SurgiSense

What is the spleen?

The spleen is a soft, oval-shaped, dark red to blue-black organ that sits high on the upper left side of your belly, tucked just under your ribs and close to your back. It lies to the left of your stomach and pancreas.

Your ribcage keeps it well protected – which is handy, because its soft, spongy texture makes it pretty fragile.
That’s why in high-speed accidents, like a car crash, a spleen rupture isn’t all that unusual.

In adults, it’s about the size of a fist, around 10–12 cm long and 150 grams in weight.

Think of it as a soft, blood-filled sponge – perfect for filtering, but easy to damage if it gets hit.

It is part of the reticuloendothelial system (RES), which also includes the liver and bone marrow. This system’s main job is to filter your blood and help your body fight infections.

Interestingly, even though the spleen is so important for filtering blood and supporting immunity, it doesn’t have its own lymphatic drainage.

What is it made of?

The spleen is made of soft tissue, blood vessels and immune cells.

It is wrapped in a protective capsule connected to trabeculae, like little supportive beams reaching into its pulp-like interior.

Both the spleen’s capsule and the trabeculae have special contractile cells, known as myoepithelial cells. These cells can squeeze the spleen like a sponge, pushing stored blood into your circulation when your body needs it – for example, during heavy bleeding. This process is controlled by the sympathetic nervous system – the fight-or-flight system – which tells the spleen to release extra red blood cells and platelets to help keep you going.

Inside the capsule is the parenchyma, which is composed of:

• Red pulp – blood filtering component
  Contains little lakes of blood (sinuses) and a network of soft tissue and immune cells (splenic cords) that work as a filtering net removing old or damaged red blood cells, clotting leftovers and toxins from your blood. It also stores red cells and platelets.
• Marginal zone – check point
  Sitting between the red and white pulp, it screens the blood for germs. It also contains macrophages that play an important role in clearing out microbes and viruses.
• White pulp – immune component
  Packed with immune cells like macrophages, T cells and B cells. It spots germs in your bloodstream and starts a defensive reaction.

The spleen’s special blood flow

Blood moves through the spleen in a unique way and that’s key to how the organ does its job.

Blood enters the spleen through the splenic artery, which splits into smaller arteries running along supportive beams called trabeculae. These branch into tiny arterioles that feed the red and white pulp.

Eventually, blood flows into small veins, leaves the spleen through the splenic vein, passes through the liver and then makes its way back to the heart and lungs to keep circulating.

In most human organs, blood stays inside tiny tubes lined with special cells, traveling from arteries into smaller arterioles and then into veins to return to the lungs and heart. This fully enclosed route is called closed circulation. But the spleen is different – its red pulp has open spaces that aren’t lined by anything and blood actually flows through them. No other human organ does this!”

For centuries, scientists believed the spleen used a mix of two systems, known as the classic ‘open and closed circulation‘ model.

What makes the spleen truly unique is its open circulation, where blood leaves the small arteries and gently flows into open spaces packed with immune cells inside the spleen. Think of it like water flowing from pipes into a garden bed – the water (blood) touches all the plants (immune cells) before returning to the main drain (veins).

Because blood moves through open spaces rather than enclosed tubes, it comes into direct contact with the immune cells without any barriers. This direct exposure makes the spleen incredibly effective at:

• Removing old or damaged red blood cells.
• Capturing bacteria and other harmful particles.
• Storing platelets and red cells when needed.

Recent research now shows that this open circulation isn’t just one option – it appears to be the main, and possibly only, way blood moves through the human spleen.

Its strategic arrangement ensures that a very effective filter where pathogens can't escape.

Even though we now understand this open system better, it still remains a mystery how the spleen keeps blood from clotting in these open spaces.

The spleen as a blood filter

As your blood flows through the spleen, it also checks for old or damaged red blood cells. When it finds them, special cleanup cells, called macrophages, gobble them up and break them down. The iron from those old red blood cells isn’t wasted – it gets recycled and sent back to the bone marrow to help make new blood cells.

For centuries, it was thought that spleen filters blood in two different ways: the closed and open systems.

In the open system, blood leaves the small arteries and gently moves through a sponge-like tissue full of specialised cleanup cells before entering the veins. This allows the spleen to filter the blood more efficiently, remove damaged cells and store red cells and platelets.

Earlier ideas suggested that a closed system, where blood rushes straight from arteries into veins, made up only about 10% of splenic blood flow. But recent evidence shows that the spleen mostly, or even entirely, relies on the open circulation.

Thanks to its special open circulation, the spleen can filter blood very efficiently, with every drop touching immune cells directly.

The spleen as a blood storage

The spleen stores blood in the open spaces of the red pulp, like a sponge holding water. It holds about 8% of your red blood cells, a quarter of all your lymphocytes (the cells that help fight infections) and roughly 30% of your platelets.  Altogether, that adds up to almost a cup of blood – ready to be released if your body ever needs it fast, which can be a real lifesaver.

Normally, the spleen doesn’t make new blood – that is the job of the bone marrow. But in serious situations, like severe infections or certain diseases, the spleen can step in as a backup blood factory.

It can produce red blood cells, white blood cells and platelets to help the body keep going when extra support is needed.

The spleen as infection patrol

The spleen is the largest lymphoid organ in the human body, which makes it your body’s biggest ‘immune hub’.

Thanks to its unique open circulation, blood flows into open spaces where every drop comes into direct contact with immune cells. With no barrier between the blood and these cells, no pathogen slips by unnoticed. This direct exposure makes the spleen extremely effective at catching bacteria and other harmful microorganisms, allowing it to kickstart immune responses quickly and coordinate your body’s defense.

The spleen plays several key roles in supporting your immune system:

• Maturing lymphocytes
  It helps lymphocytes – your infection-fighting cells – mature and stay ready to respond.
• Antibody production
  Inside the spleen, B-cells make antibodies that tag invaders for destruction.
• Direct attack
  T-cells, trained in the thymus and sent to the spleen, recognize and attack pathogens directly.
• Priming the immune response
  Dendritic cells and B-cells help ‘prime’ T-cells, making sure the immune system reacts quickly and effectively.
• Cleanup crew
  Macrophages engulf and digest invaders, dead cells and debris.

The immune cells spot pathogens in the blood, especially certain tricky bacteria with protective capsules and kick off an immune response. That’s why people without a spleen are more vulnerable to certain bacterial and protozoan infections.

The spleen keeps producing lymphocytes throughout your life to help fight infections, but the body has other ways to make these infection-fighting cells. So even though people without a spleen may be a bit more prone to certain infections, they can still live perfectly healthy lives