CANINE MEDICINE
Gallbladder Physiology Bile Bile is required for the successful digestion and absorption of nutrients . It aids in fat digestion and absorption by emulsifying large fat particles into smaller ones that are more susceptible to pancreatic lipase . Bile also enhances intestinal absorption of digested fats and aids in the excretion of waste products , including cholesterol and bilirubin .
Bile is produced by hepatocytes and collected in the hepatic canaliculi . From the canaliculi , bile drains into interlobular ducts . These ducts progressively merge to form hepatic ducts , which join the cystic duct to form the common bile duct .
The common bile duct enters the duodenal lumen at the major duodenal papilla ; this junction is commonly referred to as the sphincter of Oddi . The pancreatic duct also empties into the small intestine at this point .
The pH of bile ranges from 5.9 to 7.8 . Bile is made of water , bile acids , bilirubin , cholesterol , lecithin , and electrolytes . 3 Although some bile flows directly into the small intestine from the liver , most of it is temporarily stored in the gallbladder .
nerve , sympathetic inhibition of the splanchnic nerve , neurotensin , and substance P . 6-8
Gallbladder relaxation The gallbladder relaxes in response to somatostatin , vasoactive intestinal polypeptide , nitric oxide , and pancreatic polypeptide . 3 , 6 , 9 The relaxation phase is associated with bile storage and modification . During this period , the sphincter of Oddi is closed .
Gallbladder Mucocele Pathophysiology A gallbladder mucocele is an abnormally distended gallbladder containing a buildup of luminal mucus . Grossly , the gallbladder appears enlarged ( unless it is ruptured ) and contains gelatinous material that may be congealed mucus , a mucus cast , or inspissated bile ( Figures 1 & 2 ).
Before 2000 , mucocoeles were considered rare . 10 In fact , most were noted as incidental necropsy findings . 11 Given the diagnostic tools available at the time , the true clinical impact and incidence of mucocoeles before the 1990s are unknown .
Predisposition to mucocoeles may be associated with dyslipidemias in particular breeds such as Shetland
Substantial bile modifications occur within the gallbladder : sodium , chloride , and water are removed and hydrogen ions are added . After a meal , the gallbladder also secretes bicarbonate-rich mucin , which mixes with the stored bile . The gallbladder contents become inspissated ( sludgelike ) when more fluid is reabsorbed or more mucin is added .
Biliary Sludge Biliary sludge is a mobile mixture of precipitated cholesterol crystals , bile pigments , bile salts , and mucin . In people , inspissated bile is considered abnormal and is associated with cholestasis , choleliths , cholecystitis , biliary infection , fasting , and partial parenteral nutrition . 4 In dogs , biliary sludge may be associated with disease but is also often seen in clinically normal geriatric dogs . 5 At present , the significance of biliary sludge in dogs is unknown .
Gallbladder Contraction The gallbladder musculature contracts in response to cholecystokinin ; this hormone is released from enterocytes after the ingestion of a fatty meal . In particular , proteoses , peptones , and long-chain fatty acids stimulate cholecystokinin release . Cholecystokinin concentrations peak about 20 minutes after a meal and may remain elevated for almost two hours . Once the gallbladder contracts , its contents are emptied into the duodenum within about 60 minutes . 3 Cholecystokinin also causes relaxation of the sphincter of Oddi and the release of pancreatic digestive enzymes .
Other factors that contribute to gallbladder contraction include parasympathetic stimulation by the vagus
Figure 1 . A surgically removed canine gallbladder with a mucocele . The cystic duct has been ligated and removed , and the resultant opening allows visualisation into the distended gallbladder . ( Image courtesy of Dr . Roy R . Pool , Texas A & M University ' s Department of Veterinary Pathobiology .)
Figure 2 . A longitudinal cross section of the gallbladder shown in Figure 1 . The yellow material is the mucocele , and the black material is bile . The pink area is a hematoma that developed during surgical excision . ( Image courtesy of Dr . Roy R . Pool , Texas A & M University ' s Department of Veterinary Pathobiology .) vet360
Issue 03 | JUNE 2016 | 28