The spleen's primary functions are to filter the blood and help defend the body against pathogens. In this article, we will explain its anatomy, what it does, and what happens when it goes wrong.

Although in medieval times, people thought that the spleen was the source of anger, hence the phrase "venting your spleen," it is nothing to do with anger or any other emotions for that matter.

The spleen sits in the upper left of the abdomen, protected by the rib cage. It is the largest organ of the lymphatic system — the circulation of the immune system. It recycles old red blood cells and stores platelets (components of the blood that help stop bleeding) and white blood cells.

Below is a 3D model of the spleen, which is fully interactive.
Explore the 3D model, using your mouse pad or touchscreen, to understand more about the spleen.

Although it varies in size between individuals, a spleen is typically around 3–5.5 inches long and weighs 5.3–7.1 ounces (oz). The spleen is a soft organ with a thin outer covering of tough connective tissue, called a capsule.

There is a handy rule to remember the rough dimensions of the spleen, called the 1x3x5x7x9x11 rule:

It measures approximately 1 inch by 3 inches by 5 inches, weighs around 7 oz, and is positioned between the 9th and 11th ribs.

Anything that relates to the spleen is referred to as splenic; the spleen receives blood through the splenic artery, and blood leaves the spleen through the splenic vein. Although the spleen is connected to the blood vessels of the stomach and pancreas, it is not involved in digestion.

The spleen contains two main regions of tissue called white pulp and red pulp.

Red pulp: Contains venous sinuses (cavities filled with blood), and splenic cords (connective tissues containing red blood cells and white blood cells).

White pulp: Mostly consists of immune cells (T cells and B cells).

The spleen's primary job is to filter the blood. As blood flows into the spleen, it performs a quality control service, detecting any red blood cells that are old or damaged. Blood flows through a maze of passages in the spleen. Healthy cells flow straight through, but those considered to be unhealthy are broken down by large white blood cells called macrophages.

Once the red blood cells are broken down, the spleen stores useful leftover products, such as iron, which it eventually returns to the bone marrow, which makes hemoglobin (the iron-containing part of blood).

The spleen also stores blood — the blood vessels of the spleen can expand significantly. In humans, around 1 cup of blood is kept in the spleen, ready to be released if there is a significant loss of blood, after an accident, for instance. Interestingly, when a racehorse is at rest, up to half of its red blood cells are kept in the spleen.

The spleen also plays a role in the immune response by detecting pathogens (bacteria, for instance), and producing white blood cells in response.

Around one-quarter of our lymphocytes (a type of white blood cell) are stored in the spleen at any one time.

The spleen clears out old platelets from the blood; it also acts as a reservoir for platelets.

As a fetus is developing, the spleen makes red blood cells, but after the fifth month of gestation, it stops.

The spleen also produces compounds called opsonins, such as properdin and tuftsin, that help the immune system.

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The spleen is positioned in the top right of this image, above the kidney.

There are some conditions that can involve the spleen, these include:

Accessory spleen: An estimated 10–15 percent of people have an additional spleen. The second spleen is usually much smaller — around 1 centimeter (cm) in diameter. Generally, it causes no health problems.

Ruptured spleen: This can occur following an injury and cause life-threatening internal bleeding. Sometimes, the spleen will burst at the time of the injury; other times, it will burst days or weeks later. Certain diseases, such as malaria and infectious mononucleosis, make a ruptured spleen more likely because they cause the spleen to swell and the protective capsule to become thinner.

Enlarged spleen (splenomegaly): This can occur due to a variety of conditions, such as infectious mononucleosis (mono), blood cancers (such as leukemia), bacterial infections, and liver disease. Sometimes, the spleen is carrying out its regular work, but it is overactive (hypersplenism); it may, for instance, be destroying too many red blood cells or platelets.

Sickle cell disease: This is an inherited form of anemia; the condition is characterized by a dysfunctional type of hemoglobin. In this form of anemia, red blood cells are abnormally shaped (crescent-shaped) and block the flow of blood, causing damage to organs, including the spleen.

Thrombocytopenia: If the spleen becomes enlarged, it may store too many platelets, meaning that there are not enough in the rest of the body's circulatory system. Without platelets available to help blood clot, the primary symptom of thrombocytopenia is bleeding.

Spleen cancer: If cancer starts in the spleen, it is known as primary spleen cancer; if it spreads to the spleen from another site, it is called secondary. Both types of cancer are rare.

Splenic infarction: If the blood supply to the spleen is reduced, it is known as splenic infarction. This occurs if blood supply through the splenic artery is cut off by, for instance, a blood clot. This is often very painful, and treatment depends on the underlying cause.

Some people need to have their spleen surgically removed (splenectomy). Most commonly, this is due to a ruptured spleen, but it can also be because of an enlarged spleen, certain blood disorders, some cancers, infection, or noncancerous growths.

Although this modestly sized organ carries out a range of important tasks, it is possible to live without it. Other tissues, such as the lymph nodes and liver, can step in and carry out the spleen's tasks.

However, people who have had their spleen removed are more susceptible to infections.

As an aside, if a racehorse has its spleen removed, it will be significantly less athletic.

In a nutshell

The spleen is an important organ involved in cleaning out old blood cells and helping to mount the immune response. Although it is relatively small, it carries out a variety of roles. Despite this, if it is removed, a person can carry on without it.