Water And Electrolyte Movement In The GI Tract Flashcards
Why is only a small amount of water absorbed by the stomach?
Only small amounts of water move across the gastric mucosa, because of the high osmotic load of secreted hydrochloric acid.
How is water absorbed in the SI?
Water moves freely across the small intestine in accordance with osmotic gradients (solute driven) or possibly via channels called aquaporins (solvent driven) although the precise mechanisms or their relative importance are not known.
Na+ is actively transported from the lumen by pumps located in the cell membranes in the ileum and jejunum.
This can decrease the water potential and hence aid osmosis into the enterocytes
How is water absorbed by the colon?
The contents are iso-osmotic in the colon, Na+ is actively pumped into the lumen and water follows.
Also note:
Chloride & Bicarbonate reabsorption in the ileum and colon Cl- is actively reabsorbed in exchange for bicarbonate, this results in the intestinal contents becoming more alkaline.
Simple cellular shape
Vs multicellular
Simple = large SA:volume for absorption
Multicellular = smaller SA:volume may need a transport system
Function of the GI:
Take large solids and digest them into smaller molecules that can be absorbed, whilst acting as a barrier to toxins, bacteria and parasites.
Daily gut fluid balance:
Saliva, gastric secretions, pancreatic juices, bile, intestinal secretions all produced throughout the day and the water is reabsorbed by the SI, absorbs 7.5L per day
Colon absorbs 1.5L
How is water moved?
Water moves by osmosis (down an osmotic gradient)
Electrolytes move dow an electrochemical gradient.
Moving against an conc grad requires energy energy is supplied by the sodium gradients generated by the pumps.
They decrease the water potential inside cells and hence cause osmosis into the cell.
Trans cellular pathway =
Paracellular pathway =
Apical membrane =
Base lateral membrane =
Trans cellular pathway = through the cells
Paracellular pathway = route between cells
Apical membrane = membrane facing towards lumen of gut, kidney etc
Base lateral membrane = membrane facing away from the lumen
The small intestine is arranged into villi and crypts, what are the jobs of these?
Villi with microvilli increase the SA for absorption
The crypts are important in secretions
How does NA+ couple nutrient absorption?
Na+ are AT out of the cell by NA+/K+ ATPase
Na+ move into the cell from the lumen
As they move in they carry a glucose molecule, this glucose molecule or a.a is carried against their concentration gradient (cotransport)
This also decreases the water potential of the cell and hence water also moves in by osmosis
What factors affect absorption and secretions of enterocytes?
X 4
Number of enterocytes
Structure of villi and crypts
Nutrient intake
Bile
Overview of intestinal secretions:
Cl- into enterocyte via NA+/K+/2Cl- cotransporter
This leads to increased levels of cAMP
cAMP influence Cl- channels causing more Cl- into the lumen of the GI tract
This causes an electro gradient so NA+ follow and due to a decreased osmotic pressure so will water.
Cholera will increase this process leading to diarrhoea
What is coeliac disease?
This is when you are gluten intolerant
Gluten is found in wheat, barley etc
When it is ingested it leads to damage to the villi and hence patients can present with weight loss, diarrhoea and deficiencies due to malabsorption
What is oral rehydration what does it contain?
It is a solution of glucose and salt, as this is absorbed into cells it rings water in due to its osmotic effect and therefore it is a better rehydrated.
What is the definition of digestion?
It is the breakdown of large complex molecules into smaller ones that can be absorbed
2 x types = mechanical and chemical
Examples =
Where does digestion occur?
Occurs in the lumen of the GI tract by secreted enzymes and also by enzymes attached to the surface of enterocytes (membrane bound enzymes)
How is the SA of the small intestine increased?
Extensive folding and also the villi and microvilli
Explain the digestion of CHOs:
Starch or glycogen will be ingested
Amylase from the salivary glands will start the breakdown of these polymers to maltose, sucrose, lactose. Pancreatic amylase also does this.
Maltese, sucrase, lactase are then responsible for the further breakdown of these disaccharides.
2 x glucose
Glucose and fructose
Glucose and galactose
Lactose intolerant don’t have lactase
How is glucose, galactose and fructose absorbed?
Glucose and galactose = Cotransport
Fructose = facilitated diffusion
How are proteins digested and absorbed?
Stomach — pepsin produces polypeptides
SI - trypsin, chymotrypsin, carboxypeptidases — short polypeptides and a.as
Intestinal mucosa — peptidases — a.a
Absorbed via facilities diffusion and cotransport into the blood stream
How are fats digested and absorbed?
Bile emulsifies fat as it enters the duodenum.
Lipase, cholesterol esterase and phospholipase — breakdown lipids into fatty acids, glycerol, cholesterol and monoglycerides
Absorbed into cell…….
Amylase Lipase Pepsin Lipase Amylase Lipase Phospholipase Trypsin Chymotrypsin Enterokinase Diasaccharidases Peptidases
Location and targets
FULL LIST TO LEARN
Salivary Gland = SG
Stomach = S
Pancreas = P
Intestine = I
Amylase - SG and starch Lipase - SG and triglycerides Pepsin - S and proteins Lipase - S and triglycerides Amylase - P and starch Lipase - P and triglycerides Phospholipase - P and phospholipids Trypsin - P and peptides Chymotrypsin - P and peptides Enterokinase - I and activates trypsin Diasaccharidases - I and complex sugars Peptidases - I and peptides
GI tract secretions in stomach:
Mucous neck cells
Parietal cells
Enterochromaffin like cells
Chief cells
D cells
G cells
Secretions and function
Mucous neck cells - mucus = physical barrier between lumen and epithelium. Also secretes bicarbonate = buffers gastric acid to prevent damage to epithelium
Parietal cells - gastric acid = kills bacteria and activates pepsin also produces IF = complexes with vitamin B12 to permit absorption
Enterochromaffin like cells - histamine production - stimulate gastric acid secretions
Chief cells - pepsin - digest protein and gastric lipase = digest fats
D cells - somatostatin = inhibits gastric acid secretions
G cells -gastrin = stimulate gastric acid secretions
What is the endocrine function of the pancreas? What is the exocrine function of the pancreas?
Endocrine = production of insulin and glucagon (from islets of langerhans)
Exocrine = secretion of pancreatic juice containing enzymes
What regulates pancreatic acid secretions?
Presence of acid in stomach causes the release of secretin, presence of fats cause release of cholecystokinin
Secretin causes release of bicarbonate secretions
CCK = secretions of enzymes
A gal stimulation causes the release
