Circulation through the cord is congested and slow. This delay exposes blood cells, bacteria, and particulate matter to the red pulp. As much as 6% of the cardiac output goes to the spleen. This delay causes hemoconcentration of blood. In addition, the erythrocytes have to traverse narrow slits, which require more time than usual.
The red pulp cord comprises specialized fibroblasts, numerous macrophages, and a tridimensional network of reticular fibers, collagen type IV, and laminin. Its special microcirculation allows filtration. Blood enters the red pulp cord through the sinuses and is phagocytosed by red pulp macrophages. This process is essential for red blood cell turnover and iron recycling. It is possible only because of the unique structure of the red pulp.
The first layer of the red pulp cord or sinus comprises capillary endothelial. The endothelium in the red pulp is composed of specialized cells called capillary endothelial cells and pericytes. These cells form a sheath, which mimics the cuboidal epithelium. The sheath covers up to four sequential bifurcations of the post-arteriolar capillary.
The red pulp capillaries are characterized by variable-sized lateral openings, which are the ends of the capillary vessels. The open end of a vein can have a funnel or trumpet-like structure. Lateral openings are more prominent in cross-sections and may be lateral extensions of a different lot.
The capillary endothelial of the red pulp cord and sinuses are composed of cells called capillary endothelial cells. Blood cells are present at the capillary endothelium’s opening, and macrophages and plasma cells are in contact with them. The open spaces of the capillary endothelium serve as a reservoir for platelets, reticulocytes, and mature erythrocytes in some animal species.
Perifollicular capillary sheaths
Perifollicular capillary sheathing occurs within the red pulp cord and sinuses. This network of capillaries is characterized by its CD271+ and CD34+ open ends. The sinus network impedes the majority of red pulp capillaries. In addition, these capillaries are localized after dichotomous branching. In this image, the left capillary stem carries a tiny sheath.
Red pulp cords have many capillaries, whereas the sinuses are filled with smaller veins. These capillaries contain a more significant number of open ends than red pulp capillaries. These capillary networks are the primary bloodborne antigen and oxygen source for the red pulp cord.
The red cord or sinus capillary sheaths are the first immune cells to encounter bloodborne antigens. They are composed of three types: CD271+ stromal sheath cells switched B-lymphocytes and macrophages. The capillary sheaths are a characteristic of almost all post-arteriolar capillaries. These sheaths are associated with B cells predisposed to recognize and react to foreign materials.
The red pulp cord is supported by a network of stromal cells and sinus wall cells. The red pulp contains numerous reticular cells and many macrophages and lymphocytes. The red pulp cord also has large numbers of red pulp macrophages and is closely associated with the sinus wall.
The anastomosing network between capillaries and sinuses
The anastomosing network between capillaries, sinuses, and veins connects the blood vessels in different body areas. It is a network of blood vessels similar in size to capillaries, with a semi-permeable wall that permits small molecules to pass through. Oxygen and nutritive material travel through the capillary unit to tissues, while waste products move through the membrane and into the venous bed. In addition, white blood cells travel through capillaries by either passive diffusion or pinocytosis.
The capillaries in the nasal mucosa are interconnected with the sinuses to form a dense, anastomosing network. The veins in the sinuses are interlinked by the venous sinusoids, which can undergo vasoconstriction and vasodilatation. In addition, the anastomosing network between capillaries can facilitate rapid blood flow, thereby contributing to air-conditioning and filtering functions.
The capillary bed consists of two main parts: the metarteriole and the venule. Each component contains a precapillary sphincter, which controls blood flow through the true capillary. A precapillary sphincter can be opened to let blood flow into the true vein. It can also dilate, allowing blood to travel through the capillary bed.
Capillaries also have a network of endothelial cells that are flat and thin. These cells help the lumen of the capillaries exchange fluid with the tissue. Similarly, the sinusoids have specialized veins. They exchange blood cells between blood vessels and organs.
Corrosion casts are caused when the blood enters the red pulp cord or sinus first. This is controversial, but recent experiments have shown that a large percentage of blood enters the red pulp cord or sinus. A portion of the blood then passes to the venous sinus or white pulp cord.
There is some evidence that fibronectin functions as an adhesion molecule in B cells. Fibronectin and von Willebrand factor bind a4b1, which may be involved in B cell migration. However, it has yet to be discovered how these two molecules interact. However, the two molecules have overlapping staining patterns.
Sequestration of abnormal red cells in the cordal compartment
The red pulp in the spleen consists of splenic sinuses and cords, and red blood cells can squeeze through these capillaries. However, older and more fragile red blood cells may rupture during the passage. The spleen also contains phagocytic macrophages, which engulf and destroy cellular debris. Lymphocytes are also present in the spleen, helping the immune system defend against infection.
The red pulp contains specific microcirculatory structures that are used to sense the deformability of RBCs. The best-known of these structures is the inter-endothelial slit, a dynamic slit between two string-like endothelial cells. This structure squeezes RBCs to enter the sinus lumen, the most stringent deformability-sensing structure in the body.
Sequestration of abnormal red cells in the red pulp cord has several causes. One is hereditary spherocytosis. In hereditary spherocytosis, abnormal red blood cells are accumulated in the cordal compartment. Spherocytes are particularly sensitive to adverse conditions found in the cordal compartment, including a high level of permeability and reduced hemoglobin. In addition, the immigration of monocytes contributes to splenomegaly.
Another cause of abnormal red cell sequestration in the red pulp cord is anemia. The spleen helps to filter blood and remove old blood cells and other foreign material. It also plays a role in the immune response to antigens in the blood. Among different parts, the spleen has lymphoid cells and T and B lymphocytes.
The red pulp comprises reticular tissue and sinusoids, which are large irregular venous spaces. The reticular tissue contains numerous macrophages, lymphocytes, polymorphs, and platelets. It also includes groups of plasma cells along the trabeculae and marginal zones. There are also strands of reticular tissue between the sinusoids called Billroth cords.
Generally, the red pulp capillary network has open ends and is blue, green, or brown. Most of the capillaries are impeded by the sinus network. End processes may be fixed to surrounding structures using reticular fibers, but this may vary depending on the position of the end processes. In some cases, capillary end processes displace from surrounding structures, mainly if they are located near red pulp venules.
The red pulp cord or sinus has a unique microcirculation, with organ-specific microvessels that form the initial part of venous circulation. These microvessels eventually fuse to form venules and veins. However, the precise morphology of the splenic red pulp capillaries needs to be better understood.
As the red pulp becomes more crowded, it shifts from an open to closed circulation. As cellular traffic increases, the red pulp is increasingly congested. Its open circulation is influenced by the rise in cellular traffic downstream, whereas the splenic blood flow remains unaffected. In addition to red blood cells, platelets are trapped within the red pulp.