Digestion Flashcards
What is digestion
The breaking down of ingested food into useable smaller nutrient molecules that can enter the vascular or lymphatic system so body can make energy out of it
How is digestion achieved
Mainly through GI tract
What does the digestive system consist of
- Gastrointestinal tract ( one long tube from mouth to anus which breaks down food and absorbs it )
- The accessory structures (digestive organs)- not part of the tract - (e.g. teeth, tongue, liver, pancreas that help by mechanical or chemical means - things which we can absorb into the vascular or lymphatic system)
What is our food composed off?
The major nutrients:
Vitamins- act as co-enzymes - make enzymes work in body - they can be either fat soluble A,D,E,K) OR water soluble. (B AND C)
Minerals/trace elements- body requirements of 7 minerals- calcium, phosphorous, potassium, sulphur, sodium, chloride, magnesium) and trace amounts of. about a dozen others. They serve other functions such as CA: bone, Fe- haemoglobin, Ca/Na/K- nerve/muscle)
What are the major nutrients
- Carbohydrates - mono-,di-,&poly-saccharides) - short and long chain sugar molecules
- Protein - amino acid chains
- Fats (glycerol and fatty acids)
What are the 6 processes of digestion consist of?
Ingestion- take food in Propulsion- propel food through GI tract Mechanical Breakdown-in the stomach and teeth (accessory structure) Digestion- chemically Absorption- into vascular system Defecation- get rid off it
How do the processes of digestion happen
At the same time, together. Not one after the other
What are the 4 layers of a general GI tract
Tube consisting of 4 layers:
- Inner layer known as mucosa - inside
- Under sub mucosa - outside
- Layer of muscle
- Outer serosa - tough outer protective connective tissue
Where does GI tract start and end and what is in middle
Starts at mouth and ends at anus ( comes out as faeces )
Stomach and intestine in middle
What does mucosa contain
- Mucous secreting goblet cells to protect and lubricate the passage of food and gut
- Cells secreting digestive enzymes and hormones (in the stomach and small intestine)
- Epithelial cells (simple columnar epithelium)
- Lamina propria
Properties of lamina propria
- Lies underneath the epithelium which is loose aerolar connective tissue
- Have lymphatic vessels of lymphatic system- protect body, protective lymphoid follicles.
What type of epithelium in mouth and anus
Stratified squamous epithelium, not simple
Function of mucous in GI tract
- Protect and lubricate the gut
- Stomach has HCL - don’t want that destroying GI tract
What is the submucosa?
- External to mucosa
- Made of loose aereolar connective tissue
- Contains lymphatic and blood vessels and abundant nerve supply
- elastic fibres/tissue .
Why is their elastic tissue/fibres in GI tract
Whole system has to be able to expand when food goes in and down
What is the muscularis layer of the GI tract made up of
Inner circular ( round ) and outer longitudinal layer along GI tract of smooth muscle innervated by ANS
What is function of muscle in GI tract
- Shove food along by the process of peristalsis.
- Food turned into a bolus (balls) and that is then pushed through GI tract by contraction of the muscle behind bolus and relaxation of muscle infront of bolus
What is surrounding the whole GI tract
Protective serosa
What is function of mouth
Responsible for ingestion
Structure of mouth
- Lined with thick stratified squamous epithelium - protection
- Has tongue which mixes the food with saliva, forming a bolus which is shoved along by peristalsis
- tongue contains taste buds
- teeth macerate the food (mastication- crush food)
- mouth is connected to a number of glands whose secretions form saliva - produce digestive enzymes and protective mucous
Why do we need teeth in mouth
Digest things chemically - smash into small things
Digestive enzymes cant act on big things so you need teeth to crush it down
What are the 3 salivary glands called and what do they do
-Parotid glands
-Submandiular gland
-Sublingual gland
produce saliva
What do these secretions of saliva serve to do
- dissolve the food so we can taste it
- help form the food into a bolus
- lubricate the mouth
- begin digestion of starch mostly and fats by producing the enzymes amylase ( digest starch ) and lipase ( digest fats)
What type of gland is salivary gland and what do they contain
Exocrine
Contain mucous and serous secreting cells
What is the main digestive enzyme in mouth
Amylase
Where does the food/fluid and air end up once gone through mouth?
- Back of throat- Pharynx- which can be divided into various regions
What is the Pharynx?
Back of throat where food/fluid and air leave via the mouth
What is role of epiglottis
Normally its open to allow air to pass down trachea
Opens and shut so food goes down to stomach and air down to lungs
What happens to epiglottis when swallowing
Closes thus lowering the diaphgram ensuring food and liquids do not enter the trachea but instead enter oesophagus
Dont want food down trachea as it could cause pneumonia and could die
What is the process when food and air goes in
Food in mouth Air in through nose Mix air taken in and food in the pharynx Need to seperate - food down to stomach and air down to lungs - cant get it other way = MIX AIR AND FOOD FIRST THEN SEPARATE
What is the oesophagus?
A 25cm tube connecting the mouth to the stomach
What happens when you chew bread for a long time
Changing long chains of glucose molecules into shorter chains of monosachharides and disaccharides so it becomes sweeter = amylase is secreted by salivary glands
How many layers in oesophagus
4 - same as GI tract
What does the submucosa contain in the oesophagus?
contains muscous secreting oesophageal glands, that aid the passage of the food.- LUBRICATE
What doenst happen in oesophagus
No digestion - just a tube/passage way
How long does it take for food and fluid to be propelled to stomach
Bolus is propelled by peristalsis to stomach in about 8 seconds from mouth
For fluid - 2 seconds - quicker as its trickled by gravity - dont have to squeeze along
What is function of stomach
- store food
- mixed food into’chyme’ (semi fluid)- continually going through contractions and mixed with digestive enzymes
- begin the digestion of proteins and fats. Mostly proteins, not carbs - that happens in moth
What do you produce when you mix food with digestive enzymes
Chyme
What is structure of stomach
15-25cm long
Has a volume of 500ml
- When had a meal a volume can be up to 4L
- Is guarded at either end by a sphincter muscle
-An empty stomach has multiple longitudinal folds (rugae) -Folds in stomach increases S.A
- Has the usual 4 layersof the GI tract
What are the two circular sphincter muscles in the stomach either end
- oesophageal sphincter
- Pyloric sphincter
What is on the surface on the stomach?
- Gastric pits- these are connected to gastric glands - opening to glands
What does the gastric glands secrete?
- Mucous
- HCL ( form parietal cells )
- Pepsinogen ( from chief cells )
- Lipase ( from chief cells )
What does mixing HCL and pepsinogen
Pepsin
What is role of pepsin
Digestive enzyme that can digest and break down proteins.
HCL and pepsinogen cant alone digest proteins so you need to mix them to form pepsin.
You dont want to store pepsin itself as it would digest stomach
What do lipases digest
Fat
What is stomach covered with and why
Thick layer of mucous
It protects it from acidity and protein digesting enzymes
What are the 3 layers of smooth muscle in the stomach
circular
longitudinal
oblique
What is role of circular, longitudinal and oblique layers
Mix, grind and propel food in stomach
Why is stomach always contracting and churning
To produce chyme
What does the longitudinal muscle layer contain ?
Enteric pacemaker cells
What is the role of enteric pacemaker cells
- They depolarise spontaneously, causing 3 slow waves of contraction (causing a.p) in the stomach every minute.
- generates its own action potential
- Leaky to Na ions = depolarisation
- Pacemaker of heart works in same way as enteric pacemaker - pacemaker of gut
How is enteric pacemaker connected to surroundings of smooth muscle and why
By gap junctions
This ensures efficient transmission of the signal
A.p spreads evenly throughout stomach
Where is the basic rhythm of contraction modulated by
- The autonomic system - sympathetic system slows down digestion whereas parasympathetic speeds up digestion.
- as wells as by local tissue factors such as stomach stretch receptors and gastric secretions
- Rhythm of contraction generated in stomach - dont want brain to tell stomach when to contract - stomach itself has ANS - not reliant on brain
What is the small intestine?
Where food leaves the stomach and enters in the small intestine - coiled and stored in smaller area here
What does the small intestine consist of?
Duodenum (25cm)
jejunum (2.5m)
ileum (3.6m)
How long is small intestine in man
6metres
What does the small intestine do?
- Produces digestive juices so most digestion happens here
- Liver and gall bladder produce bile which emulsifies fats
- Pancreas produces enzymes that break down all categories of foodstuffs (e.g amylase., lipase, nuclease)-
What is emulsification
Breaking down of fats into smaller bits by bile salts which are produced by liver and stored in gall bladder
Where do ducts from gall bladder, liver and pancreas empty into
Duodenum
What is small intestine specialised for
Absorption
How much is surface are of small intestine increaed
X 600
How is surface area of small intestine increased
- 1cm tall circular folds of the mucosa and submucosa
- 1mm tall villi(containing blood and lymph vessels)
- microvilli on the absorptive epithelial cells(brush border) - on the villi
What is the total absorptive surface area of the small intestine
200m2 - size of tennis court
What is the structure of the small intestine?
- same 4 layers as other areas of the GI tract - mucosa, subucosa, muscular layer and outer serosa
- simple columnar epithelium= specialised for absorption
- Surface of these cells contain brush border enzymes that involved in carb and potein digestions
- the epithelium also contains many mucous secreting goblet cells
- Pits on surface of epitelium are linked to tubular glands ( intestinal crypts ) that secretes intestinal juice
- intestinal juice contain mucous, antibacterial agents and other protective components of the immune system
What is the large intestine?
it frames the small intestine on 3 sides
What is the size of the large intestine?
-it is wide (cm) -larger in diamter but shorter than the small intestine (1.5m)
What is main function of large intestine
To remove water from the food and store solid faces until defecation through the rectum and anal canal.
- retain water in body - reabsorb water
- little digestion occurs here
- Contains bacteria that ferment any remaining carbs - also sythesize vitamins B and K
What is the structure of the large intestine?
- Made of the same 4 layers as the GI tract
- No villi to increase SA as little digestion occurs here - no absorption apart from H20
- Intestinal glands contain goblet cells that produce mucous- to ease the path of the faeces and protect the intestinal wall.
- Most of the large intestine is lined by simple columnar- however the anal canal has stratified squamous epithelium
What is the digestion of carbohydrates?
- either monosaccharides, disaccharides or polysacchruides.
- Need to be broken down into monosaccharides to be absorbed in the vascular system
Process of digestion of carbs?
- Happens in mouth
- Amylase from the salivary gland begins the breakdown of starch in the mouth, forming shorter oligosaccharides- taste sweet
- This process is completed by amylase from the pancreas
- Remaining carbs are digested by amylase from pancreas in small intestine
- These short chain sugar molecules are then converted to monosaccharides by the brush border enzymes (produced on microvilli) in the small intestine, where they are absorbed.
- –Glucose and fructose get absorbed by the wall of the small intestine and into vascular system.
What is the digestion of proteins?
-Proteins are chains of amino aids which need to be converted into dipeptides or single a.a before they can be absorbed.
Process of digestion of amino acids
- doesnt occur in mouth, occurs in stomach
- pepsin (pepsinogen+hcl) in the stomach breaks proteins into polypeptides and free a.a
- pancreatic enzymes continue this breakdown
- which is completed by brush border enzymes in the small intestine.
- individual a.a absorbed in the epithelial cells in the small intestine and end up in the vascular system.
What is the digestion of fats?
-most ingested fats are triglycerides that need to be broken down into monoglyercides and fatty acids
Process of digestion of fats?
Limited digestion of fats occurs in mouth and stomach
- As fat is not soluble in water it would form big globules in water that could not be digested by enzymes
- It is therefore emulsified (broken into smaller bits) by bile salts
- These small fat spheres are split into monoglycerides and fatty acids by pancreatic lipases which form ‘micelles’ , that are absorbed.
What is different about the digestion of fats?
fats absorbed in small intestine BUT put into lymphatic system.
- While the breakdown products of carbs and protein digestion are absorbed into blood capillaries in the villi o the small intestine, monglycerides and fatty acids enter lacteals (lymphatic vessels)
What is lacteal
One of the villi in the small intestine lymph vessels in middle
