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abdomen
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Two anatomical illustrations of the human abdomen: the left shows labeled abdominal organs including liver, stomach, intestines, and rectum, while the right shows the same area covered by the greater omentum. Encyclopædia Britannica, first edition, art: human thorax and abdomen Detailed anatomical illustration of the human abdominal wall from an anterior view, showing labeled muscles, ligaments, blood vessels, nerves, fascia, and the diaphragm at the top, with the urinary bladder at the bottom. Illustration comparing three beetle species-a deathwatch beetle, a click beetle, and a great silver water beetle-using colored arrows to show how body parts scale differently as overall beetle size increases, demonstrating allometry in evolution.
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Two anatomical illustrations of the human abdomen: the left shows labeled abdominal organs including liver, stomach, intestines, and rectum, while the right shows the same area covered by the greater omentum.
Abdominal organs The organs located in the abdominal cavity are supported and protected by the bones of the pelvis and rib cage and are covered by the greater omentum, a fold of peritoneum that consists mainly of fat.

abdomen

anatomy
Written by
Kara Rogers
Kara Rogers is the senior editor of biomedical sciences at Encyclopædia Britannica, where she oversees a range of content from medicine and genetics to microorganisms. She joined Britannica in 2006 and...
Kara Rogers
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Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
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Top Questions

What is the abdomen in human anatomy?

What muscles support the abdomen?

What are common abdominal disorders?

abdomen, in human anatomy, the portion of the trunk situated between the thorax (chest) and the pelvis. The abdomen contains most of the organs of the digestive system, along with the spleen, kidneys, adrenal glands, and major blood vessels. The abdominal cavity is lined by a thin serous membrane known as the peritoneum, which supports and protects many abdominal organs while allowing them to move smoothly during digestion and respiration.

Anatomical features

Encyclopædia Britannica, first edition, art: human thorax and abdomen
Encyclopædia Britannica, first edition, art: human thorax and abdomenIllustration of the organs in the human abdomen and thorax from the first edition of the Encyclopædia Britannica, vol. 1, plate XIX, figure 1.

Unlike the thoracic cavity, which is enclosed by the rib cage, the abdomen has relatively little bony protection, relying instead on layers of skeletal muscle, connective tissue, and fascia that form the abdominal wall. The rectus abdominis, external and internal oblique, and transversus abdominis muscles support and protect the abdominal organs, stabilize the trunk, and assist in movements such as bending and twisting. They also generate intra-abdominal pressure, which aids in breathing, coughing, vomiting, urination, defecation, and childbirth.

Many abdominal organs are suspended by folds of the peritoneum called mesenteries, which support the organs while carrying their blood vessels, nerves, and lymphatic vessels. Others, including the kidneys, adrenal glands, and most of the pancreas, occupy the retroperitoneal space behind the peritoneum.

Illustration showing a variety of adult insects, each labeled with its common name and order, including walkingstick, earwig, stonefly, tree cricket, dragonfly, female scale, cat flea, thrips, sucking louse, thysanuran, ant, scarab beetle, stinkbug, mosquito, and cabbage butterfly, highlighting their diverse shapes and sizes.
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insect: Abdomen

Anatomical divisions

The location of organs, injuries, or pain in the abdomen typically is described based on the division of the abdominal cavity into either four quadrants or nine anatomical regions. The quadrant system is used most commonly in clinical practice for rapid assessment, whereas the nine-region system provides more precise anatomical localization.

The four abdominal quadrants
  • Right upper quadrant (RUQ): Contains most of the liver, the gallbladder, the duodenum, the head of the pancreas, the right kidney, and portions of the small and large intestines
  • Left upper quadrant (LUQ): Contains the stomach, spleen, left lobe of the liver, body and tail of the pancreas, left kidney, and portions of the colon
  • Right lower quadrant (RLQ): Contains the cecum, appendix, terminal ileum, right ureter, and, in females, the right ovary and uterine tube
  • Left lower quadrant (LLQ): Contains the descending and sigmoid colon, left ureter, and, in females, the left ovary and uterine tube
The nine abdominal regions
  • Right hypochondriac: Right lobe of the liver, gallbladder, and upper portion of the right kidney
  • Epigastric: Stomach, left lobe of the liver, pancreas, and duodenum
  • Left hypochondriac: Spleen, fundus of the stomach, tail of the pancreas, and upper portion of the left kidney
  • Right lumbar (flank): Ascending colon, right kidney, and loops of the small intestine
  • Umbilical: Small intestine, transverse colon, and portions of the pancreas
  • Left lumbar (flank): Descending colon, left kidney, and loops of the small intestine
  • Right iliac (inguinal): Cecum, appendix, and lower portion of the small intestine
  • Hypogastric (pubic): Urinary bladder, loops of the small intestine, sigmoid colon, and reproductive organs, depending on sex
  • Left iliac (inguinal): Sigmoid colon, lower descending colon, and lower portion of the small intestine

Functional integration

The abdomen functions as an integrated physiological compartment in which multiple organ systems interact to maintain homeostasis. Digestive organs break down food and absorb nutrients, the liver processes and stores nutrients while synthesizing essential proteins and detoxifying foreign substances, the pancreas regulates digestion and blood glucose through digestive enzymes and hormones, the kidneys regulate body fluids and remove metabolic wastes, and the spleen monitors the blood for damaged cells and invading microorganisms. Together, these organs support metabolism, immune function, and chemical balance throughout the body.

Clinical significance

Because the abdomen contains multiple organ systems within a relatively confined space, diseases affecting different organs can produce similar symptoms, making accurate diagnosis dependent on careful clinical evaluation. Physicians assess the location, character, and timing of abdominal pain, often using the four abdominal quadrants or nine anatomical regions to help identify the affected organ. Physical examination may also reveal tenderness, guarding, rigidity, distention, or abnormal bowel sounds, which can provide important clues to the underlying condition.

Abdominal disorders encompass a wide range of conditions involving the digestive, urinary, vascular, and lymphatic systems. Common examples include appendicitis, gallstones, peptic ulcers, inflammatory bowel disease, pancreatitis, liver disease, hernias, intestinal obstruction, peritonitis, abdominal aortic aneurysm, and traumatic injury. Diagnosis typically combines clinical findings with laboratory testing and imaging techniques such as ultrasound, computed tomography, magnetic resonance imaging, and, in some cases, endoscopy, allowing physicians to evaluate both the structure and function of abdominal organs.

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Inflammation or perforation of abdominal organs may involve the peritoneum, producing severe pain, involuntary muscle rigidity, and potentially life-threatening infection that requires urgent medical treatment.

Kara Rogers