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True or false

  • A. In the human, starch digestion begins in the stomach.
    • F, starts in mouth with salivary amylase.
  • B. Amino acids enter the lacteal by diffusion.
    • F, enter blood stream, fat enters the lacteal.
  • C. In humans the major digestive and absorptive processes take place in the small intestine.
    • T, this has the greatest surface area for absorption
  • D. The swallowing reflex facilitates closure of the glottis by raising the larynx.
    • T, closure of the glottis is aided by muscles of the neck, which raise the larynx.
  • E. Trypsinogen is converted to its active form trypsin by HCl.
    • F, activated by enterokinase (now called enteropeptidase), an enzyme in the cells that line the duodenal mucosa; pepsinogen is activated by HCl.
  • F. Pancreatic juice is an aqueous acidic solution.
    • F, it is an alkaline solution, rich in bicarbonate that is secreted mainly by the ducts.
  • G. Vitamin B and C are fat soluble.
    • F, these are water soluble vitamins. A, D, E and K are fat soluble vitamins
  • H. Calcium is absorbed only by simple diffusion.
    • F, up to 50% is an active process involving a binding protein and a calcium stimulated ATPase pump. The remaining 50% is simple diffusion.

Enzymes of digestion

Food group Enzyme Source Site of action Breakdown product
Sugar Salivary amylase Salivary glands Mouth/Stomach Disaccharides (i.e., maltose)
Pancreatic amylase Pancreas Small intestine Disaccharides
Disaccharidases (i.e., maltase, sucrase, lactase) Small intestine Small intestine Monosaccharides (e.g., glucose)
Protein Pepsin Stomach Stomach small peptide fragments
Trypsin Pancreas Duodenum small peptide fragments
Chymotrypsin Pancreas Duodenum small peptide fragments
Carboxy- peptidase Pancreas Duodenum single amino acids
Enteropeptidase Intestinal brush border (mucosa) Small intestine small peptide fragments
Fat Lipase / co-lipase Pancreas Small intestine free fatty acids, monoglycerides
Phospholipase A2, cholesterol esterase Pancreas Small intestine cholesterol, lyso‐ lecithin (+fatty acid)


Water balance

  • Ingest 2000 mL/day
  • Saliva:1500 mL/day
  • Gastric secretions: 2000 mL/day
  • Bile: 500 mL/day
  • Pancreatic secretions: 1500 mL/day
  • Intestinal secretions: 1500 mL/day
  • Small intestine absorbs 8500 mL/day
  • Colon absorbs 400 mL/day
  • 100 mL/day water excreted

True or false

  • A. The Na+/Ca2+ exchanger is responsible for establishing the Na+ gradient by which many nutrients are transported across the enterocytes.
    • F, a sodium gradient is important but it is established by a Na+/K+-ATPase.
  • B. Water soluble vitamins are absorbed with the help of micelles.
    • F, if they are water soluble they can not enter the hydrophobic/fatty micelle. Instead they are absorbed with the aid of transport proteins.
  • C. The enzyme pepsin is required for activation of chymotrypsinogen
    • F, pepsin is a stomach enzyme which is inactivated by normal pH, thus it can not activate chymotrpsinogen which is located in the neutral duodenum - enterokinase is responsible.
  • D. Reduced absorption of vitamin B12 in the stomach can result in anaemia
    • F, vitamin B12 malabsorption does cause anaemia, but this takes place in the terminal ileum.
  • E. Lactose build up in the colon causes symptoms of lactose intolerance.
    • T, lactose acts as an osmotic laxative and as a nutrient for bacteria with produce gas. Together this causes irritation, bloating and diarrhoea.
  • F. The accommodation of the stomach is initiated by the enteric nervous system.
    • F, the ENS contracts the stomach, but the vagus controls relaxation as the stomach fills.
  • G. The enteric nervous system contains sympathetic and parasympathetic neurons.
    • F, the ENS contains enteric neurons, with some terminals from symp and parasymp. The cell bodies for these are in peripheral ganglia.
  • H. The secondary peristaltic wave during swallowing requires the enteric nerves.
    • T, the ENS is responsible for peristalsis after the the initial peristaltic wave is started by the swallowing centre in the brain stem.

Control of the GIT

Three motility reflexes Function
1. Peristalsis Propulsive
2. Segmentation Mixing
3. Migrating Motility Complex (MMC) Housekeeping
Two nerve layers Function
1. Myenteric plexus (Auerbach's) Motor and integrative functions
2. Submucosal plexus (Meissner's) Secretion, vasodilation
Three types of enteric neuron Function - Transmitter
1. Intrinsic sensory neurons Detect stretch/nutrients - ACh / SP
2. Interneurons Integrate reflexes - ACh / 5-HT, SOM, NO
3. Motor neurons Relax smooth muscle - ATP, NO, VIP Contract smooth muscle - ACh, SP

VIP = vasoactive intestinal peptide

Matching

  1. Carboxypeptidase
  2. Enterokinase
  3. Lacteal
  4. Microvilli
  5. Pyloric sphincter
  6. Hydrolysis
  7. Bile
  8. Pancreas
  9. Parotid
  10. Colon
  11. Disaccharidase
  • A. increases intestinal surface area.
  • B. absorbs fat.
  • C. splits molecules by the addition of water.
  • D. emulsifies fat.
  • E. contains bacteria.
  • F. secretions empty into the mouth.
  • G. secretes watery alkaline fluid.
  • H. sucrase and maltase.
  • I. activates trypsin.
  • J. guards exit of the stomach to small intestine. K. splits polypeptides into amino acids.

Answers

  1. K
  2. I
  3. B
  4. A
  5. J
  6. C
  7. D
  8. G
  9. F
  10. E
  11. H

Hormones of digestion

Hormone and class Source Stimulus Action
Gastrin (polypeptide) endocrine (G) cells of the stomach entry of food into the stomach stimulates secretion of HCl from the parietal cell of the stomach
Histamine (amine) endocrine (ECL) cells of stomach gastrin or ACh stimulates secretion of HCl from the parietal cell of the stomach
Somatostatin (polypeptide) endocrine (D) cells of stomach low luminal pH or Ach inhibits release of gastrin, histamine and acid
Chole-cystokinin (CCK) (polypeptide) endocrine cells of the duodenum arrival of food into the duodenum contracts gall bladder and releases bile, stimulates pancreas secretion of enzymes
Secretin (polypeptide) endocrine cells of the duodenum acid in the duodenum stimulates pancreas secretion of bicarbonate, stimulates liver duct cell secretion of bicarbonate

Further images

There were two additional slides included here:

Questions

  • What's the difference between somatostatin from the pancreas and the stomach