W10 GI System Refresher & Nutrients and Drug Absorption

Question 1

What are the key functions of the digestive system?

Answer 1

1. Ingestion
2. Digestion (mechanical: gut motility &
   chemical: gut secretions)
3. Absorption
4. Elimination

Question 2

What are the two major anatomical components of the GI system?
What makes up these components?

Answer 2

1. Gastrointestinal tract
   (Mouth, Oral Cavity, Orapharynx, Oesophagus, Stomach, Small & Large intestines, Rectum, Anus)
2. Accessories components (Teeth, Tongue, Salivary glands, Liver, Gall bladder, Pancreas)

Question 3

What is the gastrointestinal tract?

Answer 3

One long tube (also known as alimentary canal/tract)
~30 feet (~ 9 meter) long
Consists of: (Mouth, Oral Cavity, Orapharynx, Oesophagus, Stomach, Small & Large intestines, Rectum, Anus)

Question 4

What are the different layers of the gastrointestinal tract? (5

Answer 4

Lumen
Mucosa
Submucosa
Circular muscular layer
Longitudinal muscle layer
Serosa

Question 5

What is the Lumen?

Answer 5

Epithelial tissue; food move from mouth to anus

Question 6

What is the Mucosa layer?
What is its function?

Answer 6

-Thin layers of epithelium, lamina propria (connective tissue) and muscularis mucosa (smooth muscle).
-Secretes mucus, digestive enzymes, and hormones

Question 7

What is the Submucosa layer?
What does it contain?

Answer 7

-Thick layer of loose connective tissue.
-Contains blood vessels, lymphatic vessels, and nerves.
-Glands may be embedded in this layer. Enables peristalsis

Question 8

What is the muscular layer composed of?

Answer 8

The circular and longitudinal muscle layers.

Question 9

What is the circular muscle layer?

Answer 9

Smooth muscle layers, blood vessels and plexus nerves and innervation

Question 10

What is the Longitudinal muscle layer?

Answer 10

Peristalsis- Involuntary contraction and
relaxation- movement of food

Question 11

What is the Serosa layer?

Answer 11

A thin sheet of connective tissue that contains blood and lymphatic vessels, and is covered by the peritoneum, Secretes serous fluid to allow lubricated sliding movements between opposing surfaces

Question 12

Regulation of GIS (nervous and hormonal control)
What are the digestive phases? (3)

Answer 12

• The enteric nervous system (exclusive for GIS) can integrate information without input from the CNS and facilitates GI secretions and motility.
• Generally, parasympathetic innervation is excitatory for GI function, and sympathetic innervation is inhibitory for GI function

1. Cephalic Phase
   anticipatory mechanisms (senses): saliva secretions
2. Gastric Phase
   Gastric secretions and motility: stomach
3. Intestinal Phase
   Intestinal secretions and motility: intestine, liver and pancreas.

Question 13

What components make up the GI components in the mouth?

Answer 13

Salivary glands
Parotid
Sublingual
Submandibular
Oesophagus

Question 14

What is Mastication?

Answer 14

• Chopping and grinding of food in the mouth
  (physical digestion)
• Teeth (bone) and tongue (skeletal muscle) aid the
  physical breakdown of food
• Salivary glands in the mouth secrete amylase enzyme (amylase digest the carbohydrates- chemical digestion breaks down into simple/smaller molecules)
• Water in the saliva helps to make the food moist and bolus for easy swallowing and passage into the propharynx
• Lysozyme is an antibacterial salivary enzyme, and salivary immunoglobulins disable bacteria and viruses (defence)

Question 15

What is the Saliva composed of?
What is the pH of saliva?

Answer 15

99.5% of water
0.5% amylase, bicarbonate ion and other electrolytes
pH of saliva at 6.5-7.5

Question 16

What is the Oesophagus?
What is the structure like?

Answer 16

Swallowed food passes into the oesophagus, a narrow tube that travels through the thorax to the abdomen (~ 8 inch long in adult)

The walls are skeletal muscle initially but transition to smooth muscle about two-thirds of the way down the length.

The muscles are under conscious control, used when breathing, eating, belching, and vomiting.

Question 17

Stomach:
What is the cardiac sphincter?

Answer 17

-A thin ring-like muscle helps to prevent
stomach contents from going back up into the oesophagus.
-Also, vents air out (burping)

Situated on top of stomach-opens to allow food bolus to enter from oesophagus into stomach. no relation to heart.

Question 18

Stomach:
What are the three regions of the stomach?
What is the function?

Answer 18

1. Fundus
2. Body
3. Antrum
   -mixing of food bolus with acid & enzyme
   - largely digestion but some absorption (like alcohol, aspirin, some lipid-soluble’s

J-shaped- Size~12x6 inches, capacity: 1-4 L
(food stays 2-4 hrs; pH: 1-3, Strongly acidic

Chyme (Stomach acids + enzymes + food) moves to the intestine after being formed in the stomach

Question 19

Stomach:
What is the Rugae?

Answer 19

Series of ridges by folding of the stomach walls; contains secretory pits (acid and others); provides a large surface area and expansion to a full capacity

Question 20

Stomach:
What is the Pylorus sphincter?

Answer 20

-A valve/sphincter that connects the stomach to the small intestine.
-Prevents the backflow of contents from the small intestine to the stomach

Question 21

Gastric Mucosa
- What is it made up of?
Gastric secretion in stomach:

Answer 21

Acids, Enzymes, Hormones & Factors

Mucous surface cell
Mucous neck cell
Parietal cells
Enterochromaffin-like cell
Chief cells
D cells
G cells

Question 22

What does Acid in the stomach lumen cause?

Answer 22

-release and activation of pepsin, an enzyme that digests proteins.
-somatostatin release from D cells.
-denatures proteins (more accessible pepsin)-
-kill bacteria and other ingested microorganisms.
-inactivates salivary amylase, stopping carbohydrate digestion that began in the
mouth

Question 23

Gastric acid secretion in stomach:
Enzymes-

Answer 23

Pepsin- activates enzymes, converts pepsinogens into pepsin

Question 24

Chief cells:

Answer 24

Secrete pepsin(ogen) and gastric lipase
Digests proteins and fats

Question 25

What Hormone is secreted from Enterochromaffin-like cells?

Answer 25

Secretes histamine (hormonal stimulus for proton pump) =Helps modulate the level of acid secreted in the stomach. Stimulates gastric acid secretion. Acetylcholine and gastrin are stimuli for histamine release from ECL cells ..which then in turn stimulates parietal cells to secrete HCl

Question 26

Gastric acid secretion- **intrinsic factors**

Answer 26

Parietal cells secrete intrinsic factor This complexes with vitamin B12 to permit absorption Presence of Gastrin and Histamine are the stimulus for release

Question 27

What occurs in the small intestine? How long is it? What are its 3 parts? What does its cross-section look like?

Answer 27

Most digestion and absorption 3 m to 5 m long (~10 feet to 20 feet) Duodenum- Jejunum- Ileum Cross section of SI: Intestinal mucosa has finger-like projections, Villi. Multi-folded (plicae), the invaginations are known as crypt After SI, Waste/unabsorbed moves to the large intestine (caecum

Question 28

Small intestine: What occurs in the duodenum?

Answer 28

Mixing of chyme and secretions (digestive acids and enzymes) from gall bladder & pancreas ducts (Oddi-tonically controlled sphincter). Absorption of minerals, vitamins and other micronutrients begins here.

Question 29

What occurs in the jejunum?

Answer 29

* Further (segmentation) mixing of chyme and secretions (digestive acids and enzymes). * Absorption of minerals, vitamins and other micronutrients, digested carbs, fats and proteins.

Question 30

What is the ileum? What processes occur?

Answer 30

Last part (largest part) of the small intestine, several folds-thin walls; mixing of digested, and non-digested (segmentation) and secretions (digestive acids and enzymes). A Absorption of leftover digested carbs, fats and proteins from the jejunum

Question 31

Intestinal secretions (Mucus, Enzymes and Hormones)

Answer 31

Mucus is secreted by duodenal glands and goblet cells, and protects the duodenum from gastric acids and digestive juices Sucrase, lactase, maltase and amylase digest carbohydrates and are absorbed via glucose transporters (SGLT, GLUTs)

Question 32

Hormones

Answer 32

Hormones: (a complex interaction between satiety and appetite, more detail in Dr Healy’s lectures) Cholecystokinin: stimulate gallbladder contraction, and pancreatic enzyme secretions and inhibits gastric emptying and secretions Secretin: stimulate bicarbonate secretion and inhibits gastric emptying and secretions Gastric inhibitory peptide- inhibits gastric emptying and secretion Glucagon-like peptide-1: inhibits gastric emptying and secretion (satiety

Question 33

What occurs in the large intestine? How long is it? What do the bacteria in the LI produce? (4)

Answer 33

* Water & electrolyte absorption * 1.5 metres, or 5 feet (20-30 hrs holding) * Composed of Caecum, Rectum, Anal Sphincter - Completion of absorption of nutrients. 2-3 times a day of gastrocolic reflux (bowel movement). - The bacteria in the LI produce vitamins (B12, thiamin, and riboflavin) as well as vitamin K.

Question 34

Accessories- Liver (A vital organ for metabolism) What is special about the liver? What are the functions of the liver? (6)

Answer 34

Largest internal organ (1.5 kg) The liver receives 30% of the resting cardiac output * Glucose and fat metabolism * Protein synthesis * Hormone synthesis * Urea production * Detoxification * Storage

Question 35

Liver secretions (bile) & fat absorption

Question 36

Accessories- Pancreas What are Exocrine glands?

Answer 36

95 percent of the pancreas is exocrine tissue. It produces pancreatic enzymes to aid digestion. -makes about 2.2 pints (1 litre) of these enzymes every day (healthy human)

Question 37

What is contained in the pancreas? What is digested?

Answer 37

Alkaline clear/milky liquids (containing several enzymes) Amylase- to digest carbs Lipase- to digest fat Trypsin- to digest proteins

Question 38

How long is the liver? What are endocrine glands?

Answer 38

Gland organ 6-8 inches long (leaf-shaped) 5 percent comprises hundreds of thousands of endocrine cells known as islets of Langerhans. (glucose control by insulin and glucagon)

Question 39

What does the gastric mucosa contain?

Answer 39

Gastric acids for gastric secretion Enzymes Hormones and factors

Question 40

What does acid in the stomach cause?

Answer 40

-release and activation of pepsin, an enzyme that digests proteins. -somatostatin release from D cells. -denatures proteins (more accessible pepsin)- -kill bacteria and other ingested microorganisms. -inactivates salivary amylase, stopping carbohydrate digestion that began in the mouth

Question 41

GI Motility: How do peristaltic waves work?

Answer 41

1. Moves the bolus forward movement/Propulsive 2. Coordinated involuntary contraction and relaxation of circular and longitudinal muscles 3. Adjacent segments alternatively contract and relax 4. Sphincters prevent the back flow Epiglottis opens, when food is swallowed it shuts to prevent food going into airway (Hiccups- food goes into airway) Epiglottis sphincter contracts involuntary, food moves from mouth to stomach

Question 42

Immune functions of the GI tract: What is the function of M cells??

Answer 42

* Sense pathogens M cells are a type of epithelial cells, most abundant in the epithelium overlying lymphoid follicles such as Peyer's patches in the intestine. The apical or lumen-facing surface has few or no microvilli . Mucosal immune response and immune surveillance M-cells have the unique ability to take up a variety of antigens - bacteria, viruses, parasites and proteins - from the lumen, transport them by transcytosis across the cell and deliver them to immune cells lying underneath, particularly dendritic cells

Question 43

Spacer between lectures

Question 44

Nutrient digestion & absorption: Carbohydrates

Answer 44

Diet contains disaccharides and complex polymers (starch and sucrose) All carbs are broken down into monosaccharides & absorbed in the small intestine 1. Starch Glycogen 2. Disaccharides 3. Monosaccharides

Question 45

Decreased lactase activity is associated with a condition known as..? What If ..... take milk/milk products?

Answer 45

Lactose intolerance Disaccharide: (dairy products) - Glucose + Galactose= Lactose Broken down by Lactase Galactose produced (monosaccharide) * Diarrhoea * Bloating and flatulence. Remedy: remove milk products from the diet, although milk predigested with lactase is available

Question 46

Nutrient digestion & absorption Proteins:

Answer 46

All proteins are broken down into di/tri-peptides & absorbed in the small intestine Small peptides also transported across the membrane by the process of transcytosis

Question 47

Nutrient digestion & absorption Lipids & Cholesterol Bile salts coat lipids to make emulsions and form..?

Answer 47

Most lipids are hydrophobic and must be emulsified to facilitate digestion in the aqueous environment of the intestine 1. Bile salts coat lipids to make emulsions and form micelles -Micelles are small disks with bile salts, phospholipids, fatty acids, cholesterol, and mono-/diglycerides. 2. Lipase and colipase digest triglycerides

Question 48

Nutrient digestion & absorption

Answer 48

Nucleic acid polymers (DNA & RNA) digest into amino bases and monosaccharides by pancreatic and intestinal enzymes and are absorbed via active transporters. The fat-soluble vitamins (A, D, E, and K) are absorbed in the small intestine along with fats. The water-soluble vitamins (C and most B vitamins) are absorbed by mediated transport. (Except, vitamin B12, also known as cobalamin because it contains the element cobalt; Dietary supply of B12 from seafood, meat, and milk products. The intestinal transporter for B12 is found only in the ileum and recognizes B12 only when the vitamin is complexed with a protein called intrinsic factor, secreted by the same gastric parietal cells that secrete acid) Minerals, the intestinal absorption of Iron and Calcium is regulated by hormonal and facilitated transport pathways Electrolytes (Sodium, potassium, chloride) and water are absorbed through various pathways (paracellular diffusion, active transport, channels and others)

Question 49

Drug Absorption How are drugs absorbed?

Answer 49

1. Passive diffusion (MAIN) 2. Active transport/diffusion

Question 50

What factors affect passive diffusion of drug transpo

Question 51

How do drugs through the lipid bilayer?

Answer 51

They need to be in their Un-ionised form for the drugs (only) to cross lipid bilayers by passive diffusion

Question 52

Factors influencing G. I. absorption (physiological)

Answer 52

- G.I. motility - G.I. secretions & enzymes - Drug-food/supplement interactions - Differences in luminal pH along the GI tract - Surface area per luminal volume - Blood perfusion - Presence of bile and mucus - The nature of epithelial membranes

Question 53

What are the Advantages of G. I absorption? (physiological)

Answer 53

– Large surface area for passive diffusion – Range of pH environments promotes the uptake of weak acids/bases – Richly vascularised (high blood supply) – Long tract and long dwell time – Some active transport (e.g. Levodopa taken up by phenylalanine transporter) – Small intestine is a major site for drug absorption