Pancreatic Anatomy, Relations, and Exocrine/Endocrine Function

Pancreatic Anatomy, Relations, and Exocrine/Endocrine Function

The pancreas is an elongated, retroperitoneal gland situated obliquely in the epigastrium and left hypochondrium. Because it is non-encapsulated, it lacks a distinct outer border and can be difficult to delineate from surrounding retroperitoneal fat. Therefore, a sonographer must rely on surrounding vascular landmarks to identify the gland and evaluate its size, echogenicity, relationship to major abdominal structures, and exocrine and endocrine functions.

Anatomical Divisions of the Pancreas

The pancreas is divided into five distinct segments, progressing from right (inferior) to left (superior):

1. Head: The most caudal portion of the pancreas. It lies to the right of the superior mesenteric vein (SMV), anterior to the inferior vena cava (IVC), and posterior to the antrum of the stomach. It is nestled within the C-loop of the duodenum. Two small vascular structures lie within the head: the gastroduodenal artery (GDA) lies anterolaterally, and the common bile duct (CBD) lies posterolaterally.
2. Uncinate Process: A posteromedial hook-like extension of the head. It is situated directly posterior to the SMV and SMA, and anterior to the IVC and aorta. This is a critical landmark to recognize on ultrasound, as tumors or lymphadenopathy in this region can compress the mesenteric vessels.
3. Neck: The narrow segment between the head and body. It lies directly anterior to the portal-splenic confluence (the junction where the splenic vein and SMV merge to form the main portal vein).
4. Body: The largest, longest portion of the pancreas. It extends horizontally to the left, anterior to the splenic vein, superior mesenteric artery (SMA), left renal vein, and abdominal aorta. The celiac axis arises from the aorta just superior to the border of the pancreatic body.
5. Tail: The most superior and lateral portion of the gland, extending toward the splenic hilum. It lies anterior to the upper pole of the left kidney and posterior to the stomach. The splenic vein courses along its posterior border, while the splenic artery runs along its superior border.

Critical Vascular Relations and Anatomical Landmarks

Understanding the vascular relationships of the pancreas is the most frequently tested anatomical concept on the ARDMS Abdomen exam. Because the pancreatic parenchyma can be highly echogenic and variable, these vascular landmarks serve as definitive markers:

• Splenic Vein: Courses along the posterior margin of the pancreatic tail and body. It serves as the primary posterior border of the pancreatic body and tail.
• Superior Mesenteric Vein (SMV): Runs posterior to the pancreatic neck and anterior to the uncinate process.
• Portal-Splenic Confluence: Located directly posterior to the pancreatic neck.
• Superior Mesenteric Artery (SMA): Arises from the anterior wall of the aorta, posterior to the pancreatic body, and runs anterior to the uncinate process and left renal vein.
• Celiac Axis: Arises from the anterior wall of the aorta, immediately superior to the pancreatic body. It branches into the splenic artery, common hepatic artery, and left gastric artery.
• Splenic Artery: Courses along the superior border of the pancreatic body and tail, following a highly tortuous path.
• Gastroduodenal Artery (GDA): Courses inferiorly along the anterolateral aspect of the pancreatic head, serving as a key landmark.
• Common Bile Duct (CBD): Courses inferiorly along the posterolateral aspect of the pancreatic head, joining the main pancreatic duct.

Pancreatic Physiology: Exocrine and Endocrine Functions

The pancreas is a heterocrine (dual-function) gland with distinct exocrine and endocrine functions:

Exocrine Function (99% of Glandular Tissue)

The exocrine pancreas is composed of acinar cells arranged in lobules. These cells produce pancreatic juice, which is alkaline due to high sodium bicarbonate content (neutralizing acidic stomach chyme) and contains essential digestive enzymes:

• Amylase: Digests carbohydrates by converting starches into maltose.
• Lipase: Digests fats by converting triglycerides into fatty acids and glycerol. Lipase is highly specific to pancreatic tissue, and its elevation in blood correlation is the primary marker for acute pancreatitis, remaining elevated longer than amylase.
• Trypsin, Chymotrypsin, Carboxypeptidase: Digest proteins by breaking down peptide bonds. These are secreted as inactive zymogens (e.g., trypsinogen) to prevent autodigestion of the pancreas.
• Ductal System: These enzymes drain into small ductules, which empty into the Main Pancreatic Duct (Duct of Wirsung). The main duct runs the length of the pancreas and joins the CBD at the Ampulla of Vater. An accessory duct, the Duct of Santorini, drains the upper head and empties into the duodenum at the minor papilla, approximately 2 cm proximal to the Ampulla of Vater.

Endocrine Function (1% of Glandular Tissue)

The endocrine pancreas consists of the Islets of Langerhans, which secrete hormones directly into the blood:

• Beta Cells (approx. 60-70% of islet cells): Produce insulin, which lowers blood glucose by facilitating glucose transport into cells and promoting glycogen storage in the liver.
• Alpha Cells (approx. 15-20%): Produce glucagon, which increases blood glucose by stimulating the liver to convert glycogen to glucose (glycogenolysis).
• Delta Cells (approx. 5-10%): Produce somatostatin, which acts locally to inhibit the release of both insulin and glucagon.

Sonographic Dimensions and Technical Optimization

• Dimensions: Normal AP measurements on ultrasound are:
  • Head: < 3.0 cm
  • Body: < 2.5 cm
  • Tail: < 2.5 cm
  • Main Pancreatic Duct (Duct of Wirsung): < 2.0 mm in the body, measured inner-to-inner wall.
• Echogenicity: Typically equal to or slightly hyperechoic relative to the liver. In older adults, it becomes more hyperechoic and smaller due to fatty infiltration and fibrotic changes. In children, it is relatively hypoechoic.
• Optimization: Bowel gas from the stomach and duodenum often obscures the pancreas. Optimization techniques include:
  • Performing the exam after a 6-to-8-hour fast (NPO).
  • Using graded compression to displace gas.
  • Having the patient drink 12 to 24 ounces of water through a straw in the upright/sitting position to fill the stomach, using the fluid-filled stomach as an acoustic window.