Abdominal Organs: Positions, Functions, and Species Differences

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

The abdominal organs are the liver, spleen, stomach, intestines, pancreas, kidneys, adrenal glands, bladder, and the reproductive tract, and they sit in predictable positions that differ sharply between dogs, cats, horses, and humans. Mapping those organs to the four quadrants and nine regions of the abdomen gives a common language for anatomy, imaging, and physical examination across species.

What Counts as an Abdominal Organ

The abdomen is the body cavity between the diaphragm and the pelvis. It is lined by the peritoneum, a thin serous membrane that also wraps most of the organs inside it. Organs that are pushed against the abdominal wall and covered by peritoneum on one surface only are called retroperitoneal, and the kidneys and adrenal glands are the classic examples. Organs that hang from a mesenteric stalk and are almost fully wrapped in peritoneum are intraperitoneal, and the spleen, stomach, and most of the small intestine fall into that group.

The abdominal cavity contains the caudal part of the esophagus, the stomach, the small intestine (duodenum, jejunum, ileum), the large intestine (cecum, colon, rectum), the liver, the biliary tree, the pancreas, the spleen, the kidneys, the adrenal glands, the ureters, the bladder, and the internal reproductive organs. The abdominal aorta and its branches, the caudal vena cava, the portal vein, and the lymphatic cisterna chyli also run through this space.

In dogs and cats the liver is the largest solid organ in the abdomen. In horses the cecum and colon dominate the space. In humans the arrangement is closer to the dog than to the horse, but the liver lobation and the position of the spleen still differ in ways that matter for imaging and surgery.

The Four Quadrants and Nine Regions

Clinicians divide the abdomen with two imaginary planes. A vertical line through the midline and a horizontal line through the umbilicus create four quadrants: right cranial (right upper), left cranial (left upper), right caudal (right lower), and left caudal (left lower). A finer grid uses two vertical planes (midclavicular lines) and two horizontal planes (subcostal and intertubercular) to create nine regions: right hypochondrium, epigastrium, left hypochondrium, right lumbar, umbilical, left lumbar, right iliac, hypogastrium, and left iliac.

The quadrant map is a clinical shorthand, not an anatomical boundary. In quadrupeds the "cranial" and "caudal" directions replace the human "upper" and "lower," and the diaphragm sits at an angle, so the liver and stomach extend further cranially than the same organs do in a standing person. In horses, the cranial abdomen is dominated by the liver on the right and the spleen on the left, and the cecum occupies a large part of the right caudal quadrant.

The nine-region grid is useful for describing pain, masses, and imaging findings. A mass in the left cranial quadrant of a dog is most likely splenic. A mass in the right cranial quadrant is more likely hepatic or biliary. A mass in the right caudal quadrant in a horse is likely cecal or colonic.

Organ-by-Organ Positions and Functions

Liver

The liver sits in the cranial abdomen, immediately behind the diaphragm, and is the largest gland in the body. It receives oxygenated blood from the hepatic artery and nutrient-rich blood from the portal vein, and it drains into the caudal vena cava through the hepatic veins. The liver stores glycogen, synthesizes plasma proteins and bile, detoxifies ammonia into urea, and metabolizes drugs and hormones.

Liver lobation differs by species. The dog liver has six lobes: right lateral, right medial, left lateral, left medial, quadrate, and caudate. The cat liver is similarly divided but the quadrate lobe is split into anterior and posterior parts in the mature cat, and the caudate lobe has a papillary process and a caudate process [1]. The horse liver is not lobed in the same way. It is a single, relatively unlobed mass with a deep notch, and it lacks a gallbladder.

Gallbladder and Biliary Tree

The gallbladder stores and concentrates bile. Dogs, cats, and humans have a gallbladder. The horse does not. In the horse, bile flows continuously from the liver into the duodenum through the common bile duct, which is wider and more distensible than in species with a gallbladder. This difference matters for interpreting equine liver disease, because horses cannot develop gallbladder disease and their bile is more dilute.

Spleen

The spleen filters blood, removes aged red cells, stores platelets, and mounts immune responses to blood-borne antigens. In the dog it is large, flat, and strap-like, and it lies in the left cranial quadrant against the greater curvature of the stomach. In the cat it is smaller and more elongated. In the horse it is sickle-shaped, lies entirely on the left side, and is oriented dorsoventrally along the dorsal body wall. In humans the spleen is a small, fist-sized organ in the left upper quadrant. The equine spleen can be imaged by ultrasound through the left 10th to 16th intercostal spaces in neonatal foals [2].

Stomach

The stomach is a muscular sac that mixes food with acid and enzymes. It sits in the cranial abdomen, mostly on the left side, and empties into the duodenum through the pylorus. The dog and cat stomach is a simple monogastric sac. The horse stomach is also simple but small relative to body size, and it is divided into a nonglandular squamous region and a glandular region. The human stomach is a simple J-shaped organ in the left upper quadrant.

Small Intestine

The small intestine is the duodenum, jejunum, and ileum. It is the main site of digestion and absorption. In dogs and cats the small intestine is short and lies in the central abdomen, suspended by the mesentery. In horses the small intestine is about 22 meters long and occupies the central and cranial abdomen. In humans the small intestine is about 6 meters long and fills the central and lower abdomen.

Large Intestine and Cecum

The large intestine absorbs water and electrolytes and houses a dense microbial population. The cecum is a blind-ended pouch at the junction of the small and large intestine. It is prominent in the horse and rabbit and small in the dog and cat. In the horse the cecum is a large, comma-shaped organ in the right caudal quadrant, and it is a major site of microbial fermentation. In the rabbit the cecum is also large and fermentative. In the dog and cat the cecum is a small, vermiform appendage. In humans the cecum is a small pouch and the appendix is a narrow extension from it.

Pancreas

The pancreas sits in the cranial abdomen, close to the duodenum, and has both exocrine and endocrine functions. The exocrine pancreas releases digestive enzymes and bicarbonate. The endocrine pancreas releases insulin, glucagon, and other hormones from the islets of Langerhans. In dogs and cats the pancreas is a V-shaped organ with a right and left lobe. In horses it is a compact, lobulated organ near the duodenum. In humans it is a retroperitoneal organ that crosses the midline.

Kidneys and Adrenal Glands

The kidneys filter blood, produce urine, regulate fluid and electrolyte balance, and secrete erythropoietin and renin. They are retroperitoneal. The right kidney sits slightly cranial to the left in most species because the liver pushes it forward. In neonatal foals the right kidney is located in the 15th to 17th intercostal space and the left kidney in the 16th and 17th intercostal spaces [2]. In dogs the kidneys are bean-shaped and the left is more mobile than the right. In cats the kidneys are also bean-shaped and the cat has a unique ability to concentrate urine. In humans the kidneys are bean-shaped and lie at the level of the T12 to L3 vertebrae.

The adrenal glands sit on the cranial pole of each kidney. In dogs, CT measurements of the adrenal glands are consistently larger than ultrasound measurements, and the difference is significant for both poles of the left adrenal gland across weight categories [3]. This matters when a clinician compares a CT report with an ultrasound report.

Bladder and Reproductive Organs

The bladder is a muscular sac in the caudal abdomen. In neonatal foals it is consistently found in the caudal aspect of the abdomen and extends into the middle or left and right segments depending on filling [2]. The uterus, ovaries, and testes are also in the abdomen or inguinal region depending on species and sex. In the horse the uterus is in the caudal abdomen and pelvis. In the dog and cat the uterus is in the caudal abdomen.

Blood Supply of the Abdominal Organs

The abdominal organs are supplied by three unpaired branches of the abdominal aorta: the celiac artery, the cranial mesenteric artery, and the caudal mesenteric artery. The celiac artery supplies the liver, spleen, stomach, pancreas, and proximal duodenum. The cranial mesenteric artery supplies the distal duodenum, jejunum, ileum, cecum, and ascending colon. The caudal mesenteric artery supplies the descending colon and rectum.

The paired branches of the abdominal aorta supply the kidneys, adrenal glands, and gonads. The renal arteries supply the kidneys. The phrenicoabdominal arteries supply the diaphragm and cranial abdominal wall. The external iliac arteries supply the pelvic limbs and the caudal abdominal wall.

Venous drainage is equally important. The portal vein carries blood from the stomach, intestines, spleen, and pancreas to the liver. The hepatic veins drain the liver into the caudal vena cava. The renal veins drain the kidneys into the caudal vena cava. The caudal vena cava then carries blood back to the heart.

The lymphatic drainage of the abdomen converges on the cisterna chyli, a sac-like structure dorsal to the aorta at the level of the first two lumbar vertebrae. From there, lymph flows into the thoracic duct and then into the venous system.

Comparative Table: Organ, Quadrant, Function, Blood Supply, and Species Notes

Species Differences That Matter

The liver is the clearest example of species variation. In dogs and cats the liver is deeply lobed and the gallbladder is present. In horses the liver is a single, relatively unlobed mass and the gallbladder is absent. In humans the liver has four lobes and a gallbladder. These differences affect how a clinician interprets a liver ultrasound, how a surgeon plans a liver biopsy, and how a pathologist reads a liver specimen.

The spleen is another clear example. In dogs the spleen is large, flat, and strap-like, and it can be imaged easily through the left abdominal wall. In horses the spleen is sickle-shaped and lies against the dorsal body wall. In cats the spleen is smaller and more elongated. In humans the spleen is a small, fist-sized organ in the left upper quadrant.

The cecum is a third example. In horses and rabbits the cecum is large and fermentative. In dogs and cats it is small and vermiform. In humans it is a small pouch with an appendix. This difference reflects the different digestive strategies of these species. Horses and rabbits are hindgut fermenters, and dogs and cats are simple-stomached carnivores or omnivores.

The pig is a useful comparative model because its abdominal anatomy is closer to humans than to dogs or horses. A review of porcine comparative anatomy notes that pigs share substantial anatomical and physiological similarities with humans, which makes them useful for transplantation research, but that critical species-specific differences exist in the gastrointestinal tract, hepatobiliary system, genitourinary tract, and vascular structures [4]. These differences matter for surgical planning and for interpreting preclinical data.

Clinical Relevance, Limitations and Common Mistakes

The most common mistake is to assume that the quadrant map is the same in quadrupeds and humans. In dogs and cats the liver and stomach extend further cranially than in humans, and the spleen sits more dorsally. In horses the cecum occupies a large part of the right caudal quadrant, which changes the differential diagnosis for a right-sided abdominal mass.

A second mistake is to forget that the kidneys are retroperitoneal. This matters for ultrasound and surgery, because a retroperitoneal approach is different from a transperitoneal approach. The right kidney sits more cranially than the left in most species because the liver pushes it forward.

A third mistake is to assume that a gallbladder is present in all species. The horse has no gallbladder, and this changes how bile flows and how liver disease presents. A fourth mistake is to compare CT and ultrasound measurements of the adrenal glands without accounting for the modality. CT measurements are consistently larger than ultrasound measurements in dogs, and the difference is significant for the left adrenal gland across weight categories [3].

A fifth mistake is to forget that organ size and position change with age, body condition, and pregnancy. In neonatal foals the umbilical vein is in the middle cranial segment and the urachus is in the middle caudal segment [2]. In mature horses these structures are no longer present. In cats the number of lumbar vertebrae can vary, and cats with six lumbar vertebrae have significantly longer vertebral bodies at L1 to L5 and lower height-to-length ratios than cats with seven [5]. This variation can affect ratio-based assessments of abdominal organ size.

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

Frequently Asked Questions

What are the main organs in the abdomen?

The main organs in the abdomen are the liver, spleen, stomach, small intestine, large intestine, cecum, pancreas, kidneys, adrenal glands, bladder, and the internal reproductive organs. The abdominal aorta, caudal vena cava, and portal vein also run through this space.

Which abdominal organ is largest in the dog?

The liver is the largest solid organ in the dog abdomen. The spleen is also large and flat, but the liver is heavier and occupies more of the cranial abdomen.

Does the horse have a gallbladder?

No. The horse has no gallbladder. Bile flows continuously from the liver into the duodenum through the common bile duct.

Why is the cecum large in horses and rabbits?

The cecum is large in horses and rabbits because they are hindgut fermenters. The cecum houses a dense microbial population that breaks down plant fiber into volatile fatty acids that the animal can absorb.

Where is the spleen located in the horse?

The spleen in the horse is sickle-shaped and lies on the left side of the abdomen, oriented dorsoventrally along the dorsal body wall. In neonatal foals it can be imaged through the left 10th to 16th intercostal spaces [2].

Are the kidneys in front of or behind the peritoneum?

The kidneys are behind the peritoneum, which means they are retroperitoneal. This is why they are approached differently in surgery than intraperitoneal organs like the spleen or stomach.

How does the cat liver differ from the dog liver?

The cat liver is divided into the same lobes as the dog liver, but the quadrate lobe is split into anterior and posterior parts in the mature cat, and the caudate lobe has a papillary process and a caudate process [1].

Why do CT and ultrasound measurements of the adrenal glands differ in dogs?

CT measurements of the adrenal glands are consistently larger than ultrasound measurements in dogs. The difference is significant for both poles of the left adrenal gland across weight categories, so the two modalities should not be used interchangeably without accounting for this bias [3].

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Further Reading

• Mid-Term Outcomes of Posterior Pelvic Reconstruction Combined with Lateral Abdominal Wall Suspension versus Sacrocolpopexy for Advanced Pelvic Organ Prolapse: A Retrospective Comparative Study.

Sources

1. Comparative morphological study of the liver in immature and mature local breed cat ( Felis catus ) in Iraq.
2. [[Sonographic topography of abdominal organs and structures in equine neonates].](https://pubmed.ncbi.nlm.nih.gov/31434116/)
3. Comparison of canine adrenal gland size assessment using ultrasonography and computed tomography.
4. A review of the comparative anatomy of the domestic pig (Sus scrofa domestica) and its relevance to translational research in abdominal transplantation.
5. Radiographic comparison of lumbar vertebral morphology between cats with six and seven lumbar vertebrae.