BIOL 1800 Lecture 10 The Digestive System - Stomach PDF

REMINDER NO lectures or labs during Review week (week 7) MCQ2!! MCQ2 will take place in week 8 at the beginning of the lab class – either Thurs 7th/ Friday 8th November MCQ2 will include Lectures from weeks 4, 5, 6 AND Labs 3, 4 & 5 – Lectures: Skin and Thermoregulation, The Nervous Syst...

REMINDER NO lectures or labs during Review week (week 7) MCQ2!! MCQ2 will take place in week 8 at the beginning of the lab class – either Thurs 7th/ Friday 8th November MCQ2 will include Lectures from weeks 4, 5, 6 AND Labs 3, 4 & 5 – Lectures: Skin and Thermoregulation, The Nervous System, The Digestive System – Intro and Oesophagus, The Digestive System – The Stomach – Labs: Skin Histology, Nervous Tissue & Reflexes, Gastrointestinal Tract & Digestive Organs Lecture 10: The Digestive System – The Stomach BIOL 1800 Dr Marie Conway Learning Objectives At the end of this lecture you should be able to do the following: Describe the anatomy and location of the stomach Understand the main functions of the stomach Understand the secretions of the stomach and the cells that produce them Describe how gastric activity is regulated The Digestive System Functions of the stomach Stores ingested food Mechanical breakdown Secretes enzymes and acid to break chemical bonds of ingested food Secretes intrinsic factor (IF) - Baz absorbtion, → viscous acidic = chyme Location of the stomach Located in the upper left abdomen, inferior to the diaphragm The stomach is an expanded section of the digestive tract between the Oesophagus and Small intestine Anatomy of the stomach - extended tube The most distal and narrow section of the stomach is termed the Pylorus As food is liquefied in the stomach it passes through the pyloric canal into the small intestine Anatomy of the stomach The wall of the stomach is structurally similar to other parts of the digestive tube HOWEVER, the stomach has an extra, oblique layer of smooth muscle inside the circular layer- strengthen the wall + helps mixing the chyme This aids in the performance of complex grinding motions Anatomy of the stomach Anatomy of the stomach - smallest, abandent mocus glands, cardiac sphincter Cardia – superior, medial portion Fundus – portion superior to stomach- oesophageal junction Body – area between the fundus and the curve of the J - largest, mixing, gastric glands Pylorus – antrum and pyloric canal adjacent to the duodenum pyloric sphincter regulates emptying to dead. - glands - pyloric canal + autumn Anatomy of the stomach Fundus Cardia Body Pylorus Anatomy of the stomach Pyloric Sphincter – Guards exit from stomach Rugae - folds of mucosa – Ridges and folds in relaxed stomach – In the empty state, the stomach is contracted and its mucosa and submucosa are thrown up into distinct folds called Rugae; – when distended with food, the rugae are "ironed out" and flat - can stratch to 507 Image of stomach when full, Rugae are not visible Stomach histology Epithelial cells in the mucosa of the stomach are: o Simple Columnar cells A simple columnar epithelium is a single layer of columnar cells attached to the basement membrane, with oval-shaped nuclei located in the basal region. Epithelium is towards the top of the mucosal layer of the stomach. Mucosa of the stomach contains many other types of secretory cells. Stomach histology Secretory cells are found in mucosal area mocus from - alcalide layer-protects agains the acid the columns" Histology of the stomach Stomach layers 1. Mucosa Simple columnar epithelium 2. Submucosa 3. Muscularis externa – Oblique fibres – Circular fibres – Longitudinal fibres 4. Serosa  Pyloric glands Histology of the stomach – Mucous secretion containing several hormones - function of GIT Gastric glands fundus, body, gastric digestion – Parietal cells Intrinsic factor and HCl – Chief cells Pepsinogen Enteroendocrine cells – G cells secrete gastrin P/D1 cells (fundus) – Produce ghrelin; ghrelin increases before meals to initiate hunger, and decreases shortly after eating to curb hunger Histology of the stomach – gastric pits/glands Parietal cells – Intrinsic factor (IF) and HCl – IF – secreted in stomach; glycoprotein that helps absorb vitamin B12 across intestinal lining – HCl – denatures proteins, breaks down cell walls, connective tissues, protection, activation of pepsin - intrinsic factor → hydrochloric a. - bra-kid. protein, cell walls Chief cells - acidic e, necessary for them – Secrete pepsinogen – inactive proenzyme; acid in gastric lumen converts pepsinogen to pepsin (protein digesting enzyme) G cells (enteroendocrine cells) – Produce gastrin – stimulates secretion by parietal and chief cells, and contractions of the gastric wall that mix and stir the gastric contents  after eating a real → hydrochloride a. Formation of gastric juice (HCL) – aka Alkaline Tide Gastric juice pH 1.5 1. H+ produced in parietal cell by reduction of CO2 + H2O 2. H+ transported into gastric gland 3. HCO-3 diffuses into interstitial fluid via counter transport with Cl- 4. HCO-3 diffuses into circ, raising blood pH (alkaline tide) 5. Cl- diffuses into gastric gland via Cl- channels Martini Fig 24.14 Stomach secretions Mucous: The most abundant epithelial cells are mucous cells, which cover the entire lumenal surface – These cells secrete a bicarbonate-rich mucous that coats and lubricates the gastric surface – protecting the epithelium from acid and other chemical insults Acid: Hydrochloric acid is secreted from parietal cells into the lumen where it establishes an extremely acidic environment – Activates pepsinogen – inactivation of ingested microorganisms such as bacteria Stomach secretions Proteases: Pepsinogen, is secreted into gastric juice from both mucous cells and chief cells – Once secreted, pepsinogen is activated by stomach acid into the active protease pepsin, which is largely responsible for the stomach's ability to initiate digestion of proteins – In the young, chief cells also secrete chymosin (rennin) a protease that coagulates milk protein allowing it to be retained more than briefly in the stomach Hormones: The principle hormone secreted from the gastric epithelium is Gastrin – a peptide that is important in control of acid secretion and gastric motility – Stimulates secretion by parietal and chief cells, and contractions of the gastric wall that mix the gastric contents Regulation of gastric activity Cephalic phase Gastric phase Intestinal phase Cephalic phase prepares Gastric phase begins with the Intestinal phase controls the rate stomach to receive ingested arrival of food in the stomach of gastric emptying material Cephalic phase Cephalic phase Seeing, smelling and anticipating food perceived in the brain and the brain informs the stomach that it should prepare for receipt of a meal – directed by the central nervous system (CNS) Parasympathetic nervous system sends a stimulus to the vagus nerve: – Resulting in release of acetylcholine (Ach) in the vicinity of G cells and parietal cells. – Binding of Ach to its receptor on G cells induces secretion of the hormone Gastrin (enteroendocrine) – Acetylcholine, histamine and Gastrin, stimulates parietal cells to secrete small amounts of acid Cephalic phase prepares stomach Additionally, a low level of gastric motility is induced to receive ingested material Gastric phase When a meal enters the stomach: Gastric phase – Stomach wall distension and mucosal irritation Activation of the enteric nervous system and release of gastrin cause vigorous smooth muscle contractions. – The net result is that secretory and motor functions of the stomach are fully turned on – lots of acid and pepsinogen are secreted – pepsinogen is converted into pepsin – vigorous grinding and mixing contractions take place However, there is a mechanism in place in the stomach to prevent excessive acid secretion - if lumenal pH drops Gastric phase begins with the low enough (less than about 2) arrival of food in the stomach – motility and secretion are temporarily suspended Acid travels up towards lumen of stomach Intestinal phase As food is liquefied in the stomach, it is emptied into the Intestinal phase small intestine The small intestine needs to be able to slow down gastric emptying, to allow it time to neutralize the acid and efficiently absorb incoming nutrients Hence, this phase of gastric function is dominated by the small intestine sending inhibitory signals to the stomach to slow secretion and motility – Two types of signals are used: nervous (neural responses) and endocrine (hormonal responses). Distension of the small intestine, as well as irritation of the mucosa leads to gastric-inhibitory impulses in the nervous system - this nervous pathway is called the enterogastric reflex. Secondly, enteric hormones are released from cells Intestinal phase controls the rate in the small intestine and contribute to suppression of gastric emptying of gastric activity. Regulation of Gastric Activity - Summary Brain alerts the stomach that it should expect arrival of a meal Stomach comes out of its inter-digestive resting and begins low level motor and secretory activity (cephalic phase) After a meal is consumed, the gastric motor and secretory activity is fully turned on (gastric phase) If the meal is at all substantial, the gastric phase is periodically suppressed by signals from the small intestine (intestinal phase) Eventually, the meal is fully liquefied and emptied, and the stomach falls back into a state of very low motor and secretory activity, where it remains until the next cephalic phase Digestion and absorption in the stomach Ingested food reaches stomach - salivary amylase and lingual lipase continue the digestion of carbohydrates and lipids Preliminary digestion of proteins – Pepsin – breaks down proteins into smaller peptide and polypeptide chains – Protein digestion not completed in the stomach Carbohydrates, lipids, and proteins only partially broken down Very little absorption of nutrients because: (1) the mucus covering the epithelial cells means that they are not directly exposed to chyme in stomach (2) the epithelial cells lack the specialized transport mechanisms of cells that line the small intestine (3) the gastric lining is somewhat impermeable to water (4) digestion has not been completed by the time chyme leaves the stomach Some drugs (aspirin, anti-inflammatory) are absorbed lipid solvable → promote stomach damage → stomach bleeding The Digestive System https://www.youtube.com/watch?v=pqgcEIaXGME