Digestive system-3 Flashcards
What is the process that the stomach starts? What does the stomach convert bolus into?
How much volume does an empty stomach have and how large can it expand? What kind of folds does the stomach make when it’s empty?
Stomach is a temporary storage tank that starts chemical breakdown of protein digestion
Converts bolus of food to paste-like chyme (bolus + gastric juice)
Empty stomach has ~50 ml volume but can expand to 4 L
When empty, stomach mucosa forms many folds called rugae
What are the 6 major regions of the stomach?
- Cardial part (cardia): surrounds cardial orifice
- Fundus: dome-shaped region beneath diaphragm
- 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) - Greater curvature: convex lateral surface of stomach
- Lesser curvature: concave medial surface of stomach
Difference between greater and lesser omentum?
– Lesser omentum: from lesser curvature to liver
– Greater omentum: inferiorly from greater curvature over intestine, spleen, and transverse colon; blends with mesocolon, mesentery that anchors large intestine to abdominal wall
• Contains fat deposits and lymph nodes
How is the stomach innervated? How is blood supplied to the stomach?
o Sympathetic fibers via splanchnic nerves via the celiac plexus o Parasympathetic (resting and digesting) via vagusnerve
o Celiac trunk
o Veins of the hepatic portal system
How many layers are in the stomach? How many layers of smooth muscle in the muscularis externa? What is the mucosa layer dotted with? What do these things do?
Four tunics/layers but the muscularis & mucosa are modified(layers are the mucosa, submucosa, muscularis externa, serosa)
Three layers of smooth muscle
Dotted with gastric pits, which lead into gastric glands that produce gastric juice
Four different types of glandular cells in the stomach? what do they do?
Mucous neck cells:
• Secrete thin, acidic mucus of unknown function
• different from mucus of the surface epithelium
Parietal cells:
▪ Hydrochloric acid (HCl) denatures protein, activates pepsin, breaks down
plant cell walls, and kills many bacteria
▪ Intrinsic factor: Glycoprotein required for absorption of vitamin B12 in small
intestine
Chief cells:
▪ Pepsinogen (activated to pepsin by HCl & by pepsin itself (+ve feedback
mechanism)
▪ Lipases (digests ~15% of lipids)
Enteroendocrine cells:
▪ gastrin (needed for HCL secretion; opening of pyloric sphincter, stomach’s
motility)
▪ ghrelin (stimulates appetite, gastric motility and opening) & somatostatin
▪ Paracrines: histamine and serotonin
What is the mucosal barrier? 3 steps to how it is created?
▪ Harsh digestive conditions require stomach to be protected
- Thick layer of bicarbonate-rich mucus
- Tight junctions between epithelial cells
• Prevent juice seeping underneath tissue - Damaged epithelial cells are quickly replaced by division of intestinal stem cells (ISC)
• Surface cells replaced every 3–6 days
7 processes carried out by the stomach?
– Carries out breakdown of food
– Serves as holding area for food
– Delivers chyme to small intestine
– 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 and 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
Treated with B12 injections
How many L of gastric juice is released per day? what are 2 mechanisms that regulate gastric secretion?
Gastric mucosa secretes >3 L of gastric juice/day
Regulated by:
– Neural mechanisms
• Vagus nerve stimulation increases secretion
• Sympathetic stimulation decreases secretion
– Hormonal mechanisms
• Gastrin stimulates enzyme and HCl secretion
• Gastrin antagonists are secreted by small intestine
What is the 1st phase of gastric secretion and what happens? what happens in each phase? what is gastrin? How is the gastric phase inhibited?
- Cephalic (reflex) phase:
reflexes initiated by sensory receptors in the head (sight, smell, taste), or by thought - Gastric phase:
-Lasts 3–4 hours and provides 2/3 of gastric juice released
-Initiated by stretch & chemoreceptors in the
stomach
-stretch receptors – detect distention of stomach
-Initiate neural (both long & short) reflexes
-Chemical stimuli, e.g. peptides, caffeine, & low acidity → enteroendocrine (G) cells to secrete GASTRIN - Intestinal phase
Gastrin:
▪ Buffering action of ingested proteins causes pH to rise, which activates
more gastrin secretion
▪ initiates HCl release from parietal cells & activates enzyme secretion
▪ Prods parietal cells to secrete HCl by:
1. Binding to receptors on parietal cells
2. Stimulating enteroendocrine cells to release histamine
Inhibition of gastric phase
▪ Low pH inhibits gastrin secretion
▪ Occurs between meals
▪ Occurs during digestion as negative feedback mechanism
▪ The more protein, the more HCl acid is secreted, causing decline in
pH, which inhibits gastrin secretion
What is the 2nd phase of gastric secretion and what happens? What is gastrin? how is the gastric phase inhibited?
- Gastric phase:
-Lasts 3–4 hours and provides 2/3 of gastric juice released
-Initiated by stretch & chemoreceptors in the
stomach
-stretch receptors – detect distention of stomach
-Initiate neural (both long & short) reflexes
-Chemical stimuli, e.g. peptides, caffeine, & low acidity → enteroendocrine (G) cells to secrete GASTRIN - Intestinal phase
Gastrin:
▪ Buffering action of ingested proteins causes pH to rise, which activates
more gastrin secretion
▪ initiates HCl release from parietal cells & activates enzyme secretion
▪ Prods parietal cells to secrete HCl by:
1. Binding to receptors on parietal cells
2. Stimulating enteroendocrine cells to release histamine
Inhibition of gastric phase
▪ Low pH inhibits gastrin secretion
▪ Occurs between meals
▪ Occurs during digestion as negative feedback mechanism
▪ The more protein, the more HCl acid is secreted, causing decline in
pH, which inhibits gastrin secretion
What is the 3rd phase of gastric secretion and how is it stimulated/inhibited? how does inhibition protect the stomach?
▪ Begins with a brief stimulatory component followed by inhibition
Stimulation of intestinal phase: ▪ Partially digested food enters small intestine, causing a brief release of intestinal (enteric) gastrin ▪ Encourages gastric glands of stomach to continue secretory activities ▪ Stimulatory effect is brief and overridden by inhibitory stimuli as intestine fills
Inhibition by intestinal phase:
Four main factors in duodenum cause inhibition
of gastric secretions:
▪ Distension of duodenum due to entry of chyme
▪ Presence of acidic chyme
▪ Presence of fatty chyme
▪ Presence of hypertonic chyme
Inhibition: protects intestine from being overwhelmed by too much chyme or
acidity and is achieved in two ways:
▪ Enterogastric reflex (Neural)Duodenum inhibits acid secretion in stomach by:
ENS short reflexes & SNS and vagus nerve long reflexes
▪ Enterogastrones (hormonal)
Duodenal enteroendocrine cells release two important hormones that inhibit gastric secretion:
secretin & cholecystokinin
(CCK)
How does pressure in stomach stay constant when it is stretched?
▪ Two factors cause pressure to remain constant until 1.5 L of food is
ingested:
• Receptive relaxation: reflex-mediated relaxation of smooth muscle coordinated by swallowing center of brain stem
• Gastric accommodation: intrinsic ability of smooth muscle to exhibit stress-relaxation response, which enables hollow organs to stretch without increasing tension or contractions
what is the rate of peristaltic waves per minute? how is the electrical rhythm of the stomach initiated? Where are stomach contractions most powerful? what are 2 things that increase force of contraction?
▪ Peristaltic waves move toward the pylorus at the rate of 3/minute
▪ Basic electrical rhythm (BER) initiated by pacemaker cells
Contractions are most vigorous and powerful near pylorus region
Distension and gastrin increase force of contraction
How does the duodenum prevent overfilling?
How long does it take for stomach to empty?
– Duodenum can prevent overfilling by controlling how much chyme enters
• Duodenal receptors respond to stretch and chemical signals
• Enterogastric reflex & enterogastrones inhibit gastric secretion & duodenal filling
– Stomach empties in ~4 hours, but increase in fatty chyme entering duodenum can
increase time to 6 hours or more
• Carbohydrate-rich chyme moves quickly through duodenum
