The Spleen

The shape and size of the spleen varies amongst the domestic animals (see Figure 10.1) although its location is reasonably constant. The spleen is a flattened, elongated, dark reddish-brown abdominal organ.

The fibrous capsule extends by trabeculae into the parenchyma, which consists of two types of tissue identified as red pulp and white pulp. Most of the spleen is red pulp and consists of mature blood cells, macrophages, plasma cells and lymphocytes. The white pulp consists of lymphoid tissue in discrete nodules, each surrounding a central arteri­ole. T (thymus) cells are adjacent to the arterioles, and then a marginal sinus separates the T cells from macrophages.

B lymphocytes predominate in the nodules where they produce antibodies to coat pathogens and facilitate their removal by scavenger cells; 25% of the body's lymphocytes are stored within the spleen.

The splenic artery supplies the splenic sinusoids in the red pulp, which act as a filter, preventing old or damaged red blood cells from entering the bloodstream. Bacteria, fungi, viruses and haemoglobin debris are also discarded and phagocytosed. The red pulp also acts as a reservoir for storing blood and blood components, e.g. phagocytes, platelets, haemoglobin and iron. Blood can be released in emergency and sent to sites of injury to facilitate healing. The spleen is a site for haematopoiesis in foetal and neonate animals. The dog's spleen is able to store one-third of the total amount of red cells. In dogs and cats this enables the release of 10-20% of the body's red cells in an emergency.

Despite the apparent important functions of the spleen, it can be removed without serious consequence.

10.3