Digestive System Part 3

Digestive System Part 3

Digestive System Part 3 Honors Anatomy & Physiology Chapter 23 Regulation of Gastric Secretion Neural and hormonal mechanisms Gastric mucosa up to 3 L gastric juice/day Vagus nerve stimulation secretion Sympathetic stimulation secretion Hormonal control largely gastrin Enzyme and HCl secretion Most small intestine secretions - gastrin

antagonists 2013 Pearson Education, Inc. Regulation of Gastric Secretion Three phases of gastric secretion Cephalic (reflex) phase conditioned reflex triggered by aroma, taste, sight, thought Gastric phase lasts 34 hours; gastric juice released Stimulated by distension, peptides, low acidity, gastrin (major stimulus) Enteroendocrine G cells stimulated by caffeine, peptides, rising pH gastrin

2013 Pearson Education, Inc. Stimuli of Gastric Phase Gastrin enzyme and HCl release Low pH inhibits gastrin secretion (as between meals) Buffering action of ingested proteins rising pH gastrin secretion Three chemicals - ACh, histamine, and gastrin - stimulate parietal cells through second-messenger systems

All three are necessary for maximum HCl secretion 2013 Pearson Education, Inc. HCl Formation Parietal cells pump H+ (from carbonic acid breakdown) into stomach lumen K+ goes into cells to balance charge HCO3 from carbonic acid breakdown blood (via Cl and HCO3 antiporter)

blood leaving stomach more alkaline Alkaline tide Cl (from blood plasma via antiporter) follows H+ HCl 2013 Pearson Education, Inc. Figure 23.18 Mechanism of HCl secretion by parietal cells. Gastric gland Blood capillary Chief cell CO2 CO2 + H2O

H2CO3 H+ K+ Stomach lumen Carbonic anhydrase HCO3 Alkaline tide Parietal cell H+-K+ ATPase H+ K+

HCI HCO3 Cl HCO3- Cl Interstitial antiporter fluid Cl 2013 Pearson Education, Inc. Cl Regulation of Gastric Secretion Intestinal phase

Stimulatory component Partially digested food enters small intestine brief intestinal gastrin release Inhibitory effects (enterogastric reflex and enterogastrones) Chyme with H+, fats, peptides, irritating substances inhibition 2013 Pearson Education, Inc. Enterogastric Reflex Three reflexes act to Inhibit vagal nuclei in medulla Inhibit local reflexes Activate sympathetic fibers tightening of pyloric sphincter no more food entry to small

intestine Decreased gastric activity protects small intestine from excessive acidity 2013 Pearson Education, Inc. Intestinal Phase Enterogastrones released Secretin, cholecystokinin (CCK), vasoactive intestinal peptide (VIP) All inhibit gastric secretion If small intestine pushed to accept more chyme dumping syndrome Nausea and vomiting Common in gastric reduction for weight loss

2013 Pearson Education, Inc. Figure 23.17 Neural and hormonal mechanisms that regulate release of gastric juice. Inhibitory events Stimulatory events Cephalic phase Gastric phase 1 Sight and thought of food Cerebral cortex Conditioned reflex 2 Stimulation of taste and smell

receptors Hypothalamus and medulla oblongata 1 Stomach distension activates stretch receptors Vagovagal reflexes Intestinal phase Stimulate

Vagus nerve Local reflexes 2 Food chemicals G cells (especially peptides and caffeine) and rising pH activate chemoreceptors 1 Presence of partially digested foods in duodenum or distension of the duodenum when stomach begins to empty

Medulla Vagus nerve Gastrin release to blood Intestinal (enteric) gastrin release to blood Cerebral cortex 1 Loss of appetite,

depression Gastrin secretion declines G cells Overrides parasympathetic controls Sympathetic nervous system activation 1 Excessive acidity (pH < 2)

in stomach 2 Emotional stress Lack of stimulatory impulses to parasympathetic center Stomach secretory activity Enterogastric reflex Local reflexes Vagal nuclei

in medulla Brief effect Pyloric sphincter Release of enterogastrones (secretin, cholecystokinin, vasoactive intestinal peptide) Inhibit 2013 Pearson Education, Inc. 1 Distension of duodenum; presence of

fatty, acidic, or hypertonic chyme; and/or irritants in the duodenum 2 Distension; presence of fatty, acidic, partially digested food in the duodenum Gastric Contractile Activity Peristaltic waves move toward pylorus at rate of 3

per minute Basic electrical rhythm (BER) set by enteric pacemaker cells (formerly interstitial cells of Cajal) Pacemaker cells linked by gap junctions entire muscularis contracts Distension and gastrin increase force of contraction 2013 Pearson Education, Inc. Gastric Contractile Activity Most vigorous near pylorus Chyme is either Delivered in ~3 ml spurts to duodenum, or Forced backward into stomach

2013 Pearson Education, Inc. Figure 23.19 Deglutition (swallowing). Pyloric valve closed 1 Propulsion: Peristaltic waves move from the fundus toward the pylorus Slide 1 Pyloric valve slightly opened

2 Grinding: The most vigorous peristalsis and mixing action occur close to the pylorus. The pyloric end of the stomach acts as a pump that delivers small amounts of chyme into the duodenum. Pyloric valve closed 3 Retropulsion: The peristaltic wave closes the pyloric valve, forcing most of the contents of the pylorus backward into the stomach.

2013 Pearson Education, Inc. Regulation of Gastric Emptying As chyme enters duodenum Receptors respond to stretch and chemical signals Enterogastric reflex and enterogastrones inhibit gastric secretion and duodenal filling Carbohydrate-rich chyme moves quickly through duodenum Fatty chyme remains in duodenum 6 hours or more

2013 Pearson Education, Inc. Figure 23.20 Neural and hormonal factors that inhibit gastric emptying. Presence of fatty, hypertonic, acidic chyme in duodenum Duodenal enteroendocrine cells Chemoreceptors and stretch receptors Secrete Enterogastrones (secretin, cholecystokinin, vasoactive intestinal peptide) Target

Via short reflexes Enteric neurons Duodenal stimuli decline Via long reflexes CNS centers sympathetic activity; parasympathetic activity Contractile force and

rate of stomach emptying decline Initial stimulus Stimulate Physiological response Inhibit Result 2013 Pearson Education, Inc. Homeostatic Imbalance Vomiting (emesis)

Extreme stretching Intestinal irritants, e.g., bacterial toxins, excessive alcohol, spicy food, certain drugs Chemicals/sensory impulses emetic center of medulla Excessive vomiting dehydration, electrolyte and acid-base imbalances (alkalosis) caused by 2013 Pearson Education, Inc. Intestinal Juice

1-2 L secreted daily in response to distension or irritation of mucosa Slightly alkaline; isotonic with blood plasma Largely water; enzyme-poor (enzymes of small intestine only in brush border); contains mucus Facilitates transport and absorption of nutrients 2013 Pearson Education, Inc. Large Intestine colon larger in diameter but shorter than small

intestine (which it frames) functions: 1. dry out indigestible food residue by absorbing water 2. elimination Parts of the Large Intestine Cecum Appendix Colon

Ascending Transvere Descending Rectum Anal Canal Cecum sack-like 1st part of large intetine site of ileoceal valve: empties chyme into large intestine from small intestine Appendix

Vermiforme appendix vestigial organ easily twists small amounts food & bacteria get trapped appendicitis Signs of Appendicitis Appendicitis Appendectomy Colon Colonoscopy Colon

Descending colon enters pelvis in LLQ where it becomes S-shaped = sigmoid colon Rectum Wall of the Colon muscularis externas longitudinal layer in 3 bands called teniae coli ribbons of the colon are normally partially contracted colon wall puckers forming haustra Histology of Colon

no villi (absorption mostly in small intestine) many Goblet cells mucus (aids movement of feces) Rectum storage area for feces Anal Canal 2 sphincters:

1. external sphincter skeletal muscle (voluntary) internal sphincter 2. smooth muscle (involuntary)

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