L3 - The Digestive System - Topic 1

Question 1

What are the characteristics of the stomach ?

Answer 1

• 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

Question 2

Wha is the gross anatomy of the stomach ?

Answer 2

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)

Question 3

T / F - The sides of the stomach grow at an equal steady rate

Answer 3

FALSE

During development, one side grows faster than the other
Greater curvature : Convex lateral surface of stomach
Lesser curvature : Concave medial surface of stomach

Question 4

Mesenteries of the Stomach

Answer 4

• 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

Question 5

Innervation of the Stomach

Answer 5

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)

Question 6

Histology of the Stomach

Answer 6

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)

Question 7

What are the glandular cells of the stomach ?

Answer 7

Mucous neck cells, parietal cells, chief cells & enteroendocrine cells

Question 8

Mucous neck cells

Answer 8

• Secrete thin, acidic mucus of unknown function
• Different from mucus of the surface epithelium

Question 9

Parietal cells

Answer 9

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 ]

Question 10

Chief cells

Answer 10

• Pepsinogen – Activated to pepsin by HCl & by pepsin itself (+pos feedback mechanism)
• Lipases – Digests ~15% of lipids / triglycerides / fats

Question 11

Enteroendocrine cells

Answer 11

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 ]

Question 12

How does the mucosal barrier protect the stomach ?

Answer 12

Acidic environment presents danger to stomach walls

1. Thick layer of bicarbonate-rich mucus (”gel”) builds up on stomach wall → “sops” up H ions
2. Tight junctions between epithelial cells
   - Prevent gastric juice leaking into underlying tissue
3. 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

Question 13

What are the digestive processes carried out by the stomach ?

Answer 13

• 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 ]

Question 14

What processes do we NOT see in the stomach ?

Answer 14

ingestion & compaction / defecation

Question 15

Regulation of Gastric Secretion

Answer 15

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

Question 16

Gastric secretions are broken down into 3 phases :

Answer 16

1. Cephalic (reflex) phase – only neural
   - Nerves take command from vagal center of medulla tells stomach what to do
   - PNS excites pepsin + acid production
2. Gastric phase – Stomach takes control of itself
   - Vagus nerve reflexes
   - Local ENS secretory reflexes
   - Gastrin-histamine stimulation
3. Intestinal phase – Uses both neural + hormonal stimulation
   - Stomach releases chyme into duodenum → duodenum tells us briefly to keep going → after tells to slow things down to allow for processing of chyme

Question 17

1. Cephalic Phase

Answer 17

few mins long

• Reflexes initiated by sensory receptors in the head (sight, smell, taste), or by thought
• Take info → triggers vagus nerve (PNS) → stimulates salivary / gastric glands & stomach to begin secretion production in preparation of food

nerve impulses to :
- mucous cells, chief cells, parietal cells to stimulate secretions
- G-cells / enteroendocrine cells to release gastrin
- submucosal nerve plexuses

Question 18

2. Gastric phase

Answer 18

Lasts 3–4 hours & provides 2/3 of gastric juice

• Stretch receptors in stomach – detect distention of stomach
• Chemoreceptors in stomach – Sense chemical stimuli (eg. peptides, low acidity / elevated pH, caffeine), will trigger enteroendocrine / G cells to secrete GASTRIN
  – [ protein in foods causes ↑ in pH bc they are “sopping up” H ions –> sensed by chemoreceptors to stim gastrin secretion & HCl release from parietal cells to provide more acidic digestive conditions to accommodate ]

MORE PROTEIN = MORE GASTRIN & HCl RELEASED (neg feedback mechanism)

Will initiate neural (both long & short) reflexes – Tell ur head (medulla) + ENS to trigger cells to produce more secretions

Question 19

2 mechanisms of HCl secretion by parietal cells :

Answer 19

DIRECT : Binding to gastrin receptors on parietal cells
INDIRECT : Stimulating enteroendocrine cells to release histamine (which will act on parietal cells to tell them to produce more HCl)

Question 20

Inhibition of Gastric Phase

Answer 20

Low pH inhibits gastrin secretion sign of nothing being in stomach, nothing “sopping up” hydrogen ions
Sympathetic stimulation also inhibits bc SympaNS overrides PNS

• 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

Question 21

3. Intestinal Phase

Answer 21

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 & overridden by inhibitory stimuli as intestine fills to give it time to process the food before more comes in

Inhibition by intestinal phase : Important to protect SI against excessive acidity & prevent massive influx of chyme
4 main factors in duodenum cause inhibition of gastric secretions :
- Distension of duodenum due to entry of chyme (mechano / stretch receptors)
- Presence of acidic / fatty / hypertonic chyme

Question 22

Response to the Stomach Filling

Answer 22

2 factors cause internal stomach pressure to remain constant until 1.5 L of food is ingested :

1. Receptive relaxation
   - Smooth muscles will reflexively relax bc of swallowing center of brain stem
   - SM can expand to accommodate particular V without the P increasing without muscles snapping back together
2. Gastric accommodation :
   - smooth muscle can exhibit stress-relaxation response, enabling hollow organs to stretch without increasing tension or contractions

Question 23

Gastric Contractile Activity

Answer 23

• Peristaltic waves move toward the pylorus at the rate of 3/ min

Basic electrical rhythm (BER) initiated by pacemaker cells (formerly interstitial cells of Cajal) located between SM layer - Can depolarize/repolarize spontaneously
- Set frequency of contractions / peristaltic waves

Question 24

Contractions are most vigorous & powerful near ____________ region

Answer 24

pylorus

[ walls of pyluric area thicker than walls in fundus of stomach to accommodate holding 30 mL of chyme ]

• Delivered in ~3 ml spurts to duodenum & rest (27 ml) forced back into stomach
• Only liquids & small particles are allowed to pass through small pyloric valve
• Retropulsion → Anything not big enough to go through bounces back & continues mixing
• Process will slow down between meals& come back up to pace as trying to process food

Question 25

______________ & _________ increase gastric motility + force fo contraction

Answer 25

Distension, gastrin

Question 26

***Regulation of Gastric Emptying***

Answer 26

Duodenum can prevent overfilling by controlling how much chyme enters : - ***Duodenal receptors*** respond to stretch & 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 vs protein-rich / fat-rich

Question 27

What are the hormones & paracrines acting in digestion ?

Answer 27

- CCK - GIP - Gastrin - Histamine - Intestinal gastrin - Motilin - Secretin - Serotonin - Somatostatin - VIP

Question 28

*Cholecytoskinin / CCK*

Answer 28

**Site of production :** Duodenal mucosa **Stimulus for production :** Fatty chyme, partially digested proteins **Target organ :** - Stomach [ inhibits secretory activity ] - Liver / pancreas [ enhances secretin's actions ] - Pancreas [ increases secretion of enzyme-rich pancreatic juice ] - Gallbladder [ stimulates contraction & release of stored bile ] - Hepatopancreatic Sphincter [ relaxes to allow entry of biile & pan juice into duodenum ]

Question 29

*GIP / Gastric Inhibitory Peptide*

Answer 29

**Site of production :** Duodenal mucosa **Stimulus for production :** Fatty chyme **Target organ :** - Stomach [ inhibits HCl production (minor effect) ] - beta cells of Pancreas [ stims insulin release ]

Question 30

*Gastrin*

Answer 30

**Site of production :** G cells of stomach mucosa **Stimulus for production :** Food in stomach (esp proteins), ACh released by nerves **Target organ :** - parietal cells of Stomach [ Increases HCl secretion + stim gastric emptying ] - Small Intestine [ stims contraction of intestinal muscle ] - Ileocecal Valve [ relaxes ] - Large Intestine [ stim mass movement ]

Question 31

*Histamine*

Answer 31

**Site of production :** Stomach mucosa **Stimulus for production :** Food in stomach **Target organ :** - Stomach [ Activates parietal cells to release HCl ]

Question 32

*Intestinal Gastrin*

Answer 32

**Site of production :** Duodenal mucosa **Stimulus for production :** Acidic + partially digested foods in duodenum **Target organ :** - Stomach [ stim gastric glands + motility ]

Question 33

*Motilin*

Answer 33

**Site of production :** Duodenal mucosa **Stimulus for production :** Fasting (neural stimuli causes periodic release every 1.5 - 2 h) **Target organ :** - Proximal duodenum [ stim migrating motor complex ]

Question 34

*Secretin*

Answer 34

**Site of production :** Duodenal mucosa **Stimulus for production :** Acidic chyme + partially digested proteins / fats **Target organ :** - Stomach [ inhibits gastric gland secretion + gastric motility ] - Pancreas [ inc in release of bicarbonate ion rich pancreatic juice, enhances CCK ] - Liver [ inc bile output ]

Question 35

*Serotonin*

Answer 35

**Site of production :** Stomach mucosa **Stimulus for production :** Food in stomach **Target organ :** - Stomach [ contraction of muscle ]

Question 36

*Somatostatin*

Answer 36

**Site of production :** Stomach & duodenal mucosas **Stimulus for production :** Food in stomach + stim from sympa NS **Target organ :** - Stomach [ inhibits all gastric secretions ] - Pancreas [ inhibits secretions ] - SI [ inhibits GI blood flow --> inhibits intestinal absorption ] - Gallbladder & liver [ inhibits contraction + bile release ]

Question 37

*VIP / Vasoactive Intestinal Peptide*

Answer 37

**Site of production :** Enteric neurons **Stimulus for production :** Chyme containing partially digested foods **Target organ :** - SI [ stim buffer secretion, inhibits blood flow through intestinal capillaries, relaxes intestinal smooth muscle ] - Stomach [ inhibits acid secretion ] - Pancreas [ increases secretions ]