Digestive System (pancreas, liver, gallbladder)

Describe endocrine control of bile secretion.

Secretin, released from the duodenum during the intestinal phase of stomach control, also stimulates the liver to increase its production of bile.

Describe the cell arrangement of a liver lobule.

The liver is composed of thousands of functional units called lobules. Each consists of specialized epithelial cells called hepatocytes arranged as plates of cells about the central vein. Adjacent plates of hepatocytes are separated by blood sinusoids lined with endothelium and reticuloendothelial cells (destroy RBCs and blood-borne bacteria).

Describe the bile canaliculi.

The liver is connected to the ampulla of Vater at the duodenum by a series of ducts through which the main liver secretion associated with digestion, bile, flows into the small intestine. Bile is secreted by rows of hepatic paren-chymal cells into tiny bile canaliculi to begin the journey out of the liver.

Describe the location of the liver.

The liver, the second largest organ of the body, is located in the right upper quadrant (right hypochondriac and part of the epigastric regions), tucked under the diaphragm

Describe the blood supply of a liver lobule.

The blood supply of the liver, which comes from two different sources, flows in the direction opposite to that of bile within the lobule (from the periphery towards the central vein in the center of the lobule). The hepatic artery, coming from the celiac artery that rises from the aorta, brings oxygenated blood into the liver. The hepatic portal vein carries the blood previously found in the small and large intestines where it absorbed nutrients from the diet.

Describe the common bile duct.

The common hepatic duct is joined by the cystic duct from the gallbladder, forming the common bile duct. The common bile duct joins the main pancreatic duct at the ampulla of Vater, as described earlier. In this way bile makes its way to the small intestine.

Describe the control of pancreatic secretions via endocrine mechanisms.

The hormones CCK and secretin, released during the intestinal phase of stomach control, affect the pancreas as well. CCK stimulates the secretion of pancreatic enzymes. Secretin stimulates the release of NaHCO3.

Describe plasma protein production.

Virtually all plasma proteins are produced by hepatocytes.

Describe glycogenesis.

hepatocytes convert glucose ( a monosaccharide) to glycogen (a polymer of glucose) when blood glucose level is high. This is one of the major means by which blood glucose is lowered back to within homeo-static range after a meal.

Describe the location of the pancreas.

oblong gland about 5 inches long and 1 inch thick. It consists of a "head" end nestled into the bend of the duodenum and a "tail" lying next to the spleen. The gland lays posterior to the greater curvature of the stomach.

Describe the gallbladder. Include its location, basic histology, how it is filled with bile, and how and when bile is removed from the gall bladder.

see notes

Give the names of the three inactive protein-digesting enzymes found in pancreatic juice.

trypsinogen, chymotrypsinogen, and procarboxypeptidase

Describe the histology of the pancreas.

, 99% of the pancreas consists of small clusters (acini) of exocrine secretory cells that produce enzymes and sodium bicarbonate, then release them into the organ's duct work. The other 1%of the cells are clusters of endocrine cells (islets of Langerhans) that secrete the hormones insulin and glucagon into the bloodstream.

List 9 functions of the liver.

1. carbohydrate metabolism 2. protein metabolism 3. lipid metabolism 4. removal of drugs, hormones, and other chemicals from the blood for detoxification 5. synthesis of bile salts 6. excretion of bile 7. storage (particularly glycogen, ferritin, certain vitamins, and toxic heavy metals that cannot be detoxified or excreted) 8. phagocytosis of worn-out RBCs, WBCs, and some bacteria 9. activation of vitamin D

Describe nervous control of bile secretion.

As with the pancreas, increased vagal stimulation during the cephalic and gastric phases of stomach control, also leads to increased production of bile by two-fold over resting levels

Why are these enzymes secreted in an inactive form?

As with the stomach and pepsinogen, these protein-digesting enzymes are secreted in an inactive form to prevent self-digestion of the pancreas.

Describe the hepatic ducts.

Bile canaliculi join larger ducts, which eventually fuse to form ducts that exit the liver as the right and left hepatic ducts. These two fuse outside the liver to form the common hepatic duct.

What is bile?

Bile is a yellowish-brown to olive-green liquid consisting of water, bile salts, bile pigments, cholesterol, and other substances. It is NOT an enzyme. Rather, it is a detergent.

Describe the bile flow of a liver lobule.

Bile is secreted from the basal surfaces of the hepatocytes, between two juxtaposed rows of cells, and into the bile canaliculi. It then flows outwards to the periphery of the lobule to eventually enter the hepatic ducts.

How is bile used in the digestive process of the small intestine?

Bile is used in the small intestine for the process of fat emulsification. That is, the breakdown of large fat globules into smaller fat droplets, resulting in increased surface area for chemical digestion by the fat-digesting enzymes.

Describe the blood flow in a liver lobule.

Blood flows from the vessels of the portal triads, into the sinusoids, and towards the central vein, located in the center of each lobule. All central veins join to form the hepatic vein that leaves the liver and enters the inferior vena cava.

Describe the control of pancreatic secretions via nervous mechanisms.

During the cephalic gastric phase of stomach control, parasympathetic impulses are conveyed to the pancreas via the vagus nerve, resulting in increased secretion of pancreatic juice

How, and where, are these enzymes activated?

Enterokinase, an enzyme secreted by the small intestinal mucosal cells, converts trypsinogen to trypsin, the active form. Trypsin, in turn, converts chymotrypsinogen to chymotrypsin and procarboxy-peptidase to carboxypeptidase. The activation of the enzymes occurs in the small intestine where the protein substrates for the active enzymes are found. In this way, self-digestion of the pancreas is avoided.

Describe the ducts of the pancreas.

Exocrine pancreatic secretions pass from the secreting cells into small ducts which unite to form ever larger ducts. Eventually 2 large ducts are formed: the main pancreatic duct (of Wirsung) and the smaller accessory duct ( of Santorini).

Describe urea formation.

Hepatocytes convert toxic ammonia (NH3) into the much less toxic urea which is excreted from the body in urine.

Describe deamination.

Hepatocytes remove the amino group (-NH2) from amino acids (deamination) so they can be used for ATP production or be converted to carbohydrates or fats. The amino groups are then converted to ammonia (NH3). Without this deamination process, death would occur within just a few days.

Describe gluconeogenesis.

In gluconeogenesis, hepatocytes convert amino acids and lactic acid to glucose when blood sugar level is low. This is another means used to raise blood glucose back into the homeostatic range during times of fast.

Describe glycogenolysis.

In glycogenolysis, stored liver glycogen is broken down into its component glucose molecules and liberated into the blood for circulation in the body. This is one of the major means by which blood sugar level is brought back into homeostatic range during times of fasting.

Describe the hepatopancreatic ampulla.

In most people the main pancreatic duct exits the pancreas, joins with the common bile duct, and enters the small intestine about 4 inches distal to the pyloric sphincter as the hepatopancreatic ampulla(a flask with a globular body, a saccular anatomic swelling) (of Vater) The smaller accessory duct, if present, opens into the duodenum about 1 inch above the ampulla of Vater.

Describe the lobes of the liver.

It is divided into two primary lobes, right and left, by the falciform ligament, a fold of the peritoneum, and two smaller lobes, the caudate and the quadrate, located between the right and left lobes.

Describe the portal triad.

Located in several locations around the periphery of a given lobule are distinct areas that contain a branch of the hepatic artery and a branch of the hepatic portal vein (delivering blood to the lobule), and a branch of a bile duct (taking bile away from the lobule). Each arrangement of these three tubes is called a portal triad.

Describe the function of carbohydrate-digesting enzymes in pancreatic juice.

Pancreatic amylase is the same as salivary amylase. It functions in the degradation of starch to maltose.

Describe the function of lipid-digesting enzymes in pancreatic juice.

Pancreatic lipase and cholesterol esterase are fat-digesting enzymes.

Describe the function of sodium bicarbonate in pancreatic juice.

(NaHCO3) buffers the pH of the small intestinal contents (chyme) to 7.1 - 8.2. This stops the action of pepsin and creates the proper environment for the enzymes that function in the small intestine.