Video: Microcirculation of the spleen

This might look like a steak to most of us, but what if I told you this is actually the spleen? Oh yes! The resemblance is uncanny. Today we're not teaching you how to cook a spleen though – that's something you might see on another creepy website. Instead, we'll take a closer look at the spleen as there's actually a whole lot going on. See? Look at all those little vessels. There's a whole circulatory system in there. I think it's time we find out a bit more about these little structures as we explore the microcirculation of the spleen in today's tutorial.

Before we kick off, here's an overview of what we'll cover today. We'll start by taking a quick look at the spleen as a whole. Then we'll zoom in and explore some of the microscopic structures of the spleen, namely, the red and white pulp before diving right into its microcirculatory system starting with its arterial branches and venous components. Here, we will explore two main circulatory pathways – open circulation and closed circulation. We'll then finish off by taking a look at some clinical correlates relevant to the microcirculation of the spleen. How does that sound? Let's dive in and begin this tutorial with a brief introduction to the structure of the spleen.

The spleen is the largest lymphoid organ in the body and functions to filter and store blood. It is an intraperitoneal structure entirely surrounded by peritoneum except at the splenic hilum. The spleen is located in the left upper quadrant of the abdomen and is protected by ribs 9 and 10. The spleen is approximately the size of your fist but can vary from individual to individual. The external structure of the spleen is composed of two surfaces – a diaphragmatic and visceral surface. The image on the right shows the spleen's smooth lateral diaphragmatic surface that lies adjacent to the diaphragm as its name suggests. The visceral, or medial surface of the spleen, lies in contact with other abdominal viscera.

Located on the visceral surface of the spleen is the hilum. This is where important structures such as the splenic artery and vein enter and exit the spleen. These vessels are responsible for its main arterial supply and venous drainage, respectively. The superior pole of the spleen receives an additional arterial supply from the short gastric arteries which travel through the gastrosplenic ligament to reach the spleen. If you'd like to find out more about the external structure of the spleen, feel free to check out our study unit specifically dedicated to the gross anatomy of the spleen. But for this video, we need to get deeper, right into the internal structure of the spleen itself.

So as you can see here, this is a sagittal section of the spleen. The part of the spleen that we're looking at is magnified from the area shown by this wee box here. We are looking at the internal structure of the posterior anatomy of the spleen. The spleen is covered by a fibrous capsule which we can see highlighted now. The parenchyma of the spleen is formed by the splenic pulp which is functionally and morphologically divided into two distinct components – the red pulp and the white pulp.

The red pulp constitutes approximately 90 percent of the total splenic volume. It is made up of a number of venous sinusoids which function to filter particulate material from the blood as it perfuses the organ. The white pulp consists entirely of lymphatic tissue and constitutes approximately 5 to 20 percent of the total splenic volume. It is subdivided into the periarterial lymphoid sheath, the splenic lymphoid nodules, and the marginal zone. Important vessels of the spleen which we'll meet in just a second travel through the white pulp. Lymphatic drainage begins in the white pulp and is carried along the blood vessels which travel through this region. Let's take a quick look at the subdivisions of the white pulp now.

The component of the white pulp that is highlighted here is called the periarterial lymphoid sheath. As its name suggests, it is a bulky sheath composed of lymphoid tissue surrounding one of the arterial structures of the spleen known as the central arterioles which we'll meet in a bit. This sheath, often referred to as PALS, mainly consists of T-lymphocytes. Lymphocyte aggregation and plasma cell formation occurs within this region of the white pulp.

Formed by expansions of periarterial lymphoid sheath and housing the B-lymphocytes are the splenic lymphoid nodules which are now highlighted in green. These nodules make up most of the remaining portion of the white pulp and are typically found at bifurcation sites of the central arterioles. Similar to the periarterial lymphoid sheaths, splenic lymphoid nodules are centers of lymphocyte aggregation and proliferation and can be divided into primary and secondary splenic lymphoid nodules. Secondary splenic lymphoid nodules are made up of two distinct parts – the central germinal centers and the mantle zones which surrounds the germinal centers. The germinal centers of the white pulp serve as sites for lymphocyte production.

Finally, the marginal zone, also known as the perifollicular zone, is situated at the interface between the red pulp and the periarterial lymphoid sheath of the white pulp. This zone of white pulp functions to screen the systematic circulation for pathogens and is involved in antigen processing. A band of macrophages known as the metallophilic macrophages can be found along the inner portion of the marginal zone. The marginal zone sinus is located adjacent and peripheral to the metallophilic macrophages and is continuous with the vessels that supply the capillary beds of the PALS and splenic lymphoid nodules.

Now that we've explored the red and white pulp of the spleen, let's take a look at another structure that traverses the parenchyma of the spleen. Extending from the fibrous capsule and projecting onto the pulp of the spleen are connective tissue trabeculae. The trabeculae provide internal support and function as a transport route for blood vessels in and out of the spleen. Traversing through the splenic pulp along the trabeculae are the trabecular arteries and the trabecular veins. How the blood passes between these vessels is what we're going to take a closer look at today.

Now that we have a general understanding of the function and microstructure of the spleen, we can focus our attention on the blood flow through these structures.

As we've just mentioned, traveling within the connective tissue trabeculae are the trabecular arteries of the spleen. The trabecular arteries arise as branches of the splenic artery after it enters the spleen via the hilum. As the trabecular arteries traverse through the splenic pulp along the connective tissue trabeculae, they taper into arterioles which extend into the white pulp. Once these arterioles reach the white pulp, they become known as the central arterioles of the spleen. The periarterial lymphoid sheath that we met earlier surrounds these arterioles and helps to fight invading bacteria within the bloodstream as it is transported through the microcirculation of the spleen. The central arterioles extend throughout the white pulp of the spleen and give off smaller arterial branches which feed the white pulp capillary beds.

Within the white pulp of the spleen, the central arterioles give rise to the penicillar arterioles. These arterioles exit the white pulp and traverse the marginal zone between the red and the white pulp to enter the red pulp. These arterioles are surrounded by a small collection of splenic macrophages which make up the periarteriolar macrophage sheath. This collection of splenic macrophages are located within the red pulp of the spleen and function to clear and remove blood-borne particles which travel through the penicillar arterioles. From here, it is thought that the blood travels through one of two systems.

The first and what is supported as the dominant system is the open circulatory system. Blood is transported through the penicillar arterioles and is emptied into the reticular tissue of the splenic cords of the red pulp rather than within the lumen of the vessels. Within the open circulatory system, the blood suffuses the reticular tissue of the splenic cords and is no longer contained within the arteries or arterioles.

Before we move on to the closed circulatory system, let's take a few moments to look at some of the components of the open circulatory system.

The splenic cords, also called the Cords of Billroth are located within the red pulp between the venous sinusoids. They are the cellular aggregates consisting of macrophages, plasmocytes, and blood cells. The reticular formation of the splenic cords contains numerous openings or spaces which allow for the passage of blood. Macrophages located within these spaces filter blood as it passes through removing any damaged or worn-out erythrocytes. After percolating through the splenic cords, blood enters venous sinusoids, also within the red pulp. The venous sinusoids form an irregular tubular space for the passage of blood and take the place of both capillaries and venules. Slits within the walls of the venous sinusoids aid in further filtering blood as it passes through. Blood emerges through the slits within the walls of the venous sinusoids to reenter a vascular lumen known as the red pulp vein. The red pulp vein functions to transport blood back towards the trabeculae, specifically, to the trabecular vein of the spleen. At the trabeculae, the red pulp vein drains into the trabecular vein of the spleen. These veins travel along the trabeculae towards the hilum and eventually unite to form the splenic vein.

Okay, so now for a little recap on the open circulation. Blood travels through the penicillar arterioles and empties into the reticular tissue of the splenic cords of the red pulp where it is no longer contained in a vessel. It then enters the venous sinusoids to reach the red pulp vein. The red pulp vein transports this blood back towards the trabecular vein of the spleen and onto the splenic vein.

So now that we have an understanding of the open circulation of the spleen, let's take some time to explore the other circulatory system.

Blood can also travel within the spleen through a closed system of vessels. This is known as closed circulation. The main difference here is that the blood never travels through the splenic cords. It always stays within a vessel, hence, the description, "closed." Blood flows through the spleen following the same pathway until it reaches the penicillar arteries. Within the closed system, the penicillar arteries which we can see here highlighted in green are continuous with the venous sinusoids. Blood passes through the penicillar arteries and continues through the venous sinusoids as it makes its way through the spleen. Once in the venous sinusoids, blood in the closed circulation follows the same path to exit the spleen. From the venous sinusoids, it passes to the red pulp vein which empties into the trabecular vein before finally draining into the splenic vein. The splenic vein is the terminal point of the microcirculation of the spleen. It collects blood from both the open and closed circulatory systems of the spleen, leaves the spleen via the hilum, and unites with the superior mesenteric vein to form the hepatic portal vein.

Let's quickly go over closed circulation one more time. So, blood passes through the penicillar arteries and continues through its venous sinusoids as it makes its way through the spleen. Once in the venous sinusoids, it passes to the red pulp vein which empties into the trabecular vein before finally draining into the splenic vein. The splenic vein is the terminal point of the microcirculation of the spleen.

Now that we have a good understanding of the microcirculation of the spleen, it's time to take a look at clinical notes to help consolidate our knowledge.

In the open circulatory system, we identified that blood flows through the splenic cords. Due to its fine framework and mesh-like structure, it functions to sift all damaged or abnormally-shaped erythrocytes. Disorders of erythrocytes such as hereditary spherocytosis may affect the size and shape of erythrocytes within blood. When erythrocytes are too large or lack flexibility, they are unable to pass through the splenic cords. Erythrocytes that fail to pass through are subject to phagocytosis and destruction by monocytes and macrophages resulting in what is known as extravascular hemolysis, so extravascular hemolysis is the removal of abnormal erythrocytes from the circulation by the spleen and liver.

Hemolysis is a fancy word for breakdown or destruction of erythrocytes. This type of hemolysis is classified as extravascular as opposed to intravascular hemolysis which occurs within the blood vessels. Depending on the level of severity, extravascular hemolysis can result in hemolytic anemia, and in certain cases, may be accompanied with splenomegaly which is an enlarged spleen. Signs and symptoms of hemolytic anemia as a result of extravascular hemolysis may include pallor and fatigue from anemia and abdominal pain from splenomegaly. Treatment depends on the condition underlying extravascular hemolysis and may involve performing a splenectomy or removal of the spleen.

And now you're an expert on the microcirculation of the spleen. Before you run away, let's quickly review what we learned today.

We started by looking at the spleen as a whole identifying its lateral diaphragmatic surface and its hilum on the medial surface. We then looked at a sagittal section of the spleen highlighting the splenic capsule making up the outer surface. Internally, we saw the splenic pulp. We then highlighted the two types of pulp making up the parenchyma of the spleen – first, the red pulp highlighted now, and the white pulp. Within the white pulp, we identified the periarterial lymphoid sheath which mainly houses T-lymphocytes. We also looked at the splenic lymphoid nodules where B-lymphocytes are housed. We identified the two parts of the secondary splenic lymphoid nodules – the central germinal center and the peripheral mantle zone. Lastly, we identified the marginal zone and the marginal zone sinus, situated at the interface with the red pulp. Extending from the fibrous capsule and projecting into the pulp of the spleen are connective tissue trabeculae which are fibrous extensions of the capsule, and within that, the trabecular arteries and trabecular veins.

Next, we got into the nitty-gritty of the microcirculation of the spleen starting with branches of the splenic artery – the trabecular arteries. Arising from those and traveling into the white pulp were the central arterioles. Exiting the white pulp, we identified the penicillar arterioles which we saw were surrounded by periarteriolar macrophage sheaths. We then followed the pathway of blood through the open circulation and into the splenic cords where cellular aggregates of macrophages, plasmocytes, and blood cells are located. From there, we identified the venous sinusoids in the red pulp – the red pulp vein and the trabecular veins. We then went back to the penicillar arterioles to describe the closed circulation and saw that in this circuit, these arterioles drain directly into the venous sinusoids and the blood always remains within a closed vessel. To finish, we explored why extravascular hemolysis occurs, how it presents, and its associated treatment options.

And that brings us to the end of our tutorial on the microcirculation of the spleen. I hope you enjoyed it and happy studying!