Spleen
A horse keeps roughly 30 percent of its red blood cells stored in a single organ, ready to release them on demand. That organ is the spleen, found in almost all vertebrates. In humans, it holds up to a cup of red blood cells, about 240 milliliters. It releases them when blood volume or oxygen drops. In structure, the spleen resembles a large lymph node, though it works mainly as a blood filter. For centuries, people credited it with something quite different, including sadness, bad temper and laughter. This documentary asks what the spleen really does, what happens to a body that loses it, and how an organ became a word for melancholy.
Inside the spleen, red pulp and white pulp sit side by side, and each runs a different kind of defense. Red pulp surrounds the white pulp and is built from venous sinuses and splenic cords, called the cords of Billroth. Blood moving through it meets macrophages that swallow old or damaged red blood cells, along with antibody-coated cells and pathogens. This fast, non-specific cleanup is a form of innate immunity. Hemoglobin from worn-out cells is broken down on the spot. The globin portion is reduced to amino acids, and the heme portion becomes bilirubin, which the liver removes. The spleen also recycles the iron.
T lymphocytes gather in the white pulp inside periarteriolar lymphoid sheaths, known as PALS, which wrap around central arterioles. B lymphocytes wait in follicles right beside them. That position lets B cells meet antigens carried in the blood. Once activated, they turn into plasma cells, which release IgM antibodies first and IgG antibodies later. This targeted response is the spleen's contribution to adaptive immunity.
After a heart attack, monocytes stored in the red pulp can travel to the injured heart. There they become dendritic cells and macrophages and help the tissue heal. The spleen also releases neutrophils following a myocardial infarction. It stores platelets for emergencies and clears old ones from circulation. At any moment, up to a quarter of the body's lymphocytes are held there.
Even the way this organ forms in an embryo sets it apart from its neighbors in the gut.
In humans, the spleen is purple and sits in the left upper quadrant of the abdomen, beneath the left side of the diaphragm. The ninth, tenth and eleventh ribs lie above it. Its outer face is smooth and convex. The inner face is divided by a ridge into two regions.
The front region, called the gastric surface, is broad and concave and touches the back wall of the stomach. The tail of the pancreas lies just below it. The rear region, the renal surface, is flatter and narrower. It rests against the upper part of the left kidney and sometimes the left adrenal gland. Four ligaments hold the organ in place: the gastrosplenic, splenorenal, colicosplenic and phrenocolic.
A healthy adult spleen measures roughly 7 to 14 centimeters in length. It weighs between 1 and 8 ounces, tracking height, body weight and degree of congestion rather than sex or age. Ultrasound reference ranges shift upward as people get taller.
Near the middle of the organ runs a long fissure called the hilum. The gastrosplenic ligament attaches there, and the splenic artery and splenic vein pass through it. Short gastric arteries add a second blood supply, and openings let lymphatic vessels and nerves through. Like the thymus, the spleen has only efferent lymphatic vessels. Small arterioles called penicilliary radicles feed its germinal centers.
The splenic plexus, which links a branch of the celiac ganglia to the vagus nerve, supplies its nerves. The brain's control of the spleen seems to involve the hypothalamic-pituitary-adrenal axis and the brainstem, especially the subfornical organ.
Most gut organs grow from endoderm, but the spleen forms from mesenchymal tissue within the dorsal mesentery. Even so, it draws blood from the celiac trunk, just as the foregut organs do. Before birth, it acts as a blood factory, producing all types of blood cells during fetal life. It keeps important blood-forming duties until the fifth month of gestation. After birth, red cell production stops except in some blood disorders, though the spleen can still make lymphocytes.
That early role fades, but medicine learned the spleen's value most clearly by studying people who lost it.
A 28-year follow-up of 740 World War II veterans, whose spleens were removed on the battlefield, put numbers on that loss. Deaths from pneumonia reached 6, where only 1.3 were expected. Deaths from ischemic heart disease reached 41, against an expected 30. Other causes of death did not rise compared with veterans whose spleens were intact.
Asplenia describes a spleen that does not work at all. It can be congenital, or it can follow traumatic injury, surgical removal called splenectomy, or a disease such as sickle cell anaemia. Hyposplenia means the spleen works only in part. Both conditions can cause a modest rise in white blood cells and platelets. They can also weaken the response to some vaccines and raise the risk of infection.
The greatest danger is sepsis from polysaccharide-encapsulated bacteria. Their capsules block complement from binding, or stop assembled complement from reaching macrophage receptors. Clearing them needs natural antibodies, which are made by IgM memory B cells in the spleen's marginal zone. A splenectomy leaves far fewer of those memory B cells.
About 10 percent of people carry an accessory spleen, a small nodule outside the main organ, usually around 1 centimeter across. It typically forms in early embryonic life. Splenosis is a related condition in which displaced pieces of splenic tissue, often after trauma or splenectomy, graft themselves into the abdominal cavity as new accessory spleens.
Polysplenia is a congenital disease with several small spleens in place of one full-sized organ. It sometimes appears alone. It often comes with intestinal malrotation, biliary atresia or a heart on the right side called dextrocardia. These small spleens do not function.
Absence is only one way the organ can cause trouble, because an enlarged spleen sends its own warnings.
Splenomegaly, the medical term for an enlarged spleen, has a long list of causes. They include sickle cell anemia, sarcoidosis, malaria, bacterial endocarditis, leukemia, polycythemia vera, pernicious anemia, Gaucher's disease, leishmaniasis, Hodgkin's disease, Banti's syndrome, hereditary spherocytosis, cysts and tumours. Glandular fever, including infectious mononucleosis from Epstein-Barr virus or human cytomegalovirus, can cause it too. Primary tumors of the spleen include hemangiomas and hemangiosarcomas. In marked cases, the spleen fills a large part of the left side of the abdomen.
The spleen is the largest collection of lymphoid tissue in the body. Doctors can normally feel it in preterm infants, in 30 percent of healthy full-term newborns, and in 5 to 10 percent of infants and toddlers. A spleen that is easy to feel below the ribs in a child over three to four years old is abnormal until proven otherwise.
Swelling has four broad routes. Infection can stimulate it, a blocked portal vein can obstruct blood flow, and hemolytic anemia can reflect a functional fault. Leukemia or a storage disease such as Gaucher's disease can infiltrate the organ. In children, the most common cause of sudden enlargement is a viral infection, which is usually moderate and passes. Doctors start with a complete blood count, platelet count, and counts of reticulocytes and atypical lymphocytes. Rising IgM antibodies to viral capsid antigen confirm Epstein-Barr virus or cytomegalovirus.
Road traffic collisions and other trauma can rupture the spleen, which is a medical emergency. Splenic infarction is a different crisis. It happens when the splenic artery or one of its branches is blocked, for example by a blood clot, and the tissue dies for lack of oxygen. The typical symptom is severe pain in the left upper quadrant, sometimes spreading to the left shoulder. Some patients also develop fever and chills, and others have no symptoms at all. Doctors must tell it apart from other causes of acute abdomen.
A rarer condition, hyaloserositis, coats the spleen in fibrous hyaline. Calculators now estimate spleen size from CT, ultrasound and MRI findings.
Long before any of these conditions were understood, physicians in ancient Greece held a very different theory of what the organ was for.
In ancient Greek medicine, the spleen was believed to be a reservoir for black bile, one of the four humours. That doctrine, humourism, tied the organ to melancholia in many cultures. The idea also reflected a long history of misconceptions that treated the spleen as a store of juices linked to digestion.
Traditional Chinese medicine sees the spleen as a key organ displaying the Yin aspect of the Earth element, with the stomach as its Yang counterpart. The Talmud, in tractate Berachoth 61b, calls it the organ of laughter, while possibly hinting at the humoral view as well.
The Ancient Greek word σπλήν, written splḗn, was once the idiomatic equivalent of the heart in modern English. The Roman writer Persius, in his satires, linked the spleen to immoderate laughter.
William Shakespeare used the word often, usually for melancholy but also for caprice and merriment. In Julius Caesar, he uses it for the irritable nature of Cassius. The speaker warns him: "You shall digest the venom of your spleen, Though it do split you."
The word was about to carry an even heavier meaning in the century that followed.
In the early 18th century, the physician Richard Blackmore ranked the spleen among the two most prevalent diseases in England, alongside consumption. Here it meant a mood disorder, not the organ. In 1701, Anne Finch, later Countess of Winchilsea, published a Pindaric ode titled The Spleen. She drew on her own experience of an affliction with a reputation as a fashionable, upper-class English complaint.
Blackmore and George Cheyne treated the malady as the male equivalent of the vapours. They preferred the more learned terms hypochondriasis and hysteria. By the late 18th century, the German word Spleen described the eccentric and hypochondriac tendencies thought typical of English people.
In French, the adjective "splénétique" names a state of pensive sadness. Charles Baudelaire, who lived from 1821 to 1867, popularized that sense through his poems and his collection Le Spleen de Paris. The usage already appeared in earlier 19th-century Romantic literature.
The old English word for the organ is milt, which today mostly describes the organ in animals raised for meat. On the plate, the spleen is one of many organs counted as offal. It is rarely a main ingredient, but cow spleen sandwiches are eaten in Sicilian cuisine. Chicken spleen is a main ingredient of Jerusalem mixed grill.
The animals themselves hold the last surprises, because their spleens take many shapes.
Lampreys and hagfishes, the jawless fishes of the group Cyclostomata, are the only vertebrates with no spleen at all. Even so, they carry a diffuse layer of blood-forming tissue in the gut wall. Its structure resembles red pulp, and scientists presume it is homologous with the spleen of higher vertebrates.
In cartilaginous and ray-finned fish, the spleen is made mostly of red pulp. It is usually elongated and lies inside the lining of the intestine. In many amphibians, especially frogs, it is more rounded and often holds more white pulp. In reptiles, birds and mammals, white pulp is always plentiful. Birds and some mammals have a rounded spleen, though it bends to fit the organs around it.
Marsupials have y-shaped spleens, and theirs develops after birth. In most vertebrates, the spleen keeps making red blood cells for life. Only mammals lose this ability in middle age. Many mammals also have tiny spleen-like structures called haemal nodes scattered through the body, presumed to do the same work.
Aquatic mammals have spleens that are generally bluish. In cetaceans and manatees they tend to be small. In deep-diving pinnipeds they can be massive, because the organ stores red blood cells.
In mice, the spleen holds half the body's monocytes, keeping a ready supply of healing cells close to any wound.
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Common questions
What does the spleen do in the human body?
The spleen acts primarily as a blood filter. It removes old red blood cells, holds a reserve of blood, recycles iron, and supports the immune system through red pulp and white pulp.
Where is the spleen located in humans?
The spleen sits in the left upper quadrant of the abdomen, beneath the left side of the diaphragm. It lies under the ninth, tenth and eleventh ribs and touches the stomach and left kidney.
What happens if a person has no spleen?
A person without a spleen has a diminished response to some vaccines and an increased susceptibility to infection. The main danger is sepsis from polysaccharide-encapsulated bacteria. A splenectomy also leaves far fewer memory B cells.
What causes an enlarged spleen?
Splenomegaly can follow infection, blocked blood flow, hemolytic anemia, or infiltration by leukemia or storage diseases such as Gaucher's disease. Malaria, sickle cell anemia and infectious mononucleosis are also on the list. In children, viral infection is the most common cause of acute enlargement.
Why is the spleen linked to melancholy and laughter?
Ancient Greek humourism held that the spleen stored black bile, one of the four humours, which tied it to melancholia. The Talmud and the Roman writer Persius linked it to laughter instead. Shakespeare used the word spleen for melancholy, caprice and merriment.
Which animals have no spleen?
Lampreys and hagfishes, the jawless fishes called Cyclostomata, are the only vertebrates that lack a spleen. They have a diffuse layer of blood-forming tissue in the gut wall that resembles red pulp.
All sources
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