Abdomen

Abdomen. Portion of the trunk of the body located between the thorax and the pelvis , in mammals , which contains the abdominal cavity inside.

Summary

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• 1 Anatomy of the abdominal wall
• 2 Groin region
• 3 Inguinoabdominal region
  • 1 Inguinal canal
  • 2 Inguinal hole
• 4 Inguinocrural region
  • 1 Femoral conduit
• 5 Spermatic cord
  • 1 Structure
• 6 Posterior wall of the abdomen
• 7 Arterial vascularization of the abdominal viscera
  • 1 Celiac trunk
  • 2 Superior mesenteric artery
  • 3 Lower mesenteric artery
  • 4 Arterial anastomoses
• 8 Venous drainage of the abdominal viscera
  • 1 inferior mesenteric vein
  • 2 Superior mesenteric vein
  • 3 Portosystemic anastomoses
• 9 Relations of the abdominal viscera
  • 1 esophagus
  • 2 Stomach
  • 3 Duodenum
  • 4 Jejunoileal handles
  • 5 Large intestine
  • 6 Liver and bile duct
  • 7 Pancreas
  • 8 Kidney, urinary tract and adrenal gland
• 10 Other structures
  • 1 Omental bag
  • 2 Root of the mesentery and sigmoid mesocolon
  • 3 Lymphatic system
  • 4 Azygos Vein System
• 11 See also
• 12 Sources

Anatomy of the abdominal wall

The anterior wall of the abdomen is made up of four pairs of muscles that extend from the rib cage to the pelvis.

Three of them are flat muscles (from superficial to deep: external or major oblique, internal or minor oblique and transverse abdomen) that form the flanks of the abdomen, and the fourth (rectus abdominis) has a vertical disposition and is located on the face. previous. The transverse and internal oblique muscles have a common insertion through the joint tendon.

These muscles have the main function of protecting the abdominal viscera . They also participate secondarily in the movements of flexion and rotation of the trunk and are auxiliary in the movements of breathing , digestion , urination and childbirth .

The rectus abdominis muscle is the main muscle of the anterior aspect of the abdomen. Most of the muscle is wrapped by a case formed by the fascia of the three flat muscles of the flanks, which is called the rectus sheath. The fibers of the fascia that form it intermingle with those on the other side, forming the alba line in the midline, separating the two portions of the rectus abdominis muscle.

At the level of the lower third, the muscle is not completely enveloped by the sheath, but the fascia of the flat muscles passes only in front, creating an “eyelet” through which the rectus muscle passes. This buttonhole is semicircular in shape and is called the Douglas Arch or arcuate line and is located on the imaginary line that exists between the navel and the iliac crest. Below this arch, the dorsal (or posterior) sheet of the rectus sheath is only formed by the transversalis fascia and the peritoneum.

At this level, the muscle thins to its insertion in the pubis , which, together with the lack of posterior reinforcement, creates a weak area of ​​the wall where hernias can appear.

In 80% of the general population there is a small triangular muscle that originates from the pubic ridge called the pyramidal muscle of the abdomen, which is located in front of the lower portion of the anterior rectum. Its function is to tighten the alba line, although it is insignificant or null.

Region inguinal

The inguinal region communicates the abdominal cavity with the genital region and with the lower limb, allowing the exit of intra-abdominal structures to the outside. This region is limited by the space between the joint tendon of the flat muscles of the abdomen and the bony rim that extends from the anterior superior iliac spine to the pubic symphysis .

It is divided into two halves by the inguinal, Poupart, or Fallopian ligament (also called the femoral arch), which is a fibrous cord that originates from the anterior superior iliac crest and is inserted into the pubic symphysis. From the imitaporencima a region that allows the exit of structures that travel to the genitals, which is why it receives the name of inguinoabdominal region or inguinal canal and below an area where the structures leave on their way to the lower limb, which is called the region inguinocrural.

Inguinoabdominal region

Groin duct

The inguinal canal is an oblique passage, approximately 4 cm in length, located in the lower part of the anterior wall of the abdomen, above the inguinal ligament. It is occupied in the male by the spermatic cord and by the round ligament of the uterus in the female. It contains the ilioinguinal or minor abdominogenital nerve in both sexes, but in the male it does not form part of the spermatic cord but it accompanies it on the outside, that is, it is extrafunicular.

• Posterior wall: it is formed by the following structures (from deep to superficial): peritoneoparietal, transverse fascia and aponeurotic muscle sheets of the transverse muscle of the abdomen and the minor oblique.
• Anterior wall: formed by the aponeurosis of the greater oblique muscle.
• Ceiling: made up of the muscle and joint tendon (aponeurotic muscle bundles of the transverse oblique muscle)
• Soil: it is formed by the inguinal ligament.

The external oblique muscle is the most superficial element and covers the area limited by the inguinal canal. It has a hole that allows the spermatic cord to exit and is limited by fibers that reinforce it inside and outside (medial and superficial pillars) and above (fiber-like fibers).

Apart from these limiting elements, in the inguinal space are other structures that have their own name:

• Hesselbach interfoveolar ligament. It is a thickening of the fascia transversalis, which is the caudal continuation of the Douglas arch. It fixes there gamentoinguinal and divides the inguinal region into two halves, in which the peritoneum is depressed and forms the external and internal inguinal fossils. The interfoveolar ligament houses the inferior epigastric artery, which ascends through it to anastomose with the internal mammary artery, which comes from the subclavian artery.
• Henle’s Ligament. It is a thickening of the fascia transversalis together with fibers of the tendon of the rectum of the labdomen, which reinforces the posterior wall of the inguinal canal in its medial area.
• Colles Ligament. They are fibers coming from the tendon of the superior contralateral lobule that cross the midline and that reinforce behind the medial wall of the superficial inguinal hole.

They are what is also called the reflex ligament, third pillar or posterior pillar.

The arrangement of the inguinal canal in the horizontal plane is like an italic “s”. It runs from top to bottom, inside to outside and from deep to shallow. The entrance is the deep inguinal hole and its exit is the superficial inguinal hole or inguinal ring. The deep inguinal opening is located at the level of the external inguinal fosilla and is lateral to the deep inferior epigastric vessels (included in the Hesselbach ligament) and is where the indirect inguinal hernias originate. Since they travel through the inguinal canal, they are included in the spermatic cord and are called intrafunicular.

Groin hole

The superficial inguinal opening is medial to the Hesselbach ligament and the epigastric vessels and is in the internal inguinal fosilla, comprised between bundles of the tendon of the greater oblique, the medial and lateral pillars of the inguinal ring joined by the arciform fibers, and the Colles ligament.

Among these structures the fascia transversalis is very weak and there is no musculature ahead, so this region is a favorite site for direct inguinal hernias, which does not exit through the inguinal ring, but protrudes through the joint tendon until reaching the inguinal ring, so they are extrafunicular (MIR 00-01, 237).

Inguinocrural region

Femoral conduit

The inguinocrural region is the space delimited superomedially by the inguinal ligament and inferolaterally by the pelvic bone rim. It is divided in two by the ileopectineal arch or band in two parts. The most lateral is the muscular lacuna and the iliac psoas muscle and the femoral (also called crural nerve) and musculocutaneous nerves run through it. The medial is the so-called vascular lagoon and contains (from lateral to medial) the external iliac artery, which is called the common femoral artery, the external iliac vein and the Cloquet or Rosenmüller lymph node. At this level are the following structures:

• Gimbernat’s lacunar ligament: sickle-shaped expansion of the oblique muscle aponeurosis that is inserted in the ileopectineal line and reinforces the crural duct inside.
• Cooper’s Ligament: These are fibers that reinforce the dorsal border of the lacunar ligament at its insertion in the pectineal crest.

Through the vascular lagoon the crural hernias make their way, between the lacunar ligament of Gimbernat and the femoral vein. It is what is known as a fecal duct. Being a very small space they have a great facility to strangle themselves.

Spermatic cord

The inguinal duct is formed in the male when the testicle migrates from the abdominal interior to the scrotum, taking with it its excretory duct and vessels. This trip is done by pushing all the structures of the abdominal wall that he finds on his way, like a glove finger.

The spermatic cord is made up of the vas deferens, the deferential, spermatic, and funicular (or cremasteric) arteries, lymphatic vessels, autonomic nervous system fibers, and the pampiniform venous plexus. The latter is responsible for the venous drainage of the testicle, which drains into the gonadal vessels, which in turn drain into the right inferior vena cava and the left renal vein.

Structure

All these components are surrounded by structures that surround it, which, from deep to superficial, are the following:

• Peritone or vaginal duct: it comes from the peritoneum and is the duct that connects the testicle with the peritoneum. It usually disappears shortly before or after birth. If not, it is responsible for the testicular hydrocele.
• Internal spermatic fascia: it comes from the fascia transversalis.
• Cremasteric fascia: formed from the fascia of the lesser oblique.
• Cremaster muscle: it is located anterior to the vas deferens and cord vessels and comes from transverse and minor oblique fibers. This muscle is striated in nature, unlike scrotal dartos. Its function is to retract the testicle before thermal or tactile stimuli (cremasteric reflex).
• External spermatic fascia: it comes from the aponeurosis of the greater oblique muscle.

The ilioinguinal nerve pierces the minor oblique very laterally to the inguinal canal and is included in it, accompanying the spermatic cord immediately deep to the external cremasteric fascia, which it finally pierces to reach the skin.

Posterior wall of the abdomen

It is made up of three muscle groups: anterior (square of loins and psoas), middle (posterior insertion of the transverse abdomen) and posterior (spinal muscles, serratus minor posterior inferior, latissimus dorsi and lumbar aponeurosis). In this region, the most important thing is to know the weak points of the wall, where lumbar hernias can break through:

• Petit’s triangle or lower lumbar trigone. Formed by the latissimus dorsi, the external or major oblique and the iliac crest. The bottom of this triangle is formed by the internal or minor oblique muscle.
• Grynfelt quadrilateral or superior lumbar trigone. Limited by the minor oblique, serratus poster or inferior, iliocostal muscle and XII rib. The bottom of this quadrilateral is occupied by the transverse muscle of the abdomen.

Arterial vascularization of the abdominal viscera

It is performed through three arterial trunks from the abdominal aorta: celiac trunk, superior mesenteric artery and inferior mesenteric artery.

The celiac trunk is responsible for supplying the distal esophagus, half the duodenum, liver, gallbladder, spleen, and part of the pancreas and omentum. The distal half of the duodenum and the proximal two thirds of the transverse colon, along with the remaining portion of the pancreas, jejunum, ileum, cecum, appendix, and ascending colon, are supplied by the superior mesenteric artery.

The inferior mesenteric artery supplies the distal third of the transverse, descending, sigma, and upper third of the rectum. The remaining two thirds of the rectum are supplied by the internal or hypogastric iliac artery. The middle third is supplied directly through the middle hemorrhoidal artery and the distal third through the lower hemorrhoidal artery, which is a branch of the pudendal artery, which in turn is the branch of the hypogastric artery. The external iliac artery does not branch until it leaves the inguinal space (MIR 01-02, 237).

Celiac trunk

It originates at a right angle from the anterior aspect of the aorta at the level of L1 in relation to the diaphragmatic abutments, the hepatic caudate lobe, and the upper border of the pancreas. It has a very small path, about 1 cm, and gives the following branches:

• Left stomach or gastric coronary artery: it is directed from the posterior wall of the abdomen towards the cardia, to descend towards the hepatogastric ligament (minor omentum) and minor curvature.
• Common hepatic artery: in turn it gives rise to two fundamental branches: gastroduodenal artery (divided in turn into right gastroepiploic and superior pancreaticoduodenal) and its own liver (its natural continuation). The latter is anterior to the portal vein and is to the left of the bile duct. It is in charge of perfusing the liver parenchyma through two branches that do not anastomose to each other. The right hepatic artery is divided into two branches that embrace the right hepatic duct or the cystic duct (MIR 2004-2005, 238). The liver itself also has two important branches: the right pyloric or gastric artery (anastomosing with the left gastric) and the cystic one, which supplies the gallbladder. The right gastroepiploic artery emits branches for the greater curvature of the stomach, upper portion of the duodenum, and greater omentum.
• Splenic artery: runs along the upper edge of the pancreas until it reaches the spleen through the splenic pancreatic meso. It is the most voluminous branch of the celiac trunk and is the most frequent location of visceral aneurysms. Its most important branch is the left gastroepiploic, which runs through the greater curvature to end up anastomosing with the right gastroepiploic. The short gastric arteries, branches of the splenic, supply the proximal third of the greater curvature.

Superior mesenteric artery

It originates from an acute angle on the anterior aspect of the abdominal aorta at the L2 level. In its origin it is related to the third portion of the duodenum and pancreatic uncus, which it embraces on its anterior face while descending to the root of the mesentery, forming the Rokitansky aortomesenteric clamp. Its main branches are:

• Lower pancreaticoduodenal artery: supplies part of the pancreas and duodenum.
• Jejunoileal loops: approximately twelve branches, which come from the left edge of the AMS, which anastomose to each other forming between two and five vascular arches, the last of which originate the straight vessels supplying the jejunum and ileum.
• Ileobiceoapendiculocholic or ileocolic artery: vascularizes the ileum, cecum and appendix.
• Right colic artery: it is divided into upper and lower branches to give arterial blood to the right colon.
• Middle colic artery: it is divided into left and right branches, supplying the transverse colon.

Lower mesenteric artery

It originates at an acute angle on the anterior aspect of the aorta at the level of L3, where it is related to an accumulation of chromaffin cells known as the Zuc¬kerkandl organ and is directed obliquely to the left. It is smaller in size than the previous ones and originates the following branches:

• Left colic: it divides into lower and upper branches and supplies the splenic angle and descending colon.
• Sigmoids: generally there are three, which distribute the sigma irrigation.
• Upper hemorrhoidal: irrigates the upper third of the rectum.

Arterial anastomoses

To guarantee the vascularization of the digestive tract, anastomoses are established between the three arterial trunks.

The celiac trunk is anastomosed with the superior mesenteric through the superior and inferior pancreaticoduodenal arteries. The superior and inferior mesenteric systems anastomose to each other through superior branches of the right and left colic, thus forming the Riolano arterial arcade, which is the largest-caliber anastomosis of the digestive tract and especially important in cases of ischemia. intestinal or hemicolectomies. The inferior mesenteric is anastomosed through the superior hemorrhoidal with the hypogastric artery (internal iliac) system.

Venous drainage of the abdominal viscera

Venous drainage of the gastrointestinal tract and adnexa is carried out through the portal venous system. This system lacks valves in its main vessels in the adult and 60% of the blood that this viscera enters to the liver. It is formed from the convergence of the superior mesenteric, splenic, and inferior mesenteric veins. Portal vein formation occurs behind the junction between the head and neck of the pancreas (MIR 94-95, 249).

The superior and inferior mesenteric veins collect blood from the territories irrigated by the arteries of the same name. The splenic vein drains blood from the territories vascularized by the celiac trunk (MIR 00-01F, 206).

Inferior mesenteric vein

It is the prolongation of the superior haemorrhoidal vein and collects blood from the upper third of the rectum, sigma, descending colon and left half of the transverse, being located on the left peritoneal posterior wall. The middle hemorrhoidal vein empties into the internal iliac vein and the inferior hemorrhoidal vein flows into the internal pudendal vein which in turn drains into the internal iliac vein. The inferior mesenteric vein ascends to meet behind the tail of the pancreas with the splenic vein, with which it forms the splenomesalaic trunk.

Superior mesenteric vein

It receives blood from the small, blind intestine, ascending colon and right half of the transverse colon and is located to the right of the superior mesenteric artery to which it accompanies, forming part of the aorto-mesenteric clamp until it converges with the splenomesalaic trunk (level the limit between the head and neck of the pancreas) to form the portal vein (MIR 94-95, 249). This vein is introduced into the lesser omentum or hepatoduodenal ligament, located behind the common bile duct and the hepatic artery itself, to go towards the liver and on its way it constitutes the anterior limit of the Winslow hiatus .

Portosystemic anastomoses

Main article: Portosystemic anastomoses
The portal system has numerous anastomoses with the systemic venous return system. These anastomoses are important in cases of increased portal flow, such as portal hypertension in liver cirrhosis. The most important ones are:

• The esophageal branches of the left gastric vein are anastomosed with the esophageal veins that drain into the azygos vein. They can give rise to esophageal varices.
• The superior rectal vein anastomoses with the middle and inferior rectal veins, which drain into the iliac vein.
• The paraumbilical veins of the sickle ligament (which is usually obliterated in the adult) establish communication with the subcutaneous veins of the periumbilical region in the anterior wall of the abdomen.
• The tributaries of the splenic vein anastomosed with the left renal vein. There are also short veins that anastomose the lumbar veins with the splenic and colic veins.

Relationships of the abdominal viscera

Esophagus

The esophagus descends from the pharynx to the stomach following the curvature of the spinal column . As it passes through the posterior mediastinum, it is associated with the tracheobronchial lymph nodes, the pericardium, the left atrium, and the left main bronchus (MIR 01-02, 238). It is inserted into the abdomen through the diaphragm on the left side, very close to the middle axis of the body. At this level it is covered by the peritoneum and surrounded by the esophageal nervous plexus.

Stomach

The anterior aspect is partially covered by parietal peritoneum and it is related to the inferior aspect of the left lobe of the liver, the diaphragm and the anterior abdominal wall. The posterior aspect is related to the transcavity of the omentum and through it, with the so-called “gastric bed”, formed by the Diaphragm , Spleen , upper portion of the left Kidney, left adrenal, body and tail of the Pancreas , transverse mesocolon and a variable extent of transverse colon .

Duodenum

The first (upper) portion is related anteriorly to the square lobe of the Liver and Gallbladder . Its posterior aspect is separated proximally from the head of the pancreas by the omental pouch and distally it is in contact with the common bile duct and the gastroduodenal artery and, through them, with the portal and inferior vena cava, located posteriorly. . The upper edge of this first portion provides insertion to the lesser omentum forming the lower limit of the Winslow hiatus.

The second (descending) portion is anteriorly crossed by the transverse colon. Above it is related to the right lobe of the liver and gallbladder and below with the jejunal loops. Its posterior aspect is related to the right kidney and its vessels, right ureter, right psoas and inferior vena cava. On its medial side it receives the mouth of the pancreatic and common bile duct.

The third (horizontal) portion is located in the aortomosenteric clamp. It is related ahead with the superior mesenteric vessels and with the root of the mesentery. Its posterior aspect is related to the right ureter, right psoas, right gonadal vessels and inferior cava to finish in front of the aorta.

The fourth portion (ascending) is related in front with the root of the mesentery and its posterior aspect with the left psoas and the left gonadal vessels. Its lateral edge is related to the left kidney while its medial edge is related to the hook of the pancreas.

Jejunoileal handles

Ahead they relate to the anterior abdominal wall through the greater omentum and behind to the stomach, spleen, and liver through the mesocolon and transverse colon.

Large intestine

The cecum rests on the right psoas-iliac and the right lateral femoral and cutaneous nerves . It is previously related to the anterior abdominal wall.

The ascending colon is joined behind the right psoas, right iliac crest, and square muscles of the right and transverse loins of the right abdomen. When it becomes a transverse colon, it is related to the anterior aspect of the right kidney and the descending portion of the duodenum. Ahead is related to the small intestine, omentum and anterior abdominal wall handles.

The transverse colon is related superiorly to the liver, gallbladder, stomach and spleen, previously with the greater omentum and later with the descending portion of the duodenum, head of the pancreas, small intestine and left kidney. When transforming into a descending colon, it is located behind the stomach and contacts the spleen, left kidney and tail of the pancreas.

The descending colon is previously related to the intestinal loops and the greater omentum, later with the left kidney and the square loins and the left iliopsoas.

Sigma is subsequently related to the piriformis muscle, sacral plexus, left ureter, and internal iliac vessels and inferiorly to the urinary bladder in men and the vagina in women. The main relations of the rectum are the above: in the male with the bladder, seminal vesicles, vas deferens and prostate, and in the female with the lower part of the vagina. Superiorly it is related to the intestinal loops or sigma through the peritoneum.

Liver and bile duct

The liver is the largest intra-abdominal organ. Ahead it is related to the rib flange and the diaphragm except for the epigastric region, which is related to the anterior abdominal wall. On the posterior aspect it is related to the diaphragm, thoracic vertebrae ten and eleven, the abdominal esophagus, the inferior vena cava, the right adrenal gland, and the minor omentum. On the right side it is related to the seventh to eleventh ribs and the diaphragm, and through it to the right pleura and lung .

On the upper or diaphragmatic side it is related to the dome of the diaphragm and through it to the pleura, lung, pericardium and heart. On the posteroinferior or visceral aspect, it is related, from left to right, with the fundus, the upper portion of the gastric body and the lesser omentum through its left lobe; pylorus and upper portion of duodenum through square lobe; It is also related to the gallbladder, descending portion of the duodenum, upper portion of the right kidney, and right colic angle.

The visceral aspect is covered by the peritoneum except at the level of the gallbladder and the hepatic fluid, which measures approximately 5 cm and contains the portal vein, the hepatic artery itself, the hepatic ducts, the lymphatic vessels and the hepatic nervous plexus. . The hepatic ducts join to form the common hepatic duct, which descends between the layers of the lesser omentum and joins the cystic duct of the gallbladder to form the common bile duct.

It crosses the portal vein, passes behind the upper portion of the duodenum and remains on the posterior wall of the head of the pancreas, where it contacts the pancreatic duct, forming the hepatopancreatic ampulla. In the most distal part, circular fibers surround the common bile duct wall before and after it penetrates the duodenal wall, forming the bile duct sphincter (MIR 01-02, 236; MIR 00-01F, 208).

The reflection zones of the peritoneum originate the suspensory ligaments of the liver. The sickle ligament is formed by the peritoneal fold that originates the embryonic umbilical vein on its way from the anterior wall of the abdomen to the liver. At the free edge of the sickle ligament is the round ligament, which is the obliterated portion of the umblical vein after birth, near the navel. The two sheets of peritoneum that make up the sickle ligament divide at the top and leave a portion of the liver without the peritoneum, called the bare area. At this level, the coronary ligament and the left and right triangular ligaments originate (MIR 00-01, 240).

Pancreas

In the duodenal framework, the head of the pancreas is related to the duodenum and the pancreaticoduodenal arteries on the right. Behind it is related to the inferior vena cava, right pillar of the diaphragm, terminal portion of the renal veins and common bile duct; anteriorly with the transverse colon and jejunal loops through the peritoneum. Pancreatic uncus is trapped between the aorta posteriorly and the superior mesenteric vessels anteriorly.

The pancreatic body is related on its anterior side to the pylorus and the posterior wall of the omental pouch. The posterior aspect is related to the portal, aorta, celiac trunk, superior mesenteric, left pillar of the diaphragm, left adrenal and left kidney with its vessels. On its lower face it is supported by the duodenojejunal angle, jejunal bowel loops and left colic angle.

The tail of the pancreas is located in the thickness of the spleen-meso and contacts the lower portion of the spleen.

Kidney, urinary tract and adrenal gland

The kidneys are retroperitoneal organs, located in the lumbar fossae at the level of T12 to L2. Its size is about 10-12cm long and 5-7cm wide. They are bean-shaped and in their concave part, located medially, is the renal hilum, from which the renal pelvis and renal vessels emerge.

The kidneys are surrounded by two fibrous capsules, between which is the perirenal space, occupied by fat. Behind the outermost capsule there is another space called the pararenal, also occupied by fat and limited by Gerotta’s fascia. Ahead, the right kidney is related to the visceral aspect of the liver, descending portion of the duodenum, right colic angle, and small intestine, while the left kidney is related to the pancreas, stomach, spleen, jejunum, and descending colon.

The renal arteries originate from the aorta at the L1-L2 level. The right renal artery crosses behind the inferior vena cava and each artery divides near the renal hilum into 5 segmental arteries, which cross the renal pelvis and divide into lobar and interlobar arteries (MIR 00-01, 236).

The renal pelvis is formed by the confluence of the larger calyces and continues caudally with the ureter. The ureters measure approximately 30-35 cm, the left one being somewhat longer. They descend along the anterior edge of the psoas muscle and cross in front of the common iliac vessels.

The right ureter is previously related to the root of the mesentery. Both empty into the urinary bladder, located in the retropubian space (MIR 02-03, 154). The wall of the bladder is made up of smooth muscle that forms the detrusor muscle and, at the level of the bladder neck, the internal sphincter, of involuntary control.In the lower part, the internal urethral hole is observed, which with the holes of the ureters forms a triangle called a trigone. Male Lauretra is divided into three parts: prostatic, membranous and spongy.

The prostatic urethra passes through the urogenital diaphragm, where the voluntarily controlled striated or external sphincter is located (MIR 97-98, 200). A hole called a seminal colliculus appears on the posterior wall of the prostatic urethra leading to a small glove-finger cavity called the prostatic utricle, which is a vestige of the embryonal uterovaginal canal. The membranous urethra passes through the urogenital diaphragm to continue with the spongy urethra, contained in the spongy body of the penis. The female urethra is shorter and in its lower part it is related to the vagina, crossing the urogenital diaphragm to empty into the vaginal vestibule.

On the upper pole of each kidney is the corresponding adrenal gland. Between both adrenals are the celiac diaphragm, trunk and plexus pillars, aorta (on the left) and inferior vena cava (on the right).

Other structures

Omental bag

The omental pouch or transcavity of the omentum is a recess in the peritoneal cavity located between the stomach and the posterior wall of the abdomen. This cavity allows great mobility of the stomach and allows it to glide freely during digestion. It is connected to the main peritoneal cavity through the omental orifice (epiploic or Winslow hiatus). The limits of this hiatus are as follows:

• Upper: caudate lobe of liver.
• Lower: first portion of the duodenum.
• Posterior: inferior vena cava and right abutment of the diaphragm.
• Anterior: the portal vein and (in front of it) the common bile duct and the hepatic artery, all located on the free edge of the lesser omentum.

The omentums collaborate to form the anterior wall of the omental pocket. The gastrohepatic omentum (omentum minor) houses the portal vein, the hepatic artery, and surrounds the common bile duct on its way from the liver to the duodenum. The gastrocolic omentum (greater omentum) joins the stomach with the transverse colon and hangs down in front of it and the intestinal handles forming the apron of the omentum.

Root of the mesentery and sigmoid mesocolon

The root of the mesentery is a double sheet of peritoneum that allows access of the mesenteric vessels to the intestinal loops. It measures about 15 cm and runs obliquely from the left side of L2 to the right iliac fossa, crossing the third and fourth portion of the duodenum, aorta, inferior vena cava, right gonadal vessels, right ureter and right psoas.

The sigmoid mesocolon is inserted in an inverted V shape with its apex near the division of the common iliac and the access of the left ureter to the pelvis. His left or upper arm ascends on the medial side of the psoas and his right arm is directed to S3.

Lymphatic system

It allows the drainage of tissue fluid, absorbs and transports fat and intervenes in the body’s defense mechanisms. Filters excess fluid in the tissues to the lymph nodes and is transported to the systemic circulation by the lymphatic vessels. After passing through one or more lymph nodes, it is distributed to larger vessels (lymphatic trunks) that join to form the thoracic duct, which receives lymphatic return from the infraradiaphragmatic, posterior mediastinal and intercostal vessels. It passes over the chyle cistern through the aortic hiatus of the diaphragm and ascends through the posterior mediastinum between the thoracic aorta and the azygos vein.

The thoracic duct drains lymph from the entire body, except for the lymph from the right side of the head and neck, the right upper limb, and the right half of the rib cage. These structures drain into the right lymphatic duct that will lead to the junction between the right internal jugular and subclavian veins. The chyle (Pequet) cistern is a dilated sac located in the lower part of the thoracic duct where the intestinal and lumbar lymphatic trunks end. The superficial lymphatic vessels usually have valves. They are found under the skin and superficial fascia and drain into the deep vessels, between the muscles and the fascia accompanying the main vessels.

Azygos Vein System

It is made up of veins on either side of the spinal column that drain the back of the trunk and the thoracic and abdominal walls. The azygos vein and hemiazygos originate from the posterior aspect of the inferior and renal cava, respectively, although it is a system with multiple variations.

• The azygos vein connects the superior and inferior vena cava and collects blood from the posterior walls of the chest and abdomen, ascending the right side of the spine. In the posterior mediastinum it forms a crook over the right pulmonary hilum that empties into the superior vena cava. On its way, it receives blood from the mediastinal, esophageal and bronchial veins.
• The hemiazygos vein originates on the left side from the subcostal and ascending lumbar veins. It ascends on the left side of the spine and at T9 level it crosses to the right side to empty into the azygos vein. It receives the last three posterior intercostal veins, the lower esophageal veins, and several mediastinal veins.
• The accessory hemiazygos vein is located on the left side and descends from the fourth and fifth intercostal spaces, draining blood from the intercostal veins, to T7-T8 where it crosses and empties into the azygos vein. Sometimes it binds to the hemiazygos and flows into the azygos vein together.