Hepatic portal system

GI tract veins to liver sinusoids to hepatic veins (Figure III-3-36).

• The hepatic portal vein is formed by the union of the superior mesenteric and splenic veins (posterior to the neck of the pancreas).

• The inferior mesenteric vein enters near the area of the junction of the superior mesenteric and splenic veins.

• The hepatic portal vein also receives gastric veins from the stomach.

The portal vein drains into the liver sinusoids, which drain to the hepatic vein, which then goes into the inferior vena cava and ultimately into the right atrium (Figure III-3-37).

Portosystemic Anastomoses

If there is an obstruction to flow through the portal system (portal hypertension), blood can flow in a retrograde direction (because of the absence of valves in the portal system) and pass through anastomoses to reach the caval system. Sites for these anastomoses include the esophageal veins, rectal veins, thoracoepigastric veins, and retroperitoneal veins. Enlargement of these veins may result in esophageal varices, hemorrhoids, and a caput medusae.

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Portal vein

Portal vein

Wood Burning Fish Patterns
Figure 111-3-36. Hepatic Portal System
Table III-3-3. Sites of Anastomoses Between the Portal and Caval Systems and Clinical Signs of Portal Hypertension

Sites of anastomoses

Portal

Caval

Clinical signs

1. Umbilicus

Paraumbilical veins

Superficial veins of the anterior abdominal wall

Caput medusa

2. Rectum

Superior rectal veins (inferior mesenteric vein)

Middle and inferior rectal veins (internal iliac vein)

Internal hemorrhoids

3. Esophagus

Gastric veins

Veins of the lower esophagus which drain into the azygos system

Esophageal varices

4. Retroperitoneal organs

Tributaries of the superior and inferior mesenteric veins

Veins of die posterior abdominal wall

Not clinically relevant j

Hepatic Portal System

Liver

^^ Hepatic Veins

Heart

Inferior Vena Cava

Figure 111-3-37. Comparison ot Normal Caval and Portal Blood Flow

Ascending Descending

Colon Psoas Major Colon

Ascending Descending

Colon Psoas Major Colon

Sacroiliac Joint

Figure III-3-38. Anteroposterior View of Abdomen

Duodenum

Duodenum

Ileum

Figure 111-3-39. Abdomen: Upper Gl, Small Bowel

Ileum

Figure 111-3-39. Abdomen: Upper Gl, Small Bowel

Ascending

Descending

Aorta

Colon

Liver

Stomach

Colon

Spleen nferior vena

Diaphragm

Left Kidney

Cava

Ascending

Descending

Aorta

Colon

Liver

Stomach

Colon

Spleen nferior vena

Diaphragm

Left Kidney

Cava

Figure III-3-43. Abdomen: CT, T12

Mesenteric Splenic

Pancreas

Artery

Liver

Spleen

Portal

Right Kidney

Inferior

Aorta

Left Kidney vena

Left Adrenal

Cava

Gland

Superior

Mesenteric Splenic

Pancreas

Artery

Liver

Spleen

Portal

Right Kidney

Inferior

Aorta

Left Kidney vena

Left Adrenal

Cava

Gland

Figure

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Inferior

Left Common

Vena Cava

Ureter

Iliac Arterv

Psoas

Right Common

Ureter

Iliac Artery

Inferior

Left Common

Vena Cava

Ureter

Iliac Arterv

Psoas

Right Common

Ureter

Iliac Artery

Figure III-3-47. Abdomen: CT, L4

Sigmoid

Left Common

Left Common

Coon

I hac Vein

Iliac Artery

Ureter

Gluteus

I lacus

Medius

Maximus

Sigmoid

Left Common

Left Common

Coon

I hac Vein

Iliac Artery

Ureter

Gluteus

Gluteus

I lacus

Medius

Maximus

Liver Spleen Kidney

Liver Spleen Kidney

Figure 111-3-49. Abdomen: MRI, Coronal

Chapter Summary

Abdomen

The abdominal wall consists primarily of three flat muscles (external oblique, internal oblique, and transversa abdominis muscles), rectus abdominis muscle, and the transversalis fascia. The inguinal canal contains the round ligament in the female and the spermatic cord in the male. The inguinal canal is an oblique canal through the lower abdominal wall beginning with the deep inguinal ring laterally and the superficial inguinal ring medially. Weakness of the walls of the canal can result in two types of inguinal hernias: direct and indirect. A direct hernia emerges through the posterior wall of the inguinal canal medial to the inferior epigastric vessels. Indirect hernias pass through the deep inguinal ring lateral to the inferior epigastria vessels and courses through the inguinal anal to reach the superficial inguinal ring. A persistent processus vaginalis often results in a congenital indirect inguinal hernia.

The gastrointestinal system develops from the primitive gut tube formed by the incorporation of the yolk sac into the embryo during body foldings. The gut tube is divided in the foregut, midgut, and hindgut. Defects in the development of the gastrointestinal tract include annular pancreas, duodenal atresia, Meckel diverticulum, and Hirschsprung disease.

The foregut, midgut, and hindgut are supplied by the celiac trunk, superior mesenteric artery, and the inferior mesenteric artery, respectively. These arteries and their branches reach the viscera mainly by coursing in different parts of the visceral peritoneum. Venous return from the abdomen is provided by the tributaries of the inferior vena cava, except for the Gl tract. 8lood flow from the Gl tract is carried by the hepatic portal system to the liver before returning to the inferior vena cava by the hepatic veins. Diseases of the liver result in obstruction of flow in the portal system and portal hypertension. Four collateral portal-aval anastomoses develop to provide retrograde venous flow back to the heart: esophageal, rectal, umbilical, and retroperitoneal.

(Continued)

Chapter Summary (continued)

The viscera of the Gl system are covered by the peritoneum, which is divided into a parietal layer lining the body wall and the visceral layer extending from the body wall and covering the surface of the viscera. 8etween these iayers is the potential space called the peritoneal cavity. The peritoneal cavity is divided into the greater peritoneal sac and the lesser peritoneal sac (omental bursa). Entrance into the omental bursa from the greater sac is the epiploic foramen that is bound anteriorly by the lesser omentum and posteriorly by the inferior vena ava.

The kidneys develop from intermediate mesoderm by three successive renal systems: pronephros, mesonephros, and metanephros. The mesonephric kidney is the first functional kidney that develops during the first trimester. The final or metanephric kidney develops from two sources: the ureteric bud that forms the drainage part of the kidney and the metanephric mass that forms the nephron of the adult kidney. The urinary bladder develops from the urogenital sinus, which is formed after division of the cloaca by the urorectal septum.

The kidneys are located against the posterior abdominal wall between the T12 and L3 vertebrae. Posterior to the kidneys lie the diaphragm and the psoas major and quadratus lumborum muscles. The superior pole of the kidney lies against the parietal pleura posteriorly. The ureters descend the posterior abdominal wall on the ventral surface of the psoas major muscle and cross the pelvic brim to enter the pelvic cavity.

Pelvis

The pelvic cavity contains the inferior portions of the Gl and urinary systems along with the reproductive viscera. The pelvic viscera and their relationships are shown for the male and female pelvis in Figures III-3-23 and III-3-24, respectively. There are two important muscular diaphragms related to the floor of the pelvis and the perineum: the pelvic diaphragm and the urogenital diaphragm, respectively. Both of these consist of two skeletal musde components under voluntary control and are innervated by somatic fibers of the lumbosacral plexus. The pelvic diaphragm forms the floor of the pelvis where it supports the weight of the pelvic viscera and forms a sphincter for the anal canal. The urogenital diaphragm is located in the perineum (deep perineal space) and forms a sphincter for the urethra. Both diaphragms are affected by an epidural injection.

The broad ligament of the female is formed by three parts: the mesosalpinx, which is attached to the uterine tube, the mesovarium attached to the ovary, and the largest component the mesometrium, attached to the lateral surface of the uterus. In the base of the broad ligament, the ureter passes inferior to the uterine artery just lateral to the cervix. The ovarian ligament is a lateral extension of the broad ligament extending upward to the lateral pelvic wall. This ligament contains the ovarian vessels, lymphatics, and autonomic nerves.

Perineum

The perineum is the area between the thighs bounded by the pubic symphysis, ischial tuberosity, and coccyx. The area is divided into two triangles. Posteriorly, the anal triangle contains the anal canal, external anal sphincter, and the pudendal canal that contains the pudendal nerve and internal pudendal vessels. Anteriorly is the urogenital triangle containing the external and deep structures of the external genitalia. The urogenital triangle is divided into two spaces. The superficial perineal space contains the root structures of the penis and clitoris, associated muscles, and the greater vestibular gland in the female. The deep perineal space is formed by the urogenital diaphragm and contains the bulbourethral gland in the male.

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