diet, digestion and absorption Flashcards
diet
provides energy
- supplies building blocks for growth and maintenance of tissues
- other nutrients:
- water - transport, solvent
-vitamins - chemical reactions, blood clotting
-minerals - muscle contraction, bone strength, haemoglobin
weight loss
catabolism of glycogen, muscle protein, fat stores
weight gain
storage of fat
carbohydrates
-polysaccharides (starch, glycogen)
- disaccharides (sucrose, lactose)
- monosaccharides (glucose, fructose
we require amylases to breakdown the glycosidic bond or hydrolyze the glycosidic bond there to break down larger structures into smaller ones.
proteins
Proteins are long chains of amino acids, which are joined by peptide bonds.
- 20 different amino acids - 11 non essential
daily intake should be 1g/ 1kg body weight
- If you have too much protein going through the digestive tract, it increases the osmotic potential. And that will draw water out into the into the Lumen and that can lead to dehydration and diarrhea
- can also increase the risk of cancer and coronary heart disease because by taking an excessive protein we tend to take in more fat as well.
how can the liver be damaged by excess proteins
- The liver can be damaged because excessive proteins are used to make glucose and byproduct of that is ammonia, which damages the liver because it’s very toxic
how can the kidney be damaged by excessive protein intake
- in terms of the kidney, We produce more renal acid
- renal acid secretion can damage the kidneys and it can also cause precipitation of uric acid.
fats - triglycerides
triglycerides havea glycerol backbone - this is the hydrophilic part and then it’s got three fatty acid side chains which are hydrophobic
function of fats
they are storage molecules in plants and animals
- are also a good source of insulation in animals
- and fatty acids can be classed as saturated if they don’t contain double bonds or unsaturated if they do
- saturated fatty acids are bad fatty acids because they increase the LDL whereas unsaturated ones will reduce it, So they are classed as good fats
importance of cholesterol
cholesterol is very important in the diet because it contributes to the fluidity of the cell membrane and it’s also important in the production of steroid hormones.
essential fatty acids
linoleic omega-6 - essential for that for production of arachidonic acid
○ Linoleic omega 3 fatty acid is actually an inhibitor of the cyclooxygenase and lipoxygenase enzymes.
what is arachidonic acid broken down into
- arachidonic acid Is broken down into leukotrienes and prostaglandins, thromboxane - so these are inflammatory mediators that are important part of the inflammatory process
role of Linoleic omega 3 fatty acid
it reduces the production of inflammatory mediators.
- So it’s got it in important anti-inflammatory actions
- and it’s also thought to be important in reducing the risk of coronary heart disease
vitamins and minerals
required in trace amounts in humans
- often toxic in excess - can cause Wilsons disease
- water-soluble vitamins (B+C) easily absorbed
- fat-soluble vitamins (A, D, E + K) -require bile or pancreatic lipase
the GI tract
the GI tract could be considered as a long tube.
- there is a mucosal layer that’s surrounded by layers of smooth muscle To propel it along the tube.
the Gi tract - the mouth
- the mouth is the start
- where mechanical digestion occurs and Chewing and swallowing
- food then passes into the oesophagus which is literally used as a connecting tube and pushes the food down to the stomach which is a storage organ
the gi tract - the small intestine
the food passes through to the small intestine where the majority of digestion and absorption occurs in the duodenum and the jejunum
○ this is aided by secretions from the pancreas - pancreatic juice and bile will provide the optimal conditions for digestion and absorption to occur
the GI tract - large intestine
- When the food passes through to the large intestine it moves a lot slower So it’s considered a storage organ - this is any undigested material
- further absorption of water occurs in the large intestine before it passes into the rectum - which is the Final storage points and pass this out through the anus
transmit time from the mouth to the anus
Transit time from the mouth to the anus can vary from 12 to 48 Hours
digestion
overall digestion is the breakdown of large into smaller molecules in order for absorption to occur.
functions of the digestive tract
- Mechanical breakdown of food into smaller pieces
- Increasing surface area for enzymatic action
- Mixing of food with acid/ enzymes
- Provision of optimal conditions for enzymatic action (e.g. pH)
- Barrier to infective organisms
- Absorption of nutrients/ water
- Movement of food towards the anus
- Egestion of indigestible/ toxic material
mechanical digestion
the temporomandibular joint it allows for the lateral side-to-side movements and translation of the mandible
The teeth and jaw are designed for the teeth to fit together and interlock - the shape of the crown, and position determine the function
mastication cuts and grinds the food
Then we’ve got the tongue - moves the food around and shapes it into a bolus mixes it with mucus, which is important for the swallowing and with saliva.
periodontal ligaments
periodontal ligaments here between the roots of the tooth and the alveolar socketprovides cushioning and that’s really important because there’s a lot of lateral impact
saliva
Saliva is composed largely of water. It’s also contains proteins such as mucins
which are slippery - So that provides lubrication
and that’s also important for speech and swallowing.
also contains alpha amylase
and antibacterial agents - important for the prevention of tooth decay
why is saliva pH slightly acidic and why is it hypotonic
improves taste
salivary glands - parotid gland
- they are in front of the ears and they produce a watery secretion
submandibular glands
produce most of the saliva - around 70%
sublingual salivary glands
the small ones which are under the tongue and they provide the mucus.
saliva gland structure
it’s divided up into acinus - which are small and then they feed into the ductules which then feed into the salivary duct, which is a larger duct
and the two parts of the saliva glands have different functions
acinus function
The acinus Uses active transport so it allows sodium and chloride into the aciners from the surrounding epithelial cells and then water follows it down its concentration gradient.
○ So that produces an isotonic fluid
○ And then the acinar feeds into the ductule
ductule of the salivary glands
We have active reabsorption of sodium and chloride from the saliva and in exchange, We have secretion of potassium and bicarbonate.
○ So this is largely impermeable
○ So what that means is that the faster it passes through, the less hypotonic or more hypertonic the saliva is
○ so that means that the composition of the saliva is largely determined by the rate at which it passes through from the saliva gland
○ this is under autonomic control.
basal rate of digestion
So the basal rate is only about 0.5 mils per minutes
and that will be increased by the presence of food in the mouth and Also the taste and the smell of food.
cerebral cortex - conditioned reflex on digestion
higher centres such as the cerebral cortex, even if we’re thinking about food, the sight or the sound of food produces a conditioned reflex.
autonomic control of digestion
it’s under parasympathetic and sympathetic control.
○ parasympathetic control of salivary production increases the blood flow.
It produces a very watery saliva and it can increase the rate up to 7 milliliters per minute.
Whereas sympathetic control - you tend to produce a very thick enzyme-rich saliva, and because it’s thick it dries out the mouth
Sjogren’s syndrome
Sjogren’s syndrome is an autoimmune disease in which the body produces antibodies against the glands and saliva glands are a target of this so sufferers have a very dry mouth.
- It’s also a side effect of drugs.
- So sympathetically acting drugs produce a dry mouth
what is acinar secretion driven by
driven by the Na+/k+ exchange pump and that’s Important to pump sodium out of the cell into the blood on the basolateral side in exchange for potassium.
-this Gives us a Na gradient for Na to diffuse in passively alongside two Cl- ions, and also a k+ ion.
- K+ then leaks out of the cell down its concentration gradient.
- that chloride will cross the apical membrane utilizing cftr channels also calcium-activated chloride channels.
- causes a more negative membrane potential And sets up the electrochemical gradient for sodium to pass passively down a para cellular pathway between the cells.
aquaporin 5
this is a protein which is inserted into the apical membrane
- and it’s been shown in experiments on mice that sympathetic stimulation will cause the insertion of aquaporin 5 into the membrane.
- And also if you knockout aquaporin 5, it means it reduces the saliva production again, this has been shown in mice.
gastric function of body of stomach
storage 2-4 hours
- secretion
muscle in the stomach
3 layers of smooth muscle
- Circular
- Longitudinal
- Oblique
- Allows churning
pylorus of stomach function
emptying via pyloric sphincter = regulated to avoid exceeding digestive and absorptive capacity of small intestine
what forms chyme
mechanical digestion - mixing - waves of peristalis - churning
rugae in stomach
capacity for stretching
- storage potential
mechanisms of absorption
-mouth, oesophagus, stomach - limited diffusion
- duodenum and jejunum - major site
- ileum - vit B12 & bile salts
colon - some sodium and water
- rectum - limited diffusion
why is the small intestine the main site of absorption
- expansion of absorptive surface
- foldings in wall, villi, microvilli
- total area - 300 m2
- and rich supply of blood vessels and lacteals
how is sodium absorbed
primarily through Na+/ H+ exchange (electroneutral)
- 3 NHE isoforms identified in enterocytes (NHE2, NHE3 and NHE8)
- in colon, ENaC also involved in sodium absorption
chloride absorption
absorbed via paracellular pathway (passive), Cl-/HCO3- Exchange couple to Na+/ H_ (electroneutral)
- SLC26A3 - gene mutations causes congenital chloride diarrhoea
amino acids and peptide absorption
specialised apical membrane transporters for different classes of amino acid, e.g. neutral, acidic and basic
- most Na+ coupled (like glucose transport)
- some coupled to H+ gradient
fat digestion and absorption
- fat globules in small intestine
- fat droplets coated with bile salts (emulsification)
lipases - FA/ MG as micelles
- FA/MG leave micelles and pass through brush border by diffusion
- FA/ MG packaged into chylomicrons + enter lacteals
- chylomicrons taken away by lymphatic system
absorption of water
water moves down an osmotic gradient
- osmotic gradient created by the absorption of nutrients
- most nutrients absorbed in the small intestine
- so most water absorbed in small intestine
large intestine functions
storage of faecal matter
- absorption of some sodium and water via ENaC
- faeces - 150g
defaecation
- reflex, including. relaxation of internal sphincters
- external sphincters under voluntary control
