D2 Digestion Flashcards
What mechanisms control the secretion of digestive juices? How?
Nervous and hormonal mechanisms
* nerves and hormones ensure resources are devoted to digestion only when needed
What does the smell, sight, thought of food causes to the brain?
Cause the brain to send nerve impulses via the vagus nerve from the medulla
What are gland cells in the stomach wall stimulated to do?
to secrete components of gastric juice
What do chemoreceptors and stretch receptors have to detect to send impulses to the brain?
Chemoreceptors in the stomach wall detect peptides in the stomach contents
Stretch receptors detect distension of the stomach
Where are chemoreceptors?
in the stomach wall
How does the brain respond to impuleses sent to the brain from chemoreceptors and stretch receptors?
respond by sending impulses via the vagus nerve to endocrine cells in the wall of the duodenum and the part of the stomach nearest to the duodenum, stimulating them to secrete gastrin
What does the hormone gastrin stimulate?
secretion of acid (HCL) and pepsinogen by two types of exocrine gland cell in the stomach wall
What 2 types of exocrine gland cell in the stomach wall is HCL and pepsinogen secreted by?
HCL: Parietal cells
Pepsinogen: Chief cells
What is secreted if pH in the stomach falls too low?
Secretin and somatostatin hormones are secreted to inhibit gastrin secretion
What is the alimentary canal?
the passage thorugh which food passes from mouth to anus
What is the alimentary canal?
the passage thorugh which food passes from mouth to anus
What do exocrine glands do?
Secrete digestive juices
What are examples of exocrine glands?
- salivary glands
- pancreas
- gland cells in the stomach and intestine wall
What is the difference between endo and exocrine glands?
Endo: secrete directly into the bloodstream
Exo: secrete into ducts
What is the arrangement of cells in an exocrine gland?
- secretory cells are in groups (acinus) around the duct branch
What is an acinus?
A group of secretory cells
What is an acinus?
A group of secretory cells
What are the features of an exocrine gland? (3)
- extensive endoplasmic reticulum for synthesis of enzymes
- numerous mitochondria to povide ATP for protein synthesis and other cell activities
- large numbers of secretory vesicles containing enzymes
When can the process of exocytosis be seen in progress in an exocrine gland?
When plasma membrane of vesicles containing enzymes is in contact with the duct
What are the features of a inner surface of the ileum?
- numerous folds
- each folds is covered in tiny projections called villi
Where does absorption take place?
through the epithelial cells covering each villus
What are the adaptations of the villus? (5)
- Tight junctions - between each epithelial cell covering the villus, which ensures that most materials pass into the blood vessels lining the villi through the epithelial cell
- microvilli - extensions that increased cell surface area of membrane on the intestinal lumen side. The collection of microvilli on the intestinal side of the epithelial cells is termed the brush border.
- Large numbers of mitochondria - high amount of ATP to drive active transport processes
- Large numbers of pinocytic vesicles - due to absorption of some foods by endocytosis
- Different types of proteins - on the apical and basal surface for material transport
What is the apical and basal surface?
Apical: the surface facing the lumen of the intestine
Basal: the surface facing the blood vessels
What is acid secreted by in the stomach?
Parietal cells
What does the acid do in the stomach?
disrupts the extracellular matrix that holds cells together in tissues
What does stomach acid do to proteins?
- Denatures the protein
- exposing the polypeptide chains
- so that the enzyme pepsin can hydrolyse the bonds within the polypeptides
What is pepsin released by?
chief cells as the inactive pepsinogen
How is inactive pepsinogen converted to pepsin?
the acid conditions within the stomach
Why is pepsin released as inactive pepsinogen?
To ensure that the cells that produce pepsinogen are not digested at the same time as the protien in the diet
What are stomach ulcers?
open sores
What is stomach ulcer caused by?
partial digestion of the stomach lining by the enzyme pepsin and HCL in gastric juice
What is stomach cancer?
the growth of tumours in the wall of the stomach
Until recently, what was beleived to be a major contributory factor in the development of stomach ulcers?
- emotional stress
- excessive gastric juice secretion
What is now known to be a more significant cause of stomach ulcers?
A bacterium, Helicobacter pylori
What is helicobacter pylori?
A bacterium that cause stomach ulcers and is also associated with stomach cancer
How does the body protect itself from stomach acid?
Stomach acid is corrosive
* body produced a natural mucus barrier which protects the lining of the stomach
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How is the symptom heart burn caused?
When the circular muscle at the top of the stomach is not functioning, and the acid escapes and irritates the esophagus (acid reflux)
What is the circular muscle at the top of the stomach used for?
prevents fluid from escaping the stomach
How is the acidic environment of the stomach produced?
By a proton pump called the H+,K+-ATPase
How does H+,K+-ATPase work to produce an acidic environment?
uses one ATP molecule to exchange two protons from the cytoplasm for two potassium ions in the lumen surrounding the parietal cell
What is a therapy that is increasingly prescribed for gastric diseases?
Proton pump inhibitors or PPIs
How do PPIs work?
- consumed in an inactive form
- bind irreversibly to a single pump (not permanent)
- Acid conditions in the vicinity of the parietal cells convert them to the active form close to their target
How do PPIs work?
- consumed in an inactive form
- bind irreversibly to a single pump (not permanent)
- Acid conditions in the vicinity of the parietal cells convert them to the active form close to their target
How is the effect of PPIs on the overall acid production system not permanent?
pumps are normally recycled and replaced with new pumps
How is the effect of PPIs on the overall acid production system not permanent?
pumps are normally recycled and replaced with new pumps
What is deitary fibre?
The edible part of plants that are resistant to being digested and are not absorbed from the small intestine
What are examples of dietary fibres?
Cellulose and lignin
What is the consequence of dietary fibre not being digested and absorbed?
there is a fraction of ingested food which never leaves the digestive tube
= secretion into the digestive tube occurs
What are excretory products from the breakdown of red blood cells?
Bilirubin
What has to happen to water in the process of digestion?
has to be reclaimed in the large intestine
What materials are egested as faeces?
- the excretory products (bilirubin)
- unabsrobed water
- undigested dietary fibre
What are the two categories of dietary fibre?
soluble and insoluble
What are the two categories of dietary fibre?
soluble and insoluble
Why does a healthy balanced diet contaian fibre?
- it increases the bulk of material passing through the intestines
- helps prevent constipation as it draws water into the intestine
What is the relationship of water content and movement of fecal matter?
The higher the water content of the intestine, the faster the movement of fecal matter (rate of transit)
What are the benefits of fibre in the diet besides for transit?
- risk of various diseases of the large intestine may be reduced i.e. bowel cancer, hemorrhoids, appendicitis
- presence of bulky material in the stomach and intestine may increase feelings of satiety, reducing the desire to eat and the risk of obesity
- Absorption of sugar may be slowed down, helping to prevent the development of type II diabetes
What are the sources of fibre?
Foods of plant origin
* whole-grain bread and cereals
* vegetables i.e. cabbage
* salads i.e. celery
* foods made from cultured fungi (mycoprotein)
What is cholera?
a diseases caused by infection by the bacterium Vibrio cholera
How is the bacterium vibrio cholera brought into the cell?
the bacterium releases a toxin that binds to a receptor on intestinal cells
it is then brough into the cell by endocytosis
How does vibrio cholera cause dehydration?
- once inside the cell, it triggers a cascade response that ultimately leads to the efflux of Cl- and HCO₃- ions from the cell into the intestine
- water follows by osmosis leading to watery diarrhoea
- water is drawn from the blood into the cells to replace the fluid loss from the intestinal cells
- can cause severe dehydration and then death
