Session 9: Digestive System Flashcards
The alimentary canal consists of what?
Mouth, tongue, pharynx, esophagus, stomach, small intestine, colon of the large intestine, appendix, rectum, anus
What are the accessory organs of the digestive system?
Salivary glands, Liver, Gall bladder, Pancreas
What is the mesentery?
The double fold of the peritoneum that attaches the intestines to the wall of the abdomen
What are the four layers of the gut wall?
The mucosa (innermost) The submucosa The external muscle layers (muscularis externae) The serosa (outermost)
The mucosa of the gut wall lines the lumen and is made up of what three layers?
Epithelium
Lamina propria
Muscularis mucosae
What structures are often present in the lamina propria of the gut mucosa?
Peyer’s patches
The submucosa of the gut contains what important features?
Layer of connective tissue Glands Arteries Veins Nerves
The external muscle layers of the gut wall contain what?
How are these seen histologically?
2 layers of smooth muscle:
1) Outer longitudinal layer (cells with cigar-shaped nuclei)
2) Inner circular layer (cells with central nuclei)
Which layer of the gut wall is responsible for the successive peristaltic waves required to move luminal contents along the gut?
Muscularis externa (External muscle layers)
The outermost layer of the gut wall is known as what?
The serosa (a serous membrane)
The serosa of the gut wall is made up of what layers?
Connective tissue
Mesothelium (simple squamous epithelium)
The mesentery contains what?
Arteries
Veins
Nerves
List some of the major functions of the GI tract
Port of entry for food into the body Mechanically disrupt food Temporarily store food Chemically digest food Kill pathogens Move food along the tract Absorb nutrients from the resultant solution Eliminate residual waste
What is digestion?
The conversion of what we eat, by physical and chemical disruption into a solution from which we can absorb our nutrients
The solution brought about by chemical and physical disruption from which we absorb our nutrients is relatively _________, _______ in pH and __________
sterile
neutral
isotonic
Saliva starts digestion with the aid of which enzymes?
Amylase and lipase
What two features of saliva are bacteriostatic?
IgA
Lysozyme
What features of saliva helps to protect our teeth?
High in calcium
Alkaline
How does the digestive system contribute to physical disruption of food?
By the action of teeth, tongue and muscles of mastication (chewing)
During mastication, the mouth forms a _____ which enters the oesophagus
bolus
Which section of the oesophagus is under voluntary control?
Upper third
The lower section of the oesophagus is under ___________ control
involuntary
The upper end of the oesophagus contains some ________ _______ muscle as well as smooth muscle
Striated skeletal
The lower end of the oesophagus contains only which types of muscle?
Smooth
What mechanism is used to transport bolus to the stomach?
Rapid peristaltic transport
How long does it take for the bolus to travel from the mouth to the stomach?
8-9 seconds
Where does the fastest of the GI transport take place?
On entry (oesophagus) On exit (rectum/acus)
The oesophagus is made up of what four layers?
It is lacking what layer that is found in the gut wall?
Mucosa Submucosa Muscularis externa Adventitia There is no serosa, not intraperetaneal
What are the name of the two major nerve centres of the digestive system?
Myenteric plexus
Submucosal plexus
The stomach undergoes receptive relaxation, what is meant by this?
The walls relax so that pressure in the stomach does not increase as it swells (up to a point)
The stomach secretes what to break down tissues and disinfect?
Acid and proteolytic enzymes
How does the stomach protect its epithelium?
By secreting mucus onto its surface
The stomach produces hypertonic chyme, what is meant by hypertonic in this case?
The osmotic pressure of chyme increases and it therefore wants to draw in fluid from the rest of the body
What is chyme?
Broken down, partially digested stomach contents
Where is the chyme delivered to?
The duodenum
The stomach is lined with simple columnar epithelium with openings to various what?
Gastric pits
What extend down into the gastric pits?
Long, straight tubular gastric glands
Shallow gastric pits are lined with what cells?
Where are these cells infrequently found?
Surface mucous cells
Tubular gastric glands
The mucous secreted by gastric pits is released in response to what?
Distention
Stomach contents
Acid secretion from gastric glands
Secreted mucus is resistant to what?
Pepsin
What can cause damage to the mucous cells?
How are these damaged cells replaced?
Alcohol and aspirin
By mitosis in deeper cells in the neck of the gastric pit
Secreted mucus contains what which neutralises the effect of H+ ions?
Bicarbonate ions (HCO3-)
What is the isthmus?
The region of the gastric gland in which stem cells divide to populate the gland by upward or downward migration
What do parietal cells secrete?
H+ ions into the lumen
HCO3- ions into nearby capillaries which move it to surface mucous cells
What do chief cells secrete?
Pepsinogens which are converted into pepsins which partly hydrolyse proteins
Enteroendocrine cells include _____ cells which secrete what?
G cells
Gastrin
What does gastrin secretion by G cells cause in the wider gastric mucosa?
Causes the gastric mucosa to secrete acid
What is the name given to the proximal part of the small intestine?
Duodenum
What shape is the duodenum?
How long is the duodenum?
C-shaped
20-25cm
The walls of the duodenum contain ___________ _______ which secrete what?
Why?
Brunner’s glands
Bicarbonate
To neutralise acidic chyme
Which two organs connect with the duodenum?
Liver
Pancreas
__________ and __________ enter the duodenum at from ampulla of Vater?
Bile
Pancreatic juice
What is the ampulla of Vater?
The joining of the pancreatic duct and the common bile duct
_____ is drawn in from the ECF to render the hypertonic chyme _________.
Water
Isotonic
The liver releases a substance called _______, generally via what organ?
Bile
Gall bladder
What does bile contain?
Water
Alkali
Bile salts
What is the function of bile salts?
To emulsify fat
The _______ and ________ secrete alkali to neutralise acidic chyme, this is precisely controlled
Liver
Pancreas
Which organs secrete specific enzymes that come to lie in an “unstirred layer” and complete digestion
Liver
Pancreas
Intestine
What are the roles of enzymes secreted from the pancreas and intestine?
Cleave peptides into amino acids
Cleave polysaccharides to monosaccharides
Break down and re-form lipids
Break down nucleic acids
True or false: Absorption is a passive process
FALSE, it is an active process as it requires energy
Is absorption a slow or fast process?
Slow
How does the small intestine achieve such a large surface area for absorption?
Many folds
Villi and micro-villi on simple columnar cells
True or false: The intestine has a good blood supply/drainage system?
TRUE TRUE TRUE
How is blood drained from the small intestine and colon?
Via the hepatic portal vein
Blood containing products of digestion is drained from the gut and passes through the ________ before entering venous circulation
Liver
The entire small intestine is how long?
22 feet (7 metres)
With its length along with the folds and villi/micro-villi, the small intestine has a surface area of what?
> 25cm^2
The duodenum absorbs what mineral?
Iron
What is absorbed by the jejunum?
Most of sugars, amino acids and fatty acids
What is absorbed by the ileum?
Vitamin B12, bile acids and remaining nutrients
By the terminal ileum, ________ have been absorbed, but ________ still needs to be absorbed
Nutrients
Water (Lots of it)
The large intestine is made up of what?
Caecum Ascending colon Transverse colon Decending colon Sigmoid colon
The whole colon is approximately how long and how wide?
1.2m long
6-9cm wide
The large intestine is made up of what three layers?
Mucosa
Submucosa
Muscularis externa
The surface epithelial cells of the large intestine are responsible for what vital function?
Absorption of water and electrolytes
Which cells are responsible for the production of mucus and for supplying cells to the surface of the crypts?
The simple columnar epithelium of the crypts
Intestinal glands of the colon are known as what?
Crypts of Lieberkuhn
The large intestine continues a water recovery over a ______ hour transit
20
By the end of the large intestine the contents are in what state?
Semi-solid
Where do the semi-solid contents of the large intestine await expulsion?
In the colon, NOT the rectum
What happens if higher centre control overrides the reflex to defaecate?
Reverse peristalsis
Which part of the GI tract contains the most bacteria?
The colon
Of the bacteria contained in the colon, most are anaerobic or aerobic?
Anaerobic
True or false: Most of the bacteria of the GI tract are lost in faeces
True
Bacteria in the large intestine are important for what processes?
Synthesis of vitamins K, B12, thiamine and riboflavine
Breakdown of primary to secondary bile acids
Conversion of bilirubin to non-pigmented metabolites
Decreased absorption or increased secretion of fluid in the gut can result in what?
Life threatening dehydration
Life threatening electrolyte imbalance
How is motility and secretion in the gut kept under precise control?
Overlapping neural, paracine and endocrine mechanisms
The digestive system is under what kinds of neural control?
Somatic for ingestion (mouth, first 1/3 oesophagus) and excretion (last sphincter of anus)
Autonomic NS for rest
The somatic nervous system innervates what type of muscle in the digestive system?
Striated
_____ ___________ neurones form plexuses as part of the autonomic nervous system for the control of resting digestion
Post ganglionic
Where are plexuses of post ganglionic neurones found in the digestive system?
What are the names given to these plexuses?
One between circular and longitudinal muscle of the gut wall (Auerbach’s “Myenteric” plexus)
One between submucosa and muscularis externa (Plexus of Meissner)
True of false: The “gut nervous system” develops with the rest of the nervous system during development?
False, it develops separately and connects up
Which substances contribute to the paracrine control of the digestive system?
Histamine which controls production of acid in the stomach
Vasoactive substances which affect blood flow in the gut
Do substances that are released in a paracrine fashion in the gut act locally or systemically?
Locally
What aspects of digestion are under endocrine (hormonal) control?
Secretion of stomach acid
Alkali secretion from the liver and pancreas
Enzyme secretion
Generally the gut is controlled by hormones that it makes itself or elsewhere?
Makes itself
What hormone is responsible for the promotion of HCl production by parietal cells?
Gastrin
What cells are responsible for secreting Gastrin?
Where are they found?
G cells
Pyloric antrum of the stomach
Gastrin is released by..?
G cells of stomach
Pancreas
Duodenum
What is the role of secretin?
Promotes bicarbonate secretion from duct cells of the pancreas
Promotes bile production by the liver
Inhibits secretion of acid by parietal cells of the stomach
Where is cholecystokinin (CKK) synthesised and secreted from?
Enteroendocrine cells of the duodenum
What is the role of CKK?
Promotes release of digestive enzymes from the pancreas
Promotes release of bile from gall bladder (stimulates it to contract)
True of false: CKK causes hunger
FALSE, it is a hunger supressant
What are the three phases of swallowing?
1) Oral
2) Pharyngeal
3) Oesophageal
What happens during swallowing to prevent the solids/fluids from entering the trachea?
The epiglottis goes down and sits over the trachea to stop it from moving into the airway
What happens during swallowing to prevent the solids/fluids from entering the nose?
The soft palette moves up when we swallow to protect the nose
True or false: the main mechanism that moves food/liquid down our oesophagus is gravity
False!!! The main mechanism is peristalsis, gravity only plays a small part
What mechanisms do we have to prevent the stomach from digesting itself?
Mucus production
Bicarbonate production
Both act as a buffer to neutralise acid
Zymogens are not activated until acid is present in response to food
Without bile we would not be able to do what?
Absorb fats and therefore fat soluble vitamins: A,D,E and K
Does the small intestine normally contain bacteria?
No, not normally
What mechanisms are in place to prevent the bacteria in the colon from invading the colonic wall and underlying tissues?
Peyer's patches Mucus Tight junctions IgA High turnover of cells
