The Spleen: A Small Organ With a Big Role in the Body

The spleen — a fist-sized organ tucked behind the stomach on the left side of the abdomen — is often overlooked. Yet it performs multiple vital functions: filtering blood, supporting the immune system, and helping maintain the normal composition of blood cells. Without it, the body becomes more vulnerable to certain infections. Despite its modest size, the spleen plays an outsized role in blood health and immune defense.

📍 Location & Anatomy: Where Is the Spleen?

The spleen is a soft, purple, fist-sized organ located in the upper left part of the abdomen (left hypochondrium), just behind the stomach and below the diaphragm. It sits near the 9th, 10th, and 11th ribs — which provide some protection against injury.

In healthy adults, the spleen is about 4-5 inches (10-12 cm) long and weighs approximately 150-200 grams (5-7 ounces). It is part of the lymphatic system along with the lymph nodes and thymus. Despite its modest size, it plays an outsized role in maintaining blood health and immune defense.

🩸 Blood Filtration: The Spleen’s Primary Job

🛡️ Immune Function: The Spleen as a Defender

🏃 Why Does Your Side Hurt When Running? (The Side Stitch)

Many runners experience a sharp, stabbing pain in the left upper abdomen (or right side) during intense exercise — commonly called a “side stitch” or exercise-related transient abdominal pain (ETAP). The spleen plays a role in this phenomenon:

• During exercise, the body directs blood away from internal organs (splanchnic vasoconstriction)
• The spleen responds by contracting and releasing stored blood into circulation — providing extra oxygen-carrying capacity to working muscles
• This contraction can cause irritation of the diaphragm and surrounding tissues, leading to the classic stitch pain

While uncomfortable, a side stitch is usually harmless. It tends to occur more often in beginner runners, during high-intensity intervals, or when exercising soon after eating.

⚠️ Enlarged Spleen (Splenomegaly): A Sign of Underlying Disease

An enlarged spleen (splenomegaly) is not a disease itself — it is a sign of an underlying condition. Common causes include:

Symptoms of an enlarged spleen include a feeling of fullness without eating (due to the spleen pressing on the stomach), left upper abdominal pain, and pain in the left shoulder (referred pain). If you experience these symptoms, consult a doctor — an enlarged spleen can rupture easily with trauma.

⚕️ Life Without a Spleen: Splenectomy

While the spleen performs critical functions, the human body can survive without it. Splenectomy (surgical removal of the spleen) may be necessary for trauma (ruptured spleen), certain blood disorders, or cancer. However, the absence of the spleen comes with important consequences:

• Increased risk of infection — particularly from encapsulated bacteria (S. pneumoniae, H. influenzae, N. meningitidis)
• Overwhelming post-splenectomy infection (OPSI) — rare but life-threatening sepsis that can develop within hours
• Recommended vaccination — patients should receive vaccines against pneumococcus, meningococcus, and Haemophilus influenzae type b (Hib)
• Daily antibiotics — sometimes prescribed for children or immunocompromised individuals
• Lifelong vigilance — seek immediate medical attention for fever or signs of infection

Despite these risks, most people without a spleen live normal lives with appropriate preventive measures.

🔬 Scientific References & External Resources

• NCBI StatPearls — Spleen Anatomy
• PubMed — Spleen function research
• PubMed — Splenomegaly causes
• PubMed — Overwhelming post-splenectomy infection (OPSI)
• Wikipedia — Spleen