Abdomen Anatomy: Organs, Regions & Functions

The abdomen is the part of the body between the chest and the pelvis. It contains many of the organs responsible for digestion, nutrient absorption, blood filtration, fluid balance, and other essential processes. It also houses major blood vessels, nerves, muscles, and connective tissues that support the body and protect its internal structures.

Understanding abdominal anatomy begins with knowing where the abdomen starts and ends, how its organs are arranged, and how those organs work together. Although the abdomen is often thought of as the stomach area, the stomach is only one of many organs located there.

What is the abdomen?

The abdomen lies between the diaphragm, the broad muscle that separates the chest from the abdominal cavity, and the pelvic inlet, the opening that leads into the pelvis. The diaphragm forms the upper boundary of the abdominal cavity, while the abdominal and back muscles, spine, and surrounding connective tissues help define its other boundaries.

The abdominal cavity contains most of the digestive system, including the stomach, small intestine, and much of the large intestine. It also contains the liver, gallbladder, pancreas, spleen, kidneys, and adrenal glands. Large blood vessels, lymphatic structures, and nerves pass through the region to supply these organs and connect them with the rest of the body.

The abdomen and pelvis are closely connected. The small intestine and portions of the large intestine occupy both regions, and the urinary bladder and internal reproductive organs are primarily located in the pelvis. The boundary between the abdominal and pelvic cavities is therefore an anatomical reference rather than a complete physical barrier.

The abdominal cavity is lined by a thin membrane called the peritoneum. This membrane helps support the organs and allows many of them to move against one another with minimal friction. Some abdominal organs lie within the peritoneal cavity or are largely covered by the peritoneum, while others sit behind it or outside its main lining.

The nine regions of the abdomen

Anatomists and health professionals divide the abdomen into nine regions to describe the location of organs, pain, swelling, and other findings more precisely. These regions are arranged in three rows and three columns, using imaginary vertical and horizontal lines across the front of the abdomen.

The right and left sides refer to the person’s own right and left, not the observer’s perspective.

The right hypochondriac region, beneath the right side of the rib cage, contains much of the liver and the gallbladder. The epigastric region, in the upper middle abdomen, includes part of the stomach, the liver, and the pancreas, which lies mostly behind the stomach. The left hypochondriac region contains much of the stomach, the spleen, and portions of the pancreas and large intestine.

The middle row contains the right lumbar region, the umbilical region, and the left lumbar region. The lumbar regions lie on either side of the navel and include portions of the ascending and descending colon, respectively, as well as parts of the small intestine and deeper structures. The umbilical region surrounds the navel and commonly overlies loops of the small intestine and part of the transverse colon.

The lower row includes the right iliac region, the hypogastric region, and the left iliac region. The right iliac region contains the cecum and appendix, along with portions of the small intestine. The hypogastric, or pubic, region lies below the navel and may contain loops of small intestine, part of the sigmoid colon, and the urinary bladder when it is enlarged. The left iliac region commonly contains the sigmoid colon and portions of the small intestine.

These regions are useful guides, not rigid maps. Organs vary in size and position, and some extend across several regions. Pregnancy, bladder filling, body position, and individual anatomical differences can also affect where structures lie.

The four quadrants of the abdomen

In clinical practice, the abdomen is also divided into four quadrants: the right upper quadrant, left upper quadrant, right lower quadrant, and left lower quadrant. An imaginary vertical line through the navel and a horizontal line through it create this simpler system.

The right upper quadrant contains most of the liver and the gallbladder, along with parts of the duodenum, pancreas, right kidney, and large intestine. The left upper quadrant contains much of the stomach, the spleen, part of the liver, the body and tail of the pancreas, and portions of the colon and left kidney.

The right lower quadrant includes the appendix, cecum, portions of the small intestine, and the right ureter. In people with ovaries, the right ovary and fallopian tube are also located in this general area of the lower abdomen and pelvis. The left lower quadrant contains portions of the small intestine, the descending and sigmoid colon, and the left ureter. The left ovary and fallopian tube, when present, lie in the adjacent lower abdominal and pelvic region.

The four-quadrant system is especially useful when describing abdominal pain or recording findings during a physical examination. For example, discomfort in the right lower quadrant can occur with appendicitis, while pain in the right upper quadrant may be associated with gallbladder or liver conditions. However, location alone cannot establish a diagnosis because pain can originate in a nearby organ, spread from another region, or be felt somewhere other than its source.

The major organs of the abdomen

The abdominal organs perform different tasks, but their functions are closely connected. The digestive organs break food down and absorb nutrients, the liver processes absorbed substances, the kidneys regulate the body’s internal chemical environment, and supporting organs contribute to digestion, immunity, and circulation.

The stomach

The stomach is a muscular, expandable organ in the upper abdomen, primarily on the left side. It receives swallowed food through the esophagus and temporarily stores it while mixing it with gastric secretions.

Cells in the stomach lining produce hydrochloric acid and enzymes, including pepsin, which begins the digestion of proteins. The acidic environment also helps destroy many microorganisms swallowed with food. A protective mucus layer and other mechanisms help prevent the stomach from digesting its own lining.

Muscular contractions churn food and combine it with digestive fluids to form a thick liquid called chyme. The stomach gradually releases chyme into the duodenum, the first section of the small intestine. Most nutrient absorption occurs later in the digestive tract rather than in the stomach itself.

The small intestine

The small intestine extends from the stomach to the large intestine and consists of three sections: the duodenum, jejunum, and ileum. It is the principal site of chemical digestion and nutrient absorption.

The duodenum receives chyme from the stomach, along with bile from the liver and gallbladder and digestive enzymes from the pancreas. These secretions help break carbohydrates, proteins, and fats into smaller components that the body can absorb.

The inner surface of the small intestine contains folds, fingerlike projections called villi, and microscopic projections called microvilli. Together, these structures create a large surface area for absorbing nutrients. Sugars and amino acids enter the bloodstream, while most absorbed dietary fats enter lymphatic vessels before eventually reaching the blood.

The ileum also absorbs vitamin B12 bound to intrinsic factor, a protein produced by the stomach, and reabsorbs bile salts. The remaining intestinal contents pass into the large intestine.

The large intestine

The large intestine includes the cecum, colon, rectum, and anal canal. Most of the colon forms a frame around the small intestine, with the ascending colon on the right, the transverse colon across the upper abdomen, the descending colon on the left, and the sigmoid colon leading toward the rectum.

Its main functions are to absorb remaining water and electrolytes, accommodate intestinal bacteria, and prepare waste for elimination. The gut microbiome—the community of microorganisms living in the intestine—ferments certain undigested carbohydrates and produces substances such as short-chain fatty acids, which can be used by intestinal cells and influence broader aspects of health.

As material moves through the colon, it becomes more solid and forms stool. The rectum stores stool until nerves and muscles coordinate a bowel movement.

The liver

The liver is the body’s largest internal solid organ and occupies much of the upper right abdomen, extending toward the upper middle region. It performs hundreds of interrelated functions essential to digestion, metabolism, and maintaining the body’s chemical balance.

One of its most important roles is processing nutrients absorbed from the digestive tract. Blood from much of the stomach and intestines travels to the liver through the hepatic portal vein before returning to the general circulation. This arrangement allows the liver to modify, store, or remove substances absorbed from food and medications.

The liver helps regulate blood glucose by storing glucose as glycogen and releasing it when needed. It also produces many blood proteins, including albumin and several proteins involved in clotting. It breaks down or modifies medications and other chemicals, processes fats and amino acids, and converts ammonia produced during protein metabolism into urea, which the kidneys can eliminate.

The liver also produces bile, a fluid that helps the body digest and absorb fats. Some components of bile are waste products, including bilirubin derived from the breakdown of hemoglobin in aging red blood cells.

The gallbladder

The gallbladder is a small, pear-shaped organ tucked beneath the liver. It stores and concentrates bile between meals.

When food containing fat enters the small intestine, hormonal signals stimulate the gallbladder to contract. Bile then flows through the bile ducts into the duodenum, where bile salts help disperse large fat droplets into smaller ones. This process, called emulsification, makes fats more accessible to digestive enzymes.

The gallbladder does not produce bile; the liver does. A person can live without a gallbladder because bile continues to flow from the liver into the intestine, although its delivery pattern changes after removal.

The pancreas

The pancreas lies deep in the upper abdomen, mostly behind the stomach. It serves both digestive and hormonal functions.

Its exocrine cells produce digestive enzymes that enter the small intestine through pancreatic ducts. These enzymes help break down proteins, fats, and carbohydrates. The pancreas also releases bicarbonate, which helps neutralize acidic material arriving from the stomach and creates a more suitable environment for intestinal enzymes.

Clusters of endocrine cells, known as the islets of Langerhans, release hormones directly into the bloodstream. Insulin helps lower blood glucose by promoting its uptake and storage, while glucagon helps raise blood glucose when levels fall. Together with other hormones and organs, these signals help keep blood sugar within a range that supports normal body function.

The spleen

The spleen sits in the upper left abdomen, close to the stomach and beneath the left side of the rib cage. Although it is near the digestive organs, it is primarily involved in blood filtration and immune defense rather than digestion.

The spleen removes many aging or damaged red blood cells from circulation and helps recycle components such as iron. It also stores and monitors certain blood cells and contains immune cells that respond to microorganisms and other foreign material circulating in the blood.

The spleen is vulnerable to injury because it contains a rich blood supply. Significant abdominal trauma can damage it and cause internal bleeding. Although people can survive without a spleen, its removal increases the risk of certain serious infections, making vaccination and other preventive measures important.

The kidneys and adrenal glands

The kidneys are two bean-shaped organs located toward the back of the upper abdomen, one on each side of the spine. They sit behind the peritoneum, a position described as retroperitoneal. The right kidney is generally slightly lower than the left because of the liver above it.

Each kidney filters blood through microscopic structures called nephrons. Rather than simply removing waste, the kidneys selectively return needed water and substances to the bloodstream while excreting excess water, electrolytes, and metabolic waste in urine.

The kidneys regulate fluid volume, blood pressure, electrolyte concentrations, and acid-base balance. They also produce a hormone that stimulates red blood cell production and activate vitamin D, which helps the body absorb calcium.

The adrenal glands sit on top of the kidneys. Despite their proximity, they are separate organs with distinct functions. They produce hormones such as cortisol, aldosterone, adrenaline, and noradrenaline, which help regulate stress responses, metabolism, blood pressure, and salt and water balance.

Blood vessels, nerves, and lymphatic structures

The abdomen contains major blood vessels that supply the digestive organs and transport blood between the heart and lower body. The abdominal aorta carries oxygen-rich blood downward from the chest and gives off branches to the abdominal organs, kidneys, and pelvic region. The inferior vena cava returns oxygen-poor blood from the lower body to the heart.

The hepatic portal vein carries nutrient-rich blood from much of the digestive tract to the liver. This is distinct from the hepatic veins, which carry blood away from the liver and into the inferior vena cava.

The abdominal nervous system coordinates digestion, blood vessel activity, and communication between organs and the brain. The autonomic nervous system, which includes sympathetic and parasympathetic pathways, helps regulate intestinal movement and secretions without requiring conscious control. The enteric nervous system, a network of neurons within the digestive tract, can coordinate many digestive processes locally.

Lymphatic vessels transport lymph, a fluid containing immune cells and other substances, and help return excess tissue fluid to the bloodstream. Lymph nodes and other immune structures monitor this fluid for signs of infection or abnormal cells. Lymphatic vessels in the small intestine also absorb much of the fat transported from meals.

How the abdominal cavity protects its organs

The abdominal cavity is not an empty space filled with freely floating organs. Its contents are arranged and supported by layers of tissue, membranes, ligaments, and connective structures.

The peritoneum has two main layers. The parietal peritoneum lines the inner surface of the abdominal wall, while the visceral peritoneum covers many abdominal organs. A thin film of fluid between these layers reduces friction as organs shift during breathing, digestion, and changes in body position.

The mesentery is a double layer of peritoneal tissue that supports much of the small intestine and connects it to the back of the abdominal cavity. It carries blood vessels, lymphatic vessels, and nerves to and from the intestine. Related peritoneal folds support other organs and provide pathways for their blood and nerve supply.

Not all abdominal organs are suspended by the same arrangement. The kidneys, adrenal glands, and much of the pancreas lie behind the peritoneum. This position affects their relationships to neighboring structures and can influence how diseases in these organs present.

The abdominal wall also provides protection. It consists of skin, layers of connective tissue, muscles, and fascia, which is a sheet or band of fibrous tissue. The abdominal muscles support the trunk, help maintain posture, and increase pressure inside the abdomen during coughing, lifting, urination, defecation, and childbirth.

Unlike the rib cage, the abdominal wall is relatively flexible. This flexibility allows the abdomen to expand after meals and during pregnancy while still supporting and protecting its contents.

How the abdominal organs work together

The digestive system illustrates how abdominal organs depend on one another. Food enters the stomach, where it is mixed and partially digested. It then moves into the small intestine, where pancreatic enzymes and bile support further digestion. Nutrients cross the intestinal lining and enter blood or lymphatic vessels, while the liver processes many of the absorbed substances before they reach the general circulation.

Material that cannot be digested or absorbed continues into the large intestine. There, water and electrolytes are recovered, bacteria act on certain remaining substances, and stool forms for eventual elimination.

The liver and kidneys provide complementary forms of waste management. The liver chemically transforms many substances and converts ammonia into urea. The kidneys then filter urea and other waste products from the blood and excrete them in urine. Both organs also help maintain the body’s internal chemical stability, but they do so through different mechanisms.

The endocrine functions of the pancreas and adrenal glands add another layer of coordination. Insulin and glucagon regulate how the body uses and stores energy, while adrenal hormones influence the body’s response to stress and help maintain circulation and fluid balance. These processes continue even when a person is not actively eating or exercising.

The nervous system coordinates many of these activities. Signals from the brain, spinal cord, and enteric nervous system adjust digestive movement and secretion in response to meals, stress, and changes in the body’s needs. Hormones provide additional signals that can act on several organs at once.

Common abdominal pain patterns and what they mean

Abdominal pain can arise from the digestive tract, urinary system, blood vessels, abdominal wall, or organs in the pelvis. It may also originate outside the abdomen and be felt there. The location, character, duration, and accompanying symptoms can help clinicians narrow the possible causes, but no single pain pattern is definitive.

Pain from an inflamed or stretched organ often differs from pain caused by irritation of the peritoneum. The internal organs have relatively limited ability to produce sharply localized pain, so discomfort from the intestines or other visceral structures may feel dull, cramping, or poorly defined. Pain from irritation of the lining of the abdominal cavity may be sharper and more precisely localized, particularly when movement or coughing makes it worse.

Some conditions produce recognizable patterns. Gallbladder problems may cause pain in the right upper abdomen, sometimes extending toward the back or right shoulder. Appendicitis often begins as poorly localized discomfort near the navel and later shifts toward the right lower abdomen, although presentations vary. Kidney stones may cause intense pain in the side or back that travels toward the lower abdomen or groin. Diverticulitis commonly causes pain in the left lower abdomen, particularly in adults, but the location can vary.

Bloating and abdominal distension also have several possible explanations. Gas, constipation, changes in intestinal movement, fluid accumulation, and enlargement of an organ can all change how the abdomen feels or looks. The sensation of bloating does not necessarily mean that the abdomen has physically enlarged.

Severe or worsening abdominal pain, a rigid or very tender abdomen, vomiting blood, bloody or black stools, fainting, persistent vomiting, or abdominal pain accompanied by fever and significant illness warrants prompt medical evaluation. Sudden severe pain after an injury is also an emergency because internal bleeding or organ damage may not be visible from the outside.

Why abdominal anatomy matters in medicine

Knowledge of abdominal anatomy helps healthcare professionals interpret symptoms, conduct physical examinations, and choose appropriate diagnostic tests. When a person reports pain, clinicians consider which organs lie in that region, how those organs connect with surrounding structures, and whether the symptoms suggest a problem within or outside the abdomen.

A physical examination may include inspection for swelling or asymmetry, listening for bowel sounds, and gently pressing different areas to assess tenderness, masses, or guarding. The findings are interpreted alongside the medical history and other symptoms rather than in isolation.

Imaging tests provide additional information when needed. Ultrasound can examine structures such as the liver, gallbladder, kidneys, and abdominal blood vessels. Computed tomography, commonly called CT, produces detailed cross-sectional images that can help identify inflammation, obstruction, bleeding, or masses. Magnetic resonance imaging, or MRI, can provide detailed soft-tissue images in selected situations.

The location of an organ also influences surgery and other procedures. Surgeons must account for the paths of blood vessels, the position of the peritoneum, and the relationships among neighboring organs. Anatomical differences between individuals can affect both the presentation of disease and the safest approach to treatment.

The abdomen is therefore best understood as an interconnected system rather than a collection of isolated organs. Its structures cooperate to digest food, distribute nutrients, eliminate waste, regulate hormones and body fluids, and protect essential internal processes. Understanding their positions and functions provides a foundation for making sense of both normal bodily activity and many common medical conditions.