University:
Glendale Community College
Course:
BIO160 | Introduction to Human Anatomy and Physiology
Academic year:

2019
Views:

134

Pages:

20
Author:

Alfarolivy

Esophagus • Straight muscular tube from pharynx to stomach – nonkeratinized stratified squamous epithelium of mucosa – esophageal glands in submucosa release mucous during swallowing – skeletal muscle in upper 1/3 (voluntary control) – smooth muscle in lower 1/3 (involuntary control) – mix of muscle types in middle 1/3 • Esophagus passes through a hiatus of the diaphragm muscle fibers which forms part of the lower esophageal sphincter Constriction of diaphragm and the muscularis externa layer of the esophagus helps close cardiac orifice of stomach – failure can result in gasroesophageal reflux of stomach contents into the esophagus. Weak esophageal sphincter can lead to a hiatal hernia. X-ray of Esophagus during Swallowing Stomach • Mechanically breaks up food particles, liquifies the food and begins chemical digestion of protein and fat resulting in a soupy mixture called chyme. • Stomach absorbs some water, alcohol, and some drugs including aspirin. • Stomach does not absorb a significant amount of nutrients. Anatomy of the Stomach • Expandable muscular sac – internal volume from 50 ml to 4,000 ml • Regions of the stomach – – – – cardiac region just inside cardiac orifice fundus is domed portion superior to esophageal opening body is main portion of organ pyloric region is narrow inferior end that opens into the duodenum • region consists of the antrum, pyloric canal and pyloric sphincter • pyloric sphincter is a thick ring of smooth muscle – Rugae are wrinkles and folds of stomach lining that increase surface area for secretions and allow for expansion – Gastroesophageal Junction • abrupt transition from stratified squamous epithelium of the esophagus to simple columnar epithelium of the stomach Gross Anatomy of the Stomach Stomach Innervation and Circulation • Innervation by parasympathetic fibers and sympathetic fibers. – parasympathetic action (through acetylcholine) increases digestive processes and blood flow to digestive organs – sympathetic action (through epinephrine and norepinephrine) decreases digestive processes and blood flow to digestive organs (stomach ache) • Venous blood from the stomach flows into the hepatic portal system of the liver before returning to the heart and the general circulation. Features of the Stomach Wall • Mucosa – epithelium of simple columnar glands (gastric pits and glands) – lamina propria of loose connective tissue containing vessels and nerves – muscularis mucosae – thin layer of smooth muscle undulates epithelium • Submucosa is loose CT with vessels and nerves • Muscularis Externa has 3 layers of smooth muscle for mixing chyme – outer longitudinal, middle circular, inner oblique • Serosa is a thin smooth connective tissue capsule. Features of the Stomach Wall Opening of a Gastric Pit Surrounded by Mucous Cells Gastric Pit and Gastric Gland Cells of the Gastric Pit and Gastric Glands • Mucous Cells secrete mucus that protects the mucosa. • Parietal Cells secrete HCl and Intrinsic Factor • Chief Cells secrete inactive pepsinogen which must be converted into the active enzyme pepsin by HCl or more pepsin • Enteroendocrine Cells (G Cells) secrete the hormones gastrin and histamine into blood and paracrine messengers that regulate digestion. • Regenerative Cells in base of pit and neck of gland divide rapidly to produce new cells that migrate upwards towards surface and replace old cells. These are not shown in this illustration. Functions of Gastric Hydrochloric Acid (HCl) • Stomach pH can be as low as 0.8 • HCl cleaves inactive pepsinogen into the active enzyme pepsin. • HCl converts ingested ferric ions (Fe+3) to nutritious ferrous ions (Fe+2) that can be absorbed and used for hemoglobin synthesis. • HCl breaks up connective tissues in meat and the cellulose cell walls of plants. • HCl destroys many ingested bacteria and other pathogenic microbes. (one E. coli bacterium ingested in the morning can divide into 2 billion cells by 2:00 in the afternoon) Gastric HCl Secretion • Parietal cells contain the enzyme carbonic anhydrase (CAH) that speeds up production of carbonic acid: CAH CO2 + H2O  H2CO3  HCO3- + H+ • H+/K+ ATPase (proton pump) exchanges H+ (out) and K+ (in). • HCO3- exchanged for Cl- (chloride shift) • Cl- pumped into stomach combines with H+ to form HCl •  HCO3- in blood causes alkaline tide ( blood pH) • Parietal cells are inhibited by proton pump inhibitors (Prilosec and Nexium, Prevacid) and some antihistamines (H2 receptor blockers like Pepcid, Tagamet, Zantac) CAH Gastric Enzymes and Intrinsic Factor • Pepsin (from chief cells) – pepsin is an enzyme for protein digestion – pepsin is secreted as pepsinogen (an inactive enzyme) – HCl cleaves pepsinogen into to pepsin (active form) • pepsin then activates more pepsinogen • Gastric Lipase (from chief cells) – lipase digests fats (important in milk digestion in infants) • Chymosin (from chief cells) – chymosin coagulates proteins (curdles milk) • Intrinsic Factor (from parietal cells) binds to Vitamin B12 – essential for absorption of B12 by small intestine – necessary for RBC and Hb production – pernicious anemia results when intrinsic factor is lacking Production and Action of Pepsin in the Stomach 3 Phases of Gastric Function 1. Cephalic Phase – sight, smell, taste or thoughts of food trigger vagus nerve to stimulate gastric secretions and motility 2. Gastric Phase – activated by presence of food in stomach that stretches the stomach and starts to dilute the stomach acid (chyme becomes more alkaline) – gastric secretion is stimulated by activation of parietal cells and chief cells that bind to histamine and gastrin from gastric enteroendocrine cells – histamine and gastrin have a paracrine effect on chief cells and parietal cells 3. Intestinal Phase – duodenum regulates gastric activity through hormones and nervous reflexes – at first, when the duodenum is stimulated by chyme, it stimulates the stomach by releasing gastrin into the blood stream and the gastrin stimulates chief cells and parietal cells – later at the end of a meal, the last chyme entering the duodenum stimulates duodenal cells to release 3 hormones that inhibit the stomach and suppress gastric secretions: • secretin • gastric inhibitory peptide (GIP) • cholecystokinin (CCK) Neural and Hormonal Control of the 3 Phases of Gastric Function Food moves from the Stomach into the Small Intestine • Most chemical digestion and nutrient absorption occurs in the small intestine