Digestive System. - Food is ingested

Transcription

1 11 V. Digestive Processes in the Mouth - Food is ingested - Mechanical digestion begins (chewing) - Salivary amylase begins chemical breakdown of starch - Propulsion is initiated by Deglutition (Swallowing) * Involves the coordinated activity of the tongue, soft palate, pharynx, esophagus and 22 separate muscle groups * Buccal phase - bolus is forced into the oropharynx * Pharyngeal & esophageal phase(s) - controlled by the medulla and lower pons ** All routes except into the digestive tract are sealed off ** Peristalsis moves food through the pharynx to the esophagus

2 12 VI. Stomach - OVERVIEW: Chemical breakdown of proteins begins and food is convertedd to chyme. Also, storage while waiting to move food into the rest of the tract. * Holds (stores) ingested food * Degrades this food both physically and chemically * Delivers chyme to the small intestine * Begins digestion of proteins. HCl begins the process. Enzymatically digests proteins with pepsin. * Secretes intrinsic factor equired for absorption of vitamin B12 A) Gross Anatomy - Cardiac region - surrounds the cardiac orifice - Fundus - dome-shaped region beneath the diaphragm - Body - midportion of the stomach - Pyloric region - made up of the antrum and canal which terminates at the pylorus * The pylorus is continuous with the duodenum through the pyloric sphincter - Greater curvature - entire extent of the convex lateral surface - Lesser curvature - concave medial surface - Lesser omentum - runs from the liver to the lesser curvature - Greater omentum - drapes inferiorly from the greater curvature to the small intestine - Muscularis layer - has an additional oblique layer that: * Allows the stomach to churn, mix, and pummel food physically * Breaks down food into smaller fragments

3 13 B) Microscopicc Anatomy of the Stomach - Stomach Lining * The stomach is exposed to the harshest conditions in the digestive tract * Epithelial lining is composed of: 1) Epithelial cells that are joined by tight junctions 2) Goblet cells that produce a coat off alkaline mucus - The mucous surface layer traps a bicarbonate-rich fluid beneath it.

4 14 C) Digestion in the Stomach. Both mechanical and chemical digestion (of proteins) occurs. Gastric juice is secreted. HCl and pepsin act to digest proteins. 1) Intrinsic Control of Gastric Secretion: Gastric pits contain gastric glands that secrete gastric juice, mucus, and gastrin - Glands of the Stomach Fundus and Body (we will only talkk about this region) * Gastric glands that have cells impermeable to HCl. Gastric glands of the fundus and body have a variety of secretory cells: * Mucous neck cells - secrete acid mucuss * Parietal cells - secrete HCl and intrinsic factor * Chief cells - produce pepsinogen Pepsinogen is activated to pepsin by: HCl in the stomach Pepsin itself via a positive feedback mechanismm * Enteroendocrine cells - secrete gastrin, histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin into the lamina propria

5 15 What to know about Intrinsic Control off Stomach Digestion: Putting food (protein) in the stomach starts the process of producing HCl and pepsin (an enzyme which digests proteins). After all, you do not want HCl and protein-digesting enzymes floating around all the time, or it would destroy the stomach lining! Proteins are slightly basic; as ph rises, this starts a positive feedback loop wheree G cells release gastrin, which causes parietal & chief cells to releasee HCl and pepsinogen (an inactive enzyme,, or zymogen). HCl helps to denature the proteins, and kill any invaders (bacteria, etc.) that have entered with the food. AND.HCl activates pepsinogen into pepsin, which 1) Digests the polypeptides into smaller peptides, and 2) Activates itself! Of course, this causes the ph to rise even further (buffering), which stimulates the G cells even more, and the cycle continues until all the small-sized proteins have passed into the small intestines. At that point, the buffering goes away, and the whole cycle shuts down. 2) Extrinsic Regulation of Gastric Secretion (summaryy at end) * Neural and hormonal mechanisms regulate the release of gastric juice * Stimulatory and inhibitory events occur in three phases:

6 16 What to know regarding Extrinsic Controls of Stomach Digestion: 3 Phases, named after what is controlling the stomach. Each has an excitatory & inhibitory phase. (parasympathetic) control. Vagus nerve Cephalic phase: Brought on by the thought/sight/smell of food. Inhibited by losss of appetite, depression. Gastric phase: Brought on by stomach distension & chemoreceptors. Inhibited by a ph of less than 2 (acidic). Intestinal phase: Brought on by acidity in the small intestines (up to a point! After ph of 2, it inhibits! See below). Gastric glands are excited. Inhibited as local hormones are released (see CCK later), initiated by the distension of the duodenum, plus presence of FAs, irritants, etc.

7 17 3) Regulation and Mechanism of HCl Secretion Remember: This also activates pepsin! Relate this to earlier section on Gastric Glands and Juice! 1. HCl secretion is stimulated by ACh, histamine, and gastrin through second-messenger 2. Also, reflex-mediated events which include stretching of the stomach. systems: * Is low if only one ligand binds to parietal cells * Is high if all three ligands bind to parietal cells * Antihistamines block H2 receptors and decrease HCl release 4) Mechanical Digestion: Gastric Contractile Activity - Peristaltic waves move toward the pylorus at the rate of 3 per minute - This basic electrical rhythm is initiated by pacemaker cells (cells of Cajal) - Chyme is either: * Delivered in small amounts to the duodenum or * Forced backward into the stomach for further mixing

8 18 D) Regulation of Gastric Emptying complicated! - Gastric emptying is regulated by: * Pressure from mixing opens valve. * The neural enterogastric reflex short reflexes, no detail * Hormonal (enterogastrone) mechanisms no detail, CCK is involved in closing it (inhibits emptying). - We will see later, areas further down the tract stimulate the pyloric sphincter glands to move things out of the stomach. In general, substances will stay in the stomach until we have moved chyme out of the small intestines, giving it time to fully digest and absorb. - Some mechanisms inhibit gastric secretion and duodenal filling: * Carbohydrate-rich chyme quickly moves through the duodenum * Fat-laden chyme is digested more slowly causing food to remain in the stomach longer E) Absorption in the Stomach - Although the absorption is mainly a function of the small intestine, some absorption of certain small molecules nevertheless does occur in the stomach through its lining. This includes: * Water, if the body is dehydrated * Medication, like aspirin * 10 20% of ingested ethanol (e.g. from alcoholic beverages) * Caffeine (possible) - Parietal cells make Intrinsic Factor, which is necessary for the absorption of Vitamin D. F) Clinical Considerations - Peptic Ulcers and Gastric Cancer: Ulcerations: Tear in a membrane. Gastric Ulcer: Helicobacter pylori is one of the most common causes of peptic ulcer. Ulcers can also be caused or worsened by drugs such as aspirin, ibuprofen, and other NSAIDs. Gastric cancer: Fourth most common worldwide, althoughh incidence is fairly low in the western world. The most common cause is infection by the bacteria Helicobacter pylori, which accounts for more than 60% of cases.