The venous system is irregular in its development and in its anatomy. Much less attention is devoted to the venous system of the various body structures, and seldom are diseases of the endocrine venous system encountered. The endocrine system is divided into two groups—the pure (ductless) group, which includes the thyroid, pituitary gland, thymus, and adrenal gland, and the dual group (both endocrine and exocrine), which includes the testes or ovaries, and pancreas. The glands may be developed from the ectoderm, mesoderm, or entoderm. Pharyngeal pouch endocrine organ derivatives include the thyroid, thymus gland, and parathyroid glands. Being developed from the cephalic part of the foregut, these buds separate and form into separate glands. The parathyroid glands develop from the third pouch. The thymus gland may also develop from caudally directed cylinder cells of the third pouch. The floor of the pharynx may develop a minute diverticulum known as the median thyroid diverticulum and become the tuberculum, which is eventually incorporated in the thyroid. All of the glands—both the arterial and the venous systems—may be intimately related to the vascular system. The endocrine glands routinely secrete chemical compounds or hormones that control the function of other cellular tissues. The glands themselves, histologically, may be well differentiated, and chemical analysis of the glands may be performed. The arterial supply as well as the venous drainage may be involved in different aspects of the endocrine system and of the specific gland. The venous drainage of these glands may be used for diagnosis more frequently than for disease-specific considerations.

The pancreas is both an exocrine and an endocrine organ, and it develops as a gastrointestinal structure with insulin-secreting characteristics. A large arterial and venous system may develop about the gland. In general, the veins parallel the arteries and lie superficial to them. Both lie posterior to the ducts in the body and tail of the pancreas (Figure 28-1). They drain into the portal, splenic, superior, and inferior mesenteric veins. The veins of the head of the pancreas include four pancreaticoduodenal veins that form arcades draining the head of the pancreas and duodenum. The anterior vein joins the right gastroepiploic vein, which joins the superior mesenteric vein (SMV). The posterior vein enters the portal vein above the superior margin of the pancreas. The anterior and posterior veins separately or together form a large vein that enters the dorsal aspect of the SMV. The pancreatic tributaries do not enter the anterior surface of the portal or SMVs. Other small, unnamed veins drain independently into the SMV and the right side of the portal vein.

The veins of the left portion of the pancreas form two large venous channels, the splenic vein above and the transverse (inferior) pancreatic vein below; a smaller superior vein can sometimes be identified as well. The portal vein is formed behind the neck of the pancreas by the union of the superior mesenteric and splenic veins. The inferior mesenteric vein opens at this junction or joins the SMVs.¹

The portal vein lies behind the pancreas in front of the inferior vena cava, rarely anterior to the pancreas and duodenum, and may be associated with an annular pancreas, malformations, and biliary tract anomalies. At surgery, the veins should be carefully dissected, especially the small, thin-walled veins about the head of the pancreas draining into the SMV, which should be exposed at the base of the mesenteric pedicle; then one may proceed carefully upward.

Of particular concern is the portal venous system, where portal vein thrombosis or portal vein hypertension may occur. When this happens, the pancreatoduodenal veins as well as the gastroepiploic veins may become large, dilated, and tortuous. These varices then become a problem when portal hypertension is present, and surgical intervention is required. Traumatic rupture or erosion of these vessels as well as iatrogenic surgical injury with secondary bleeding may occur and be difficult to control. Individual ligation or clipping of these dilated veins will reduce blood loss and help avoid major pressure concerns.

The arterial supply of the thyroid gland comes from the superior and inferior thyroid arteries and sometimes from the thyroidea ima artery. The capillary vessels form a dense plexus in the connective tissue around the vessels between the epithelium of the vessels and the endothelium of the lymphatics (Figure 28-2).

The capillaries drain to a venous plexus on the gland in front of the trachea between the sternothyroid muscles. This venous plexus receives esophageal, tracheal, and inferior laryngeal veins and has thyroidathymic anastomosis. From the thyroid venous plexus, veins arise that do not follow the thyroid arteries. The superior thyroid veins drain the upper part of the gland and ascend along the superior thyroid arteries along the outer omohyoid border across the carotid artery to drain into the internal jugular vein.

The middle or lateral thyroid vein drains the lateral part of the thyroid lobe along the outer border of the omohyoid across the carotid artery to end in the internal jugular vein. The middle thyroid vein commonly passes straight from the anterior surface of the gland directly to the internal jugular vein and thus is easily torn and has to be ligated and divided before passing the finger between the capsule and the gland in delivering the lobe or intrathoracic extension. Sometimes this vein hugs or lies on the posterior capsule of the gland, and this is not necessarily ligated.²