(Transcribed from Dr. Kalliecharan’s lecture, 13 Mar 2000 by Brian Buschman)
Return to Semester One Goodies
Return to The Unofficial Ross Page
The pancreas is a mixed gland, meaning that it has both endocrine and exocrine cells. The exocrine cells secrete digestive enzymes into ducts. The endocrine cells, called islets of Langerhan, secrete hormones into fenestrated capillaries.
The exocrine part of the pancreas is made of compound acinar glands. They have intercalated ducts which penetrate the lumen of the acini. This is a distinctive feature of the pancreatic exocrine glandular units. The intercalated ducts dump into interlobar ducts as the pancreas has no straight ducts. At the center of the acini units there are cells with a pale staining cytoplasm called centroacinar cells. The acini have pyramidal cells with spherical nucleuses. They contain zymogen granules and the number of zymogen granules increase in the fasting state. The exocrine cells secrete trypsin, chymotrypsin, carboxypeptidase, Rnase, Dnase, lipase, amylase and elastase.
The islets of Langerhan are the collections of endocrine cells within the pancreas which secrete into fenestrated capillaries. They are made up of four types of cells:
1) A cells – In response to low blood sugar the A cells secrete glucagon hormone which stimulates tissues to break down glycogen and convert amino acids and lipids to glucose.
2) B cells secrete glucose in response to raised blood sugar levels. Type I diabetes results from the destruction of B cells.
3) D cells release somatostatin which inhibits the release of other hormones from the pancreas (glucagon and insulin).
4) F cells secrete pancreatic polypeptide which stimulates gastric chief cells to release pepsinogen.
The duodenum and pylorus secrete secretin which stimulates the release of pancreatic bicarbonate and it secretes cholecystokinin that induces gal bladder contraction and hence the release of pancreatic enzymes.
The liver is surrounded by a connective tissue capsule and has a hillus where the portal vein, hepatic artery and hepatic duct enters and the lymphatics exit.
The primary cell type of the liver is the hepatocyte. Hepatocytes are arranged in rows and the rows formed into lobules which are the functional units of the liver. The lobules are not divided by a connective tissue boarder so in section it is hard to differentiate between lobules in humans. In the middle of a lobule you can find a central vein which carries blood back to the IVC. The corners of the lobule are marked by the presence of a portal triad. The triad includes a hepatic arteriole, a portal venule and a bile duct. Since they were given the name portal triad it was discovered they also contain a lymph vessel.
Sinusoids are the spaces located between rows of hepatocytes where the blood flows. They receive blood from both the portal vein and the hepatic artery and pass the blood on to the central vein. The sinusoids are lined by endothelial cells with hepatocytes deep to them. The space that exists between the endothelial cells and the hepatocytes is the space of Disse. Most materials are able to pass the endothelial boarder and enter the space of Disse so they can come into contact with the hepatocytes. The space of Disse is also the location of the liver’s fat storing cells which also keep the liver’s store of vitamin A. The hepatocytes primarily secrete albumins, lipoproteins and fibrinogen. The sinusoids also contain Kuffer cells which are the antigen presenting cells of the liver. Their primary job is to grab and break down aged erythrocytes.
The liver gets blood from two sources, the portal vein and the hepatic artery. The hepatic artery enters at the hillus, branches into arterioles for the lobes and then for the lobules (in the portal triads) to supply the sinusoids. The hepatic artery bring nutrient poor yet O2 rich blood. On the other had the portal vein comes directly from the abdominal viscera so it contains very nutrient rich blood yet it’s blood is O2 poor. The portal vein enters at the hillus, branches into venules for the lobes and then for the lobules (the portal triads) which also supply the sinusoids. The sinusoids dump into the central vein which joins to give the hepatic vein that will terminate in the IVC.
To understand the flow of blood through the lobule you can picture the hepatic acinus which is a method of visualizing a watershed from the portal triads to the central vein. For this it is best to look at a diagram. You can draw lines to make a diamond out of two central veins and two adjacent portal triads. The line that connects the two triads is the divide and blood will run from that level to the respective central vein based on the side it is on. If you then look at the triangle formed by connecting the two triads and one central vein you can then divide that up into three parts based on distance from the triads. The first part is called zone I and is the first section supplied by the blood. When there is high blood glucose it gets first dibs on the goodies. When blood glucose level drops it is also the first to give up it’s nutrients. The middle part is zone II and is the second to get a go at the blood’s nutrients when all is good but gets to wait a while after zone I starts to give up it’s goodies when needed. The part closest to the central vein is zone III which is the last to get nutrients and the last to give them up.
When two hepatocytes are lined up right next to each other they leave a little space which is called the bile canaliculi. The bile canaliculi is where they secrete their enzymes and it drains into the bile duct in the portal triad. Hepatocytes are cuboidal cells that contain eosinophilic cytoplasm, lots of mitochondria, SER, RER, lipid droplets, glycogen and lysomes.
I) Protein synthesis. The endocrine function if to make and secrete all plasma proteins except for Igs. The plasma proteins are not made and held in granules but are synthesized on demand.
II) Bile secretion. The exocrine function of the liver is to produce bile which contains electrolytes, bile acids, cholesterol, phospholipids and bilirubin. Bilirubin is naturally insoluble so one of the liver’s jobs is to make it stay in solution. When things precipitate out of bile, such as in gal stones, cholesterol is usually the first substance to do that.
III) Metabolic storage. The liver holds onto some fat and glycogen for use in maintenance of blood glucose. It also holds vitamin A.
IV) Metabolic function. The liver plays it’s active role in metabolically regulating the level of glucose in the blood.
V) Detoxification. It metabolizes many drugs and other toxins introduced into the body.
The gal bladder is merely a sac that holds and concentrates bile (30-50ml) and releases it on demand (ie. when stimulated by cholecystokinin). It has a simple columnar (absorptive) mucosa with a lamina propria but no muscularis mucosa. It also has no submucosa but does have a muscularis externa. It has a serosa except for a small part between itself and the liver that had an adventitia. When stimulated by cholecystokinin it pumps bile into the major duodenal papilla. It’s epithelia also has invaginations called Rakitansky-Aschoff sinuses.
Return to Semester One Goodies
Return to The Unofficial Ross Page