Abdomen Anatomy: Surgical Notes

• The study notes cover the surgical anatomy of the abdomen.
• Aims to facilitate understanding of abdominal viscera, peritoneal cavity concepts, referred pain, peritoneal structure, abdominal wall surface anatomy, and related anatomical structures.

Abdomen: General Description

• Extends from the inferior thorax margin to the superior pelvis margin.
• Musculomembranous walls surround a large abdominal cavity.
• The peritoneal cavity freely communicates with the pelvic cavity.
• Abdominal viscera are suspended by mesenteries within the peritoneal cavity or positioned between the cavity and the musculoskeletal wall.
• Major components of the gastrointestinal system liver, pancreas, gall bladder, spleen, urinary system components, suprarenal glands, and major neurovascular structures are contained in the abdominal viscera.

Peritoneal Cavity: Basic Concepts

• A potential space within the abdomen.
• The cavity is lined by the peritoneum and contains a few milliliters of lubricating fluid.
• Capable of distension to accommodate abdominal contents in healthy states, including food, feces, flatus, fat, and a fetus.
• The cavity can accommodate fluids (ascites, pus, blood), fluids in cysts (ovarian), and gas (hollow viscus perforation, carbon dioxide) during abdominal surgery/exploration in disease states.

Peritoneum

• Gastrointestinal system is suspended from the posterior abdominal wall and the anterior abdominal wall via the mesenteries.
• Lined by the peritoneum similarly to the pleura and serous pericardium , it reflects off the abdominal wall to become a component of the mesenteries.
• The parietal peritoneum is associated with the abdominal wall, while the visceral peritoneum covers the organs.

Viscera: Intraperitoneal vs. Extraperitoneal

• Viscera can be classified as either intraperitoneal or extraperitoneal.
• Structures suspended by mesenteries from the abdominal wall are intraperitoneal. .
• Extraperitoneal structures are not suspended by mesenteries and lie between the parietal peritoneum and the abdominal wall; described as retroperitoneal in position.
• Some structures are initially suspended by mesenteries during development but later become retroperitoneal due to the fusion of part of the peritoneum with the abdominal wall.

Peritoneal Reflections

• Mesentery includes the stomach, ileum, jejunum, transverse, and sigmoid colon.
• The ascending and descending colon are partially retroperitoneal.
• The 2nd, 3rd, and 4th parts of the duodenum is retroperitoneal

Abdominal Pain

• Visceral pain happens when noxious stimuli affect an abdominal viscus, is poorly localized, dull, and may involve secondary autonomic symptoms.
• Somatic/Parietal pain occurs when noxious stimuli affect the parietal peritoneum, is precisely localized, intense, and aggravated by movement.
• Referred pain is felt in remote areas supplied by the same neurosegment as the diseased organ and well localized, appearing with intense visceral stimulus.

Abdomen: Surface Topography

• Topographical divisions are used to describe the location of abdominal organs and the pain associated with abdominal disorders
• The abdomen is divided using two schemes: a nine-region pattern and a four-quadrant pattern.
• The transumbilical plane passes horizontally through the umbilicus and the disc between L3 and L4, dividing the abdomen into upper and lower levels.
• The median sagittal plane passes vertically through the body, dividing it into right and left halves.
• The superior horizontal (subcostal) plane is inferior to the costal margin, runs into the 10th rib, and passes through the L2 and L3 discs.
• The inferior horizontal plane extends between the right and the left iliac crest tubercles.
• The vertical planes are the right and the left midclavicular planes, extending from the clavicle midpoint to the inguinal ligament midpoint.

Abdomen: Disposition of Viscera

• Much of the liver, gall bladder, stomach, spleen, and parts of the colon are under the domes of the diaphragm.
• The domes of the diaphragm project superiorly above the costal margin, protecting abdominal viscera by the thoracic wall.
• Superior poles of the kidneys are deep to the lower ribs.
• Viscera not under the domes of the diaphragm are supported and protected by the muscular abdominal walls.

Peritoneal Cavity

• Subdivided into the greater sac and lesser sac (omental bursa).
• The greater sac accounts for most of the space.
• The lesser sac is smaller, lies posterior to the stomach and liver, and communicates with the greater sac via the omental foramen (of Winslow)..
• The transverse mesocolon divides the peritoneal cavity into the supracolic compartment (stomach, liver, spleen) and the infracolic compartment (small intestine and ascending/descending colon).

Peritoneal Cavity: Clinical Correlation

• A large surface area facilitates disease spread through the peritoneal cavity.
• Allows easy spread of infection and malignant disease.
• Perforations of the gut tube lead to rapid spread.
• The cavity may also restrict the spread into other areas of the body.
• Intra-abdominal infections may occur below and above the diaphragm.
• Bowel perforation may allow gas to escape into the cavity, such as in a perforated gastric ulcer where gas is found under the diaphragm.
• Peritonitis refers to infection/inflammation of the peritoneum due to bacterial contamination, traumatic penetration, or bowel rupture.
• Ascites involves excess fluid, is due to mechanical injury or disorders like portal hypertension/metastasis, and leads to abdominal distension.
• Adhesions are abnormal attachments between visceral and/or parietal peritoneum.
• They result from stab wounds and infection.
• The greater omentum prevents the visceral peritoneum from adhering to the parietal peritoneum, forms adhesions to inflamed organs, and seals off inflamed organs.
• This is known as 'policeman of the abdomen'.

Fluid Flow in Peritoneum

• Infracolic spaces and paracolic gutters determine fluid flow in the upright position and infection spread.
• Fluid typically collects in pelvic recesses (rectouterine/rectovesicle pouch).
• Because toxins are slowly absorbed in the pelvic cavity, peritonitis patients are placed in a sitting position.
• In the supine position, fluids can spread or accumulate in subphrenic recesses (spaces) and subphrenic abscesses are common on the right side due to ruptured appendices and duodenal ulcers.
• Perforations on the posterior stomach wall cause fluid to enter the lesser sac.