Digestive system - Cell biology Semester 2
How big is the gastrointestinal tract
It is around 30 feet long from the mouth to the anus and there are smooth muscles in the wall that push the material from 1 end to another end of the digestive tract
In the abdominal cavity it is curled up, in the chest cavity it runs straight and the smooth muscles are involuntary
What are the major organs of the digestive tract
Oral Cavity, Pharnxy and oesphagus, stomach, small and large intestimne and the rectum and anal cavity
What are accessory organs of the digestive tract
These are all connected by the GI tracr and are used to assist in the digestion of the food
Teeth and tongue, peritonium and mesenteries to hold the tract in place, salivary glands, liver and biliary system, gall bladder and the pancreas
What are the walls of the digestive tract
Each layer has evolved to have different functions to help the overall function
The mucusa- is the inner most layer and is an epithelial layer and so is lamina of loose connective tissue rich in blood as well as lymph vessels and smooth muscles to make mucosae
Submucosa - Is a dense connective tissue with blood and lymph vessels that have specialised nerves
Muscles- Contain smooth muscles that are either 2 or 3 layers thick, if 2 then there is internal circular and external longitudinal muscled with special nerve plexi
Serosa - Thin layer of connective tissue rich in blood and lymph vessels and it is an adipose tissue and single squamous epithelium layer as well
What does the oral cavity do
The function is for the manual breakdown of food with teeth and tongue via malstication
It is made up of the lips,cheek, tongue, soft palate and the mouth floor
It has the salivary glands as its accessory glands
There are also tastebuds due to neuroepithelial cells that jave receptor hairs on them being stimulated by the saliva
What are the salivary gland as an accessory gland
Their function is too secrete saliva in order to begin the digestion of starch, keep mouth moist, lubricate the food, prevent food decay and control bacteria flora of the mouth
What are the 3 main salivary glands
Serous cell- to produce amylase,lyozyme and is found in the paratoid gland
Mucous cell to secrete mucin and fluid to lubricate the mouth from the submandibular and subligual gland
Mixed cell that secrete enzyme + Mucin
What does the oesphagus do
It is musculae tube that transport fluid and food from pharnxy to the stomach
The lumen is collapsed except when we arev swallowing
The upper third of the muscle is skeletal, the middle section is skeletal and smooth and the lower third is only smooth muscle
In the lower end there are mucus secreting cardiac glands near the stomach
What does the stomach do
It is the place where food is held whilst being digested, it wil acidify the food with HCL during digestion and contain mucus secreting cells to protect the musical surface
There are Rugae that increasde surface area due to the highly ridged 3 layer of smooth muscle in the muscularis layer of the wall and this is lined with mucus secreting epithelium as well as gastric glands in the mucosa to make the acid and digestive enzymes
What does the small intestine do
There are 3 main parts- deodendum, jejunum and ileum
The main role is for the absorption of nutrients and there is a large surface area to help with this as well as many microvilli and circular folds to increase absorption
What do the 3 parts of the small intestine do
Duedenum has bunner glands and there is alkaline secretion to protect small intestine from stomach acid
Jujunum has mucosa and circular folds as well as microvilla for large SA and there are also goblet cells to secrete antibacteria lysozymes + lacteals which are lymphatic capillaries to digest fat
Ileum there is the peyer's patchh which is an area of lymphoid aggregation
What does the colon do
The colon will be used to absorb fluid and to dry out the chyme and create and store faeces
It is made up of caecum, appendix, rectum, anal canal as well as the ascending,transverse,descedning and sigmoid colon
What is the large intestine
There are mucosa cells made of cyrpts of lieberkuhn and goblet cells but there are no folds or microvilli
There is a muscle layer made of inner, circular muscle and then outer longitudianl muscle as well that form the Taenmia coli + nervous plexi between the 2 muscle layers
Serosa layer is where the appendice epiploicae is found which is a pouch filled with fat
What is the rectum and the anal
This is the exit of the digestive tract and there are internal and external anal sphincters to remove faeces
The sphincters are made up of loose mucous membrane and submucosa to allow expansion when shitting
The internal muscle is smooth and the external is skeletal striated muscles
How do smooth muscle in GI tract work
They all act as 1 unitary muscle all together
They are involuntary and they are not striated so have a central nucleus, spindle shaped fibres and poorly developed SR withy no sarcomere
There are gap junctions for communication with surrounding cells as well as for ion transfer
Internal diameter of contracting organ can change due to loose connective tissue surrouding muscle fibres
How does the GI tract contract as 1 muslce
There is a single smooth muscle unit due to many cells are interconnected by gap junctions and contract as 1 unit as they are rhythmically active and so there are very few nerve endings needed in the cells
What are the types of contraction in the gut
There are phase contractions that are produced by intersitial cells of cajal that produce slow wave potentials that then lead to cycclic relaxtions- contraction cycle
The force and duration of the conntraction is directly related to the amplitude and frequency of the action potentials
Tonic contractions = These are long term contractions in the spincters between the 2 GI organs, these occur due to tonically active contractions and the more stable phasic contractions
How is there motility in the gut
Motility will move chyme along the tract and mix the food togethert to expose it to enzymes to digest it
Smooth muscles will be controlled by the nervous system and paracrine signals and differing contraction types will control the GI as contrcations are passed from segment to segment
There is a migrating motor complex that acts in homeostasis and will clear the gut and contract the gut even when there is nothing in the gut anymore
What are the different patterns of contractions in the gut
Peristalsis- This will move chyme through the gut by smoth muscles behind contrcating and the muscles infront will relax to push the bolus through the GI tract towards the summer
Segmentation - Mix the chyme in the gut to mix it with enzymes in the stomach along with the acid by alternate segment will contract so little or no movement of chyme forward in the stomach
Convolution - Muscularis mucosa undergo to increase folding
and used to open villial vessels
Complex - This is for swallowing and will release the content from the stomach
Mass movement - In the large intestine and include mixing to move the faeces to the rectum
What controls the GI tract
It can be controlled by the enteric nervous system that can act independent from higher control input from the CNS
It is made of submucosal plexus that recive signals from lumen and control secretion as well as myenteric plexus
Gi tract can send signals to and feceive from autonomic nervsou system as well and send signals to the CNS
Hormonal control can also control the GI tract
What are short reflexes in the GI tract
These will originate in the ENS without the use of CNS as the ENS has sensory receptors and will detect the presence of food in the GI and send signals down inter neurons to the enteric neurons that send signals to exocrine cells of the smooth muscles as well as GI peptide secretory cells in stomach
This all will cause change in GI motility, release of bile and pancreatic secretion of enzymes as well as bicarbonate sysnthesis and release
What are long reflexes in the GI
This is where the gut has the ability to send signals to the brain and these are longer and under autonomic control
The ENS respond to longer relfexes from higher centre inlut like CNS such as hormonal input, and these will be over the course of a whole day rather than short period of time to ensure the gut is properly looked after and gtets the hormones it needs for longer term functionn in response to stimuli
Where do GI secretiosn go
They can be released to the blood to allow the blood to get vitamins, osmolarity change and also get countercating molecules to CNS to stop signals
They can be to the lumen to help with the digestion process
Can be to neighbouring cells in a paracrine approach as 1 cells may have ion or nutrient overflow so give some to neighbouring cells
What are the gastric secretions in the stomach
This will be mucus from mucus surface cells as well as bicarbonate from the mucus neck cells of the gatric gland mucosa
The mucus will add a barrier between lumen and epithelium and bicarbonate is to buffer the HCL in the stomach and prevent damage to epithelium cells
Parietal cells will release HCL for protein digestions by activating pepsin as well as intrinsic factors to allow nutrient absorption in the stomach
Cheif cells secrete pepsin and lipase for protein and lipid digestion
D cells secrete somatostatin to inhib HCL secretion if there become to much in the stomach
G cells secrete gastrin to strimualt HCL secretion if to little acid in stomach
What are the pancrease secretions in the GI tract
It will secrete zymogen that are precursors to enzymes as well as also enzymes and bicarbonate
Enzyme is mainly pepsin for protein digestion and the ztymogen can be activated by trypsin to then become full enzymes
Pancrease will directly secret to the duedenum at the top of the small intestine for lipid digestion
What does the liver excrete
The hepatic artery will bring oxygenated blood from the tissue to the liver and hepatic portal vein will take blood from GI tract to and HB from spleen to the liver to be detoxified in the liver
Bile ducts carry bile from the gall bladder to the intestine and will store and concentrate the bile
Bile can be secreted to the duedenum or can be passed to the gall bladder to then become more concentrated
What is the structure of the liver
There are hepatocytyes arranged in lobules around a central vein and there are triads formed of the portal vein, bile ducts and the hepatic artery
The hepatocytyes are made in the bile and then go to bile duct where water and bicarbionate is added by the epithelial cells
How are carbohydrates absorbed
Carbohydrates will be broken down into monosaccharides starting from the salviary gland secretion in the mouth
Once the enymes have started the breakdown iof carbs then the simple sugars are taken up by the cell
They need specialised transporters to do thos:
SGLT are sodium glucose transporters for moving glucose to blood from intestine lumen
GLUT2 are for hexose transporters and GLUT5 for frcutose transporters to the blood
How are proteins digested
They are a chain of amino acids that need to be broken down by peptidases
Endopeptidases break peptide bonds in the middle of the chain to make smaller peptide chains E.G trypsin and pepsin
Exopeptidases break terminal peptide bonds to release single amino acids from a peptide chain and digest them
How are proteins absorbed
For amino acids there is the use Na+ cotransporters
Di/tri peptides need H+ cotransporters
Small peptides need endo/exocytosis or transcytosis across a cell membrane
How are lipids digested
Triglycerides are cleaved to monoglycerides and this will all happen in the duodenum and small intestine to break down larger lipid molecules due to enzyme lipase
The end goals is to produce small fatty acid molecules
Bile salts are needed for lipid emulsification to then reduce the surface tension of lipids and turn them to micelles and make it easier to digest due to being smaller
This is so the free fatty acid droplets are easier to be bought into the cell and be digested easier
How are lipids absorbed
Micellar transport if lipid breakdown product bought to the surfade of the microvilli
Lacteals are lymph vessels in small intestine that absorb lipids and carry them to the Vena Cava
How are ions absorbed
- Iron
In the form of ferrous and ferric ions from food and breakdown of HB and they need specific transporters via co transportes of H+ to get into the cell and then ferroprortin to get them into the blood to get Fe2+
-Calcium
Can be paracellular or transported through calcium channels that need ATP or via a Ca2+ Na+ ion channel
- Sodium
o Can enter via sodium channels, Co transport with chloride ions or via proton pumps
Water and potassium movement in paracellular so can diffuse in without a transporter through the paracellular pathway