In humans, the abdomen stretches from the thorax at thethoracic diaphragm to the pelvis at thepelvic brim. The pelvic brim stretches from thelumbosacral joint (theintervertebral disc betweenL5 andS1) to thepubic symphysis and is the edge of thepelvic inlet. The space above this inlet and under the thoracic diaphragm is termed the abdominal cavity. The boundary of the abdominal cavity is theabdominal wall in the front and the peritoneal surface at the rear.
In vertebrates, the abdomen is a largebody cavity enclosed by the abdominal muscles, at the front and to the sides, and by part of thevertebral column at the back. Lower ribs can also enclose ventral and lateral walls. The abdominal cavity is continuous with, and above, the pelvic cavity. It is attached to thethoracic cavity by the diaphragm. Structures such as theaorta,inferior vena cava andesophagus pass through the diaphragm. Both the abdominal and pelvic cavities are lined by a serous membrane known as theparietal peritoneum. This membrane is continuous with thevisceral peritoneum lining the organs.[2] The abdomen in vertebrates contains a number oforgans belonging to, for instance, thedigestive system,urinary system, andmuscular system.
Theabdominal cavity contains most organs of thedigestive system, including thestomach, thesmall intestine, and thecolon with its attachedappendix. Other digestive organs are known as the accessory digestive organs and include theliver, its attachedgallbladder, and thepancreas, and these communicate with the rest of the system via various ducts. Thespleen, and organs of theurinary system including thekidneys, andadrenal glands also lie within the abdomen, along with many blood vessels including theaorta andinferior vena cava. Theurinary bladder,uterus,fallopian tubes, andovaries may be seen as either abdominal organs or as pelvic organs. Finally, the abdomen contains an extensive membrane called theperitoneum. A fold of peritoneum may completely cover certain organs, whereas it may cover only one side of organs that usually lie closer to the abdominal wall. This is called theretroperitoneum, and the kidneys and ureters are known asretroperitoneal organs.
View of the various organs and blood-vessels in proximity with liver.
The relations of the viscera and large vessels of the abdomen, seen from behind.
Thetransverse abdominal muscle is flat and triangular, with its fibers running horizontally. It lies between the internal oblique and the underlyingtransverse fascia. It originates from the inguinal ligament, costal cartilages 7–12, the iliac crest and thoracolumbar fascia. Inserts into the conjoint tendon,xiphoid process, linea alba and the pubic crest.
Therectus abdominis muscles are long and flat. The muscle is crossed by three fibrous bands called thetendinous intersections. The rectus abdominis is enclosed in a thick sheath, formed as described above, by fibers from each of the three muscles of the lateral abdominal wall. They originate at thepubis bone, run up the abdomen on either side of the linea alba, and insert into the cartilages of the fifth, sixth, and seventh ribs. In the region of thegroin, theinguinal canal, is a passage through the layers. This gap is where thetestes can drop through the wall and where the fibrous cord from theuterus in the female runs. This is also where weakness can form, and causeinguinal hernias.[3]
Thepyramidalis muscle is small and triangular. It is located in the lower abdomen in front of the rectus abdominis. It originates at the pubic bone and is inserted into the linea alba halfway up to thenavel.
The abdominal muscles have different important functions. They assist asmuscles of exhalation in the breathing process duringforceful exhalation. Moreover, these muscles serve as protection for the inner organs. Furthermore, together with the back muscles they provide postural support and are important in defining the form. When theglottis is closed and thethorax andpelvis are fixed, they are integral in thecough,urination,defecation,childbirth,vomit, and singing functions.[3] When the pelvis is fixed, they can initiate the movement of the trunk in a forward motion. They also preventhyperextension. When the thorax is fixed, they can pull up the pelvis and finally, they can bend the vertebral column sideways and assist in the trunk's rotation.[3]
The transverse abdominis muscle is the deepest muscle; therefore, it cannot be touched from the outside. It can greatly affect the body's posture. The internal obliques are also deep and also affect body posture. Both of them are involved in rotation and lateral flexion of thespine and are used to bend and support the spine from the front. The external obliques are more superficial and are also involved in rotation and lateral flexion of the spine. They also stabilize the spine when upright. The rectus abdominis muscle is not the most superficial abdominal muscle. The tendonous sheath extending from the external obliques cover the rectus abdominis. The rectus abdominis is the muscle that very fit people develop into "six-pack" abs, though there are five vertical sections on each side. The two bottom sections are just above the pubic bone and usually not visible. The rectus abdominals' function is to bend one's back forward (flexion). The main work of the abdominal muscles is to bend the spine forward when contracting concentrically.[4]
Social and cultural perceptions of the outward appearance of the abdomen has varying significance around the world. Depending on the type of society,excess weight can be perceived as an indicator of wealth and prestige due to excess food, or as a sign of poor health due to lack of exercise. In many cultures, bare abdomens are distinctly sexualized and perceived similarly tobreast cleavage.
The Superman exercise, involving balancing solely with the abdomen (on a folded towel) while raising the upper torso and pelvis off the floor with the limbs bent. There are numerous variations.[5]
Being key elements of spinal support, and contributors to good posture, it is important to properly exercise the abdominal muscles together with the back muscles because when these are weak or overly tight they can suffer painful spasms andinjuries. When properly exercised, abdominal muscles contribute to improved posture and balance, reduce the likelihood ofback pain episodes, reduce the severity of back pain,[6] protect against injury,[7] help avoid some back surgeries, and help with the healing of back problems, or after spine surgery. When strengthened, the abdominal muscles provide flexibility as well. The abdominal muscles can be worked by strength and fitness exercises, and through practicing disciplines of general body strength such asPilates,[8]yoga,[9]tai chi, andjogging.
Abdominal trauma is an injury to the abdomen and can involve damage to the abdominal organs. There is an associated risk ofsevere blood loss andinfection.[10] Injury to the lower chest can cause injuries to the spleen and liver.[11]
Surface projections of the organs of thetrunk, from which organ locations are derived mainly fromvertebra levels,ribs and theilium.
In the mid-line, a slight furrow extends from thexiphoid process above to thepubic symphysis below, representing thelinea alba in the abdominal wall. At about its midpoint sits the umbilicus ornavel. Therectus abdominis on each side of the linea alba stands out in muscular people. The outline of these muscles is interrupted by three or more transverse depressions indicating thetendinous intersections. There is usually one about the xiphoid process, one at the navel, and one in between. It is the combination of the linea alba and the tendinous intersections which form the abdominal "six-pack" sought after by many people.
The upper lateral limit of the abdomen is the subcostal margin (at or near thesubcostal plane) formed by the cartilages of thefalse ribs (8, 9, 10) joining one another. The lower lateral limit is the anterior crest of theilium andPoupart's ligament, which runs from the anterior superior spine of the ilium to the spine of thepubis. These lower limits are marked by visible grooves. Just above the pubic spines on either side are the external abdominal rings, which are openings in the muscular wall of the abdomen through which thespermatic cord emerges in the male, and through which aninguinal hernia may rupture.
One method by which the location of the abdominal contents can be appreciated is to draw three horizontal and two vertical lines.
Front of abdomen, showing markings for duodenum, pancreas, and kidneys.
The highest of the former is thetranspyloric line of C. Addison, which is situated halfway between thesuprasternal notch and the top of the pubic symphysis, and often cuts the pyloric opening of the stomach an inch to the right of the mid-line. Thehilum of eachkidney is a little below it, while its left end approximately touches the lower limit of thespleen. It corresponds to the first lumbar vertebra behind.
The second line is thesubcostal line, drawn from the lowest point of thesubcostal arch (tenth rib). It corresponds to the upper part of the third lumbar vertebra, and it is an inch or so above the umbilicus. It indicates roughly thetransverse colon, the lower ends of the kidneys, and the upper limit of the transverse (3rd) part of theduodenum.
The third line is called theintertubercular line, and runs across between the two roughtubercles, which can be felt on the outer lip of the crest of the ilium about two and a half inches (64 mm) from the anterior superior spine. This line corresponds to the body of the fifth lumbar vertebra, and passes through or just above theileo-caecal valve, where thesmall intestine joins thelarge intestine.
The two vertical or mid-Poupart lines are drawn from the point midway between the anterior superior spine and the pubic symphysis on each side, vertically upward to the costal margin.
The right one is the most valuable, as theileo-caecal valve is situated where it cuts the intertubercular line. The orifice of theappendix lies an inch lower, atMcBurney's point. In its upper part, the vertical line meets the transpyloric line at the lower margin of the ribs, usually the ninth, and here thegallbladder is situated.
The left mid-Poupart line corresponds in its upper three-quarters to the inner edge of thedescending colon.
The right subcostal margin corresponds to the lower limit of theliver, while the right nipple is about half an inch above its upper limit.
The abdomen can be divided into quadrants or regions to describe the location of an organ or structure. Classically, quadrants are described as the left upper, left lower, right upper, and right lower.[citation needed] Quadrants are also often used in describing the site of an abdominal pain.[15]
The abdomen can also be divided into nine regions.
These terms stem from "hypo" meaning "below" and "epi" means "above", while "chondron" means "cartilage" (in this case, the cartilage of the rib) and "gaster" means stomach. The reversal of "left" and "right" is intentional, because the anatomical designations reflectthe patient's own right and left.)
The "right iliac fossa" (RIF) is a common site of pain and tenderness in patients who haveappendicitis. The fossa is named for the underlyingiliac fossa of thehip bone, and thus is somewhat imprecise. Most of the anatomical structures that will produce pain and tenderness in this region are not in fact in the concavity of the ileum. However, the term is in common usage.
Inarthropods, the abdomen is built up of a series of upper plates known astergites and lower plates known assternites, the whole being held together by a tough yet stretchable membrane.
Ininsects, the abdomen contains the insect's digestive tract and reproductive organs. It consists of eleven segments in most orders of insects, though the eleventh segment is absent in the adult of most higher orders. The number of these segments does vary from species to species with the number of segments visible reduced to only seven in the commonhoney bee. In theCollembola (springtails), the abdomen has only six segments.
The abdomen is sometimes highly modified. InApocrita (bees, ants and wasps), the first segment of the abdomen is fused to thethorax and is called thepropodeum. Inants, the second segment forms the narrowpetiole. Some ants have an additionalpostpetiole segment, and the remaining segments form the bulbousgaster.[17] The petiole and gaster (abdominal segments 2 and onward) are collectively called themetasoma.
Unlike other arthropods, insects possess no legs on the abdomen in adult form, though theProtura do have rudimentary leg-like appendages on the first three abdominal segments, andArchaeognatha possess small, articulated "styli" which are sometimes considered to be rudimentary appendages. Many larval insects including theLepidoptera and theSymphyta (sawflies) have fleshy appendages calledprolegs on their abdominal segments (as well as their more familiar thoracic legs), which allow them to grip onto the edges of plant leaves as they walk around.
Inarachnids (spiders, scorpions and relatives), the term "abdomen" is used interchangeably with "opisthosoma" ("hind body"), which is the body section posterior to that bearing the legs and head (the prosoma orcephalothorax).
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