PBL Week 7 Flashcards
What is the basic anatomy of the GI tract?
Is basically a giant tube for carrying food from the mouth to the anus. Made up of the mouth, pharynx, oesophagus, stomach, intestines and the anus. It is also amde up of accessory organs, which aid the process of digestion. These include the teeth, tongue, salivary glands, liver, gall bladder and pancreas.
Most of the GI tract has a similar structure, with the epithelial lining of the tract being supported by connective tissue called lamina propria. These are collectively called the mucosa. Outside of this is a thin musclar layer called the musclaris mucosae, then the submucosa, then 2 layers of smooth muscle (circular and longitudinal layers) and finally the serosa on the outisde. The smooth muscle contracting causes the bowel to shorten (longitudinal) and decrease in diameter (circular).
What is the structure and function of the nervous system of the GI tract, including the enteric nervous system?
The enteric nervous system (ENS) is a division of the PNS that can control gastrointestinal behaviour independently of the CNS. The GI tract receives signals from both the sympathetic and parasympathetic nervous systems, with efferent fibres carrying info from the CNS to the gut and afferent fibres carrying info from gut receptors back.
Sympathetic stimuli excite the gut and are carried by the vagus and pelvic nerves, which sysnapse in the myenteric plexus (between muscle layers) and submucosal plexus (in the submucosa). These plexuses then send out signals to the various effector cells of the GI tract. However, sympathetic nerves are used for fight and flight, so the GI tract instead mostly uses the enteric nervous system, whose parasympathetic fibres are found in the prevertebral ganglia. These also target the plexuses and control most functions of the GI tract.
What is the role of the main organs in the GI tract?
Mouth - crushes up food, then tongue rolls it into a mass called a bolus, which is then pushed towards the pharynx.
Oesophagus - during swallowing, the muscles in the wall contract and relax (peristalsis) to push then bolus towards the stomach. The oesophagus has sphincters at the top and bottom, which contract to stop food/stomach acid re-entering.
Stomach - has a top half (orad region) which is rich is secretory glands and a bottom half (caudad region), which contracts, mixing the food and pushing it into the duodenum. At first, the orad region relaxes to accomodate the meal, then the caudad region contracts to push the food back, mixing it. Once the stomach contents are isotonic, the caudad region contracts to push the food into the duodenum.
Small intestine - made of duodenum, jejunum and ileum. Has lots of villi and microvilli for increased surface area. The food from the somach is called chyme and is slowly passed through the intestine by peristalsis. At the duodenum, enzymes from the pancreas and bile from the gallbladder are mixed with the food to break it down. At the jejunum, most of the nutrients and water are absorbed. At the ileum, bile acids and vitamin b12 is absobed, as well as any final nutrients.
Large intestine - the food is once again passed through by peristalsis, water and vitamins K and B are absorbed here. No digestive enzymes, however; bacteria instead digest the remaining nutrients. Anything left is eliminated via feces.
Anus - the end of the GI tract. Detects and pushes feces out of the body.
What is the role of the accessory organs in the GI tract?
Liver - receives blood from the digestive tract via the hepatic portal vein, is used for metabolism and detoxification. Consists of lobes suspended from the diaphragm and abdominal wall by ligaments. The liver also produces bile and degrades hormones, produces cholesterol and creates certain blood proteins.
Gall bladder - stores bile from the liver and releases it into the duodenum in the presence of lipids. Bile acids contain hydrophobic ends (which connect to the lipids) and hydrophilic ends (which point outwards). This structure is called micelle and seperates the lipid droplets (emulsification), making them easier to absorb.
Pancreas - releases water and bicarbonate to neutralise stomach contents and raise PH. Also contains lots of important enzymes to break down the stomach contents, including trypsin, amylase and lipase to break down peptides, carbohydrates and fats.
What is the physiological control for the release of digestive secretions?
Even before we eat, the 3 main salivary glands are already secreting saliva in response to signals from the medulla oblongata.
In the stomach, there are 4 secretory cells. Mucus cells produce mucus, which protects the walls of the stomach against stomach acid. Parietal cells produce this acid by converting H2O and CO2 into H+ and HCO3- ions, the latter of which transfer with Cl- ions in the blood. The Cl- and H+ ions then combine to form HCL. Chief cells secretes pepsinogen, which converts to pepsin to break down proteins. Finally, G-cells secrete the hormone gastrin, which stimulate the other 3 cells.
S-cells in the intestine create a hormone called secretin in the presence of acid, which stimulate the pancreatic cells to release water and bicarbonate (to protect the walls of the duodenum). At the same time, CCK is secreted (stimulated by stomach acid or amino acids) which stimulates the gall bladder to release bile. G-cells in the duodenum also produce gastrin, which stimulate the pancreas to release digestive enzymes.
What is the human microbiome and what is it’s role?
The human microbiome are all the organisms harboured by people. It’s incredibly varied and is found all over, with many functions including: immune education, colonisation resistance, PH regulation (in mouth), various metabolic pathways, fatty acid production (in gut), digestion, carbohydrate breakdown (in mouth) and more. Disturbance of the microbiome can lead to a diseases state, with the diversity able to be influenced by both internal and external factors. The gut microbiota has been linked to psychiatric disorders and tumour formation, too.
What are the biochemical processes associated with metabolism and the generation of energy?
Digestion - breaking down of proteins into amino acids, polysaccharides into simple sugars and fats into lipids.
Respiration - breakdown of glucose and water to create molecules of ATP. Seperated into glycolysis and krebs cycle.
What are modern techniques used to examine the workings of the GI tract?
An endoscopy is the insertion of a long, thin tube through the mouth/anus to observe the internal environment in detail. Can be used for diagnosis or some minor surgeries.
Stool tests can detect the presence of abnormal bacteria or blood in feces, which could denote a larger problem within the GI tract.
A barium meal is where the patient eats a meal containing barium (a metallic liquid), allowing a physician to watch the stomach as it digests the meal (using an x-ray). Can also determine if the time it takes to digest the meal is abnormal.
CT scans are used to make detailed images of the inside of the body and can reveal any issues with the GI tract.
Ultrasounds can be used to view internal organs and determine if they’re functioning normally or not.
Do prebiotics and probiotics have a role in modern medicine?
Prebiotics are compounds found in food that increase the activity of beneficial microorganisms in our gut, allowing the digestive system to work better. Usually found in fruit, veg and whole grains.
Probiotics are live bacteria and yeasts promoted as being beneficial to health. Are found in yogurts and supplements. They are meant to restore the balance of your gut microbiome.
There is lots of evidence that probiotics and prebiotics can be used against metabolic disorders (diabetes, CVD). Furthermore, prebiotics have been found to improve calcium absorption and probiotics on treating diarrhoea. However, they can actually make things worse in those with pre-existing conditions, such as IBS. They can also trigger allergic reactions and probiotics aren’t recommended for those with weakened immune systems.
