Absorption of ions and water Flashcards
Name the 5 cells of the small intestine wall
Absorptive cell Goblet cell Enteric endocrine cell Stem cell Paneth cell
What are villous cells mostly responsible for?
Absorption of nutrients and electrolytes
What are crypt cells responsible for?
Secretion
What mediates secretion in the colon?
Gland cells
What do all mucosa have?
A stem cell compartment
What increases the surface area of the small intestine?
Macroscopic folds of Kerckring
Villi
Microvilli
What is transcellular movement?
The movement of solute across two membranes in series.
Active transport
What is paracellular movement?
The movement of solute passively between adjacent epithelial cells via tight junctions
What is the absorption of water dependent on?
The absorption of ions, Na+ and Cl-
What is the importance of Na+ and Cl-
Transport of Na+ and Cl- causes the cell to be hypertonic and water moves in via osmosis
Where is Na+ absorbed?
Along the entire length of the intestine
Name the 4 methods of Na+ absorption
Na-Glucose transporter
Na-H exchanger
Parallel Na-H and Cl-HCO3 exchanger
Epithelial Na+ channels
Describe how the Na-Glucose transporter works
Na+ exported through basal membrane by Na+K+ATPase
Na+ crosses apical membrane down an electrochemical gradient
Provides energy for moving sugars and neutral amino acids into the cells against concentration gradients
Common postprandially
What is the meaning of postprandial?
After eating
Describe how the Na-H exchanger works
Na+ uptake is coupled with H+ export
The energy comes from the Na+ gradient created by Na+K+ATPase pump on the basal membrane
Where is the Na-H exchanger most commonly used?
In the jejunum and to a lesser extent in the duodenum
Describe how the parallel Na-H and Cl-HCO3 exchanger works
Carbonic anhydrase coverts carbonic acid it to a bicarbonate ion and a H+
These are exported from the cell in the exchange for Na+ and Cl-
This regulated by cAMP and cGMP and Ca2+
An increase of any of these reduces absorption
Common in fasted state
Explain how the Na+ epithelial channels work
Channels are highly efficient at transporting Na+
How do mineralocorticoids affect Na+ absorption?
Increase it
Increase opening of apical Na+ channels
Insertion of preformed Na+ channels into apical membrane
Increased synthesis of Na+ channels and Na+K+ATPase
Where does Cl- absorption occur?
Throughout the small and large intestine
Name the three methods of Cl- absorption
Passive Cl- absorption
Parallel Na-H and Cl-HCO3 exchanger
Cl-HCO3 exchanger
Describe passive Cl- absorption
A passive process driven by the electrochemical gradient for Cl-
Driving force is derived from Na-Glucose transport and Na+ channels
What is congenital chloridorrhea?
The congenital absence of an apical Cl-HCO3 exchanger
Affected children have diarrhoea with high Cl- concentration and are alkalotic due to reduced HCO3 secretion
What do absorptagogues and secretagogues promote?
Absorptagogues promote absorption
Secretagogues promote secretion
Name two absorptagogues
Angiotensin and aldosterone
What stimulates the release of absorptagogues?
Dehydration
A drop in effective circulating volume stimulates the renin-angiotensin-aldosterone axis
What does angiotensin stimulate?
Electroneutral NaCl absorption by upregulating apical membrane Na-H exchange
What does aldosterone stimulate?
Stimulates Na+ absorption in the colon through the epithelial Na+ channels
What are the four categories of secretagogues?
Bacterial enterotoxins
Hormones and neurotransmitters
Products of the immune system
Laxatives
How do bacterial enterotoxins induce secretory diarrhoea?
First induces intracellular concentration of cAMP to increase which stimulates Cl- and K+ secretion to increase whilst inhibiting NaCl absorption
Give an example of hormone mediated diarrhoea
Verner-Morrison syndrome
Pancreatic tumours produce vasoactive intestinal peptide which via cAMP induces diarrhoea
Describe immune mediated diarrhoea
Prostaglandins and histamine released from mast cells and macrophages induce diarrhoea via cAMP
Describe how anti-diarrhoeals work
Act on myenteric plexus of large intestine
Reduce smooth muscle motility increasing the amount of time water stays in the lumen increasing absorption
Name the 6 kinds of laxative
Bulk laxatives Stool softeners Lubricant agents Osmotic laxatives Stimulant laxatives Prokinetic agents
How is calcium absorbed?
Passively and actively
Describe the passive transport of calcium
Through the paracellular pathway, not dependent on VDR
Describe the active transport of calcium
Transcellular uptake in the epithelial cells of the duodenum, dependent of VDR
Describe the three stages of active Ca2+ absorption
Via Ca2+ channels
Cytostolic Ca2+ binds to calbindin which acts as a buffer
A Ca2+ pump and a Na-Ca exchanger on the basal membrane export Ca2+
The active form of Vitamin D1 stimulates this
What will vitamin D deficiency lead to?
Rickets, leading to hypocalcaemia
