What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Where Is the Spleen? Location, Function and Anatomy

Name: Where Is the Spleen? Location, Function and Anatomy
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The spleen is a soft, vascular organ in the left cranial part of the abdomen, tucked behind and to the left of the stomach and pressed against the diaphragm and left body wall. In people it lies roughly at the level of ribs 9 through 11, with its long axis running along the line of the 10th rib.

This article covers where the spleen sits in humans and in the common domestic species, what its two tissue compartments do, and why the spleen of a dog, horse or pig behaves differently from a human spleen when the animal is frightened, exercising or bleeding.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

The Short Answer: Position in the Body

The spleen is an intraperitoneal organ. That means it develops inside the peritoneal cavity and is covered by a layer of peritoneum, the thin membrane that lines the abdomen and wraps the abdominal organs. It is not part of the digestive tube, and food never passes through it. It is a blood-filtering and immune organ that happens to sit next to the stomach because of how it develops from the dorsal mesentery.

In a person lying on the back, the spleen occupies the left upper quadrant of the abdomen. Its superior (upper) pole touches the diaphragm. Its medial (inner) surface is molded against the greater curvature of the stomach. Its inferior (lower) pole points toward the left colic flexure, the bend where the transverse colon turns down into the descending colon. The left kidney and the tail of the pancreas lie behind and below it.

The organ is normally hidden under the rib cage, which is why a healthy spleen cannot be felt through the abdominal wall in people or in most animals. It becomes palpable only when it enlarges.

Normal Size and Shape

A healthy adult human spleen is about 12 cm long, 7 cm wide and 3 to 4 cm thick, and weighs roughly 150 g. It is roughly the shape of a fist or a thick wedge, with a convex outer surface that follows the curve of the ribs and a concave inner surface marked by impressions from the stomach, kidney and colon.

Size scales with body size across species, but the spleen is proportionally larger in many animals than in humans. The organ is also highly distensible. It can swell to several times its normal size (splenomegaly) and it can shrink dramatically within minutes when its smooth muscle contracts.

The Torso Diagram: What Sits Next to the Spleen

Because the spleen has no fixed attachments to bone, its position is defined by its neighbors. The table below lists the structures that touch or lie immediately adjacent to the spleen, and what each relationship means clinically.

Relation	Position relative to spleen	Why it matters
Stomach	Medial and ventral, against the gastric impression	Gastric dilation or torsion can displace or twist the spleen
Diaphragm	Cranial, against the diaphragmatic surface	Splenic injury can irritate the diaphragm and cause referred shoulder pain
Left kidney	Caudal and dorsal, against the renal impression	Renal and splenic disease can mimic each other on palpation
Left colic flexure	Caudal, near the inferior pole	Colonic gas can obscure the spleen on radiographs
Pancreas (tail)	Dorsal, in the splenic hilus region	Pancreatitis can inflame the splenic capsule and vice versa
Left body wall and ribs 9 to 11	Lateral and superficial	Rib fractures can lacerate the spleen
Splenic artery and vein	At the hilus, the medial indentation	The vascular pedicle is the only fixed anchor, so it can twist

The hilus is the notch on the medial surface where blood vessels and nerves enter and leave. The splenic artery comes from the celiac trunk, and the splenic vein drains into the portal vein, which carries blood to the liver. This is why splenic blood passes through the liver before returning to the heart.

Comparative Anatomy: Spleen Position and Shape Across Species

The spleen is present in all vertebrates, but its shape, position and mobility vary widely. In domestic mammals it is generally elongated and strap-like, lying along the left body wall. In birds it is small and round, and in fish it is often a dark, compact organ associated with the gut.

The table below summarizes the key comparative facts for the species most relevant to veterinary readers.

Species	Approximate size and weight	Position	Shape	Contraction ability
Human	About 12 cm long, 7 cm wide, 150 g	Left upper quadrant, ribs 9 to 11, long axis along 10th rib	Fist-shaped wedge	Limited, capsule has little smooth muscle
Dog	Varies with breed, often 10 to 20 cm long	Left cranial abdomen, dorsal to stomach, along left body wall	Elongated strap or tongue shape	Strong, thick smooth muscle capsule and trabeculae
Cat	Smaller, often 5 to 10 cm long	Left cranial abdomen, dorsal to stomach	Elongated, slightly flattened	Moderate smooth muscle
Horse	Large, often 40 to 80 cm long	Left dorsal abdomen, under the last ribs and lumbar region	Flat, triangular or sickle shape	Strong, thick smooth muscle capsule
Pig	Large, often 30 to 45 cm long	Left cranial abdomen, dorsal to stomach	Long, narrow strap	Strong, thick smooth muscle capsule
Cattle	Large, often 40 to 60 cm long	Left dorsal abdomen, partly over the rumen	Long, flat strap	Moderate to strong smooth muscle
Bird	Small, often 1 to 3 cm	Near the junction of the esophagus and proventriculus	Round or oval	Limited

The pig, dog and horse are the classic examples of species with a muscular spleen. Their capsules and trabeculae contain substantial smooth muscle, which lets the organ contract and push stored red blood cells into circulation. This is a reserve of oxygen-carrying capacity that can be released during exercise, fear or blood loss.

Red Pulp and White Pulp: Two Organs in One

The spleen is not a uniform tissue. It is built from two functionally distinct compartments, and the balance between them determines what the organ is doing at any moment.

Red Pulp: Filtration, Storage and Blood Cell Production

The red pulp makes up most of the splenic volume. It is a spongy network of splenic cords and blood-filled sinusoids. Blood enters the red pulp through small arteries and then percolates through the cords before re-entering the venous sinuses.

Three jobs happen here.

First, filtration. Macrophages lining the cords and sinuses examine every red blood cell that passes. They remove aged, damaged or antibody-coated red cells, along with blood-borne particles and microorganisms. The blood-spleen barrier is the structural filter that performs this task. In fruit bats, tracer studies show that carbon particles accumulate most heavily in the marginal zone, the ellipsoids and the periellipsoidal lymphoid sheaths, with further deposits in the splenic cords and sinuses [1]. The same general architecture, an ellipsoid-and-macrophage filtration barrier, is found in goldfish, where ellipsoids are wrapped in reticular and macrophage sheaths [2].

Second, storage. In species with a muscular capsule, the red pulp holds a reservoir of red blood cells that can be squeezed out on demand. This is why a horse's spleen can shrink by a third or more during a hard gallop, and why a dog's spleen may look small on an ultrasound after exercise.

Third, hematopoiesis. In the fetus, the spleen makes red blood cells. In adult mammals this function is normally taken over by the bone marrow, but the spleen can resume blood cell production when the marrow fails. This is called extramedullary hematopoiesis. In a mouse model of myeloproliferative neoplasm driven by the Jak2-V617F mutation, blocking integrin signaling reduced spleen volume by about 30 percent and cut erythroblast counts by roughly 30 percent, showing that the enlarged spleen in that disease is largely a factory for red cell precursors [3]. In fish, the spleen remains a major hematopoietic organ throughout life, and in yellowfin tuna the red pulp supports the high oxygen-carrying demands of long-distance swimming [4].

White Pulp: Immune Surveillance

The white pulp is the immune arm of the spleen. It is organized around small arteries as a periarteriolar lymphoid sheath, with follicles and germinal centers where B and T lymphocytes are activated.

When a pathogen enters the bloodstream, antigen-presenting cells carry it to the white pulp. There, lymphocytes that recognize the antigen multiply and differentiate. The spleen is therefore a major secondary lymphoid organ, meaning it is a site where immune responses are mounted, not where lymphocytes are first made.

White pulp activity changes with disease. In mice treated with high-dose erythropoietin, the spleen enlarged and the white pulp atrophied, with indistinct germinal centers and a blurred boundary between white and red pulp [5]. In a mouse model of psoriasis, imiquimod treatment increased spleen weight and expanded the white pulp [6]. In cattle egrets, the white pulp consists of a periarteriolar lymphoid sheath and a periellipsoidal lymphatic sheath, and proliferating cells are most numerous in the splenic cords and periellipsoidal regions [7].

The red-to-white pulp ratio is a useful index of what the spleen is doing. A shift toward red pulp suggests filtration, storage or extramedullary hematopoiesis. A shift toward white pulp suggests immune activation.

How Blood Flows Through the Spleen

Blood enters through the splenic artery at the hilus. The artery branches into trabecular arteries, then central arteries surrounded by white pulp, then penicillar capillaries. In some species the capillaries open directly into the venous sinuses (a closed circulation). In others they open into the splenic cords and blood must squeeze between endothelial cells to reach the sinuses (an open circulation). Many species have both pathways. The fruit bat spleen, for example, has open and closed microcirculation side by side [1].

The route matters because it determines how long blood cells spend in the filtration zone. Slower transit means more thorough screening by macrophages. It also means more opportunity for pathogens to encounter immune cells in the marginal zone.

Why the Spleen Contracts

Contraction is the feature that most distinguishes the spleen of a dog, horse or pig from a human spleen. The capsule and trabeculae of these species contain smooth muscle cells that respond to sympathetic nerve signals and to circulating catecholamines such as epinephrine.

During exercise, fear or hemorrhage, the spleen contracts and expels stored red cells into the general circulation. This raises the hematocrit, the proportion of blood volume made up of red cells, within minutes. It is a rapid, short-term boost to oxygen delivery, not a long-term change in red cell production.

A spleen that has just contracted is small and wrinkled. A spleen that has just relaxed is engorged and smooth. This is normal physiology, not disease, and it is one reason serial ultrasound measurements of the spleen can vary widely in the same animal.

Clinical Relevance, Limitations and Common Mistakes

Splenic disease is common in veterinary practice, and several misconceptions cause confusion.

The first mistake is assuming an enlarged spleen is always cancer. Splenomegaly has many causes, including infection, immune-mediated disease, congestion from sedation or anesthesia, and extramedullary hematopoiesis. In mice, a high-fat diet caused splenic histopathology with loss of the sharp red-white pulp demarcation, and treatment that restored that demarcation also lowered inflammatory mediators such as TNF-alpha, IL-1beta, IL-6 and iNOS while raising IL-10 [8]. This shows that splenic enlargement and inflammation are linked, and that the organ can recover when the underlying insult is removed.

The second mistake is assuming a small spleen is abnormal. In a dog or horse that has just exercised or is excited, a small spleen is expected. In a human, by contrast, the spleen does not contract much because the capsule has little smooth muscle.

The third mistake is forgetting that the spleen is not essential for survival. Animals and people can live without a spleen, but they lose a filtration and immune reserve. The organ's limited regenerative capacity is a recognized clinical problem, and research into scaffolds that support in situ spleen regeneration is ongoing [9].

The fourth mistake is overlooking the spleen's role in systemic disease. In a mouse model of Alzheimer's disease, the spleen showed red pulp degeneration and an altered red-to-white pulp ratio alongside kidney changes, and these alterations correlated with anxiety-like behavior [10]. This does not mean the spleen causes Alzheimer's disease. It means the spleen is a participant in systemic inflammatory states, and its condition can reflect what is happening elsewhere in the body.

The fifth mistake is assuming all spleens look alike. A bird's spleen is small and round and sits at the junction of the esophagus and proventriculus, as described in the cattle egret [7]. A fish spleen is often a dark, elongated organ closely associated with the gut, and in some species it contains prominent melano-macrophage centers that store pigment and participate in phagocytosis [11]. A reptile spleen, such as that of the Chinese soft-shelled turtle, can show severe lesions with loss of red-white pulp demarcation after bacterial infection [12].

A wandering spleen is a rare condition in which the suspensory ligaments are lax, allowing the organ to move abnormally. In one reported human case, the spleen was found in the pelvis and its abnormal mobility caused traction on the pancreatic tail and recurrent pancreatitis [13]. This is a human surgical case, but the underlying principle, that splenic position depends on ligamentous support, applies across species.

The spleen is also a target of therapeutic research. A normothermic machine-perfusion platform using intact pig spleens preserved red and white pulp histology and supported serial immune measurements, including glucocorticoid-responsive transcriptional programs [14]. This kind of platform is a research tool, not a clinical service, but it illustrates how central the spleen is to immune pharmacology.

Individual animals vary, and a veterinarian who can examine the patient, review imaging and run laboratory tests is the right person to interpret any splenic finding. This article is educational and is not a substitute for veterinary diagnosis or treatment.

Frequently Asked Questions

Where is the spleen located in the body?

The spleen sits in the left cranial abdomen, dorsal to the stomach and against the diaphragm and left body wall. In humans it lies at the level of ribs 9 to 11, with its long axis along the 10th rib.

How big is a normal spleen?

A healthy adult human spleen is about 12 cm long, 7 cm wide and 150 g. Size varies with body size and species, and the organ can shrink or swell rapidly depending on blood flow and contraction.

What does the spleen do?

The spleen filters blood, removes old red blood cells, stores a reserve of red cells in some species, and mounts immune responses against blood-borne pathogens. It also can resume blood cell production when the bone marrow fails.

Can you live without a spleen?

Yes. Animals and people can survive without a spleen, but they lose a filtration and immune reserve. The organ has limited ability to regrow after injury.

Why does a dog's spleen get smaller during exercise?

The capsule and trabeculae of the dog spleen contain smooth muscle. Sympathetic signals during exercise or fear cause the organ to contract and push stored red blood cells into circulation.

Which animals have a muscular spleen?

Dogs, horses and pigs have notably muscular spleens with thick smooth muscle in the capsule and trabeculae. Humans have much less smooth muscle and therefore limited splenic contraction.

What is the difference between red pulp and white pulp?

Red pulp filters blood, stores red cells and can produce blood cells. White pulp is immune tissue organized around small arteries, where lymphocytes are activated against pathogens.

What does an enlarged spleen mean in a pet?

An enlarged spleen can result from infection, immune disease, congestion, blood cell production outside the marrow, or cancer. It is a finding, not a diagnosis, and it needs veterinary evaluation.