L26 Absorption And Secretion Flashcards
What are the three primary control systems that regulate gastrointestinal (GI) actions and secretions?
Hormonal, paracrine and neurocrine control.
How many L of fluid is handle by the GI per day and what happens to them?
> 9L per day and most are absorbed in the SI and the remaining fluid is lost in the LI or lost in faeces
How does the body regulate extracellular fluid (ECF) volume and maintain blood pressure?
By integrating salt and water balance, primarily through the kidneys
What is the key factor in regulating ECF volume?
Adjusting the total body sodium chloride (NaCl) content
Describe the steady state of sodium balance.
In a steady state, the sodium (Na+) intake through the gastrointestinal (GI) tract equals the Na+ output through renal (and other) pathways
What are the three main functions of the stomach? (3)
- Secretion
- Motor - regulates food intake, mixing, decreasing food particle size
- Humoral - gastrin activity
What are the distal secretions of the stomach? (3)
- Gastrin
- Somatostatin
- Pepsinogens
What are the proximal secretions of the stomach? (4)
- HCl
- Pepsinogens
- Intrinsic factor
- Mucins/HCO ions.
Name the oxyntic glands and what they secrete? (7)
- Epithelial cells - HCO3
- Mucous neck cells - Mucus
- Parietal cells - HCl, intrinsic factors
- Enterochromaffin-like cells histamine
- Chief cells - pepsinogens
- Enterochromaffin cells – serotonin, VIP, substance P.
- D cells – somatostatin (H)
Describe the secretion/function of epithelial cells
HCO3- - combines with mucus to form protective barrier + neutralises acid
Describe the secretion/function of mucous neck cells
Mucus – protects epithelia from other agents harmful to stomach lining
Describe the secretion/function of parietal cells
HCl – activates pepsinogen, denatures proteins, inhibits pathogens
Intrinsic factor – required from B12 absorption
Describe the secretion/function of Enterochromaffin - like cells
Histamine – ↑ parietal cell HCl secretion
Describe the secretion/function of Enterochromaffin cells
Serotonin – regulates motility & secretion
VIP – motility, secretions, vasodilation, ↓HCl
Substance P – smooth muscle contractions
Describe the secretion/function of Chief cells
Pepsinogen (precursor of pepsin – cleavage results in digestion of protein)
Describe the secretion/function of D cells
Somatostatin (hormone) – inhibits gastrin release (feedback mechanism)
Describe the secretion/function of G cells
Gastrin hormone - ↑parietal cell HCl secretion, ↑motility, ↑pepsinogen release
Where is the location of the tubulovesicular membrane?
Apical side
In resting parietal cells, what enzyme is responsible for acid secretion?
H/K- ATPase
Stimulation induces cytoskeletal changes. Which membranes fuse together in the parietal cells?
Tubulovesicular and canalicular membrane
Stimulation induce cytoskeletal changes, How much does the surface area (SA) of parietal cells increase by?
50-100x
What structural changes occurs to the parietal cells on the cell surface during cytoskeletal changes?
Microvilli appearace
Describe the mechanism of gastric acid secretion in parietal cells
- H2CO3 in dissociated
- H+ will be sectreted across the apical membrane through H+-K+ ATPase.(Cl- follows)
- Bicarbonate ion is absorbed into the blood via Cl - HCO3 exchanger.
- Secretion of HCl and absorption of HCO3 occurs.
- K is recycled into lumen by K channels.
How does regulation of HCl secretion occur?
- Stimulation of H+ secretion: histamine, Ach and Gastrin
- Inhibition : leading to low pH and secretion of somatostatin and prostaglandins.
What is somatostatin?
Somatostatin is a hormone that inhibits the release of other hormones
What are the key electrolytes that the SI and LI need to absorb?
Na+, K+, Cl-, HCO3- via villus cells/surface epithelia respectively
What do Cyrpt epithelial cells do?
They secrete fluid and electrolytes
What are the two main intestines and its sections?
1) Small intestine - Dudodenum, jejunum and ileum
2) Large intestine - colon and cecum
What are the primary sites for absorption of Na+, Cl-, K+ and HCO3-?
Duodenum and jejunum
What do the crypt epithelial cells do?
They secrete fluid and electrolytes
Do SI me LI have similar structures?
Yes they do
What seperates the apical and basolateral membranes in polar cells?
Tight junctions
How do both SI and LI absorb + secrete solutes?
Via active and passive mechanisms.
Transcellular movement is usually active
Paracellular movement is generally passive
How are ions transported into the cell from the blood in the intestines?
Na+K+Cl- cotransporter
Describe the general mechanism of secretion of NaCl in epithelial cells of crypts
1) NKCC2 brings Na+, K+, 2Cl - into cell from the blood
2) Cl- ion diffuses into the lumen through Cl- channels in the apical membrane
3) Na+ follows Cl- secretion passively (paracellularly) which draws water allowing the movement/ secretion of NaCl
How is water secreted into the lumen of the intestines?
Na+ follows Cl- secretion passively (paracellularly) - water drawn following the movement of NaCl
Describe the mechanism of electrolyte absorption in the jejunum
- All Na+ transcellular absorption via Na+/K+ ATPase
- Low IC Na+, so allows Na+ entry via Na+/H+ exchanger or Na+/ glucose cotransporter
- Luminal HCO3- increases Na+ absorption by stimulating the apical Na+/H+ ion exchanger (think about previous lecture on HCO3- with NHE3 cotransporter)
Describe the mechanism of electrolyte absorption in the ileum
- Same transport mechanisms as jejunum in the top half, however have CL–/HCO3- exchanger on apical membrane
- H+ secreted into the lumen via Na+/H+ exchanger. HCO3- secreted into lumen via CL-/HCO3- exchanger
- Net effect - movement NaCl- into cell (absorbed into blood)
What is different between the mechanisms of the ileum and jejunum?
- HCO3- is on basolateral membrane in the jejunum however in the ileum, it’s on the apical membrane
- Has a chloride ion channel on basolateral membrane
Describe the mechanism of electrolyte pancreatic secretion
- HCO3- secreted into pancreatic juice (lumen) by Cl-/ HCO3- exchanger
- H+ transported into blood via Na+/H+ exchanger
- Net effect: secretion of HCO3- +absorption of H+
Describe the mechanism of absorption of the large intestine (colon)
- Synthesis of Na+ channels induced by aldosterone
- Increased Na+ channels –> Increased Na+ entry –> Increased Na+ across basolateral membrane via NaK ATPase
- Increased K+ in cell due to ATPase –> Increased K+ secretion across apical membrane
Name me 6 carbohydrate which are located across the brush border
α-amylase
α-dextrinase
Maltase
Sucrase
Trehelase
Lactase
The products absorbed into villus blood
Name me 5 proteases
Pepsin
Tryosin
Chymotrypsin
Elastase
Carboxypeptidases
- Products into villus blood
Some products may go into the lacteals instead of the villus blood within the villus. What substances are the causes of that?
The products produced are the result of lipases and bile salts
Describe the mechanism of absorption of carbohydrates in the epithelial cell of SI
- Na/K ATPase establishes a Na+ gradient within cell so that Na+ can cotransport glucose and galactose via SGLT1 cotransporters and fructose via GLUT5
- GLUT2 on the basolateral membrane allows glucose, galactose and frutose to diffuse into blood
Describe the mechanism behind the absorption of proteins in the SI
- Na/K ATPase establishes Na+ gradient inside the epithelial cell of SI
- This allows amino acids and dipeptides and tripeptides to be contransported from apical membrane into the cell
- Dipeptides and tripeptides are hydrolysed into amino acids via peptidase where it diffuses into the blood
- There is a Na+/H+ exchanger on the apical membrane
Describe the mechanism behind lipid absorption of the small intestine (7)
- Pancreatic lipase + other lipases, aided by bile salts complete lipid hydrolysis in duodenum + jejunum
- Products of lipid digestion: cholesterol, monoglycerides + fatty acids have been made soluble in mixed micelles
- Micelle exterior lined with amphipathic bile salts
- In the mucous gel layer lining the epithelial surface FAs become protonated + cross luminal surface of enterocyte by 3 methods: diffusion, incorporated into enterocyte membrane and carrier mediated transport
- Products re-esterified with FA in SER which is then pacakaged into chylomicrones
- Cross basolateral membrane and absorbed into lacteals
- Bile salts recycled
What are the 3 methods for surface Fas to become protonated and cross the luminal surface of enterocyte?
-Diffusion
-Incorporated into enterocyte membrane
-Carrier mediated transport
