⭐ PERITONEUM — 20% CONTENT → 80% EXAM MARKS

1️⃣ What the Peritoneum Is

• Serous membrane lining abdominal + pelvic cavities.
• Two layers:
• Parietal peritoneum – lines walls.
• Visceral peritoneum – covers organs.
• Made of: single flat mesothelial layer + underlying connective tissue.

Exam trigger: Parietal = wall, Visceral = organs.

2️⃣ Key Idea: Continuous Sheet

Although different parts have different names (fascia, folds), it is ONE continuous membrane.

3️⃣ Extraperitoneal Tissue

• Connective tissue between parietal peritoneum and body wall.
• Can be loose (expansile areas) or dense (non-expansile areas).

Examples:

• Loose → transversalis fascia.
• Dense → iliac fascia, psoas fascia, parietal pelvic fascia.

Exam trigger: All these fasciae = extraperitoneal connective tissue, not separate structures.

4️⃣ Peritoneal Folds & Special Names

Peritoneum makes double-layered folds that support or connect organs.

Mesenteries (transport vessels + nerves)

• Mesentery → small intestine
• Transverse mesocolon
• Sigmoid mesocolon
• Mesoappendix

Key idea: Mesenteries = two layers → suspend organs + conduct vessels.

Omenta

• Lesser omentum: stomach → liver
• Greater omentum: hangs from stomach like an apron

Function: Protection, fat storage, infection control ("policeman of the abdomen").

Peritoneal Ligaments

Not true ligaments — just folds.

Examples:

• Falciform ligament
• Ligaments of liver, stomach, spleen
• Broad ligament of uterus

5️⃣ Anterior Abdominal Wall Folds

Above umbilicus:

• Falciform ligament
• Contains ligamentum teres (obliterated L umbilical vein).

Below umbilicus — 5 folds (very high-yield):

Exam trigger:

Only lateral umbilical folds contain active vessels → inferior epigastric vessels.

⭐ PERITONEAL CAVITY — GREATER & LESSER SACS (20% → 80% MARKS)

1️⃣ The Peritoneal Cavity: Key Idea

• It is a potential space, not a real open cavity.
• Contains only a thin lubricating fluid for organ movement.
• Divided into:
• Greater sac — main part
• Lesser sac (omental bursa) — small space behind stomach

Exam trigger: In life, the peritoneal cavity is slit-like, not wide or empty.

2️⃣ GREATER SAC

• Represents most of the peritoneal cavity.
• Occupies the space between viscera or viscera and parietal peritoneum.

👉 Nothing more to memorise — extremely straightforward.

3️⃣ LESSER SAC (OMENTAL BURSA) — THE HIGH-YIELD PART

Location

• Lies behind the stomach and lesser omentum.

Connection

• Communicates with the greater sac via epiploic foramen (of Winslow).
• Lies in front of the inferior vena cava.

Exam trigger: Epiploic foramen = ONLY connection between the two sacs.

⭐ 4️⃣ Walls of the Lesser Sac — SUPER HIGH-YIELD

✔ Anterior wall

Formed by:

• Posterior layer of lesser omentum
• Posterior wall of stomach
• Posterior layer of greater omentum (front two layers)

✔ Posterior wall

Formed by peritoneum covering:

• Pancreas (neck + body)
• Left kidney
• Left suprarenal gland
• Coeliac artery + plexus + nodes
• Upper abdominal aorta
• Part of diaphragm

✔ Lower boundary

• Theoretically descends between layers of the greater omentum
• BUT layers fuse → cavity stops at transverse colon

(This is why lesser sac does NOT extend into the greater omentum in adults.)

✔ Upper boundary

• At right side of abdominal oesophagus
• Peritoneum reflects on diaphragm → forms posterior layer of lesser omentum

✔ Left boundary

• Gastrosplenic ligament
• Splenorenal ligament

(These close the sac on the left.)

⭐ GREATER OMENTUM — 20% CONTENT → 80% MARKS

1️⃣ What It Is

• A double sheet of peritoneum folded onto itself → 4 layers.
• Hangs like an apron from the greater curvature of the stomach.

👉 Exam trigger: “Four layers” and “fat-laden apron over intestines.”

2️⃣ Course & Attachments

Step-by-step (VERY high-yield):

1. Two layers descend from greater curvature of stomach.
2. Hang down in front of small intestine → “apron.”
3. Turn upward and ascend back toward transverse colon.
4. Fuse with peritoneum over:
• Anterior surface of transverse colon
• Transverse mesocolon

👉 Below the transverse colon → all 4 layers fuse into a single fatty sheet.

3️⃣ Functions

• Contains fat, macrophages, immune cells.
• Known as “policeman of the abdomen” → walls off infection.

4️⃣ Gastrocolic Omentum (high-yield subpart)

• Portion between stomach ↔ transverse colon.
• Contains:
• Right gastroepiploic vessels
• Left gastroepiploic vessels

👉 Surgeons enter the lesser sac through the gastrocolic omentum.

Other surgical access routes:

• Lesser omentum
• Transverse mesocolon

5️⃣ Relation to Spleen

• Greater omentum wraps around spleen (except hilum).
• Spleen remains in greater sac (not lesser sac).

6️⃣ Important Ligaments (all double-layered peritoneal folds)

Gastrosplenic ligament

• Connects greater curvature → spleen
• Contains:
• Short gastric vessels
• Left gastroepiploic vessels

Splenorenal ligament

• Connects spleen → anterior surface of left kidney
• Contains:
• Splenic vessels
• Tail of pancreas

👉 Super exam trigger: Pancreatic tail lies in splenorenal ligament.

⭐ LESSER OMENTUM — 20% CONTENT → 80% MARKS

1️⃣ What It Is

• A double layer of peritoneum connecting:

Liver ↔ Lesser curvature of stomach & first 2 cm of duodenum.

• Also called gastrohepatic omentum.

👉 Exam trigger: “Double-layered fold from liver to lesser curvature.”

2️⃣ Attachments (simple, must-know)

Stomach + duodenum attachment

• From right side of abdominal oesophagus
• Along entire lesser curvature
• Up to first 2 cm of duodenum

Liver attachment

• L-shaped
• Goes to:
• Fissure for ligamentum venosum
• Porta hepatis

3️⃣ Right Free Border — The Most Important Exam Point

The right free margin of the lesser omentum forms the anterior boundary of the epiploic foramen (of Winslow).

Inside this free edge lie (very high-yield triad):

⭐ Portal triad (from posterior → anterior):

1. Portal vein — most posterior
2. Hepatic artery — anterior and to the left
3. Bile duct — anterior and to the right

Also present: lymphatics + autonomic nerves.

👉 Clinically: This is where you apply Pringle manoeuvre ⟶ compress hepatic artery + portal vein to control liver bleeding.

⭐ 4️⃣ Epiploic Foramen (of Winslow)

The ONLY natural communication between greater sac ↔ lesser sac.

Boundaries:

• Anterior: Right free edge of lesser omentum
• Posterior: Inferior vena cava
• Superior: Caudate process of liver
• Inferior: 1st part of duodenum

👉 Essential exam fact: “IVC forms posterior boundary.”

5️⃣ Lesser Omentum Transitions

• Downwards → its two layers continue and form the greater omentum.
• Upwards → layers enclose liver and become:
• Coronary ligament
• Right & left triangular ligaments
• Falciform ligament

👉 So the lesser omentum is part of the same continuous peritoneal sheet.

⭐ PERITONEAL COMPARTMENTS — 20% CONTENT → 80% MARKS

The peritoneal cavity is divided into three major compartments:

1️⃣ Supracolic compartment

2️⃣ Infracolic compartment

3️⃣ Pelvic compartment

1️⃣ SUPRACOLIC COMPARTMENT

Above the transverse mesocolon.

Boundaries are defined by the liver’s peritoneal attachments

It has three important spaces:

A. Right subphrenic space

• Right of falciform ligament
• Between diaphragm & liver

B. Left subphrenic space

• Left of falciform ligament
• Closed above by left triangular ligament

C. Right subhepatic space (Hepatorenal pouch of Morison) ⭐

• Between right lobe of liver & right kidney
• Communicates with lesser sac through epiploic foramen
• MOST important clinically → fluid collects here when supine.

👉 Key exam trigger: Morison pouch = lowest intraperitoneal space when supine.

2️⃣ INFRACOLIC COMPARTMENT

Below the transverse mesocolon.

Divided into right & left infracolic spaces by the root of the mesentery.

⭐ Root of the Mesentery (very high-yield)

• Runs from left → right at 45° angle, 15 cm long
• Starts at duodenojejunal flexure
• Crosses:
• 3rd part of duodenum
• Aorta
• IVC
• Right psoas
• Ureter
• Ends in right iliac fossa

👉 Exam pearl: Mesentery length → root = 15 cm, bowel border = ~6 m.

A. Right infracolic space

• Triangular
• Apex → ileocaecal junction
• Bounded by:
• Ascending colon (right)
• Transverse mesocolon (top)
• Root of mesentery (left)

Right paracolic gutter (VERY CLINICAL):

• Lateral to ascending colon
• Conducts fluid upward → Morison pouch
• Downward → pelvis

B. Left infracolic space

• Larger, quadrilateral
• Bounded by:
• Descending colon (left)
• Transverse mesocolon (top)
• Root of mesentery (right)
• Continuous with pelvis below

Left paracolic gutter

• Lateral to descending colon
• Limited above by phrenicocolic ligament

(prevents pus from easily reaching subphrenic space)

3️⃣ SIGMOID MESOCOLON (high-yield anatomy)

• L-shaped attachment
• Located at bifurcation of common iliac vessels
• Lateral limb: along pelvic brim (over external iliac vessels)
• Medial limb: slopes into sacral hollow → ends at S3 (rectum begins)

👉 Key relation: Left ureter lies beneath the apex, over common iliac bifurcation.

Inflammation or surgery here risks ureteric injury.

⭐ PERITONEAL NERVE SUPPLY — 20% CONTENT → 80% MARKS

1️⃣ Two Completely Different Nerve Supplies

Peritoneum has two types of innervation:

✔ A. Parietal peritoneum

→ Somatic nerve supply (like skin)

→ Sharp, well-localized pain

✔ B. Visceral peritoneum

→ Autonomic afferents (visceral)

→ Poorly localized, dull pain

👉 Key exam contrast:

Parietal = sharp + localized

Visceral = dull + poorly localized

2️⃣ PARIETAL PERITONEUM — SEGMENTAL SUPPLY

⭐ Central diaphragm

• Phrenic nerve (C4)

→ Pain felt at shoulder tip (C4 dermatome)

Very high-yield:

“Shoulder pain after ruptured ectopic / subphrenic abscess = diaphragmatic peritoneum irritation.”

⭐ Peripheral diaphragm + abdominal wall

• Intercostal nerves (T6–T12)
• Lumbar nerves (L1)

⭐ Pelvic peritoneum

• Chief nerve: Obturator nerve (L2–L4)

3️⃣ VISCERAL PERITONEUM — AUTONOMIC

• Sensation carried along autonomic nerves (sympathetic + parasympathetic pathways).
• Pain mechanisms:
• Ischaemia
• Stretching of visceral peritoneum or mesentery
• Smooth muscle spasm

👉 Pain is poorly localized and midline (e.g., early appendicitis → periumbilical pain).

⭐ RETROPERITONEAL SPACE — 20% CONTENT → 80% MARKS

1️⃣ What It Is

• The area behind the parietal peritoneum and in front of the posterior abdominal wall.
• Contains major vessels, organs, nerves, and lymphatics.

👉 Exam trigger: Retroperitoneal = posterior to peritoneum, NOT inside peritoneal cavity.

2️⃣ CLASSIC LIST — ORGANS IN THE RETROPERITONEUM (must memorize)

⭐ Primary (always retroperitoneal)

• Kidneys
• Ureters
• Suprarenal glands (adrenals)
• Aorta & IVC
• Sympathetic trunks
• Lumbar plexus nerves

⭐ Secondary (became retroperitoneal after rotation)

• Pancreas (except tail)
• Duodenum (2nd–4th parts)
• Ascending colon
• Descending colon

👉 Memory tip: SAD PUCKER

Suprarenal, Aorta/IVC, Duodenum (2–4), Pancreas, Ureters,

Colon (asc/desc), Kidneys, Esophagus (thoracoabdominal), Rectum (upper 2/3).

3️⃣ Other Retroperitoneal Structures

• Cisterna chyli
• Lymph nodes and vessels
• Psoas & quadratus lumborum muscles (covered by fascia)

4️⃣ Clinical Importance (very high yield)

⭐ Retroperitoneal hemorrhage

• Blood can accumulate silently behind peritoneum.
• Causes: ruptured AAA, renal trauma, pancreatic injury.

⭐ Retroperitoneal infection

• Pus may remain confined, not entering peritoneal cavity.
• Seen in: psoas abscess, pancreatitis, renal infections.

👉 Essential concept: Peritoneum acts like a barrier, so retroperitoneal pathology may lack peritoneal signs.