases or when the bone marrow is compromised.