Physiology of Digestion Flashcards
1
Q
- What makes up the alimentary canal of the GI tract?
- What makes up the digestive accessory components?
A
- Mouth, oesophagus, stomach, duodenum , small and large intestine, rectum, anus
- Teeth, salivary glands, liver, gallbladder, pancreas
2
Q
How are the accessory components linked to the alimentary canal?
A
- Connected through ducts (apart from the teeth)
3
Q
Describe the general process of digestion.
A
- Ingestion of food
- Secretion of enzymes in salivary glands to breakdown food
- Mastication
- Peristalsis throughout alimentary canal
- Digestion (mechanical/chemical) and absorption from stomach onwards
- Anything not required is excreted as waste
4
Q
Where does most absorption occur?
A
SMALL INTESTINE
5
Q
Describe the areas of the stomach.
A
- Cardia, fundus, corpus (or body) and antrum
- All comprised of pits - different cell types across different areas
6
Q
Describe the cell types within the pits located in the corpus.
A
- Surface epithelial cells
- Mucus neck cells
- Chief cells - secrete pepsinogen.
- Parietal cells - secrete HCl and intrinsic factor (for vitamin B12 digestion)
7
Q
Describe the cell types within the pits located in the antrum.
A
- No parietal cells
- G cells - secrete gastrin - stimulate parietal cells to secrete acid
- D cells - secrete somatostatin - inhibits gastrin secretion from G cells
8
Q
Describe acid secretion in the parietal cells. PART 1
A
- NKCC1 channel permits entry of 1 sodium, 1 potassium and 2 chloride ions into the cell using the concentration gradient of sodium as the energy source
9
Q
Describe acid secretion in the parietal cells. PART 2
A
- Chloride release into gastric lumen through passive diffusion through chloride channels
- Entry of water and CO2 passively through basolateral membrane
- Turned into H+ and HCO3- through carbonic anhydrase
10
Q
Describe acid secretion in the parietal cells. PART 3
A
- HCO3- pumped out of basolateral cell membrane through AE2 channels - pump chloride ions into cell
- Hydrogen pumped out of apical membrane through hydrogen-potassium ATPase
- Interacts with chloride to form HCl
11
Q
How do proton pump inhibitors e.g omeprazole work?
A
- Block hydrogen-potassium ATPase
- Prevent entry of hydrogen ions into gastric lumen
12
Q
What stimulates acid secretion from parietal cells?
A
- Vagus nerve - direct stimulation of parietal cell. Also acts on ECL cells - secrete histamine which acts on parietal cells
- Gastrin release acts on CCK-2 receptors on both parietal/ECL cells - also stimulating histamine release
- Increased expression of hydrogen-potassium ATPase ∴ increased H+ efflux
13
Q
Describe the differences in mucus layers found in the stomach and small intestine.
A
- Stomach protected from eroding effects of acid through mucus production - rich in neutralising bicarbonate
- Small intestine - layer of mucus thinner and more loosely attached so more liable to damaging effects of stomach acid
14
Q
What happens under normal conditions to gastric acid?
A
- Neutralised on entry into small intestine through bicarbonate secreted by pancreas
- Cannot buffer all acid if gastric acid secretion is abnormally high
15
Q
What occurs during H. Pylori infections?
A
- If antrum infected, causes inflammation and stimulates G cells
- Elevated gastrin secretion ∴ elevated acid secretion
- Inflammation degrades gastric mucosa - loss of mucus producing cell. More susceptible to acid erosion
16
Q
What is Zollinger-Ellison syndrome?
A
- Extra-gastric gastrinoma (gastrin producing tumours)
- Located in pancreas/duodenum
- Secrete gastin - increasing HCl production
17
Q
With regards to the macronutrients, why is digestion important?
A
- Ingested macronutrients cannot be used for energy in non-digested form e.g proteins, carbohydrates and triglycerides
18
Q
Why do complex carbohydrates ingested through food (e.g sucrose, starch and lactose) need to be broken down to glucose before being used for energy release?
A
- Body cannot utilise any form of sugar other than glucose
19
Q
Describe carbohydrate digestion in the mouth.
A
- Ptyalin (alpha-amylase released by parotid gland
- Breaks down starch molecules through hydrolysis to maltose
- 5% of carbohydrate digestion occurs here - food doesn’t stay in mouth for long
20
Q
Describe carbohydrate digestion in the upper GI tract.
A
- Doesn’t occur in stomach - amylase denatured by acid
- Most occurs in duodenum
- Exocrine tissue (acinar cells) of pancreas secretes amylase which converts starch to maltose
21
Q
Enterocytes on the brush border of the villi of the small intestine contain carbohydrate enzymes. Identify these enzymes.
A
- Lactase hydrolyses lactose to form glucose and galactose
- Sucrase hydrolyses sucrose to form glucose and fructose
- Maltase hydrolyses maltose to form two glucose molecules
- Alpha-dextrinase breaks down extrin to form maltose, which is then further broken down by maltase
22
Q
What is special about the enterocytes (apart from the enzymes present on their surface)?
A
Site of sugar absorption, so carbohydrates are broken down and immediately absorbed
23
Q
What is the basis behind lactose intolerance?
A
- Lactase hydrolyses lactose into galactose and glucose
- In two thirds of people, lactose intolerance occurs due to non-expression of lactase gene
24
Q
Can amino acids be absorbed by the body?
A
NO
25
Describe protein digestion in the stomach.
- Gastric distension and vagal stimulation (from anticipated ingestion and mastication) stimulate gastrin secretion from G-cells
- Also stimulate parietal cells to secrete HCl
- Also stimulate chief cells to secrete pepsinogen
- HCl activates pepsinogen - converted to pepsin
- Pepsin cleaves complex proteins e.g collagen into smaller polypeptides
26
Where does the vast majority of protein digestion occur?
Duodenum and jejunum
27
Describe the major pancreatic proteases.
- Trypsin, chymotrypsin, carboxypeptidase and elastase
- Many released in precursor film - so don't digest pancreas itself
- Inside duodenum, trypsinogen cleaved to form trypsin by enteropeptidase
- Other precursor enzymes activated by trypsin in pancreas
28
Describe fat digestion in the duodenum.
- Fats from diet primarily composed of triglycerides, cholesterol and phospholipids
- Enteroendocrine cells release hormones
- CCK travels through blood to gall bladder - stimulates its contraction and bile secretion into small intestine
28
Describe protein digestion in the small intestine.
- Proteases expressed on surface of enterocytes - remaining polypeptides into amino acids, di/tripeptides
- Small peptides and amino acids taken up by enterocytes
- Intracellular proteases - breakdown into individual amino acids that transported into circulation
29
Describe the role of bile in fat digestion.
- Emulsification of fats - turn into smaller fat droplets
- Lethicin and bile salts contain fat soluble and water soluble components
- Fat soluble ends - dissolve in fat globules. Water soluble ends - dissolve in water of duodenum
- Reduced interfacial tension of fat globules which break down into smaller droplets. Surface area increases
30
Describe the role of pancreatic lipase in fat digestion.
- Lipases now enters the duodenum
- Lipases is water soluble so attacks fat droplets from outside and break down into smaller droplets
- Breaks down fatty acids at positions 1 and 3 of triglycerides - forms 2-monoglycerol and 2 fatty acids via hydrolysis
- Phospholipids and cholesterol also broken down into fatty acids
31
Describe what happens after the role of lipases in fat digestion.
- Triglyceride hydrolysis is reversible - fatty acids and monoglycerides need to be removed
- Bile salts remove these by forming micelles
- Micelles encapsulate these products and because polar group is negatively charged, allows it to dissolve in water
32
Describe micelle structure.
Small (40nm) spherical structures that have a fat soluble nucleus and a water soluble polar group
33
What happens at the small intestine in fat digestion?
- Micelles deliver fatty acids to enterocytes
- Na-H antiporters on walls of enterocytes acidify environment. Micelle breaks down - releasing products
- Products taken up inro ER - turned back into triglycerides and chloesterol before transported to Golgi
- These are packaged and absorbed into lymphatic system
