Abdomen

The abdomen or belly is a cavity of the human body located between the lower opening of the thorax (closed and separated from it by the diaphragm) and the upper opening of the thorax. the true (lesser) pelvis and lower extremities. In mammals it is separated from the rib cage by the diaphragm. Many of the viscera contained in the abdominal cavity belong to the digestive system and are located in the front two-thirds of the abdomen. Other organs, such as the kidney, adrenal gland, and female genital tract, are intra-abdominal. The posterior third of the abdomen comprises the lumbar vertebrae, the sacrococcygeal bone, and the iliac bones.

In the definition of the Dictionary of the Spanish language, the belly is the body cavity of vertebrate animals, which contains the main organs of the digestive and genitourinary systems, and is the outer region of the body, corresponding to the abdomen, which is anterior in man and inferior in other vertebrates.

The abdominal cavity is divided into two parts: the peritoneal cavity and the retroperitoneum.

• The peritoneal cavity is covered inwardly by a serous membrane, called peritoneum, which forms a virtual cavity called peritoneal cavity, which communicates freely with pelvic cavity and which contains the organs of the digestive system.
• The retroperitoneal cavity also called retroperitoneum houses the kidneys and the adrenal glands.

In the center of the abdomen is a point known as the navel, which is an important anatomical reference for its superficial topographic division.

Embryonic origin

The ventral folding of the embryonic trilaminar disc during the fourth week of gestation results in the fusion of the sheets of the lateral mesoderm, which causes the formation of the coelomic cavity (which after successive membrane fusions will form the primordium of the cavity single body). The splanchnopleural sheet surrounds the endoderm derivative and maintains its union with the posterior wall, forming the "meso" (mesentery, mesogastrium, mesocolon) which is where the vascular and nervous supply will enter.

Abdominal organs

Main organs in the abdomen

The abdominal organs are suspended in the abdominal cavity by mesenteries, or are situated between the abdominal cavity and/or embedded in the musculoskeletal wall. The abdominal viscera are:

Esophagus

The esophagus is a musculo-membranous duct (a muscular tube), located in the middle of the thorax, which extends from the pharynx to the stomach. Food passes through the esophagus to the stomach. Its function is to be the conduit between the mouth and the stomach and allow food to reach it. The esophageal wall is made up of four layers: mucosa, submucosa, muscular, and serous.

From the top to the portion where the esophagus meets the stomach is about 40 cm. The esophagus begins at the neck, traverses the entire thorax, and passes into the abdomen through the esophageal hiatus of the diaphragm. It is usually a virtual cavity (that is, its walls are joined and only open when the food bolus passes through). The esophagus has two sphincters:

• Upper esophageal sphincter: separates the pharynx from the esophagus. It is formed by a striated muscle, that is, voluntary, that initiates the swallowing.
• Lower esophageal sphincter: is located inside the abdomen and separates the esophagus from the stomach. It is not really an anatomical sphincter, but physiological, because there is no sphincter structure, but it does possess high pressure when measured at rest. This sphincter decreases its tone, usually high, in response to several stimuli as

(a) The arrival of the primary peristaltic wave,

(b) the distension of the esophagus when the food bowl (secondary period) passes;

(c) gastric distention.

Stomach

Parts of the stomach

The stomach is a muscular reservoir interposed between the esophagus and the duodenum, it is the most dilated portion of the digestive tract and is shaped like a "J", but its shape and orientation are constantly changing, depending on the times of digestion and according to the position of the body. It occupies almost the entire left hypochondrium and a large part of the epigastrium, above the transverse mesocolon, below the liver and diaphragm. The stomach is divided into four regions:

• The cardiaswhich surrounds the esophagus orifice to the stomach
• The fundus or gastric background that is the area above the level of cardia
• The body gastric, which is the widest part
• The pilric portion which is divided into a pilric antro and pilric canal and is the distal end of the stomach.

Small intestine

The small intestine is the part of the digestive system that connects the stomach to the large intestine. It is divided into three parts: duodenum, jejunum, and ileum.

Large intestine

The large intestine begins from the ileocecal valve in a cul-de-sac called the cecum from which the vermiform appendix emerges and ends in the rectum. From the cecum to the rectum it describes a series of curves, forming a frame in the center of which are the loops of the jejunum ileum. Its length is variable, between 120 and 160 cm, and its caliber decreases progressively, the narrowest portion being the region where it joins the rectum or rectosigmoid junction where its diameter does not usually exceed 3 cm, while the cecum is 6. or 7in.

Pancreas

It is a gland closely related to the duodenum, it is of mixed origin, it secretes hormones into the blood to control sugars and pancreatic juice that is poured into the intestine through the pancreatic duct, and it intervenes and facilitates digestion, its secretions are very important in the digestion of food.

Gallbladder

The gallbladder is an organ that is part of the digestive system of humans and quadruped animals (except horses and deer). It is located below the liver. Its Latin name is vesica fellea.

Spleen

The spleen is a flattened, oblong, and very friable parenchymal type organ, located in the upper left quadrant of the abdominal cavity, related to the pancreas, diaphragm, and left kidney. Although its size varies from one person to another, it usually has a length of 12 cm, a width of 8 cm and a thickness of 4 cm, as well as a weight of approximately 200 g. Its main function is to destroy old red blood cells, produce new ones, and maintain a blood reserve.

Part of the urinary system

• Kidneys
• Uréteres
• Adrenal glands

Classification of the abdominal viscera

Intra and retroperitoneal classification of abdominal viscera

Under normal conditions, the abdominal viscera completely occupy the abdominal cavity, the peritoneal cavity being a virtual space, the visceral peritoneum of the organs and the parietal peritoneum of the adjacent abdominal wall sliding over each other freely. The abdominal organs can be classified by two criteria:

• According to their position regarding the mesos can be classified into intraperitonial and retroperitonial viscera, so the intraperitonial structures by virtue of their endodermic origin and a meso long enough, such as the elements of the digestive system, are suspended from the abdominal wall by mesories. On the other hand, the structures that are not suspended in the abdominal cavity by messians and which are located between the parietal peritoneum and the abdominal wall are in retroperitoneal position. They can also be intraperitoneals that then remain fixed (e.g., second portion of the duodenum and part of the pancreas), either by the spins and rotations of the digestive system your meso sticks to the posterior cellomic wall.

• According to the location of the transverse mesocolon, they can be classified as supra-mesocholic and inframeosolic, as they are above the transverse mesocolon (stomach, liver, spleen) or below (slim instine, kidneys, adrenals).

Irrigation

Arteria aorta

The arterial supply of the abdominal viscera comes from the abdominal portion of the aorta that reaches the abdomen through the hiatus of the diaphragm, at the level of the T12 vertebra, immediately emitting the inferior diaphragmatic arteries and the middle adrenal arteries. Farther caudally it gives off three thick unpaired trunks for the intraperitoneal viscera, from which many other arteries arise. The most cranial is the celiac trunk (supplies the liver, stomach and spleen), followed by the superior mesenteric artery (small intestine) and the inferior mesenteric artery (large intestine).

Main blood vessels: Lower vena cava and aorta

To the retroperitoneal space it gives off pairs of arteries such as the renal (for the kidneys), lumbar (posterior abdominal wall), and gonadal (testicular or ovarian) arteries. It gradually divides into the two common iliac arteries (external and internal) and the median sacral artery.

Venous return from the abdomen corresponds to the inferior vena cava, which results from the fusion of the two common iliac veins. It receives the renal, lumbar, and gonadal veins, and passes through the diaphragm over the liver. The venous return of the intestinal loops -because they carry blood that incorporates the products of digestion and cannot thus pass into the systemic circulatory stream- converges in the superior and inferior mesenteric veins, which together with the gastric veins and the splenic form the trunk of the portal vein that enters the liver. The portal vein branches into the liver parenchyma (portal system, which branches twice). Once the blood from the portal vein is treated by the liver, in the hepatic sinusoids, it converges in the hepatic veins that flow into the inferior vena cava, which leads it to the right atrium. In other words, the blood that collects the products of digestion is not led directly to the vena cava, but rather through the liver through the portal system. A large part of liver problems come from an alteration in this hepatic portal system.

As for the lymphatic drainage of this region, it collects the lymph from the digestive system and the lower extremities in the so-called cistern of the chyli, located in the posterior abdominal wall, between the aorta artery and the vertebral column at the level of the T12-L1. It passes into the posterior mediastinum through the aortic orifice. In addition, they present a series of lymph nodes that generally accompany the large arteries, or are located in the hilum of the organs.

Innervation

chest and abdominal inervation, sagital view

The innervation of the abdominal viscera is carried out by the two components of the sympathetic and parasympathetic autonomic nervous system. The sympathetic nervous system of the abdominal viscera derives preferentially from the splanchnic nerves.

The splanchnic nerves are three nerves on each side. They originate in the sympathetic chain of the thorax: the largest ganglia T5-T9 or T10, the smallest ganglia T10-T11, and the inferior (also known as imo) ganglia T12. They end up, either making synapses or passing towards the viscera, in the celiac ganglia (on both sides of the exit of the celiac trunk from the aorta), mesenteric or renal. The lumbar splanchnic nerves (L1-L5) and the superior and inferior mesenteric ganglia complete the sympathetic innervation of the abdomen. They reach the viscera with the arteries, walking the fibers in the adventitia. The parasympathetic innervation of all derivatives of the embryonic foregut and midgut is from the vagus nerve, from the esophagus to the left end of the transverse colon. The distal transverse colon, descending colon, sigmoid colon, and rectum receive parasympathetic innervation from the sacral parasympathetic nucleus located in the sacral spinal cord, located between spinal segments S2 through S4.

Musculature

Illustration of the muscles of the abdomen.

The transverse abdominis muscles are long, narrow, triangular-shaped muscles located on the sides of the abdomen below the internal oblique muscle. These originate from the medial aspect of the seventh to twelfth ribs, the lumbar fascia, the iliac crest, and the inguinal ligament until they insert behind the rectus abdominis muscle, merging with it.

The rectus abdominis muscles are muscles that extend from the midline of the pubis to the lower border of the ribcage. They are inserted by means of a short, flattened tendon, which has two muscle bundles, external and internal, which are separated by a strip of connective tissue called the linea alba.

The pyramidal muscles of the abdomen are muscles that are located in the anterolateral part of the abdomen. It is a small, triangular-shaped muscle that originates from the pubis, inserting into the linea alba.

All of these muscles are covered by the external oblique muscle, which is on the anterolateral part of the abdomen; Its origin is on the external face of the 5-12 rib, and its insertion on the linea alba and symphysis pubis. Its functions are: compress and support the abdominal viscera, rotate and flex the trunk.

External anatomical regions of the abdomen

Abdominal regions and quadrants

For clinical purposes, such as describing pain, tumors, and incisions, the abdomen is divided into regions that are defined by lines on the surface of the anterior abdominal wall. Typically, nine regions cut by two horizontal and two vertical lines are delineated:

• A) and B) The 2 vertical lines go through half of each crural or femoral arc (the groin's length: between the anterosuperior iliac spine and the pubis spine). Located between the synphysis of the pubis and the anterosuperior iliac spine.
• C) Subcostal line, which passes through the lower edge of the tenth rib cartilage above, and by the body of the vertebra LIII later. However, the transpilic line is sometimes used, halfway between the jugular neckline and the top of the pubis synphysis; passing through the lower edge of the ninth rib cartilage and the lower edge of the LI vertebrae.
• D) Lower transverse line or intertubercular line, it is traced between the tubers of the iliac ridges, which are palpable 5 cm behind the anterosuperior iliac thorns and which are at the height of the body of the LV vertebra.

Using these four lines, nine anatomical regions are defined, which are:

1. Right hypochondria: In this region the right lobe of the liver, gallbladder, upper kidney pole, liver flexure of the colon, adrenal gland is located.
2. Epigastric or epigastric region: left lobe area of the liver and pilric portion of the stomach
3. Left hypochondrium: Here is the spleen, pancreas tail, upper left kidney pole, stomach, abdominal esophagus, spinal flexure of the colon.
4. Vacuum region, flank, lumbar or right side: region of the ascending colon, part of the duodenum and
5. Mesogastrio or umbilical region: region of the epiplon, mesenterio, yeyuno, ileon, transverse colon and where the navel is located.
6. Vacuum region, flank or left side: region of the descending colon.
7. Right iliac feat or right inguinal region:region of the blind, appendix, right ovary in women, right spermatic cord in man.
8. Hypogastrium or suprapubical region: region of urinary bladder, uterus
9. Left or left inguinal region: Sigmoid colon region, left ovary, left spermatic cord

This relationship between the external anatomical region of the abdomen and intra-abdominal viscera is not exact, because the abdominal viscera move and exceed the mentioned limits, but it serves as a general indicator. On the other hand, it is frequently used in the clinic to refer pain in any of the targeted regions, although it must be taken into account that the localization of visceral pain is poor and the phenomenon of referred pain can occur, in which a area away from the responsible viscera.

In other animals

comparison analogous morphology of a human being and an ant

In the working ant, the abdomen is formed by the proposition merged to the thorax and metaoma, divided into narrow petiole and bulbous gangster.

The abdomen of invertebrates is made up of a series of upper plates known as tergites and lower plates known as sternites, all held together by a tough stretchable membrane.

The abdomen contains the insect's digestive tract and reproductive organs, consisting of eleven segments in most insect orders, although the eleventh segment is absent in the adult of most higher orders. The number of these segments varies from species to species with the number of visible segments reduced to only seven in the common honeybee. In springtails the abdomen has only six segments.

The abdomen is sometimes highly modified. In Apocrita (bees, ants, and wasps), the first segment of the abdomen is fused with the thorax and is called the propodeum. In ants the second segment forms the narrow petiole. Some ants have an additional postpetiole segment, and the remaining segments form the bulbous gaster. The petiole and gaster (abdominal segments 2 and on) are collectively called the metasoma.

Unlike other arthropods, insects possess appendages on the abdomen only in the adult form, although the Protura have rudimentary appendages on the first three abdominal segments and archaeognatha possess small, jointed "needles" which are sometimes regarded as rudimentary appendages. Many larval insects including Lepidoptera and Symphyta (sawflies) have fleshy appendages called prolegs on their abdominal segments (as well as their more familiar thoracic legs), which allow them to grasp the edges of plant leaves, for walking. around.

In arachnids (spiders, scorpions, and relatives), the term "abdomen" used interchangeably with "opistosoma" ("back body"), which is the section of the body behind the one that bears the legs and head (the prosoma or cephalothorax).