Unit 09 — Peritoneum & Endocrine · Question Bank

Definition and layers

The peritoneum is a continuous sheet of simple squamous mesothelium (supported by connective tissue) lining the abdominal and pelvic cavities and covering most abdominal viscera. It has two layers:

• Parietal peritoneum: lines the inner surface of the abdominal wall, pelvis, and undersurface of the diaphragm. Supplied by somatic nerves → sensitive to pain, temperature, and touch. Irritation (peritonitis, perforated viscus) → localised guarding and rebound tenderness (somatic pain).
• Visceral peritoneum: reflected onto and covering the organs; continuous with parietal peritoneum. Supplied by autonomic nerves → insensitive to sharp pain; responds to stretch, ischaemia, and tension (dull, poorly-localised visceral pain).

Peritoneal cavity

The peritoneal cavity is the potential space between the parietal and visceral layers. It contains only a thin film of serous fluid (~50 mL) which lubricates organ movement. The cavity is divided into the greater sac (the main peritoneal cavity) and the lesser sac (omental bursa) which communicates with the greater sac only via the epiploic foramen of Winslow. The peritoneum has an absorptive capacity — fluid, bacteria, and drugs can be absorbed from it (basis of peritoneal dialysis).

Male vs female difference

• Male: the peritoneal cavity is a completely closed sac — no communication with the exterior.
• Female: the peritoneal cavity communicates with the exterior indirectly through the uterine (Fallopian) tubes, uterus, and vagina. The ostium of each uterine tube opens into the peritoneal cavity → physiological pathway, but also a route for ascending pelvic infection (PID), and rare cases of peritoneal dissemination via this route.

Peritoneal folds and recesses

The peritoneum forms reflections: omenta (greater and lesser), mesenteries (small bowel, transverse mesocolon, sigmoid mesocolon), and ligaments (falciform, coronary, splenorenal, gastrosplenic). Important recesses include Morison’s pouch (supine lowest point) and the rectouterine/rectovesical pouch (erect lowest point).

Definition

The omental bursa (lesser sac) is a peritoneal recess that lies posterior to the stomach and the lesser omentum. It is part of the peritoneal cavity but communicates with the greater sac only via the epiploic foramen of Winslow.

Boundaries

• Anterior: lesser omentum, posterior wall of the stomach, and gastrocolic ligament (upper part of greater omentum)
• Posterior: peritoneum covering the abdominal aorta, the pancreas, and the upper parts of the left kidney
• Superior: inferior surface of the liver and part of the peritoneum covering the diaphragm
• Inferior: transverse colon and its mesocolon
• Left boundary: spleen, gastrosplenic ligament, and splenorenal ligament
• Right: communicates with the greater sac through the epiploic foramen (the only opening)

Surgical entry

The lesser sac can be entered surgically by:

• Passing through the epiploic foramen (finger palpation in Pringle’s manoeuvre)
• Cutting through the gastrocolic ligament (between the greater curvature of the stomach and the transverse colon) — the standard operative approach for access to the posterior stomach wall and pancreas
• Cutting through the lesser omentum (hepatogastric ligament) above the epiploic foramen

Clinical significance

• Posterior gastric ulcer perforation: a perforated posterior wall gastric ulcer may bleed or leak into the lesser sac, causing a localised collection (not freely into the greater peritoneal cavity). This can present with back pain radiating to the left shoulder tip.
• Acute pancreatitis: the pancreas forms the posterior boundary; pancreatic enzymes released in pancreatitis initially collect in the lesser sac → pseudocyst formation. A pancreatic pseudocyst is an encapsulated fluid collection within the lesser sac.
• Lesser sac abscess: may be walled off following perforated gastric or duodenal ulcer.

Portal hypertension — background

Portal hypertension (portal venous pressure >10 mmHg; normal 5–10 mmHg) most commonly results from liver cirrhosis. Increased hepatic resistance diverts portal blood through portosystemic anastomoses into the systemic venous system, causing characteristic clinical findings.

1. Lower oesophagus

• Portal tributary: Left gastric (coronary) vein → oesophageal submucosal veins
• Systemic vein: Azygos vein
• Clinical: Oesophageal varices — dilated tortuous submucosal veins in the lower oesophagus. Risk of catastrophic haematemesis; mortality ~30% per bleed. Managed by endoscopic band ligation, TIPS, propranolol.

2. Anterior abdominal wall (umbilicus)

• Portal tributary: Paraumbilical veins (run in falciform ligament, connect to left branch of portal vein)
• Systemic vein: Superior and inferior epigastric veins
• Clinical: Caput medusae — dilated veins radiating from the umbilicus, visible on inspection. Flow is away from umbilicus in all directions (differentiates from IVC obstruction where flow in flanks is upward).

3. Rectum

• Portal tributary: Superior rectal vein → inferior mesenteric vein
• Systemic vein: Middle and inferior rectal veins → internal iliac → IVC
• Clinical: Rectal varices (distinct from haemorrhoids — varices extend up into the rectum above the pectinate line). Less likely to bleed than oesophageal varices but can cause rectal bleeding.

4. Retroperitoneum (veins of Retzius)

• Portal tributary: Mesenteric and colonic veins in the retroperitoneum
• Systemic vein: Lumbar veins, renal veins, phrenic veins
• Clinical: Usually asymptomatic; visible as collateral vessels on CT. Major risk of haemorrhage during retroperitoneal surgery in portal hypertensive patients.

5. Bare area of the liver (diaphragmatic)

• Portal tributary: Small hepatic veins in the bare area of the liver
• Systemic vein: Diaphragmatic veins → inferior phrenic veins → IVC
• Clinical: Rarely of clinical significance unless other anastomoses are exhausted. The bare area is where the liver is in direct contact with the diaphragm (no peritoneal cover).

Position and shape

The suprarenal glands are a pair of small yellowish-brown bodies situated on the superior poles of the kidneys, within the renal fascia but separated from the kidney by a layer of fat. Shapes differ: Right suprarenal = pyramidal (triangular); Left suprarenal = semilunar (crescentic). Each gland is enclosed in the renal fascia and is a primary retroperitoneal structure.

Arterial supply

Each gland has a rich, triple blood supply: superior suprarenal arteries (from inferior phrenic artery), middle suprarenal artery (directly from aorta), and inferior suprarenal artery (from renal artery). Venous drainage: right suprarenal vein → IVC directly (short, important surgically); left suprarenal vein → left renal vein.

Adrenal cortex — zones GFR (outer to inner)

• Zona glomerulosa (outer): secretes mineralocorticoids (mainly aldosterone). Aldosterone acts on the renal collecting tubule to retain Na and excrete K. Regulated by renin-angiotensin system. Conn’s syndrome = aldosterone-secreting adenoma.
• Zona fasciculata (middle, largest zone): secretes glucocorticoids (mainly cortisol). Cortisol regulates carbohydrate metabolism, stress response, immune suppression. Regulated by ACTH from the pituitary. Cushing’s syndrome = cortisol excess.
• Zona reticularis (inner): secretes androgens (mainly DHEA). Responsible for adrenarche (pubic/axillary hair at puberty). Congenital adrenal hyperplasia (CAH) → androgen excess → virilisation.

Adrenal medulla

The medulla (innermost, distinct from cortex embryologically — derived from neural crest cells) contains chromaffin cells, which are modified sympathetic postganglionic neurons. They secrete adrenaline (epinephrine) ~80% and noradrenaline (norepinephrine) ~20% directly into the bloodstream in response to preganglionic sympathetic stimulation (splanchnic nerves). Effect: fight-or-flight response (increased HR, BP, blood glucose, bronchodilation). A phaeochromocytoma is a tumour of these chromaffin cells causing catecholamine excess.

Position and shape

The thyroid gland is an H-shaped endocrine gland at the lower larynx and upper trachea. It consists of two lateral lobes connected by an isthmus. The isthmus overlies tracheal rings 2–4 in the midline. The lateral lobes extend from the oblique line of the thyroid cartilage above to tracheal ring 6 below (vertebral levels C5–T1). A pyramidal lobe (remnant of the thyroglossal duct) extends superiorly from the isthmus in ~50% of individuals. Each lobe has a true fibrous capsule and a false capsule (from pretracheal fascia) with a surgical plane between them.

Relations

• Anterior: strap muscles (sternohyoid, sternothyroid, omohyoid) covered by investing layer of deep cervical fascia and platysma
• Posterior medial: trachea, oesophagus, recurrent laryngeal nerves (in the tracheo-oesophageal groove)
• Posterior lateral: carotid sheath (common carotid artery, internal jugular vein, vagus nerve)
• Posterior surface of lateral lobes: four parathyroid glands

Blood supply

• Superior thyroid artery: first branch of external carotid artery → enters the apex of each lobe. Runs with the external branch of the superior laryngeal nerve (cricothyroid muscle; injury → loss of high-pitched voice — important for singers).
• Inferior thyroid artery: from thyrocervical trunk (subclavian artery) → enters the posterior surface. Crosses the recurrent laryngeal nerve (RLN).
• Thyroid ima artery: present in ~10%; arises from brachiocephalic trunk or aortic arch, enters the isthmus inferiorly — important in tracheostomy.
• Veins: superior thyroid vein (to IJV), middle thyroid vein (to IJV), inferior thyroid veins (to brachiocephalic veins).

Recurrent laryngeal nerve risk

The recurrent laryngeal nerve (RLN) is a branch of the vagus nerve (CN X). Right RLN loops under the right subclavian artery; left RLN loops under the aortic arch at the ligamentum arteriosum. Both ascend in the tracheo-oesophageal groove behind the thyroid to enter the larynx. The RLN supplies all intrinsic laryngeal muscles except cricothyroid → it controls vocal cord movement.

During thyroidectomy, the RLN is at greatest risk at the point where the inferior thyroid artery crosses the nerve. Surgical rule: identify the RLN before ligating the inferior thyroid artery. Unilateral RLN injury → hoarseness (cord paralysis). Bilateral RLN injury → stridor, respiratory distress, may require emergency tracheostomy.