Digestive System

Question 1

What are the functions of the digestive system?

Answer 1

1. Ingestion: material enters the digestive system through the mouth
2. mechanical digestion and pirpulsion: physical breakdown of food to allow easier propulsion along the tract, increases surface are for easier breakdown
3. Chemical digestion: chemical breakdown of food for absorption, large molecules broken down into small absorbable molecules.
4. Secretion: release of water acids, enzymes, buffers, salts to aid chemical digestion.
5. Absorption: movement of substrates, electrolytes, vitamins, and water across epithelium into interstitial fluid.
6. Defectation: Removal of waste products from the body.

Question 2

What are the organs of the digestive system and their functions?

Answer 2

Digestive organs:
- Mouth: ingestion and mechanical digestion using the accessory organs (teeth and tongue), moistening and mixing with salivary secretions.
- Pharynx: Muscular propulsion of materials into the oesophagus.
- Oesophagus: Transport of materials to the stomach
- Stomach: Chemical digestion of materials by acid and enzymes, mechanical digestion through muscular contractions.
- Small intestine: Enzymatic digestion and absorption of water, organic substrates, vitamins, and ions.
- Large intestine: Dehydration and compaction of indigestible materials in preparation for elimination.

Accessory organs: Aid the digestive process
- Teeth: mechanical digestion
- Tongue: Mechanical digestion + sensory analysis
- Salivary glands: Secretion of lubricating fluid containing enzymes that break down carbohydrates
- Liver: Secretion of bile, storage of nutrients.
- Gallbladder: storage and concentration of bile. Releases bile when required for lipid digestion.
- Pancreas: Exocrine cells secrete buffers and digestive enzymes; endocrine cells secrete hormones.
- Mesentery: Serous membrane with areolar tissue in between sheets. It provides blood supply, nerve, and lymphatic supply to the small intestine. It stabalises and attaches digestive organs to the peritoneal cavity. Stops digestive organs from entangling.

Question 3

Describe the major layers of the digestive tract walls.

Answer 3

1) Mucosa:
- inner lining, is a mucous membrane
Digestive Epithelium:
- Simple columnar (stomach, small intestine, large intestine) or stratified squamous (oral cavity, pharynx, oesophagus, and edge of rectum) epithelium
- secrete hormones
- moistened by glandular secretions
- longitudinal folds, circular folds, and villi to increase SA.
Specialised epithelial cells:
- stem cells
- enteroendocrine cells - secrete hormones
- goblet cells - secrete mucous
- paneth’s cells - secrete antimicrobial peptides

2) Submucosa:
- loose, irregular connective tissue
- surrounds muscularis mucosae
- contains large blood vessels and lymphatic vessels
- may contain exocrine glands which secrete buffers and enzymes into the digestive tract.

Submucosal plexus: innervates the mucosa and submucosa.
- sensory neurons, parasympathetic neurons and sympathetic neurons.

3) Muscularis externa:
- Smooth muscle arranged in an inner circular layer, outer longitudinal layer.
- involved in mechanical processing and movement of materials along the digestive tract.
- movements are coordinated by the enteric nervous system, but primarily innervated by the parasympathetic nervous system.

4) Serosa:
- serous membrane covering the muscular layer
- attaches to the mesentery
- In oral cavity, pharynx, oesophagus, and rectum, Adventitia (a firmer arrangement of collagen fibres) attaches the digestive tract to adjacent structures.

Question 4

What are the processes of how food moves through the digestive tract?

Answer 4

Pacesetter cells: trigger waves of contraction that spread throughout muscular sheet.

Peristalsis:
Circular muscles contract behind the bolus.
This wave of contraction in the circular muscles propels the bolus forward.
Longitudinal muscles shorten whilst the circular muscles push the bolus forward.
Peristalsis allows for efficient digestion and absorption of nutrients by ensuring food moves along the digestive tract.
One directional
Occurs in the oesophagus and the small intestine

Segmentation:
Segmentation is the mixing and churning of the bolus to mechanically break it through cycles of contraction.
This process allows the fragments to be mixed with intestinal secretions from the duodenum and small intestine, which aids chemical digestion because it is mixed with buffers and enzymes.
The fragments increase surface area, so the brush boarder enzymes and buffers have a greater amount of contact.
These secretions include bile, pancreatic enzymes, and buffers to neutralize the acid from the stomach.
Occurs primarily in the small intestine.
Non-directional
Final absorption and digestion takes place here.

Question 5

How is the digestive system regulated?

Answer 5

• Three ways in which the digestive system is regulated to make sure absorption and secretion is the most efficient.

1. Receptors:
   - chemoreceptors, osmoreceptors, stretchreceptors.
   - external such as sight and smell activate salivary glands which also triggers the stomach.
2. Control centres:
   - Local factors are only activated when something is occurring at that spot.
   - Nerves including the enteric nervous system (part of PNS), and the autonomic nervous system (sympathetic and parasympathetic).
   - hormones such as gastrin, GIP, CCK, secretin, VIP.
3. Effectors:
   - smooth muscle increase or alter movement
   - exocrine glands secrete hormones

Question 6

Describe the main structure and function of the oral cavity.

Answer 6

Structure:
- teeth, tongue, hard palate, soft palate, salivary gland.

Function:
- The tongue is responsible for sensory analysis before swallowing food
- the teeth and tongue and palatal surfaces begin mechanical digestion in addition to mastication.
- lubrication occurs when the food is mixed with mucous and saliva for lubrication when moving down the GI tract.
- salivary glands produce saliva which lubricate the mouth and pharynx, cleanse mouth and teeth, moisten food, dissolve food and chemicals, stimulate tastebuds, and begin digestion.

Chemical digestion begins:
- salivary amylase breaks down starch
- lingual lipase breaks down lipids

Question 7

Describe the structure and function of the oesophagus.

Answer 7

Structure:
- hollow muscular tube
- Pierces diaphragm at the oesophageal hiatus
- Upper and lower sphincters prevent air entering esophagus and prevents reflux from stomach.

Function:
- moves food to the stomach through peristalsis
- involved in the swallowing process.

Question 8

What are the four stages of swallowing?

Answer 8

1) buccal stage:
- voluntary
- bolus goes to oropharynx

2) pharyngeal stage
- involuntary
- bolus goes to esophagus

3) oesophageal stage
- involuntary
- bolus goes to the stomach

4) bolus enters stomach

Question 9

Structure and function of the stomach.

Answer 9

• made of three layers of smooth muscle and rugae.
• Mucosa
• submucose
• muscular layer
• serosa

Stomach mucosa:
- mucous cells secrete mucous
- parietal cells secrete HCl
- intrinsic factor required for vitamin B12 absorption
- chief cells secrete pepsinogen and gastric lipase
- G-cells secrete gastrin (hormone) which stimulates parietal and chief cells to increase secretions and motility.
- STEM cells to renew stomach lining.

Function:
- starch digestion continues
- protein digestion begins

Mechanical digestion:
- mixes every 15-25 seconds
- forms chyme (partially digested fluid)

Chemical digestion:
- HCl denatures proteins
- Pepsinogen is converted to pepsin which hydrolyses peptide bonds of proteins
- gastric lipase hydrolyses triglycerides minimally.

Question 10

What are the three phases of gastric activity?

Answer 10

• 3 overlapping phases, the names correspond to where the food is.
• Cephalic phase:
  Begins when sight, smell, taste or thought of food, therefore it prepares the stomach for its arrival by impulses to the submucosal plexus (vagus nerve), increasing gastric secretion from gastric glands, and increasing motility.
• Gastric phase:
  Food reaches stomach and triggers stretch receptors and chemoreceptors (pH). This enhances secretion for gastrin, pepsinogen, and HCl. Stimulates gastric motility.
• Intestinal phase:
  Begins when chyme enters duodenum. There is an increase of intestinal secretion including secretin, CCK, and GIP. This phase controls the rate of chyme exiting from the stomach (gastric emptying).

Question 11

Describe the vomiting reflex.

Answer 11

1. Deep inspiration closes and close or the epiglottis.
2. Diaphragm and abdominal muscles contract, compressing the stomach.
3. Gastric contents are forced out of the gastroesophageal sphincter when it relaxes.
4. Distension of the oesophagus influences peristalsis to force contents back to the stomach
5. This cycle repeats causing retching
6. Pressure in the oesophagus increases, jaw thrusts out, pharyngeal sphincter opens and gastric contents are forced out.

Question 12

What are the three histological sections of the small intestine and what are their functions respectively?

Answer 12

1. duodenum:
   - Recieves chyme from the stomach and mixes with digestive enzymes from the pancreas
   - neutralises acids to avoid damage to absorptive surfaces, avoid inactivating digestive enzymes.
2. jejunum:
   - Chemical digestion and nutrient absorption
3. Ileum:
   - nutrient absorption of vitamin B12, bile salts, left over undigested products
   - controls flow of from ileum to large intestine.

Question 13

How does the small intestine maximise surface area?

Answer 13

1. Circular folds
2. Villi
3. Microvilli

These small finger like projections, covered by simple columnar epithelium increase the SA and therefore increase nerve endings, network of capillaries, and lacteals.

Question 14

What are the two main types of pancreatic cells and what are their functions?

Answer 14

• Acini and islet cells.

Exocrine: Acini:
- Secrete pancreatic juice, digestive enzymes, and buffers.

Endocrine: Islet:
- secrete insulin and glucagon into bloodstream

Question 15

What are the three main types of digestive enzymes produced and what are their roles in digestion?

Answer 15

Amylase:
- digest carbohydrates

Proteases:
- digest proteins

Pancreatic lipase:
- digest triglycerides

Question 16

What are the 7 main functions of the liver?

Answer 16

Metabolic regulation (carbohydrates, lipids, proteins)

Waste removal

Storage of vitamins and minerals

Drug metabolism

Haematological regulation

Bile production and secretion

Immune functions via Kupffer cells

Question 17

What is the role of bile in the digestive system?

Answer 17

Bile emulsifies lipids, breaking them into smaller droplets to increase surface area for pancreatic lipase to act, enabling lipid digestion and absorption.

Question 18

What are the main functions of the gallbladder?

Answer 18

Store bile

Concentrate bile by absorbing water

Release bile into the duodenum under hormonal control

Question 19

Can you explain the 4 steps of gallbladder physiology?

Answer 19

Bile produced by liver

Stored and concentrated in gallbladder

Chyme with fat enters duodenum

CCK triggers gallbladder contraction and bile release

Question 20

What are the classes of digestive enzymes?

Answer 20

Carbohydrases (amylases): break down carbohydrates

Proteases/peptidases: break proteins into amino acids

Lipases: digest fats into fatty acids and monoglycerides

Nucleases: break nucleic acids into nucleotides

Brush border enzymes: assist with final digestion steps

Question 21

Can you describe the main steps involved in digestion and absorption of nutrients?

Answer 21

Carbohydrates: Broken into monosaccharides by amylases and brush border enzymes; absorbed via secondary active transport and facilitated diffusion into capillaries.

Proteins: Broken into amino acids and small peptides by pepsin, pancreatic, and brush border enzymes; absorbed via primary/secondary active transport into capillaries.

Lipids: Emulsified by bile salts; digested by pancreatic lipase into fatty acids and monoglycerides; absorbed via diffusion and formed into chylomicrons that enter lacteals.

Question 22

How does the structure of the large intestine differ from the small intestine?

Answer 22

It has no villi or circular folds; mucosa contains absorptive and goblet cells arranged in crypts; has teniae coli and haustra for mechanical movement.

Question 23

What are the main functions of the large intestine?

Answer 23

Reabsorption of water

Compaction of waste into feces

Absorption of vitamins produced by bacteria

Breakdown of residual material

Storage of feces before defecation

Question 24

Can you describe the two final stages of digestion in the large intestine?

Answer 24

Mechanical: Haustral churning and mass movement (strong peristalsis during/after meals).

Chemical: Bacterial fermentation of undigested carbohydrates and production of vitamins like K, biotin, and SCFAs.

Question 25

What stimulates and completes the defecation reflex?

Answer 25

Distension of the rectum activates stretch receptors, triggering peristalsis and relaxing the internal anal sphincter; voluntary relaxation of the external sphincter completes defecation.