Week 8 (gastrointestinal system) Flashcards
What are the three main components of the GI tract?
Stomach
Small intestines
Large intestines (colon)
How many layers are there in the structure of the GI tract?
4
Name the 4 layers of the GI tract
Mucosa
Submucosa
Muscularis
Serosa
Describe the mucosa
An absorptive surface that contains blood capillaries and has a metabolic active part that is below a thin muscular layer to maintain an integral structure.
Describe the submucosa
Contains large blood vessels that lead to the liver and are branches of the lymphatic system and ANS.
Describe the muscularis
A double muscle layer that produces different types of movements and constrictions and can be circular or longitudinal.
Describe the serosa
A thick layer of connective tissue for separation of the stomach.
Describe the stomach
A muscular bag that varies in size from 75ml to 1l and can expand to 2l.
It contains basolateral folds (Rugae) that stretch out and increase surface.
Highly adapted for muscular contraction and secretion.
Describe the small intestines
A 4 layered structure that is fully adapted for absorption.
Contains villi (and microvilli) which increase the surface area for absorption.
Contains lacteal which helps with immune protection and helps long-chain fatty acids to enter the bloodstream.
Name the two functional segments of the small intestines
Duodenum jejunum
Ileum
Describe the duodenum jejunum
It has a large lumen towards the stomach.
High vascularised.
Dark red (due to greater absorption)
Muscular
Contains Brunners glands which produce mucus and bicarbonate.
Has long villi to increase surface area and therefore absorption.
Describe the ileum
Smaller food volume.
Less muscular
Pale appearance due to less vessels and less absorption.
Contain Peyers patches which protect from bacteria.
Have shorter villi and a smaller surface area resulting in less absorption.
Describe the large intestines (colon)
1.5m long
Large lumen
4 layered structure, but longitudinal muscles in 3 bands.
Contains crypts instead of villi, which increase surface area to maximise absorption.
Name the 2 main accessory organs in the GI system
Liver
Pancreas
Describe the role of the liver in the GI system
Secretes bile into the gall bladder.
Describe the role of pancreas in the GI system
Endocrine = hormones in bladder
Exocrine = substrates in GI
Explain the function of motility and its regulation within the GI tract
Motility controls the movement of food from the mouth to the anus and mixes food with digestive enzymes and exposure to absorptive surfaces.
Name the mechanical motility actions that happen in the stomach
Propulsion
Grinding
Retropulsion
Describe propulsion
The food bolus is pushed towards the closed pylorus. Constriction starts in the body and moves to the antrum.
The circular muscle are the pylorus closes with each gastric contraction. The pylorus is always slightly open (1-2mm) to allow for the movement of liquids.
Describe grinding
The antrum churns the trapped material to reduce the particle size.
Describe retropulsion
Bolus is pushed into the proximal stomach
Name the mechanical motility actions that happen in the small intestines
Segmentation
Peristalsis
Describe segmentation
‘circular mixing motion’
Contractions of the circular muscle layer breaks up the bolus. Alternate contraction and relaxation of segments mixes chyme with digestive secretions. Segmentation can be altered by hormones, the enteric nervous system and the autonomic nervous system.
Describe peristalsis
Circular muscles contract just behind a mass and moves the bolus forward into the receiving segment where circular muscles are relaxed. It is affected by hormones and the autonomic nervous system. Distension in the terminal ileum relaxes the ileocecal sphincter and bushes bolus to large intestines and prevents backflow of bacterial components.
Name the mechanical motility actions that happen in the large intestines
Mass peristalsis
Describe mass peristalsis
Large wave 1/3 of the way, so 3 mass peristalsis to move all the way.
It is triggered by co-ordinated reflex governed by local distension, control reflex arcs activity and hormones from stomach.
Explain distension in different regions of the GI tract
Distension in the region of GI tract immediately before a sphincter causes it to open = feedforward contraction.
Distension (and absorption) in one ‘segment’ will inhibit contraction and motility in the preceding segment = feedback inhibition
What does secretion optimise?
Digestion
Protection
Explain the secretion process during the pre-ingestive cephalic phase
Initiated when receptors in the head (cephalic) are stimulated by though, sight, smell and taste of food.
It involves the parasympathetic nervous system (Vagus) and only lasts a few minutes.
What is stimulated during this cephalic phase?
Saliva production
Gastric acid
Gallbladder contraction (start to release bile to absorb fat)
Pancreatic exocrine (enzymes) and endocrine (hormones) function
Same responses as actually eating but normally weaker.
Why is the cephalic phase important?
Feed forward mechanism
Prepares the body for influx of food.
What are the names of the three cell types that are involved in the secretion stimulated by food in the stomach?
Parietal cells
Chief cells
Mucous cells
What do parietal cells secrete?
HCl (breaks down food)
pH 1-2 (very acidic/corrosive)
What do chief cells secrete?
Pepsinogen (protein digestion)
Lipase (lipid digestion)
What do mucous cells secrete?
Are involved in the production of the mucus bicarbonate layer which protects us from low gastric acid.
Describe the bicarbonate mucus layer
Continuously produces mucus which goes down into the gut.
Bicarbonate catches hydrogen, and releases gas and water, acting as a ‘bicarbonate buffer system’.
It is critical for protein of the mucosa (mucus barrier) and makes it watery (good for digestion).
What happens during secretion in the small intestines?
Presence of food stimulates secretion.
Digestive enzymes and bile are secreted.
HCO3 secretions neutralise acidic chyme (stomach contents).
Bicarbonate can come from the pancreas and duodenal glands.
Enzymes work in an alkali range.
Describe pancreatic exocrine secretion
The pancreas produces pancreatic juice, a watery solution of digestive enzymes and HCO3.
Proteases are secreted in an inactive form to prevent auto-digestion.
Amylase (starch) and lipases (triacylglycerol) secreted in an active form.
Describe bile
Released from the gall bladder.
A yellow/green bitter/alkaline solution.
Contains bile salts, bile pigments, cholesterol, neutral fats, phospholipids and electrolytes.
Emulsifies dietary fat.
Describe the 2 processes to regulate/release bile secretion
Cephalic phase: parasympathetic nervous system (Vagus) stimulates the bladder to constrict of smooth muscle and relaxation of sphincter.
Presence of nutrients triggers hormones (CCK) via blood to construct bladder.
Describe the process of water absorption
Water crosses lipid-rich membranes by diffusion through aquaporins.
Bi-direction diffusion.
Osmotic gradient depends on electrolytes.
Can be rapid = 3 million water molecules / second.
What happens during carbohydrate digestion?
Starch is broken down into glucose.
It begins in the mouth with the secretion of salivary alpha-amylase (1,4 bond).
Enzymatic activity ids inhibited in the stomach due to low pH.
Most of the digestion happens due to pancreatic amylase but doesn’t liberate glucose only short chain glucose.
What happens during the absorption of glucose?
Pancreatic exocrine secretion is stimulated via the release of hormones (nutrient stimulated process).
Describe the process of carbohydrate digestion and absorption at the brush border.
Enzymes liberate monosaccharides from disaccharides in the diet and glucan from starch digestion.
Monosaccharides are released in really close proximity to their transporters (efficient absorption).
They don’t have to travel far, no malabsorption.
Describe the process of protein digestion
Mechanical disruption by chewing and gastric motility which increases surface and break down structure.
Protein denaturation by HCl (break bonds and folding) leaving large polypeptides.
Name the 3 transport mechanisms
Na+ dependent amino acid transport (4 types)
H+ co-transport with small peptides
Larger peptide fragments moved by transcytosis
Describe the process of fat digestion
In the mouth: saliva containing lingual lipase.
Few digestion products stimulate the stomach to release enzymes and CCK from sl (feed forward mechanism).
In the stomach there is a release of gastric lipase and motility, which is used for emulsification and increases surface area.
Fatty acids in duodenum converts into cholecystokinin which release bile and pancreatic enzymes.
Why is bile important for emulsification?
Doesn’t contain enzymes but important for digestion.
Bile salts crucial in increasing (and maintaining) the surface area for lipase action.
Prevent droplets to merge into big one again.
How are medium and short chain fatty acids absorbed?
They are water soluble so are absorbed directly into the capillary and into the hepatic portal circulation bound to albumin.
Define digestion
Breakdown of large to small molecules.
Define absorption
Uptake of nutrients into the enterocyte and ultimately the body
