L3 - The Digestive System - Topic 1 Flashcards
What are the characteristics of the stomach ?
- Temporary storage tank that starts chemical breakdown of protein digestion
- chemical digestion begins : pepsin begins breakdown of bolus food → mixes bolus with gastric juice → converts into paste-like chyme (bolus + gastric juice)
- Connects esophagus to duodenum (proximal area of small intestine), releases food into it in small quantities
- When empty, stomach mucosa forms many folds called rugae – Allows stomach to expand as we start to eat (in empty space recoils)
- Empty stomach has ~50 ml volume but can expand to 4 L
Wha is the gross anatomy of the stomach ?
Major regions :
- Cardial part (cardia) – Surrounds cardial orifice (opening to which esophagus connects)
Fundus: dome-shaped region beneath diaphragm (most storage takes place)
Body: midportion
Pyloric part: wider and more superior portion of pyloric region, antrum, narrows into pyloric canal that terminates in pylorus
Pylorus is continuous with duodenum through pyloric valve (sphincter controlling stomach emptying)
T / F - The sides of the stomach grow at an equal steady rate
FALSE
During development, one side grows faster than the other
Greater curvature : Convex lateral surface of stomach
Lesser curvature : Concave medial surface of stomach
Mesenteries of the Stomach
- Extend from curvatures & tether stomach to other digestive organs
Lesser omentum : From lesser curvature to liver
- Anchors stomach to liver
Greater omentum : Inferiorly from greater curvature over intestine, spleen, and transverse colon; blends with mesocolon
- Anchors large intestine to abdominal wall
- Contains fat deposits, lymph nodes / MALT
Innervation of the Stomach
ANS supplies
- Sympathetic fibers via splanchnic nerves via the celiac plexus
- Parasympathetic via vagus nerve (DOMINATES over sympa)
Blood Supply
- Celiac trunk – supplies viscera (stomach), left / right gastric artery
- Veins of the Hepatic Portal System (shunts to liver before getting back into venous circulation)
Histology of the Stomach
4 tunics but the mucosa & muscularis layers modified
Mucosa layer
- Simple columnar epithelium entirely composed of mucous cells
- Secrete 2-layer coat of alkaline mucus
– Surface layer traps bicarbonate-rich fluid layer that is beneath it
- Dotted with gastric pits which lead into gastric glands that produce gastric juice
Muscularis externa
- 3 layers of smooth muscle
- Outer longitudinal, middle circular & inner _oblique_ layer allows stomach to churn, mix, move & physically break down food (oblique layer adds extra force to assist with mech digestion)
What are the glandular cells of the stomach ?
Mucous neck cells, parietal cells, chief cells & enteroendocrine cells
Mucous neck cells
- Secrete thin, acidic mucus of unknown function
- Different from mucus of the surface epithelium
Parietal cells
Parietal cells :
a ) Hydrochloric acid (HCl)
- Send H+ & Cl- ions in lumen where they combine to form HCl
- 3 prongs that have dense microvilli → Maximizes SA for secreting the ions into lumen
- HCI makes the stomach contents extremely acidic (pH l .5-3.5), necessary for activation of pepsin enzyme (digests proteins)
– Pepsin secreted as pepsinogen (inactive form) converted into pepsin under influence of HCl (active form), creates pos feedback mechanism to keep converting more pepsinogen into pepsin - Acidity helps digest food by denaturing proteins + breaking down cell walls of plant foods
- Harsh enough to kill many of the bacteria ingested with foods
b ) Intrinsic Factor
- Glycoprotein required for absorption of vitamin B12 in small intestine [ ESSENTIAL FOR LIFE ]
Chief cells
- Pepsinogen – Activated to pepsin by HCl & by pepsin itself (+pos feedback mechanism)
- Lipases – Digests ~15% of lipids / triglycerides / fats
Enteroendocrine cells
Hormones :
- Gastrin [ stimulates parietal cells to produce HCl, involved in opening / closing of pyloric sphincter (gate of SM thickening – controls how much chyme goes into duodenum), stomach motility ]
- Ghrelin [ stimulates appetite, gastric motility and opening)
- Somatostatin [ inhibits gastric secretions ]
Paracrines - Locally Acting :
- Histamine [ involved in secretion of HCl ]
- Serotonin [ contraction of stomach muscles ]
How does the mucosal barrier protect the stomach ?
Acidic environment presents danger to stomach walls
- Thick layer of bicarbonate-rich mucus (”gel”) builds up on stomach wall → “sops” up H ions
- Tight junctions between epithelial cells
- Prevent gastric juice leaking into underlying tissue - Damaged epithelial cells are shed + quickly replaced by division of intestinal stem cells (ISC) to maintain integrity of mucosal barrier
- Surface cells replaced every 3–6 days
What are the digestive processes carried out by the stomach ?
- Carries out breakdown of food (chem + mech)
- Serves as holding area for food (4 hr to process food)
- Delivers chyme to small intestine (bolus + gastric juice = chyme)
- Denatures proteins by HCl
-
Pepsin carries out enzymatic digestion of proteins
– Milk protein (casein) is broken down by rennin in infants
[ Results in curdy substance ] - Lipid-soluble alcohol & aspirin are absorbed into blood
- Only stomach function essential to life is secretion of intrinsic factor for vitamin B12 absorption
– B12 needed for red blood cells to mature
– Lack of intrinsic factor causes pernicious anemia [ B12 injections ]
What processes do we NOT see in the stomach ?
ingestion & compaction / defecation
Regulation of Gastric Secretion
Gastric mucosa secretes >3 L of gastric juice/day
Neural mechanisms
- Both long & short reflexes – ACh released to stimulate output of gastric juice
- Long ( vagus nerve, PNS ) → stimulation ↑ secretion
- Short ( local ENS )
- Sympathetic stimulation ↓ secretion
Hormonal mechanisms
- Gastrin stimulates enzyme and HCl secretion in stomach
- Gastrin antagonists are secreted by small intestine
