Digestive System

Question 1

What are the three main salivary glands, and how do their secretions differ?

Answer 1

• Parotid Gland - serous only
• Mandibular Gland - serous and mucus
• Sublingual Gland - serous and mucus

Question 2

List the four major layers, in order from the lumen outwards, that make up the structure of the GI tract wall.

Other cards will ask for more detail about these layers, simply list.

Answer 2

• Mucosa
• Submucosa
• Muscularis propria
• Adventitia / Serosa

Question 3

Describe the structure and function of the mucosa layer of the GI tract wall.

Answer 3

It is composed of 3 layers: epithelium, lamina propria and muscularis mucosae.
The muscularis mucosae contracts glands, forcing secretion.
The mucosa more generally supports local movement.

Question 4

What are the main components and functions of saliva?

Answer 4

Components:
* Mainly water.
* Electrolytes (sodium, chloride, bicarbonate).
* Amylase.
* Mucus.
* Antimicrobial agents (IgA, lysozyme).
Functions:
* Oral hygiene.
* Lubrication for swallowing.
* Initiation of carbohydrate metabolism.
* Protection against pathogens.

Question 5

Describe the mechanism of secretion from salivary glands including control by autonomic nervous system.

Answer 5

• Salivary gland secretion is mainly under parasympathetic control - causes vasodilation of blood vessels near to salivary glands.
• Increased blood flow and increased permeability results in increased salivary flow.
• Acinar cell secrete substance isotonic to plasm into duct.
• As substance travels down the duct, sodium ions and chloride ions are exchanged for potassium ions and bicarbonate ions.
• Bicarbonate ions help to neutralise acidic foods.
• Saliva becomes hypotonic because whilst salts (Na+ and Cl-) move of the duct, ductal cells are nearly impermeable to water.
• Benefit of saliva being very watery: lubrication.

Question 6

Describe the structure and function of the Submucosa layer of the GI tract wall.

Answer 6

Structure: A layer of loose connective tissue containing larger blood vessels, lymphatics, and nerves.
Function: Supporting mucosa layer.

Question 7

Describe the structure and function of the Muscularis propria layer of the GI tract wall.

Answer 7

Structure: Inner circular smooth muscle layer, outer longitudinal smooth muscle layer.
Function: Facilitation of peristalsis.

Question 8

Describe the structure and function of the Adventitia / Serosa layer of the GI tract wall.

Answer 8

Structure: Contains major blood vessels, nerves, and some adipose tissue.
Function: Mechanical support of GI tract wall.

Question 9

Explain the roles of mucus and bicarbonate in the stomach.

Answer 9

Mucus forms a physical barrier that protects the stomach lining from the corrosive effects of acid and pepsin. Bicarbonate neutralises acid near the epithelial surface, further protecting the stomach lining.

Question 10

How is hydrochloric acid (HCl) produced by parietal cells in the stomach?

Answer 10

• Carbon dioxide produced in aerobic respiration is converted into carbonic acid, which then dissociates into protons and bicarbonate ions.
• This reaction is catalysed by carbonic anhydrase.
• The H+ are actively transported from parietal cells into the stomach lumen by the H+-K+ ATPase pump, while Cl- is transported into the lumen in exchange for HCO3-, resulting in the secretion of HCl.

Question 11

What are the three phases of gastric secretion, and what stimulates each phase?

Answer 11

• Cephalic Phase: Stimulated by sensing food, mediated by the vagus nerve.
• Gastric Phase: Stimulated by stomach distension and presence of peptides.
• Intestinal Phase:

Question 12

Describe how acetylcholine, gastrin, and histamine stimulate parietal cell secretion.

Answer 12

Synergistically.
- ACh: released by the vagus nerve, stimulates parietal cells and ECL cells to release histamine.
- Gastrin: released from G cells, stimulates parietal cells and histamine release from ECL cells.
- Histamine: acts on parietal cells to further increase acid secretion.

Question 13

Explain the process of slow wave contractions in the stomach.

Answer 13

Spontaneous, rhythmic depolarisation / repolarisation of smooth muscle cells in the stomach.
Propagate from fundus to pylorus at a frequency of 3-4 per minute, mixing the stomach contents and propelling chyme towards the duodenum.

Question 14

What are the main steps involved in the vomiting (emesis) reflex?

Answer 14

Reverse peristalsis, moving intestinal contents into the stomach.
Pyloric sphincter opens, and the glottis closes, followed by a strong contraction of the abdominal muscles, increasing intra-abdominal pressure and forcefully expelling the stomach contents through the open upper oesophageal sphincter.

Question 15

Sphincter between small intestine and large intestine.

How does the ileocecal sphincter prevent retrograde movement of faecal matter?

Answer 15

Ileocecal sphincter constricts in response to distension of the colon (large intestine), preventing the backflow.
Distension of the ileum (final part of small intestine) relaxes the sphincter, facilitating forward movement of chyme into the colon (large intestine).

Question 16

Explain the consequences of chronic vomiting.

Answer 16

Dehydration (loss of blood volume). Significant loss of acid (stomach content) can lead to metabolic alkalosis.

Question 17

Using knowledge of the digestive system, what is one reason why patients are instructed to eat before taking aspirin?

Answer 17

Aspirin inhibits the production of bicarbonate and mucus, allowing stomach acid to damage the stomach.

Question 18

Explain how intestinal pH and products of digestion regulate the rate of gastric emptying.

Answer 18

pH: Stomach contents are very acidic - must be neutralised to enter the duodenum.
Gastric emptying slows in response to low (acidic) pH in the duodenum to allow time for buffering.
Products of Digestion:
- Peptides in the stomach increase gastrin release, stimulating smooth muscle contraction and promoting emptying.
- Fats High concentration in the duodenum signals for slower gastric emptying.
Osmolarity: A high osmolarity (concentration of solutes) in the duodenum inhibits gastric emptying to prevent fluid imbalances.

Question 19

What factors stimulate gastric emptying?

Answer 19

• Stomach distension: Activation of mechanoreceptors → increased gastrin secretion → more contraction of stomach muscles.
• Peptides in the stomach: Stimulate gastrin release → emptying.
• Increase in parasympathetic tone: Increases gastric motility.

Question 20

What factors inhibit gastric emptying?

Answer 20

• High pressure in the duodenum: Prevents additional food from entering.
• High [peptide] in the duodenum: Signals that digestion is still ongoing.
• High Osmolarity: Slows emptying to avoid drawing excessive water into the intestines.
• Low pH in duodenum: More time needed to neutralise acidic stomach contents.
• Fats High concentration in the duodenum signals for slower gastric emptying.
• Increased sympathetic tone (stress):Reduces gastric motility.
• Gut hormones (e.g., GLP-1): Released by the intestine to inhibit gastric emptying.

Question 21

Describe the 7 key sphincters of the GI tract.

Answer 21

1. Upper Oesophageal Sphincter: Located at the pharynx-oesophagus junction. Prevents air from entering the oesophagus during breathing. Relaxes during swallowing.
2. Lower Oesophageal Sphincter: Located at the junction of the oesophagus and stomach. Prevents reflux of stomach acid into the oesophagus. Weakness here can lead to acid reflux (GERD).
3. Pyloric Sphincter: Between the stomach and duodenum. Controls the rate of gastric emptying into the small intestine.
4. Sphincter of Oddi: Controls the release of bile and pancreatic enzymes into the duodenum. Prevents reflux of duodenal contents into the bile and pancreatic ducts.
5. Ileocecal Sphincter: Between the small and large intestine. Opens in response to high ileal pressure.Closes when food enters the large intestine to prevent backflow.
6. Internal Anal Sphincter: Autonomic Control, Normally contracted to maintain continence.
7. External Anal Sphincter:Conscious regulation of defecation.

Question 22

Describe the physiology of emesis, including the roles of the vomiting centre and chemoreceptor trigger zone.

Answer 22

• Chemical Trigger Zone (in brain but sits outside of blood-brain barrier) detects toxic chemicals, drugs, and metabolic disturbances.
• Vomiting Centre: Integrates signals from the CTZ, vestibular system, gut mechanoreceptors and chemoreceptors.
  Physiological Response:
• Increased salivation (protects oesophagus from stomach acid)
• Closure of the glottis/elevation of the soft palate - prevents vomit from entering the respiratory tract.
• Abdominal muscle and diaphragm contraction, increasing intra-abdominal pressure.
• Relaxation of the lower oesophageal sphincter, allowing contents to exit.

Question 23

How is retching physiologically different from vomiting - role of the lower oesophageal sphincter.

Answer 23

• If the lower oesophageal sphincter remains closed, retching occurs.
• If it opens, vomiting occurs.

Question 24

What is the location and function of the myenteric plexus?

Answer 24

Location: Between muscle layers.
Function: Regulation of muscular activity and motility in the GI tract.

Question 25

What is the location and function of the submucosal plexus?

Answer 25

**Location:** Submucosa-circular muscle boundary. **Function:** Regulation of secretion from mucosal glands.

Question 26

What is the incretin effect?

Answer 26

Enhanced insulin secretion in response to oral glucose compared to intravenous glucose. ## Footnote Key hormones: Glucagon-like peptide-1 (GLP-1) and Glucose-dependent insulinotropic polypeptide (GIP).

Question 27

What are the two main types of motility in the small intestine?

Answer 27

**1. Segmentation:** Alternating contractions that mix the intestinal contents, increasing the rate of digestion and absorption. **2. Peristalsis:** Coordinated contractions that propel gut contents along the GI tract.

Question 28

What is the role of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) in intestinal secretion?

Answer 28

Mediates the secretion of chloride ions by crypt cells in the small intestine, which drives the secretion of fluid into the intestinal lumen.

Question 29

What is the composition of pancreatic secretions and their importance in digestion?

Answer 29

**Composition:** Bicarbonate and enzymes. - Bicarbonate neutralises acidic chyme entering the duodenum from the stomach. - Enzymes break down macromolecules.

Question 30

What substances are contained in bile and why is bile important for digestion?

Answer 30

**Contents:** Bile salts, bilirubin, and cholesterol. **Function:** Emulsification of fats to increase rate of digestion.

Question 31

Describe the cephalic phase of digestion.

Answer 31

Initiated by sensing food. Stimulation of gastric secretion via the vagus nerve, releasing ACh, gastrin-releasing peptide (GRP), and histamine.

Question 32

What is the difference between short-loop and long-loop reflexes in the gut?

Answer 32

**Short-loop reflexes:** Contained within the enteric nervous system. Sensory neurons and interneurons directly influence effector cells in the gut wall. **Long-loop reflexes:** Sensory neurons communicate with the CNS which then sends efferent signals back to the gut via autonomic nerves.

Question 33

What are the key differences between the function of the crypt and the villus in the small intestine?

Answer 33

**Crypts:** Secretion of fluid, electrolytes, and mucus. **Villi:** Absorption of nutrients and other substances from the intestinal lumen.

Question 34

Describe the standing gradient osmotic model and its role in water absorption in the small intestine.

Answer 34

Model explaining water absorption in the small intestine. - Na+ actively transported into the lateral space between cells, ↑ [solute]. Establishes osmotic gradient: water from the lumen into the lateral space, which then moves into the bloodstream.

Question 35

What are the 5 primary causes of diarrhoea, and how does each disrupt the normal water balance in the GI tract?

Answer 35

**Cause 1:** Reduced Transit Time. **Effect 1:** Less time for water absorption in the intestines; water remains in the lumen → loose stools. **Cause 2:** Impaired Absorption (e.g., due to damaged villi). **Effect 2:** Reduces surface area for absorption → less effective uptake of water and electrolytes → excess water in the intestinal lumen. **Cause 3:** Stimulated Secretion from Crypts. **Effect 3:** Increased fluid secretion into the lumen, overwhelming the absorptive capacity of the gut. **Cause 4:** Solute Retention (Osmotic Diarrhoea). **Effect 4:** Retained solutes draw water into the intestinal lumen via osmosis, increasing water content in the stool. **Cause 5:** Leaky Epithelia due to Tissue Damage. **Effect 5:** Uncontrolled water and electrolyte movement into the lumen, contributing to fluid loss.

Question 36

Explain how oral rehydration therapy (ORT) works, highlighting the importance of sodium, sugars, and amino acids.

Answer 36

Na+, sugars and amino acids, are co-transported into epithelial cells, driving water absorption via osmosis. → More uptake of water and electrolytes, rehydrating the body more effectively than drinking water alone.

Question 37

Outline the steps involved in carbohydrate digestion, including the enzymes involved and the final products absorbed by the small intestine.

Answer 37

1. Amylase breaks down starch and glycogen into maltose. 2. Maltase breaks down maltose into glucose. 3. Glucose absorbed into epithelial cells via sodium-coupled transport. 4. Glucose enters bloodstream via facilitated diffusion.

Question 38

# Include location of secretion, it pepsin essential? Describe how proteins are digested in the stomach, and what role pepsin plays in the process.

Answer 38

Location: Stomach Gastric chief cells secrete pepsinogen, which activates to pepsin at low pH. Pepsin hydrolyses proteins into smaller peptides, initiating protein digestion. Pepsin is not essential for normal protein digestion, as pancreatic and brush border proteases can adequately digest ingested protein.

Question 39

Explain the emulsification of fats in the small intestine, including the roles of bile salts and pancreatic lipases.

Answer 39

Bile salts, produced by the liver, emulsify fats by breaking large fat droplets into smaller ones. Pancreatic lipases then hydrolyse triglycerides into monoglycerides and free fatty acids. This process increases the surface area of the fat, facilitating efficient digestion and absorption.

Question 40

Describe the role of the colonic microbiota in maintaining epithelial barrier function, vitamin synthesis, and short-chain fatty acid production.

Answer 40

- Prevent pathogens from entering the body - Aid synthesis of vitamin K and B. - Ferment fibre into short-chain fatty acids that can provide energy for epithelial cells and act as signalling molecules influencing hunger and satiety.

Question 41

Explain how Vitamin D3 increases Calcium absorption in the small intestine.

Answer 41

Vitamin D3, is a steroid hormone, acts on the nucleus to regulate gene expression. It increases the expression of genes that encode calcium-binding proteins in the intestinal membrane. These proteins transport Calcium into the epithelial cells increasing calcium absorption through the gut.

Question 42

Explain how Vitamin B12 is absorbed, mentioning the molecules involved and the location in the GI tract where it takes place.

Answer 42

1. Vitamin B12 is released from food by pepsin in the stomach. 2. Binds to R-proteins, which are degraded by pancreatic proteases. 3. B12 then binds to intrinsic factor (IF), secreted by parietal cells in the stomach, and this complex is transported to the ileum. 4. This complex can be absorbed into the liver and other tissues.