Spleen: Anatomy

Development

The spleen is the largest lymphatic organ in the body.

Embryology

• Spleen development begins during the 5th week of development.
• Originates from the embryonic mesoderm Mesoderm The middle germ layer of an embryo derived from three paired mesenchymal aggregates along the neural tube. Gastrulation and Neurulation (and not from the endoderm Endoderm The inner of the three germ layers of an embryo. Gastrulation and Neurulation like the primitive gut tube)
• Several mesenchymal buds form off the dorsal mesogastrium Dorsal mesogastrium Development of the Abdominal Organs → fuse together to form the spleen
• Fetal spleen has lobules Lobules Breasts: Anatomy that gradually regress as the organ matures → formation of the adult spleen that is divided into functional lobules Lobules Breasts: Anatomy
• The fetal spleen acts solely as a hematopoietic organ until the 8th month of gestation.

Congenital variations

• Congenital asplenia Asplenia Asplenia is the absence of splenic tissue or function and can stem from several factors ranging from congenital to iatrogenic. There is a distinction between anatomic asplenia, which is due to the surgical removal of the spleen, and functional asplenia, which is due to a condition that leads to splenic atrophy, infarct, congestion, or infiltrative disease. Asplenia: congenital absence of a spleen
• Congenital hyposplenism Hyposplenism Asplenia: congenital presence of a small, pathological spleen
• Polysplenia: occurrence of multiple similarly sized smaller spleens (varies from 2 to 16)
• Accessory spleens (supernumerary spleens):
  • Congenital occurrence of a primary spleen with 1 or more smaller spleens
  • Result from incomplete fusion of mesenchymal buds during development
  • Detected incidentally in 10%–30% of the population
  • When present, individuals will usually have 1–6 accessory spleens.
  • Tend to be about the size of cherries
  • Found in the splenic hilum Hilum Lungs: Anatomy in 75% of cases
• Splenic lobulation: persistence of embryonic lobules Lobules Breasts: Anatomy in the adult 
• Wandering spleen:
  • A rare condition in which the spleen changes positions in the abdomen
  • Caused due to malformation and excessive elasticity Elasticity Resistance and recovery from distortion of shape. Skeletal Muscle Contraction of the splenic ligaments that fix the spleen in the upper abdomen
• Splenopancreatic fusion:
  • Fusion of the spleen with the tail of the pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy
  • Usually associated with trisomy Trisomy The possession of a third chromosome of any one type in an otherwise diploid cell. Types of Mutations 13

Gross Anatomy

Location

• LUQ of the abdomen against the left dome of the diaphragm Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm: Anatomy
• In contact with the 9th, 10th, and 11th ribs Ribs A set of twelve curved bones which connect to the vertebral column posteriorly, and terminate anteriorly as costal cartilage. Together, they form a protective cage around the internal thoracic organs. Chest Wall: Anatomy. The long axis of the spleen corresponds to that of the left 10th rib.
• Positioned obliquely from posterior/superior (posterior pole) to anterior/inferior (anterior pole)
• The spleen is an intraperitoneal Intraperitoneal Peritoneum: Anatomy organ:
  • Covered by visceral peritoneum Visceral peritoneum Peritoneum: Anatomy
  • Suspended by mesentery Mesentery A layer of the peritoneum which attaches the abdominal viscera to the abdominal wall and conveys their blood vessels and nerves. Peritoneum: Anatomy folds of peritoneum Peritoneum The peritoneum is a serous membrane lining the abdominopelvic cavity. This lining is formed by connective tissue and originates from the mesoderm. The membrane lines both the abdominal walls (as parietal peritoneum) and all of the visceral organs (as visceral peritoneum). Peritoneum: Anatomy
  • Only the hilum Hilum Lungs: Anatomy of the spleen, the site through which the splenic artery and vein pass, is not covered by the peritoneum Peritoneum The peritoneum is a serous membrane lining the abdominopelvic cavity. This lining is formed by connective tissue and originates from the mesoderm. The membrane lines both the abdominal walls (as parietal peritoneum) and all of the visceral organs (as visceral peritoneum). Peritoneum: Anatomy.

Anatomic relationships

• Anterior to spleen: stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy
• Lateral to spleen:
  • Diaphragm Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm: Anatomy
  • Ribs Ribs A set of twelve curved bones which connect to the vertebral column posteriorly, and terminate anteriorly as costal cartilage. Together, they form a protective cage around the internal thoracic organs. Chest Wall: Anatomy and intercostal muscles Intercostal Muscles Respiratory muscles that arise from the lower border of one rib and insert into the upper border of the adjoining rib, and contract during inspiration or respiration. Chest Wall: Anatomy
• Medial to spleen:
  • Left kidney and adrenal gland
  • Tail of the pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy
• Posterior to spleen:
  • Diaphragm Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm: Anatomy
  • Left lung
  • 9th–11th ribs Ribs A set of twelve curved bones which connect to the vertebral column posteriorly, and terminate anteriorly as costal cartilage. Together, they form a protective cage around the internal thoracic organs. Chest Wall: Anatomy and intercostal muscles Intercostal Muscles Respiratory muscles that arise from the lower border of one rib and insert into the upper border of the adjoining rib, and contract during inspiration or respiration. Chest Wall: Anatomy
• Inferior to spleen: left colic flexure

Size

Average measurements in normal healthy adults:

• Length: approximately 10–12 cm
• Width (at its widest portion): 7–14 cm 
• Circumference: 11 cm
• Weight: 150–200 grams

Ligaments

The spleen is connected to adjacent organs via several important ligaments.

• Gastrosplenic ligament: connects the spleen to the greater curvature Greater curvature Stomach: Anatomy of the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy
• Splenorenal ligament:
  • Posterior to the splenic hilum Hilum Lungs: Anatomy
  • Connects the spleen to the left kidney
  • Contains:
    • Splenic vessels
    • Tail of the pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy
• Splenocolic ligament: connects the spleen to the transverse colon Transverse colon The segment of large intestine between ascending colon and descending colon. It passes from the right colic flexure across the abdomen, then turns sharply at the left colonic flexure into the descending colon. Colon, Cecum, and Appendix: Anatomy
• Phrenicocolic ligament:
  • A double fold of the peritoneum Peritoneum The peritoneum is a serous membrane lining the abdominopelvic cavity. This lining is formed by connective tissue and originates from the mesoderm. The membrane lines both the abdominal walls (as parietal peritoneum) and all of the visceral organs (as visceral peritoneum). Peritoneum: Anatomy acting as a mesentery Mesentery A layer of the peritoneum which attaches the abdominal viscera to the abdominal wall and conveys their blood vessels and nerves. Peritoneum: Anatomy that connects the diaphragm Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm: Anatomy and spleen
  • Part of the greater omentum Omentum Peritoneum: Anatomy

Surface anatomy

• Hilum Hilum Lungs: Anatomy:
  • The point where splenic vessels (artery, vein, and lymphatics) enter the spleen
  • Located in the middle of the medial surface
• Impressions: indentations in the medial spleen where it is adjacent to other organs
  • Gastric impression:
    • Posterior, superior aspect of the spleen
    • In contact with the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy
  • Colic impression:
    • Anterior, superior aspect of the spleen
    • In contact with the transverse colon Transverse colon The segment of large intestine between ascending colon and descending colon. It passes from the right colic flexure across the abdomen, then turns sharply at the left colonic flexure into the descending colon. Colon, Cecum, and Appendix: Anatomy
  • Renal impression:
    • Inferior aspect of the spleen
    • In contact with the left kidney

Microscopic Anatomy

The spleen consists of a capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides and inner tissue known as parenchyma. The parenchyma consists of 2 types of tissues, white pulp White pulp Secondary Lymphatic Organs and red pulp Red pulp Secondary Lymphatic Organs.

Capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides

• A fibroelastic capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides wraps the spleen.
• Relatively weak:
  • Allows for an increase in size when necessary
  • Ruptures relatively easily compared with other organs
• Numerous septa (trabeculae) extend from the capsule Capsule An envelope of loose gel surrounding a bacterial cell which is associated with the virulence of pathogenic bacteria. Some capsules have a well-defined border, whereas others form a slime layer that trails off into the medium. Most capsules consist of relatively simple polysaccharides but there are some bacteria whose capsules are made of polypeptides. Bacteroides into the parenchyma of the spleen → divides the spleen into functional lobules Lobules Breasts: Anatomy

Parenchyma: white pulp White pulp Secondary Lymphatic Organs

The white pulp White pulp Secondary Lymphatic Organs makes up 25% of the spleen, surrounds the larger arterioles Arterioles The smallest divisions of the arteries located between the muscular arteries and the capillaries. Arteries: Histology in the spleen, and contains:

• A central arteriole
• Periarteriolar lymphoid sheath ( PALS PALS Secondary Lymphatic Organs):
  • Congregations of lymphoid tissue surrounding the central arterioles Arterioles The smallest divisions of the arteries located between the muscular arteries and the capillaries. Arteries: Histology
  • Primarily composed of T cells T cells Lymphocytes responsible for cell-mediated immunity. Two types have been identified – cytotoxic (t-lymphocytes, cytotoxic) and helper T-lymphocytes (t-lymphocytes, helper-inducer). They are formed when lymphocytes circulate through the thymus gland and differentiate to thymocytes. When exposed to an antigen, they divide rapidly and produce large numbers of new T cells sensitized to that antigen. T cells: Types and Functions
  • PALS PALS Secondary Lymphatic Organs distinguishes the spleen from other lymphatic organs.
• Lymphoid follicles Lymphoid follicles Secondary Lymphatic Organs:
  • Nodules of lymphoid tissue near the central arterioles Arterioles The smallest divisions of the arteries located between the muscular arteries and the capillaries. Arteries: Histology
  • Primarily composed of B cells B cells Lymphoid cells concerned with humoral immunity. They are short-lived cells resembling bursa-derived lymphocytes of birds in their production of immunoglobulin upon appropriate stimulation. B cells: Types and Functions (antibody production)
• Marginal zone Marginal zone MALT Lymphoma:
  • Located at the edges of the white pulp White pulp Secondary Lymphatic Organs
  • Contains antigen-presenting cells Antigen-presenting cells A heterogeneous group of immunocompetent cells that mediate the cellular immune response by processing and presenting antigens to the T-cells. Traditional antigen-presenting cells include macrophages; dendritic cells; langerhans cells; and B-lymphocytes. Follicular dendritic cells are not traditional antigen-presenting cells, but because they hold antigen on their cell surface in the form of immune complexes for b-cell recognition they are considered so by some authors. Adaptive Immune Response: macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation and dendritic cells Dendritic cells Specialized cells of the hematopoietic system that have branch-like extensions. They are found throughout the lymphatic system, and in non-lymphoid tissues such as skin and the epithelia of the intestinal, respiratory, and reproductive tracts. They trap and process antigens, and present them to T-cells, thereby stimulating cell-mediated immunity. They are different from the non-hematopoietic follicular dendritic cells, which have a similar morphology and immune system function, but with respect to humoral immunity (antibody production). Skin: Structure and Functions
  • Blood to be filtered leaves the vasculature in the white pulp White pulp Secondary Lymphatic Organs → travels through the marginal zone Marginal zone MALT Lymphoma to the red pulp Red pulp Secondary Lymphatic Organs (being “cleaned” by macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation along the way)

Parenchyma: red pulp Red pulp Secondary Lymphatic Organs

The red pulp Red pulp Secondary Lymphatic Organs makes up 75% of the spleen and is present between the white pulp White pulp Secondary Lymphatic Organs surrounding the larger vessels.

• Consists of:
  • Bands of connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology known as splenic cords Splenic cords Secondary Lymphatic Organs (cords of Billroth)
  • Reticular network of capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries: Histology and venous sinuses Venous sinuses Veins: Histology
• White pulp White pulp Secondary Lymphatic Organs transitions to red pulp Red pulp Secondary Lymphatic Organs as trabecular arterioles Arterioles The smallest divisions of the arteries located between the muscular arteries and the capillaries. Arteries: Histology branch into the network of capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries: Histology and venous sinusoids Sinusoids Liver: Anatomy.
• Venous sinusoids Sinusoids Liver: Anatomy:
  • Lined with reticuloendothelial macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation
  • Splenic cords Splenic cords Secondary Lymphatic Organs help define the structure.
  • Blood flows through the venous sinusoids Sinusoids Liver: Anatomy → “cleaned” of foreign and defective components by macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation
  • Very small diameter:
    • Low flow rate Flow rate maximum flow the ventilator will deliver a set tidal volume in liters per minute Invasive Mechanical Ventilation
    • ↑ Contact with macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation
    • The structure is optimized for phagocytosis Phagocytosis The engulfing and degradation of microorganisms; other cells that are dead, dying, or pathogenic; and foreign particles by phagocytic cells (phagocytes). Innate Immunity: Phagocytes and Antigen Presentation.

Neurovasculature

Blood supply

• The spleen is a highly vascular organ.
• Arterial supply: via the splenic artery
  • Arises from the celiac trunk
  • Runs laterally along the superior aspect of the pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy within the splenorenal ligament
  • Branches into several smaller vessels called trabecular arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology upon entering the spleen
  • Each vessel supplies a different segment of the spleen:
    • The vessels do not anastomose with each other.
    • Allows for easy surgical removal of 1 of more splenic segments
• Venous drainage: via the splenic vein
  • Sinusoids Sinusoids Liver: Anatomy in the red pulp Red pulp Secondary Lymphatic Organs drain into collecting veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology → splenic vein
  • Splenic vein:
    • Runs along the posterior aspect of the pancreas Pancreas The pancreas lies mostly posterior to the stomach and extends across the posterior abdominal wall from the duodenum on the right to the spleen on the left. This organ has both exocrine and endocrine tissue. Pancreas: Anatomy
    • Combines with the superior mesenteric vein (SMV) to form the hepatic portal Hepatic portal Liver: Anatomy vein

Patterns of blood flow Blood flow Blood flow refers to the movement of a certain volume of blood through the vasculature over a given unit of time (e.g., mL per minute). Vascular Resistance, Flow, and Mean Arterial Pressure

About 10% of the blood stays within the vessels, whereas approximately 90% is filtered out into the splenic tissue. Blood flows to, through, and out of the spleen as follows:

• Aorta Aorta The main trunk of the systemic arteries. Mediastinum and Great Vessels: Anatomy → celiac trunk → splenic artery → trabecular arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology → splenic central arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries: Histology → 
• Blood is filtered out of the vessels →
• Travels through lymphoid tissue in the white pulp White pulp Secondary Lymphatic Organs (activates immune responses) → 
• Travels through splenic cords Splenic cords Secondary Lymphatic Organs/venous sinusoids Sinusoids Liver: Anatomy in the red pulp Red pulp Secondary Lymphatic Organs (blood is filtered by macrophages Macrophages The relatively long-lived phagocytic cell of mammalian tissues that are derived from blood monocytes. Main types are peritoneal macrophages; alveolar macrophages; histiocytes; kupffer cells of the liver; and osteoclasts. They may further differentiate within chronic inflammatory lesions to epithelioid cells or may fuse to form foreign body giant cells or langhans giant cells. Innate Immunity: Phagocytes and Antigen Presentation) → 
• Venous sinusoids Sinusoids Liver: Anatomy → collecting veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology → splenic vein → hepatic portal Hepatic portal Liver: Anatomy vein

Lymphatics

• The spleen acts as a large lymph Lymph The interstitial fluid that is in the lymphatic system. Secondary Lymphatic Organs node.
• Lymphocytes Lymphocytes Lymphocytes are heterogeneous WBCs involved in immune response. Lymphocytes develop from the bone marrow, starting from hematopoietic stem cells (HSCs) and progressing to common lymphoid progenitors (CLPs). B and T lymphocytes and natural killer (NK) cells arise from the lineage. Lymphocytes: Histology produced in the spleen exit via efferent Efferent Neurons which send impulses peripherally to activate muscles or secretory cells. Nervous System: Histology lymph Lymph The interstitial fluid that is in the lymphatic system. Secondary Lymphatic Organs vessels → splenic lymph nodes Lymph Nodes They are oval or bean shaped bodies (1 – 30 mm in diameter) located along the lymphatic system. Lymphatic Drainage System: Anatomy

Innervation

Innervation of the spleen takes place via the ANS ANS The ans is a component of the peripheral nervous system that uses both afferent (sensory) and efferent (effector) neurons, which control the functioning of the internal organs and involuntary processes via connections with the CNS. The ans consists of the sympathetic and parasympathetic nervous systems. Autonomic Nervous System: Anatomy.

• Sympathetic: celiac plexus
• Parasympathetic: vagus nerve Vagus nerve The 10th cranial nerve. The vagus is a mixed nerve which contains somatic afferents (from skin in back of the ear and the external auditory meatus), visceral afferents (from the pharynx, larynx, thorax, and abdomen), parasympathetic efferents (to the thorax and abdomen), and efferents to striated muscle (of the larynx and pharynx). Pharynx: Anatomy

Functions

The spleen is the largest secondary lymphoid organ in the body. The functions of the spleen are primarily immunologic and hematologic. Survival without a spleen is possible as it is not a vital organ.

Immune functions

• Detection of foreign antigens in blood
• Activation of immune responses
• Production of antibodies Antibodies Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins: Types and Functions
• Phagocytosis Phagocytosis The engulfing and degradation of microorganisms; other cells that are dead, dying, or pathogenic; and foreign particles by phagocytic cells (phagocytes). Innate Immunity: Phagocytes and Antigen Presentation (Spleen is the most effective phagocytic compartment of the body.)

Hematologic functions

• Cleanses the blood of old RBCs RBCs Erythrocytes, or red blood cells (RBCs), are the most abundant cells in the blood. While erythrocytes in the fetus are initially produced in the yolk sac then the liver, the bone marrow eventually becomes the main site of production. Erythrocytes: Histology
• Storage compartment for platelets Platelets Platelets are small cell fragments involved in hemostasis. Thrombopoiesis takes place primarily in the bone marrow through a series of cell differentiation and is influenced by several cytokines. Platelets are formed after fragmentation of the megakaryocyte cytoplasm. Platelets: Histology
• Hematopoiesis Hematopoiesis The development and formation of various types of blood cells. Hematopoiesis can take place in the bone marrow (medullary) or outside the bone marrow (extramedullary hematopoiesis). Bone Marrow: Composition and Hematopoiesis:
  • In the fetus
  • In cases of extreme anemia Anemia Anemia is a condition in which individuals have low Hb levels, which can arise from various causes. Anemia is accompanied by a reduced number of RBCs and may manifest with fatigue, shortness of breath, pallor, and weakness. Subtypes are classified by the size of RBCs, chronicity, and etiology. Anemia: Overview and Types in adults
• Acts as a blood reservoir Reservoir Animate or inanimate sources which normally harbor disease-causing organisms and thus serve as potential sources of disease outbreaks. Reservoirs are distinguished from vectors (disease vectors) and carriers, which are agents of disease transmission rather than continuing sources of potential disease outbreaks. Humans may serve both as disease reservoirs and carriers. Escherichia coli → spleen contraction can release red blood cells Red blood cells Erythrocytes, or red blood cells (RBCs), are the most abundant cells in the blood. While erythrocytes in the fetus are initially produced in the yolk sac then the liver, the bone marrow eventually becomes the main site of production. Erythrocytes: Histology and increase blood volume in periods of hypoxia Hypoxia Sub-optimal oxygen levels in the ambient air of living organisms. Ischemic Cell Damage or hypovolemia Hypovolemia Sepsis in Children

Clinical Relevance

• Splenomegaly Splenomegaly Splenomegaly is pathologic enlargement of the spleen that is attributable to numerous causes, including infections, hemoglobinopathies, infiltrative processes, and outflow obstruction of the portal vein. Splenomegaly: pathological enlargement of the spleen. Causes include infections Infections Invasion of the host organism by microorganisms or their toxins or by parasites that can cause pathological conditions or diseases. Chronic Granulomatous Disease, myeloproliferative disorders, hemoglobinopathies Hemoglobinopathies A group of inherited disorders characterized by structural alterations within the hemoglobin molecule. Anemia: Overview and Types characterized by chronic extravascular hemolysis Extravascular hemolysis Hemolytic Anemia, infiltrative processes (e.g., sarcoidosis Sarcoidosis Sarcoidosis is a multisystem inflammatory disease that causes noncaseating granulomas. The exact etiology is unknown. Sarcoidosis usually affects the lungs and thoracic lymph nodes, but it can also affect almost every system in the body, including the skin, heart, and eyes, most commonly. Sarcoidosis, neoplasms Neoplasms New abnormal growth of tissue. Malignant neoplasms show a greater degree of anaplasia and have the properties of invasion and metastasis, compared to benign neoplasms. Benign Bone Tumors), and congestive disorders (e.g., portal hypertension Portal hypertension Portal hypertension is increased pressure in the portal venous system. This increased pressure can lead to splanchnic vasodilation, collateral blood flow through portosystemic anastomoses, and increased hydrostatic pressure. There are a number of etiologies, including cirrhosis, right-sided congestive heart failure, schistosomiasis, portal vein thrombosis, hepatitis, and Budd-Chiari syndrome. Portal Hypertension, splenic vein thrombosis Splenic Vein Thrombosis Portal Hypertension). The most common symptoms are abdominal discomfort or pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways, increased abdominal girth, and early satiety Early Satiety Bariatric Surgery due to compression Compression Blunt Chest Trauma of the stomach Stomach The stomach is a muscular sac in the upper left portion of the abdomen that plays a critical role in digestion. The stomach develops from the foregut and connects the esophagus with the duodenum. Structurally, the stomach is C-shaped and forms a greater and lesser curvature and is divided grossly into regions: the cardia, fundus, body, and pylorus. Stomach: Anatomy. Anemia Anemia Anemia is a condition in which individuals have low Hb levels, which can arise from various causes. Anemia is accompanied by a reduced number of RBCs and may manifest with fatigue, shortness of breath, pallor, and weakness. Subtypes are classified by the size of RBCs, chronicity, and etiology. Anemia: Overview and Types and thrombocytopenia Thrombocytopenia Thrombocytopenia occurs when the platelet count is < 150,000 per microliter. The normal range for platelets is usually 150,000-450,000/µL of whole blood. Thrombocytopenia can be a result of decreased production, increased destruction, or splenic sequestration of platelets. Patients are often asymptomatic until platelet counts are < 50,000/µL. Thrombocytopenia are often present.
• Rupture of the spleen Rupture of the Spleen Splenic rupture is a medical emergency that carries a significant risk of hypovolemic shock and death. Injury to the spleen accounts for nearly half of all injuries to intra-abdominal organs. The most common reason for a rupture of the spleen is blunt abdominal trauma, specifically, motor vehicle accidents. Rupture of the Spleen: a medical emergency that carries a significant risk of hypovolemic shock Hypovolemic Shock Types of Shock and death. The most common cause is blunt abdominal trauma, (e.g., motor vehicle accidents Motor Vehicle Accidents Spinal Cord Injuries). For individuals with splenomegaly Splenomegaly Splenomegaly is pathologic enlargement of the spleen that is attributable to numerous causes, including infections, hemoglobinopathies, infiltrative processes, and outflow obstruction of the portal vein. Splenomegaly, however, even minimal trauma may result in splenic rupture Splenic rupture Splenic rupture is a medical emergency that carries a significant risk of hypovolemic shock and death. Injury to the spleen accounts for nearly half of all injuries to intra-abdominal organs. The most common reason for a rupture of the spleen is blunt abdominal trauma, specifically, motor vehicle accidents. Rupture of the Spleen. Affected individuals often present with LUQ abdominal pain Abdominal Pain Acute Abdomen, although the pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways may be referred to the left shoulder. Management depends on hemodynamic stability and ranges from observation to splenectomy Splenectomy Surgical procedure involving either partial or entire removal of the spleen. Rupture of the Spleen.
• Asplenia Asplenia Asplenia is the absence of splenic tissue or function and can stem from several factors ranging from congenital to iatrogenic. There is a distinction between anatomic asplenia, which is due to the surgical removal of the spleen, and functional asplenia, which is due to a condition that leads to splenic atrophy, infarct, congestion, or infiltrative disease. Asplenia: absence of splenic tissue or function. There is a distinction between anatomic asplenia Anatomic Asplenia Asplenia, which is due to surgical removal of the spleen, and functional asplenia Functional Asplenia Asplenia, which is due to conditions that lead to splenic atrophy Atrophy Decrease in the size of a cell, tissue, organ, or multiple organs, associated with a variety of pathological conditions such as abnormal cellular changes, ischemia, malnutrition, or hormonal changes. Cellular Adaptation, infarcts, congestion, or infiltrative disease. Individuals are at high risk of sepsis Sepsis Systemic inflammatory response syndrome with a proven or suspected infectious etiology. When sepsis is associated with organ dysfunction distant from the site of infection, it is called severe sepsis. When sepsis is accompanied by hypotension despite adequate fluid infusion, it is called septic shock. Sepsis and Septic Shock caused by encapsulated Encapsulated Klebsiella bacteria Bacteria Bacteria are prokaryotic single-celled microorganisms that are metabolically active and divide by binary fission. Some of these organisms play a significant role in the pathogenesis of diseases. Bacteriology, thereby requiring adherence to a strict vaccination Vaccination Vaccination is the administration of a substance to induce the immune system to develop protection against a disease. Unlike passive immunization, which involves the administration of pre-performed antibodies, active immunization constitutes the administration of a vaccine to stimulate the body to produce its own antibodies. Vaccination schedule and early antibiotic treatment when an infection is suspected. Thromboembolic events are common. 
• Hypersplenism Hypersplenism Condition characterized by splenomegaly, some reduction in the number of circulating blood cells in the presence of a normal or hyperactive bone marrow, and the potential for reversal by splenectomy. Splenomegaly: increased functioning of the spleen that occurs with or without organ enlargement. Sequestration of blood elements leads to congestion, whereas activation of the reticuloendothelial system leads to thrombocytopenia Thrombocytopenia Thrombocytopenia occurs when the platelet count is < 150,000 per microliter. The normal range for platelets is usually 150,000-450,000/µL of whole blood. Thrombocytopenia can be a result of decreased production, increased destruction, or splenic sequestration of platelets. Patients are often asymptomatic until platelet counts are < 50,000/µL. Thrombocytopenia and anemia Anemia Anemia is a condition in which individuals have low Hb levels, which can arise from various causes. Anemia is accompanied by a reduced number of RBCs and may manifest with fatigue, shortness of breath, pallor, and weakness. Subtypes are classified by the size of RBCs, chronicity, and etiology. Anemia: Overview and Types. Hypersplenism Hypersplenism Condition characterized by splenomegaly, some reduction in the number of circulating blood cells in the presence of a normal or hyperactive bone marrow, and the potential for reversal by splenectomy. Splenomegaly is associated with an increased risk of splenic rupture Splenic rupture Splenic rupture is a medical emergency that carries a significant risk of hypovolemic shock and death. Injury to the spleen accounts for nearly half of all injuries to intra-abdominal organs. The most common reason for a rupture of the spleen is blunt abdominal trauma, specifically, motor vehicle accidents. Rupture of the Spleen. Individuals may present with splenomegaly Splenomegaly Splenomegaly is pathologic enlargement of the spleen that is attributable to numerous causes, including infections, hemoglobinopathies, infiltrative processes, and outflow obstruction of the portal vein. Splenomegaly, pancytopenia Pancytopenia Deficiency of all three cell elements of the blood, erythrocytes, leukocytes and platelets. Aplastic Anemia, and compensatory bone marrow Bone marrow The soft tissue filling the cavities of bones. Bone marrow exists in two types, yellow and red. Yellow marrow is found in the large cavities of large bones and consists mostly of fat cells and a few primitive blood cells. Red marrow is a hematopoietic tissue and is the site of production of erythrocytes and granular leukocytes. Bone marrow is made up of a framework of connective tissue containing branching fibers with the frame being filled with marrow cells. Bone Marrow: Composition and Hematopoiesis hyperplasia Hyperplasia An increase in the number of cells in a tissue or organ without tumor formation. It differs from hypertrophy, which is an increase in bulk without an increase in the number of cells. Cellular Adaptation.
• Splenosis: an acquired condition characterized by auto-implantation of 1 or more deposits of splenic tissue in various compartments of the body, typically occurring after abdominal trauma or surgery. Multiple ectopic splenic foci are seen, which are typically small, sessile (growing on peritoneal surfaces), and sometimes functional. Symptoms depend on the location of the implants. Affected individuals are often asymptomatic and are usually observed for further changes.
• Splenic abscess Splenic Abscess Imaging of the Spleen: a rare infection associated with high mortality Mortality All deaths reported in a given population. Measures of Health Status if not treated properly. The etiology is usually due to the spread of infection from different sites and is commonly associated with endocarditis Endocarditis Endocarditis is an inflammatory disease involving the inner lining (endometrium) of the heart, most commonly affecting the cardiac valves. Both infectious and noninfectious etiologies lead to vegetations on the valve leaflets. Patients may present with nonspecific symptoms such as fever and fatigue. Endocarditis. Symptoms include abdominal pain Abdominal Pain Acute Abdomen, left pleuritic chest or shoulder pain Pain An unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons. Pain: Types and Pathways (due to irritation of the diaphragm Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm: Anatomy), fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever, and malaise Malaise Tick-borne Encephalitis Virus. Management includes antibiotic therapy, percutaneous drainage Percutaneous Drainage Echinococcus/Echinococcosis, and surgical excision.