Structure of the spleenThe spleen is similar to a lymph node in shape and structure but is much larger. In fact, it is the largest lymphatic organ in the body. It is enclosed in an elastic capsule of connective tissue, which extends inward to divide the organ into lobules.
The spleen consists of two types of tissue called white pulp and red pulp. The white pulp is lymphatic tissue consisting mainly of lymphocytes around arteries. The red pulp consists of venous sinuses (cavities) filled with blood and cords of lymphatic cells, such as lymphocytes and macrophages.
Blood enters the spleen through the splenic artery, moves through the sinuses where it is filtered, then leaves through the splenic vein.
Blood storageThe splenic artery enters the spleen through a fissure called the hilum and divides into numerous branches, assuming an appearance rather like a brush; they are sometimes called the brush arteries.
Each one of these arteries leads to a sinus whose function is to store the blood; when they are full the sinuses swell up and treble their volume. The spleen increases in volume while digestion is in progress and returns to its normal size when it is finished.
In emergencies such as hemorrhage, smooth muscle in the vessel walls and in the capsule of the spleen contracts. This squeezes the blood out of the spleen into the general circulation.
Production and destruction of blood cellsThe spleen filters blood in much the way that the lymph nodes filter lymph. Lymphocytes in the spleen react to pathogens in the blood and attempt to destroy them. Macrophages then engulf the resulting debris, the damaged cells, and the other large particles.
The spleen, along with the liver, removes old and damaged erythrocytes (red blood cells) from the circulating blood. An erythrocyte ceases to work effectively as an oxygen-carrier after about 15–16 weeks. Such aging cells are collected by the spleen and destroyed. The hemoglobin contained in them is broken down and used by the liver to make bile, and the iron in the recycled to make fresh hemoglobin.
The spleen can also, like the bone marrow, produce new red blood cells. It can accordingly be regarded both as a hemolytic and as a hemopoietic organ (These terms come from the Greek: hema, blood; luein, to dissolve or destroy; poiein, to produce.)
Like other lymphatic tissue, the spleen produces lymphocytes, especially in response to invading pathogens.
Related category• ANATOMY AND PHYSIOLOGY
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