Anatomy of the spleen

In this note, we are going to discuss the anatomy and function of the spleen.

The spleen is the largest organ in the lymphatic system, which is a part of the immune system.

Spleen in situ in the left hypochondrium

Its primary function is to filter blood, removing old red blood cells, but it also performs other important roles that we’ll discuss shortly.

Macroscopic anatomy

The spleen is located in the left hypochondriac region (upper left quadrant of the abdomen) at the level of the 9th, 10th, and 11th ribs, just beneath the diaphragm.

Spleen in situ in the left hypochondrium under the diaphragm

It typically does not extend below the costal margin, and therefore the spleen is non-palpable during a clinical examination unless it is enlarged.

As an intraperitoneal organ, the spleen is entirely covered by visceral peritoneum, except the so-called hilum, where the splenic artery and vein pass. Beneath the peritoneum lies a fibroelastic capsule that encases the spleen.

Peritoneal covering of the spleen in the abdominal cavity

The spleen has two surfaces:

The diaphragmatic (or lateral) surface, which fits perfectly into the concavity of the diaphragm.

The medial surface of the spleen, which has three impressions or ares:

the colic area from the left colic flexure,

the gastric area from the stomach,

and the renal area from the left kidney.

The already mentioned splenic hilum, where the splenic vessels pass, is found in the central part of this surface.

The spleen has three borders:

Superior border

Inferior border

and anterior border – as well as two extremities, anterior and posterior.

The spleen is connected to the stomach and kidney by components of the greater omentum, a double-folded layer of peritoneum that originates from the stomach:

The gastrosplenic ligament, which contains the short gastric arteries and veins, as well as the left gastroepiploic artery and vein.

The splenorenal ligament, which contains the terminal branches of the splenic artery, tributaries of the splenic vein, as well as the tail of the pancreas.

Additionally, the spleen is situated on the phrenicocolic ligament, also known as the sustentaculum lienis, which originates from the colon.

Microscopic anatomy

Numerous septa, known as trabeculae, extend from the spleen’s capsule into its parenchyma, which is referred to as the pulp. Both the capsule and trabeculae contain myoepithelial cells capable of contraction. Since the spleen stores a significant amount of blood, the contraction of these cells allows the spleen to release stored blood into the circulation, for instance in response to massive hemorrhage or during intense physical activity.

Let’s get back to the pulp. Red and white pulp can be distinguished based on color.

The white pulp of the spleen consists of lymphatic tissue, known as periarterial lymphoid sheaths (PALS), which surround the central arteries. This area is the primary site for immune and phagocytic activity.

The red pulp consists of venous sinusoids and splenic cords, also known as cords of Billroth, and is the main site of filtration of old and damaged erythrocytes.

Functions

The spleen, as a secondary lymphoid organ, performs several key functions:

Blood filtration. Recycles damaged erythrocytes and platelets;
Blood storage. Stores blood and platelets in the red pulp;
Immune response. Provides protection against infection;
Immune cell production. Matures lymphocytes and macrophages in the white pulp;
Fetal hematopoiesis. Produces blood cells until the bone marrow takes over this role.

Vasculature

The spleen is a highly vascular organ, primarily supplied by the splenic artery.

Upon reaching the spleen, the splenic artery divides into five branches, each supplying a distinct region of the organ. These branches don’t anastomose with each other, creating distinct vascular segments within the spleen.