Anatomy and Physiology II Ch. 24 - Digestive System (Part 2)

Pancreas

- A dual gland with endocrine (the islets) and exocrine (the acinar units) portions - Exocrine portion called acinar units or acini cells secrete bicarbonate and enzymes - Bicarbonate helps to raise the pH if it is too acidic - Enzymes provide chemical digestion - Secretions enter the pancreatic duct which joins the common bile duct as it enters the duodenum

Segmentation

1. A secretion introduced into the digestive tract or into food within the tract begins in one location. 2. Segments of the digestive tract alternate between contraction and relaxation. 3. Material (brown) in the intestine is spread out in both directions from the site of introduction. 4. The secretion or food is spread out in the digestive tract and becomes more diffuse (lighter color) through time.

Liver Functions

1. Bile production 2. Storage of nutrients (glycogen). 3. Processing of nutrients 4. Detoxification (recycling of RBCs and excretion of toxins like bilirubin and ammonia) 5. Synthesis of new molecules (plasma proteins such as albumin, and clotting factors).

Intestinal Phase "Slow Down"

1. Chyme in the duodenum with a pH less than 2 or containing fat digestion products (lipids) inhibits gastric secretions by three mechanisms (2-4) 2. Chemoreceptors in the duodenum are stimulated by H+ (low pH) or lipids. Action potentials generated by the chemoreceptors are carried by the vagus nerves to the medulla oblongata (green arrow), where they inhibit parasympathetic action potentials (pink arrow), thereby decreasing gastric secretions. 3. Local reflexes activated by H+ or lipids also inhibit gastric secretions (orange arrows). 4. Secretin and cholecystokinin produced by the duodenum (brown arrows) decrease gastric secretions in the stomach.

Gastric Phase "Go for it"

1. Distention of the stomach stimulates mechanoreceptors (stretch receptors) and activates a parasympathetic reflex. Action potentials generated by the mechanoreceptors are carried by the vagus nerves to the medulla oblongata. 2. The medulla oblongata increases action potentials in the vagus nerves that stimulate secretions by parietal and chief cells and stimulate gastrin and histamine secretion by endocrine cells. 3. Distention of the stomach also activates local reflexes that increase stomach secretions 4. Gastrin is carried through the circulation back to the stomach, where, along with histamine, it stimulates secretion.

Cephalic Phase "Get Started"

1. The taste, smell, or thought of food or tactile sensations of food in the mouth stimulate the medulla oblongata (green arrows) 2. Vagus nerves carry parasympathetic action potentials to the stomach (Pink arrow), where enteric plexus neurons are activated. 3. Postganglionic neurons stimulate secretion by parietal and chief cells and stimulate gastrin and histamine secretion by endocrine cells. 4. Gastrin is carried through the circulation back to the stomach (purple arrow), where, along with histamine, it stimulates secretion.

5 main pancreatic enzymes

1. Trypsin (active protease) - made as trypsinogen (inactive zymogen): Trypsin activates other zymogens & breaks down large proteins into smaller peptides 2. Carboxypeptidase (protease) - Splits off C-terminal amino acids from proteins 3. Pancreatic Lipase - splits triacylglycerols into free fatty acids (FFA's) & monoglycerides 4. Pancreatic Amylase - splits starch into glucose and maltose 5. Pancreatic Nucleases - splits polynucleotides (DNA & RNA) into mono nucleotides

Control of Bile Secretion and Release

1. Vagus nerve stimulates Gallbladder to contract- Bile releases into duodenum 2. Secretin- hormone secreted by duodenum is carried by blood to the liver and causes release of bile from liver into duodenum 3. CCK another hormone secreted by duodenum is carried by blood to the Gallbladder and causes contraction & release of bile

Large Intestine Anatomy (Cecum)

3 Divisons: Cecum- Colon- Rectum Cecum: - First portion joins at the ileocecal junction - Ileocecal valve is the opening from ileum to cecum - Vermiform appendix (or appendix) - may contribute to flora

Large Intestine Anatomy (Colon)

3 Divisons: Cecum- Colon- Rectum Colon: - Has 4 divisions (ascending, transverse, descending and sigmoid) - Ascending - from the cecum superiorly to the right colic flexure (or hepatic flexure) - Transverse - from the right colic flexure horizontally to the left colic flexure (or splenic flexure) - Descending - from the left colic flexure inferiorly to the pelvis - Sigmoid - "S" shape that extends from the pelvis into the pelvic cavity to the rectum

Large Intestine Anatomy (Rectum)

3 Divisons: Cecum- Colon- Rectum Rectum: - Goes from the colon and ends with the anal canal - Anus - opening of the digestive tract - Internal anal sphincter is smooth muscle - External anal sphincter is skeletal muscle

Small Intestine Anatomy

3 divisions: Duodenum: - First portion, about 12 inches (duodenum means 12) - The common bile duct from the liver and the pancreatic duct from the pancreas meets at the duodenum at the duodenal papilla - Secretes hormones (secretin and cholecystokinin) Jejunum: - Second/middle division Ileum - Last portion that meets the ileum of the large intestine at the ileocecal junction

Pancreas Anatomy

Head: - Near duodenum Body: - Main portion Tail: - Left end, near spleen Pancreatic duct: - Carries secretions

Small Intestine

Approximately 6.25 m long (about 20.5 feet) and about 1 inch in diameter Between the stomach and the large intestine Main site of absorption: - About 1.5 L of fluid is secreted into the small intestine lumen and reabsorbed - Ion transport helps drive osmosis to reabsorb the fluid with nutrients Also mechanical (by segmentation and bile from the liver) and chemical (by enzymes) digestion occurs

Pancreatic Secretions (Endocrine)

Endocrine: - Alpha cells secrete glucagon which stimulates glycogenolysis and gluconeogenesis and glucose release from cells to increase blood glucose - Beta cells secrete insulin which stimulate glucose entry into cells and glycogenesis or glycolysis to decrease blood glucose

Mass Movement

Every 8-12 hours the colon undergoes a mass movement: - A mass movement is similar to peristalsis but moves it much further than normal peristalsis When feces causes the wall of the rectum to distend a defecation reflex may be triggered: - The internal sphincter automatically relaxes - The external sphincter is under voluntary control and will relax and expel feces when consciously signaled

Pancreatic Secretions (Exocrine)

Exocrine: Bicarbonate: If the pH is too low the duodenum secretes secretin to stimulate bicarbonate release Enzymes: If fatty acids or amino acids are detected the duodenum secretes CCK to stimulate bile release from liver and gallbladder and enzyme release from the pancreas

Gallbladder

Has 3 layers: - Outer covering of visceral peritoneum - Middle smooth muscle - Inner mucous layer with rugae Stores and concentrates bile (removes water) Ejects bile when CCK is present Gallstones are often precipitate cholesterol - Can be very serious if the block cystic duct

Liver: Bile production

Hepatocytes produce bile - Bile flows into branches of hepatic duct into the left or right hepatic duct - Bile helps emulsify fats ( a form of mechanical digestion) - Bile may contain the pigment bilirubin which results from Hgb breakdown - 6 major ingredients: bile salts, lecithin, bicarbonate, cholesterol, bile pigments and trace metals

Bile flow through liver

If duodenum secretes CCK: -Hepatocytes -Bile -Bile canaliculi -L/R Hepatic duct -Hepatopancreatic ampulla -Duodenal Papilla -Duodenum If duodenum does not secrete CCK: -Hepatocytes -Bile -Bile canaliculi -L/R Hepatic duct -Common Hepatic duct -Cystic duct -Gall bladder (stored)

Cells in Small Intestine

Mucosa is simple columnar with four cell types: 1. Absorptive - Have microvilli, enzymes such as peptidase, sucrase, maltase, lactase, lipase, and enterokinase 2. Goblet - Located in Crypts of Lieberkuhn, Produces mucus 3. Granular - May help protect from microorganisms 4. Endocrine - Produces secretin and cholecystokinin

Liver

One of the largest glands in the body Has two lobes, left and right, separated by the falciform ligament: - Left is smaller, right has three "sub lobes") Each lobe has lobules: - The hepatic lobules are cylinders made of hepatocytes (hepatic cells) - Hepatic cords - sheets of hepatic cells in the lobules

Pancreatic Secretions & Control

Pancreatic enzymes are also secreted as zymogens to prevent auto-digestion (digestion of self)

Small Intestine cont.

Peristalsis for movement Mucosa contains plica circulares (circular folds) which are permanent folds of the mucous lining Muscularis has smooth muscle in 2 layers (inner is circular and outer is longitudinal) Brunner's glands (duodenal glands): - Small mucous glands found in the mucous lining Solitary lymph nodes and Peyer's patches are also present in the mucous lining

Increasing surface area

Plica circulares which are permanent folds of the mucous lining Villi are fingerlike projections of the mucosa - Villi contain blood capillaries and lymphatic capillaries called lacteals Most of the epithelial cells that make the villus contain microscopic hair-like structures called microvilli - Brush border - toward the top of each villus, produce enzymes - Villi and microvilli increase the surface area Crypts of Lieberkuhn - Depressions between villi

Secretin

Produced by the Duodenum Stimulation: Acidity of chyme Secretory Effect: Decreases gastric secretion; stimulates pancreatic and bile secretions high in bicarbonate ions Motility effect: Decreases gastric motility

Cholecystokinin (CCK)

Produced by the Duodenum Stimulation: Fatty acids and peptides Secretory effect: Slightly decreases gastric secretion; stimulates pancreatic secretions high in digestive enzymes; causes contraction of the gallbladder and relaxation of the hepatopancreatic sphincter Motility effect: Strongly decreases gastric motility

Gastrin

Produced by the stomach Stimulation: Distension; partially digested proteins, autonomic stimulation. ingestion of alcohol or caffeine Secretory effect: Increases gastric secretion Motility effect: Causes minor increase in gastric motility

Large Intestine cont.

Segmentation occurs at a slower rate than small intestine Motility occurs by peristalsis Muscularis has smooth muscle and 2 layers (inner is circular and outer is longitudinal) - Haustra - pouches of circular muscle - Taenia coli - parallel strips of large intestine

Large Intestine

Stores and concentrates the waste (chyme turns into feces) and eliminates the feces by defecation Secretes mucus and fluid with bicarbonate and potassium but no enzymes Microorganisms create a resident flora

Pancreatic Enzymes

Trypsinogen - converted to trypsin, activates other zymogens and break proteins (hydrolyze) into peptides Lipase - splits triacylglycerols into free fatty acids (FFA's) and monoglycerides Amylase - splits starch into glucose and maltose Carboxypeptidase - splits off C-terminal amino acids from proteins Nucleases - splits nucleotides into mononucleotides

Gastrointestinal hormones

gastrin, secretin, cholecystokinin

CCK (cholecystokinin)

increases release of bile when there is an increase of lipids or proteins