Abdomen is the region of the trunk between the thorax, or chest, and the pelvis. It contains a muscular wall, digestive and urinary organs, and major blood vessels. Its anatomy is described through surface regions and quadrants, organ relationships to the peritoneum, and arterial and venous pathways.
Where does the abdomen begin and end?
The diaphragm, a muscular partition between the chest and abdomen, forms the upper boundary of the abdominal cavity. Below, the cavity continues into the pelvic cavity through the pelvic inlet, the opening bounded by the upper pelvic bones. There is no muscular partition separating these cavities.
The wall of abdomen surrounds the cavity at the front, sides and back. Its posterior part includes the lumbar vertebrae, the bones of the lower spine, and posterior abdominal wall muscles, including psoas major and quadratus lumborum. The lower ribs overlap the upper abdominal organs, so the cavity extends above the visible lower edge of the rib cage.
What muscles form the abdominal wall and rectus sheath?
The front and side wall contains skin, fat, connective tissue and muscle. Fascia is connective tissue that surrounds or separates structures. Deep to the muscles lie transversalis fascia, a connective-tissue lining, then extraperitoneal fat and the peritoneum, the membrane lining the abdominal cavity and covering many organs.
The main muscles of the front and side wall are:
- External oblique: the superficial flat muscle, with most fibres directed downwards and towards the midline.
- Internal oblique: the middle flat muscle, with fibres generally running upwards and towards the midline.
- Transversus abdominis: the deepest flat muscle, with fibres running mainly across the wall.
- Rectus abdominis muscle: a vertical muscle on each side of the midline, extending between the pubis at the front of the pelvis and the lower chest.
Together, these muscles compress the abdominal contents, support the trunk and assist forced expiration, the active movement of air out of the lungs. The oblique muscles also rotate the trunk, while the rectus abdominis muscle bends it forwards.
An aponeurosis is a broad, flat tendon. The aponeuroses of the flat muscles form the rectus sheath around the rectus abdominis muscle and meet at the linea alba, a midline connective-tissue band. Between the costal margin and the arcuate line, the usual lower boundary of the sheath's posterior layer, the aponeuroses contribute layers both in front of and behind the muscle. Above the costal margin, there is no posterior aponeurotic sheath, and the muscle lies on the costal cartilages. Below the arcuate line, all three aponeuroses usually pass in front, leaving transversalis fascia directly behind the muscle; the level and form of this transition vary.
What are the abdominal quadrants and nine regions?
The four quadrants are formed by a vertical midline plane and a horizontal plane through the umbilicus, or navel. They are the right upper, left upper, right lower and left lower quadrants. Right and left always refer to the person's own sides.
The nine-region scheme uses two vertical midclavicular lines, descending from the midpoints of the collarbones, and two horizontal planes. The subcostal plane passes through the inferior borders of the tenth costal cartilages, near the lowest points of the costal margins, the lower borders of the rib cage. The transtubercular plane passes through the iliac tubercles, bony prominences on the upper rims of the pelvic bones.
From the person's right to left, the regions are:
- Upper row: right hypochondriac, epigastric and left hypochondriac regions, beneath the ribs and over the upper central abdomen.
- Middle row: right lumbar, umbilical and left lumbar regions, covering the flanks and central abdomen around the navel.
- Lower row: right iliac, hypogastric and left iliac regions, covering the lower sides and lower central abdomen.
These schemes describe surface locations rather than fixed organ compartments. Many organs cross their boundaries, and positions vary with breathing, posture and filling.
What is the peritoneal cavity, and which organs are retroperitoneal?
The peritoneum has a parietal layer lining the wall and a visceral layer covering organ surfaces. The peritoneal cavity is the potential space between these layers, normally containing a thin film of lubricating fluid. Organs do not lie within this space as objects inside a container.
Intraperitoneal organs are largely covered by visceral peritoneum and often suspended by a mesentery, a double layer of peritoneum carrying vessels, nerves and lymphatic vessels. Retroperitoneal organs lie behind the parietal peritoneum of the posterior wall. Secondarily retroperitoneal organs developed with a mesentery but became fixed against that wall.
| Organ or part | Peritoneal relationship | Important qualification |
|---|---|---|
| Stomach and spleen | Intraperitoneal | Connected to neighbouring structures by peritoneal folds. |
| Liver | Mostly intraperitoneal | Its bare area lacks a peritoneal covering. |
| Jejunum and ileum, the later parts of the small intestine | Intraperitoneal | Suspended by the mesentery of the small intestine. |
| Colon | Relationship varies | Transverse and sigmoid parts have mesenteries; ascending and descending parts are usually secondarily retroperitoneal. |
| Pancreas and duodenum, the first part of the small intestine | Mostly secondarily retroperitoneal | The pancreatic tail and usually the proximal approximately 2 cm of the superior part of the duodenum are intraperitoneal exceptions. |
| Kidney and suprarenal gland, the endocrine gland above it | Primarily retroperitoneal | Remain behind the posterior parietal peritoneum during development. |
Which organs lie in each abdominal quadrant?
Quadrant descriptions identify typical locations, not exclusive boundaries. Loops of the small intestine occupy several quadrants, and the liver and stomach extend across the midline.
- Right upper quadrant: most of the liver, the gallbladder beneath it, the duodenum, the head of the pancreas, the right kidney and parts of the ascending and transverse colon.
- Left upper quadrant: much of the stomach, the spleen, the body and tail of the pancreas, the left kidney and parts of the transverse and descending colon.
- Right lower quadrant: the caecum, the pouch at the beginning of the large intestine; the appendix, a narrow projection from the caecum; the terminal ileum and lower ascending colon.
- Left lower quadrant: the lower descending colon, sigmoid colon and loops of the small intestine.
The kidneys lie against the posterior wall, behind many digestive organs. Pelvic organs may extend upwards into the lower abdomen, depending on their size and filling.
In the 3D model beside the article, rotate and zoom, then select, hide or fade a structure to compare overlapping positions.
What are the main branches of the abdominal aorta?
The abdominal aorta is the continuation of the main arterial vessel after it passes through the diaphragm. It descends in front of the lumbar vertebrae, slightly left of the midline, and divides into the common iliac arteries supplying the pelvis and lower limbs.
Its major unpaired digestive branches are:
- Coeliac trunk: divides into the left gastric, splenic and common hepatic arteries, supplying the stomach, spleen, liver and parts of the pancreas and duodenum.
- Superior mesenteric artery: supplies most of the small intestine and the large intestine as far as the proximal transverse colon.
- Inferior mesenteric artery: supplies the distal transverse colon, descending colon, sigmoid colon and upper rectum, the part of the large intestine between the sigmoid colon and anal canal.
Other branches include renal arteries to the kidneys, gonadal arteries to the testes or ovaries, lumbar arteries to the posterior wall, and inferior phrenic arteries to the diaphragm. The middle suprarenal arteries supply the suprarenal glands.
What does the portal vein drain?
The portal vein carries blood from much of the digestive tract, spleen, pancreas and gallbladder to the liver. It usually forms behind the neck of the pancreas by the union of the superior mesenteric and splenic veins. The inferior mesenteric vein commonly joins the splenic vein.
Within the liver, portal blood enters sinusoids, specialised small vascular channels, where it mixes with blood from the hepatic artery. Blood then leaves through hepatic veins into the inferior vena cava, the large vein returning blood from the lower body towards the heart. The portal vein therefore enters the liver, whereas the hepatic veins drain it.