The Spleen ABOLARIN A.T BUTH, OGBOMOSO Outline
Structure and physiology
Functions
Diseases associated with the spleen The major functions of the spleen are (i) filtration and ‘quality control’ of red cells within the circulation (ii) capture and destruction of blood-borne pathogens (iii) generation of adaptive immune responses. In order to achieve these aims the Spleen has evolved a unique anatomical structure that is based on the filtering of blood through two main systems. These consist of a white pulp, which is concerned mainly with immunological function, and a red pulp, which regulates the selection of red cells for re-entry into the circulation. Structure of the spleen
The normal spleen weighsabout 150–250 g, but there is considerable variation between normal individuals and at various ages in the same individual.
At puberty it weighs about 200–300 g but after the age of 65 years this decreases to 100–150 g or less.
In the adult its length is 8–13cm, width 4.5–7.0 cm, surface area 45– 80 cm2 and volume less than 275 cm3.
A spleen greater than 14 cm long is usually palpable. Blood flow
It is enclosed by a connective tissue framework that extends inwards to form a fibrous network. Blood enters at the pelvis and the majority of vessels open into these open networks(the red pulp) before re-entering the closed venous system. There is no afferent lymphatic to the spleen and the efferent lymphatic system leaves along the route of the splenic vein. The spleen contains a large amount of lymphatic tissue that is mostly concentrated in concentric rings around the arterioles (white pulp). Between the red pulp and white pulp is an intermediate marginal zone, which lies at the periphery of the white pulp, blending into the red pulp.
The red pulp
In the red pulp there are two major forms of blood circulation: a closed system, typical of the rest of the vascular system, in which arteries and veins communicate through endothelial lined vessels an open circulation, in which arterioles terminate in free endings on splenic cords (also known as the cords of Billroth) The cords consist of numerous macrophages and erythrocytes and they are critical to the filtration function of the spleen. Red cells in the cords need to gain entry to a venous sinus of about 20– 40 mm in diameter, lined by endothelial and adventitial cells with a basement membrane and possess narrow interendothelial spaces in thesinus wall through which flexible red cells may pass. Red cells with inflexible membranes, mostly elderly, are not able to pass through these gaps and are ingested by macrophages in the cords.
Blood pooling
The normal red cell content of the spleen is 30–70 mL, which represents less than 5% of the total red cell mass.
When the spleen is enlarged, expansion of the vascular bed occurs. This results in a considerable pool with a high haematocrit and only a slow exchange of red cells with the general circulation.
In states Of massive hypersplenism, such as myelofibrosis, as much as 40% of the red cell mass may be pooled in the spleen. The normal spleen contains a reservoir of granulocytes that is in dynamic equilibrium with the circulating granulocytes. It represents 30–50% of the total marginating pool.
20–40% of the total platelet mass is pooled in the spleen and the platelets spend up to one-third of their lifespan there. The pool increases when the spleen is enlarged. Roles of the spleen – quality control
The spleen acts as the quality control organ of the red cells via sequestration and /or phagocytosis.
Sequestration is a reversible process whereby cells are temporarily trapped by adhesion to the reticular meshwork of the cords on their passage through the spleen.
Phagocytosis is the irreversible uptake by macrophages of particulate matter, non-viable cells and viable cells that have been damaged by prolonged sequestration or by antibody coating.
Removal of Heinz bodies, Howell jolly bodies, nuclear remnants and siderotic granules. Immunological functions of the spleen
Macrophages and B cells act as antigen presenting cells.
T cell destruction of foreign bodies
Bcell maturation in the splenic follicles.
T cells produce first line of defence against encapsulated organisms S.pneumonia, N, meningitidis, H, influenzae Extramedullary haemopoiesis
The spleen is an important site of haemopoiesis in utero and retains the ability to re-activate this process after birth.
This can occur as a compensatory erythroblastic hyperplasia in severe anaemia, such as chronic haemolysis, megaloblastic anaemia and thalassaemia major……
or as a more generalized haemopoiesis often seen in primary myelofibrosis or other malignant disorders in the bone marrow. Causes of Splenomegaly
Hypersplenism
Hypersplenism is a clinical syndrome and does not imply a specific causal mechanism. It has the characteristic features of:
Enlargement of the spleen
Reduction in one or more of the cell lines in the peripheral Blood.
Normal or hyperplastic cellularity of the bone marrow. Hyposplenism
Hyposplenism occurs in a range of medical conditions, In some disorders such as sickle cell disease, severe gluten induced enteropathy and untreated HIV infection as well as following surgical splenectomy or therapeutic splenic embolization.
Congenital aplasia, nephrotic syndrome, autoimmune diseases and some lymphomas are also associated with hyposplenism. In sickle cell anaemia there is functional asplenia by 1 year of age and autoinfarction leads to a state of anatomical asplenia after 6–8 years of age. Changes post splenectomy and in hyposplenism.
Howell jolly bodies Patient is at risk of Postsplenectomy Target cells sepsis Siderotic granules Acanthocytes There is a need for – Reticulocytes are increased 1. Vaccination for hemophilus, s.pneumo and nesseriae.
2. Patient will need lifelong antibiotic Neutrophils rise in the early post op prophylaxis and prompt stage and later the white cell and commencement of empirical drugs monocyte counts remain elevated at for febrile episodes. about 10-15x 109/l 3. Proper patient education. Thrombocytosis – rises afer procedure and peaks between day 7-12.