Active transport of Na, Cl, and HCO3 across the lateral plasma membrane into the intercellular (paracellular) compartment causes the concentration of electrolytes in the intercellular space to increase. The increased electrolyte concentration creates an osmotic gradient between the intercellular space and the cytoplasm and between the intercellular space and the lumen. Water moves from the cytoplasm and from the lumen into the intercellular space because of the osmotic gradient (i.e., it moves down its concentration gradient; see Fig. 18.17b). Although the intercellular space can distend to a degree often visible with the light microscope, this ability is limited. The movement of electrolytes and water into the space creates hydrostatic pressure that forces a nearly isotonic fluid out of the inter-cellular compartment into the subepithelial connective tissue (the lamina propria). The fluid that enters the lamina propria quickly passes into the numerous fenestrated capillaries and the venules that closely underlie the epithelium. Studies of fluid transport in the gallbladder first demonstrated the essential role of the intercellular compartment in transepithelial transport of an isotonic fluid from the lumen to the vasculature. Therefore, the final modification of bile is mainly the result of the active transport of Na , Cl, and HCO3 , and the passive, aquaporin-mediated transport of water across the plasma membrane of epithelial cells of the gallbladder.
FIGURE 18.18 • Photomicrograph of the Rokitansky-Aschoff sinuses in the wall of the gallbladder. This photomicrograph shows deep invaginations of the mucosa extending into the muscularis externa. These invaginations are referred to as Rokitansky-Aschoff sinuses. 120.
FIGURE 18.19 • Diagram of pancreas, duodenum, and associated excretory ducts. The main pancreatic duct (of Wirsung) traverses the length of the pancreas and enters the duodenum after joining with the common bile duct. An accessory pancreatic duct (of Santorini) is commonly present, as shown, and empties into the duodenum at a separate minor duodenal papilla. The site of entry of the common bile duct and main pancreatic duct into the duodenum is typically marked by a major duodenal papilla visible on the inner surface of the duodenum.
The pancreas is an elongate gland described as having a head, body, and tail. The head is an expanded portion that lies in the C-shaped curve of the duodenum (Fig. 18.19). It is joined to the duodenum by connective tissue. The centrally located body of the pancreas crosses the midline of the human body, and the tail extends toward the hilum of the spleen. The pancreatic duct (of Wirsung) extends through the length of the gland and empties into the duodenum at the hepatopan creatic ampulla (of Vater), through which the common bile duct from the liver and gallbladder also enters the duodenum.
The hepatopancreatic sphincter (of Oddi) surrounds the ampulla and not only regulates the flow of bile and pancreatic juice into the duodenum but also prevents reflux of intestinal contents into the pancreatic duct. In some individuals, an ac cessory pancreatic duct (of  Santorini) is present, a vestige of the pancreas’s origin from two embryonic endodermal pri mordia that evaginate from the foregut.
A thin layer of loose connective tissue forms a capsule around the gland. From this capsule, septa extend into the gland, dividing it into ill-defined lobules. Within the lob ules, a stroma of loose connective tissue surrounds the parenchymal units. Between the lobules, larger amounts of connective tissue surround the larger ducts, blood vessels, and nerves. Moreover, in the connective tissue surrounding the pancreatic duct, there are small mucous glands that empty into the duct.