Tubular function Flashcards
Where does reabsorption take place in the nephron?
All epithelial cells lining nephron carry out reabsorption but primarily in Proximal convoluted tubule
What are the types of movement from fluid to blood?
- paracellular (between cells)
- Transcellular (through cells)
- Reabsorption (filtrate to blood)
- Secretion (blood to filtrate)
- Excretion (Elimination through urine)
What are the types of transcellular movement?
- Passive diffusion
- Facilitated diffusion (with grad using transporter)
- Primary active transport (against grad using transporter)
- Secondary active transport (One molecules with grad but transporter carries another molecule with it)
Where is glucose reabsorbed?
Early in PCT
What transporters are used to reabsorb glucose into blood?
1) SGLT2 97%
2) SGLT1 3%
both use secondary active transport, sodium travels with concentration gradient and glucose is carried across with it
3) GLUT2 and GLUT1 transport from cells of PCT to blood
What factors affect the transport maximum of glucose and amino acids?
- Rate at which transporters can operate
- Na gradient substantial so doesn’t affect absorption
In what condition is the transport maximum exceeded?
Diabetes
What is the cause of glucosuria outside diabetes?
- Genetic dysfunction in reabsorption (Fanconi syndrome)
- Can be induced by action of SGLT2 inhibitors e.g. Gliflozin
Where are Amino Acids reabsorbed?
Early in PCT
What transporters are used for movement of AAs?
Secondary active transport using sodium and chlorine concentration gradients
What can cause Aminoaciduria?
- congenital disorders of AA metabolism where too many AAs (can’t all be reabsorbed)
- Transport protein defects (Hartnup disease)
Where is sodium reabsorbed?
PCT 70% Ascending Loop Henle 20% DCT 5% Collecting duct 3% Excretion of 2%
What transporters are used to move sodium in the PCT?
- sodium-glucose pump
- sodium-aminoacid pump
- sodium-hydrogen pump
- sodium potassium ATP pump (movement from PCT to blood)
What transporters are used to move sodium in the ascending loop of henle?
- Na/K/Cl co-transporter
- Na/K ATP pump still used to move into blood
What transporters are used for sodium movement in DCT?
- Na/Cl transporter
- facilitated diffusion of just sodium
- Na/K ATP pump still
What transporters are used to move sodium in the Collecting duct?
- facilitated diffusion Sodium transporter
- Na/K ATP pump
What are the limiting factors in reabsorption of sodium?
- in PCT and loop of henle limiting factor is gradient
- DCT and collecting duct (no gradient used) so aldosterone used as rate limitation
What is the function of aldosterone in sodium reabsorption?
- aldosterone increases Na/K ATP pump expression so more sodium can be reabsorbed into blood
- by increasing aldosterone can stop any excretion of sodium
Why is sodium important in treatment of hypertension and heart failure?
- If want to reduce blood pressure then reducing amount of sodium in blood will reduce the amount of water and so lowers pressure
- More excreted sodium means less sodium in blood
- heart therefore doesn’t have to work as hard
What drugs are used to block Na/K ATP transporter?
Spironolactone
What transporter do Thiazides act on?
Na/Cl transporter
What transporter do loop diuretics act on?
Na/K/Cl transporter
What transporter do SGLT2 inhibitors act on?
Na/Gluc transporter
What ion is used in reabsorption of water?
Sodium
water follows movement of sodium
What forms of movement are used in reabsorption of water?
Paracellular and transcellular
Where is water reabsorbed?
PCT 65%
Desc Loop Henle 15%
ADH dependant reabsorption in DCT and Collecting duct
What transporter molecule is used by ADH to affect water reabsorption?
AQP2 (aquaporins)
- inserted into ducts to increase movement of water back into blood
Explain the counter-current multiplier mechanism?
- Loop of Henle filtrate is flowing down descending and up ascending
- whereas the Vasa Recta blood next to the loop is flowing the opposite direction
- Na actively pumped out of ascending loop Henle and passively into vasa recta
- therefore the further along/ lower down the vasa recta the more sodium
- due to high concentrations of sodium as vasa recta moves up the descending loop of henle lots of water is reabsorbed
- this then means filtrate of descending limb is becoming more concentrated which drives the concentration gradient when its moves along to the ascending limb
How does filtrate concentration change in the loop of henle?
Getting more and more concentrated in the descending loop and getting less and less concentrated in ascending loop (opposite for blood)
Where is urea reabsorbed and secreted in the nephron?
- PCT 50% reabsorbed
- Loop of Henle all of that 50% secreted back into filtrate
- collecting duct 80% reabsorbed
- excrete 20%
By what process is urea reabsorbed in the PCT?
Passive diffusion
Why is urea reabsorbed in the Collecting duct?
Contributes to loop and medullary interstitial hyperosmolarity needed for water reabsorption
What is tubular secretion?
removal of substances from interstitium or blood to go into filtrate
What substances are secreted in the nephron in order to control blood plasma
- Potassium
- Hydrogen
- Urea
Give an example of toxins removed in the urine which therefore need to be secreted?
- Creatinine
- Uremic toxins
What substances are secreted in the nephron?
- H, K and urea
- metabolites and toxins
- Drugs cleared
What substances can be used to assess the kidney clearance?
- PAH
- Cinnamoylglycine
Often used to check how well a drug can be cleared
