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Karan - MBBS Year 1: Gastrointestinal Week 1 > Physiology of Digestion > Flashcards

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
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2
Q

How are the accessory components linked to the alimentary canal?

A
  • Connected through ducts (apart from the teeth)
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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
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4
Q

Where does most absorption occur?

A

SMALL INTESTINE

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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
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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)
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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
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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
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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
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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
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11
Q

How do proton pump inhibitors e.g omeprazole work?

A
  • Block hydrogen-potassium ATPase
  • Prevent entry of hydrogen ions into gastric lumen
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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
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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
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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
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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
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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
Q

Describe protein digestion in the stomach.

A
  • 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
Q

Where does the vast majority of protein digestion occur?

A

Duodenum and jejunum

27
Q

Describe the major pancreatic proteases.

A
  • 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
Q

Describe fat digestion in the duodenum.

A
  • 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
Q

Describe protein digestion in the small intestine.

A
  • 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
Q

Describe the role of bile in fat digestion.

A
  • 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
Q

Describe the role of pancreatic lipase in fat digestion.

A
  • 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
Q

Describe what happens after the role of lipases in fat digestion.

A
  • 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
Q

Describe micelle structure.

A

Small (40nm) spherical structures that have a fat soluble nucleus and a water soluble polar group

33
Q

What happens at the small intestine in fat digestion?

A
  • 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

Karan - MBBS Year 1: Gastrointestinal Week 1 (6 decks)

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