The spleen is a spongy, soft and very densely vascularized secondary lymphoid organ. It appears reddish-purple because of its good vascularization. It is the largest organ of the lymphatic system.

The spleen has an essential role in immune surveillance and blood filtration. Various foreign bodies and senescent and damaged red blood cells are filtered in the spleen. In addition, lymphocytes mature and proliferate in this organ.

Although the spleen varies in shape and size depending on the gender, weight, and height of a person, it usually appears somewhat oval-shaped and fist-sized. On average, it is 3.94 - 5.51 inches (10 - 14 centimeters) long, and its width varies from 2.76 to 3.54 inches (7 - 9 centimeters). The thickness of the spleen is typically 1.18 - 1.97 inches (3 - 5 centimeters). Usually, the spleen weighs around 4.9 - 7.1 ounces (140 - 200 grams).

There is a rule that can help recall these parameters of the spleen. It is the 1x3x5x7x9x11 rule. It describes the spleen as being 1 inch thick, 3 inches wide, and 5 inches long. The number 7 corresponds to its weight (7 oz), while 9 x 11 means it lies between the ribs 9 to 11.

The spleen is classified as an intraperitoneal organ - the peritoneum surrounds it from all sides except at the hilum of the spleen. Underneath the peritoneum, it is encased by a weak outer connective tissue capsule of the spleen. It has a protective function, and it helps the organ to remain in place. At the same time, the capsule also contains contractile cells called myofibroblasts. Contractions of these cells help to discharge the blood within the spleen into circulation. The splenic capsule also allows the spleen to expand if necessary.

Overall, the spleen has two surfaces, three borders, and two extremities, as described in the following slides. Also, several ligaments connect the spleen to the adjacent internal organs and structures.

Spleen functions

While the spleen is an essential organ with a wide range of functions, it is not vital. A person can live an everyday life without a spleen, as other organs can adapt to compensate for its absence. Internally, the spleen consists of two different types of tissue. It is composed of white pulp and red pulp, each providing different functions.

The white pulp forms the central portion of the spleen and is mainly composed of lymphatic tissue containing white blood cells. It plays a key role in the production and maturation of white blood cells (particularly B and T lymphocytes). It is also involved in the production of antibodies as various antigens are presented by antigen-presenting cells within the lymphoid follicles of the white pulp. In turn, this process initiates the activation of T and B lymphocytes, triggering the synthesis of antibodies.

The majority of the spleen is formed by red pulp. It consists of splenic sinusoids and cords of connective tissue (cords of Billroth). Besides large amounts of sinusoids, red pulp also contains massive amounts of macrophages. This part of the spleen filters the blood. It not only removes senescent, damaged and useless red blood cells but also eliminates various antigens such as viruses, bacteria, and fungi. Besides all the mentioned, the red pulp serves as a storage site for platelets and white blood cells.

During the fetal period, the spleen plays a large role in hemoglobin synthesis and hematopoiesis. It is one of the major sites of activity from week 10 to week 26 of gestation. Later, the red bone marrow takes over this process.

In addition, the spleen is involved in iron metabolism. Splenic macrophages recycle iron during blood filtration from old and damaged red blood cells. This process is essential for hemoglobin synthesis. Macrophages either store ingested iron or release it into the bloodstream, where it is further transported by transferrin.

Moreover, the spleen acts as a reservoir for blood. During hemorrhage, it can contract its capsule and trabeculae to increase systemic blood supply due to sympathetic stimulation. The spleen stores approximately 25 - 30% of red blood cells and 25% of platelets.

And finally, the spleen can also play a role in hematopoiesis if required. It can provide extramedullary hematopoiesis during certain pathological conditions (such as beta-thalassemia major). This helps the bone marrow to compensate for the hemolysis.