Functions of stomach Physiology Unit

Secretory and Digestive Functions of the Stomach The objective of the lecture is to discuss the functions of the stomach; parietal cells and other cells in gastric glands; cellular mechanism of gastric acid secretion by parietal cells; physiologic mechanisms protecting gastric mucosa from injury by acid and pepsin and the regulation of gastric acid secretion by neural and humoral mechanisms.

Learning Outcomes At the end of the lecture, students should be able to: 1.Briefly describe the secretory functions of the stomach. 2.List the types of cells present in gastric glands and outline their principal functions. 3.Describe the cellular mechanism of secretion of gastric acid using a labeled diagram. 4.Describe regulation of gastric secretion under the following headings – cephalic phase, gastric phase, and ininal phase. 5.State the intestinal factors inhibiting gastric secretion and its functional significance. 6.Describe the consequences of imbalance between the rate of secretion of gastric juice and the degree of protection of mucosal barrier and its underlying mechanism 7.State the functional consequences of complete gastrectomy

Stomach  Main Functions  Storage  Preparing the chyme for digestion in the small intestine  Absorption of water and lipid-soluble substances (alcohol and drugs)

Stomach Types of Gland (located in gastric mucosa):  Cardiac Glands  Pyloric glands (many G cells)  Oxyntic glands (most abundant, found in fundus and corpus)

Oxyntic gland :

Types of Cell  Parietal cells  most distinctive cells in stomach (HCl & intrinsic factor)  Chief cells  pepsinogen  Mucus neck cells: - HCO Mucus

Types of Cells  G Cells: Gastrin (hormone) ---> HCl secretion  D Cells: Somatostatin (antrum)  Enterochromaffin-like cell: Histamine

Illustration of Gastric secretion by parietal cells

Gastric juice HCL Pepsinogen Electrolytes Intrinsic factor Mucus (mucus gel layer)

Gastric motility Functions 1. allows the stomach to serve as reservoir 2. breaks food to small particles and mix it with gastric juice 3. empties gastric contents at a controlled rate

Gastric motility Reservoir part fundus + 1/3 corpus (tonic contraction) Antral pump 2/3 corpus + antrum & pylorus (phasic contraction)

Mixing & emptying of gastric contents Gastric contents may remain unmixed (1h) Fat takes longer time for empty than other Liquids are emptied easier and first Major mixing activities in the antrum Retropulsion

Constriction of pyloric sphincter Hormones promote constriction 1. CCK 2. Secretin 3. Gastrin 4. GIP Sympathetic innervation

Gastric reservoir Functions:  To maintain a continuous compression  To accommodate the received food with out significant gastric wall distention or pressure

Relaxation in gastric reservoir Receptive relaxation - triggered by swallowing reflex Adaptive relaxation - triggered by stretch receptors (vago-vagal reflex) - lost in vagotomy  threshold of fullness and pain Feedback relaxation - triggered by chyme in small intestine

Gastric juice HCL Secretion Secreted by parietal cells  Fundus  Body

HCL Secretion (cont) Mechanism of HCl production:  H/K ATPase  Inhibited by: omeprazole  H/K pump depends on [K] out  [HCl] drives water into gastric content to maintain iso-osmolality  During gastric acid secretion: amount of HCO 3 - in blood = amount of HCl being secreted  Alkaline tide

Neural & Hormonal Control of Gastric Secretion  Vagus nerve (neural effector)  Gastrin (hormonal effector)  Enterochromaffin-like cells  Histamine ---  H 2 receptor (parietal cells)  acid secretion  Cimetidine (H 2 receptor blocker)  peptic ulcer and gastroesophageal reflux

Phases of Acid Secretion  Cephalic phase(30%):  Smelling, Chewing and swallowing  Stimulates parietal G-Cells  GRP  Gastric phase (60%):  gastric distention  proteins  Intestinal phase (10%):  digested proteins

PEPTIC ULCER Definitions: Stomach or duodenal mucosal lesions Occurrence: due to imbalance between aggressive factors and mucosal protective mechanisms.

PEPTIC ULCER Aggressive factors Pepsin secretion - acid secretion Protective factors Prostaglandins (E2 & I2 ) Mucus/bicarbonate secretion Mucosal blood flow Rapid turnover of gastric mucosa

Risk factors H Pylori infections Alcohol Smoking Diet Drugs (corticosteroids). Stress Genetic factors Diseases (Zollinger Ellison Syndrome). (Zollinger-Ellison syndrome is a condition in which there is increased production of the hormone gastrin. Usually, a small tumor (gastrinoma) in the pancreas or small intestine produces the high levels of gastrin in the blood.)

SYMPTOMS: Nausea – Vomiting – Anorexia Upper abdominal pain. Weight loss. Heart burn.

COMPLICATIONS: Gastrointestinal hemorrhage Chronic iron deficiency anemia Pyloric stenosis Perforation

Gastric secretions 1.Pepsinogens (Chief cells). 2.HCl and intrinsic factor (Parietal cells). 3.Gastrin (G-cells). 4.Mucus, bicarbonate (mucus-secreting cells).

Regulation of Gastric secretions 1.Histamine (local hormone) 2.Acetylcholine (neurotransmitter). 3.Gastrin (hormone).

AIMS OF ULCER TREATMENT Promotion of ulcer healing. Symptomatic relief of pain. Prevention of recurrence (relapse). Prevention of complications

DRUG TREATMENT OF PEPTIC ULCER I. Gastric hyposecretory drugs. H2 receptor blockers Muscarinic receptor blockers Proton pump inhibitors II. Eradication of Helicobacter pylori ( H. pylori ) infections To prevent relapse

All the best for your superb preparation for GIT Quiz tomorrow – IRS.