Nakamura Human Physiology Lecture 14 Flashcards
(27 cards)
Structure of Liver
-Hepatocytes (liver cells) form hepatic plates that are 1 – 2 cells thick.
•Hepatic plates are separated by sinusoids
•Sinusoids contain phagocytic Kupffer cells (fight bacteria, keep liver environment clean)
•Liver receives blood from the hepatic artery and also the hepatic portal vein
Flow of blood to liver
-The portal system: a double blood supply;
-the right and left hepatic arteries carry oxygenated blood to the liver,
-the portal vein carries venous blood from the GI tract to the liver, where it is screened by Kupffer cells to remove any pathogens.
•Hepatic plates are arranged in lobules, and each lobule has a central vein to drain the sinusoids.
-central vein drains into the hepatic vein
•Hepatocytes produce bile and secrete it into small channels that drain into bile ductules
Gallbladder and bile secretion
-When the small intestine is empty, the bile secreted by the liver is stored in the gall bladder.
•Upon ingestion of a meal containing fats, the gall bladder contracts and secretes the bile into the duodenum (duodenal papilla)
•Bile emulsifies fats and breaks up large fat droplets into smaller droplets
Enterohepatic circulation
Means intestine and liver
Variety of exogenous compounds are secreted by the liver into the bile ducts. (Liver makes bile)
•Can excrete these compounds into the intestine with the bile (via common bile duct)
•Many compounds excreted by the liver are recycled (hepatic portal vein back to liver)
–Bile salts (endogenous)
–Many antibiotics (exogenous)
Enterohepatic circulation diagram
Hemaglobin and myoglobin to biliverdin
Biliverdin to bilirubin and albumin
To liver, makes bile salts
90% bile salts reabsorbed in ileum as urobilinogen
10% bile salts lost in feces as stercobilinogen
Liver functions
Detoxification of blood Carbohydrate metabolism Lipid metabolism Protein synthesis Secretion of bile
Detoxification of blood
- phagocytosis by kupffer cells
- Chemical alteration of biologically active molecules (hormones and drugs)
- Production of urea, Uric acid, and other molecules that are less toxic than parent compounds
- Excretion of molecules in bile
Carbohydrate metabolism
- conversion of blood glucose to glycogen and fat
- production of glucose from liver glycogen and from other molecules (Amino acids, lactic acid) by gluconeogenesis
- secretion of glucose into the blood
Lipid metabolism
Synthesis of of triglyceride and cholesterol
- Excretion of cholesterol in bile
- production of ketone bodies from fatty acids
Protein synthesis
- production of albumin (osmolarity)
- production of plasma transport proteins
- production of clotting factors (fibrinogen, prothrombin and others)
Secretion of bile
- synthesis of bile salts
- conjugation and excretion of bile pigment (bilirubin)
Pancreas
-Endocrine secretion of insulin and glucagon from islets
-endocrine cells are alpha and beta cells
•Exocrine secretion of pancreatic juice from acini into pancreatic duct which joins the common bile duct.
-exocrine cells are acinus cells
•Secretion of bile and pancreatic juice through duodenal papilla in the duodenum
•Enzymes from pancreatic acini are secreted as zymogen granules that must be activated in the duodenum
Pancreatic juice
.•Trypsinogen (trypsin is activated version) is activated by the brush border enzyme enterokinase
•Activated trypsin then activates other enzymes
Contains H20 , HC03- and digestive enzymes
•Activation of trypsin by enterokinase
Regulation of gastric function
.•GI tract is both an endocrine gland and a target for the action of hormones
•Extrinsic control of gastric function is divided into 3 phases
–Cephalic phase (brain)
–Gastric phase (stomach)
–Intestinal phase (intestine)
Cephalic phase
.Sight, smell, and taste of food.
•Activation of vagus(cranial nerve X) centers in the brain stimulates:
–Chief cells to secrete pepsinogen
–G cells to secrete gastrin
–ECL (Enterochromaffin-like) cells to secrete histamine
•Histamine secretion stimulates parietal cells to secrete HCl
Gastric phase
.Arrival of food in stomach stimulates the gastric phase
•Gastric secretion stimulated by
–Distension
–Chemical nature of chyme (amino acids and peptides)
•Activation of vagus by presence of certain amino acids
–Stimulates G cells to secrete gastrin
–Stimulates chief cells to secrete pepsinogen
–Stimulates ECL cells to secrete histamine
•Histamine stimulates secretion of HCl
Positive and negative feedback effect on gastric phase
.•Positive feedback effect: Decrease in pH converts more pepsinogen to pepsin, which then cleaves more peptides
-Secretion of HCl is also regulated by a negative feedback effect:
–HCl secretion decreases if pH
Intestinal phase
.Inhibition of gastric activity when chyme enters the small intestine
•Arrival of chyme increases osmolality and distension in duodenum
•In the presence of fat, enterogasterone inhibits gastric motility and secretion
-enterogasrerone a hormone, obtained from intestinal mucosa.
-Secretion of enterogastrone is stimulated by exposure of duodenal mucosa to dietary lipids
-other Hormone secretion to inhibit gastric activity:
–Somatostatin
–Cholecystokinin (CCK)
–Glucagon-Like Peptide-1 (GLP-1)
Secretion of bile
.When chyme from an ingested meal enters the small intestine, acid and partially digested fats and proteins stimulate secretion of the small intestine hormones
- cholecystokinin and secretin. - enteric (intestine) hormones have important effects on pancreatic exocrine secretion. - also important for secretion and flow of bile.
Cholecystokinin
.•The name of this hormone describes its effect on the biliary system - cholecysto = gallbladder and kinin = movement.
•The most potent stimulus for release of cholecystokinin is the presence of fat in the duodenum.
•Once released, it stimulates contractions of the gallbladder and common bile duct, resulting in delivery of bile into the gut.
•CCK also stimulates opening of the sphincter of Oddi
Secretin
- secreted in response to acid in the duodenum.
- Its effect on the biliary system is very similar to what was seen in the pancreas
- it simulates biliary duct cells to secrete bicarbonate and water, which expands the volume of bile and increases its flow out into the intestine
Biliary tree/ system path
Bile canaliculi → Canals of Hering → interlobular bile ducts → intrahepatic bile ducts → left and right hepatic ducts merge to form → common hepatic duct exits liver and joins → cystic duct (from gall bladder) forming → common bile duct → joins with pancreatic duct → forming ampulla of Vater → enters duodenum
Digestion and absorption of carbohydrates
.Salivary amylase begins starch digestion in the mouth
•Pancreatic amylase:
–Digests starch to oligosaccharides
–Oligosaccharides hydrolyzed by brush border enzymes
–Transported by secondary active transport with Na+
Digestion and absorption of protein (beggining)
.Digestion begins in the stomach by activation of pepsin to form polypeptides.
•In the duodenum and jejunum
–Endopeptidases cleave peptide bonds in the interior of the polypeptide:
•Trypsin
•Chymotrypsin
•Elastase
–Exopeptidases cleave peptide bonds from the ends of the polypeptide:
•Carboxypeptidase
•Aminopeptidase
