Tubular Reabsorption Flashcards
Where is the proximal convoluted tubule(PCT)?
Present in the renal cortex
What makes up the PCT?
first 60% is proximal convoluted, then the rest is proximal straight.
What is the purpose of the microvilli on the surface of the PCT?
Increases the surface area of the PCT to increase the maximal rate of transport during reabsorption
How does transport occur in the PCT?
Ion channels and exchanges/cotransporters - secondary AT
pumps - primary AT
Transcellular - movement through cells
Paracellular - movement between cells.
Movement of Na creates osmotic gradient for the movement of water trans/paracellularly.
By the end of the PCT system about 70% of the water is reabsorbed.
how does water move in the PCT?
Movement occurs via paracellular and transcellular routes through aquaporin1 (AQP1).
Why does water flow paracellularly?
Due to the net outward hydrostatic and osmotic forces.
what molecules are strongly absorbed in the PCT?
amino acids
glucose
HCO3-
What molecules are weakly absorbed in the PCT?
Insulin
Urea
Chloride ions
How is glucose absorbed in the PCT?
normal conditions:
90% of glucose is transported by the low affinity/high capacity sodium glucose cotransporter (SGLT2) [ uses 1 Na ion]
The rest is transported by SGLT1 (high-affinity/low capacity). Higher affinity is required as further along the PCT the glucose conc decreases. [ uses 2 Na ions]
Basolateral transport is by GLUT2/GLUT1
What happens to glucose filtered when the concentration in the plasma increases
A the concentration of plasma increases, so does the amount filtered.
Glucose reabsorbed is equal
Reach a point until the transported reaches a maximum rate of reabsorption (Tm) where glucose transporters are saturated
How can you work out the amount of glucose excreted in the urine?
amount filtered - amount reabsorbed.
Low conc - no glucose in urine
high conc - some glucose in urine
How are SGLT2 inhibitors being used to treat diabetes?
SGLT2 inhibitors would inhibit the transporters, meaning less glucose is reabsorbed and more can be excreted in urine from the blood reducing the serum glucose levels.
How are amino acids absorbed in the PCT?
Plasma Conc - 2.5-3.5mM
There are many different transporters but most are cotransporters that use the Na gradient
Transport is Tm limited.
how is chloride reabsorbed in the PCT?
Can occur paracellularly or transcellularly.
Paracellular:
Cl moves between the cells via bulk flow (with water) as Na creates an osmotic gradient and [Cl] increases towards the end of the PCT. The Na also creates an electrochemical gradient.
Transcellularly:
Cl ions exchange with other anions (HCOO-). The methanoate ions exchanging with Cl can receive a H+ ion in the filtrate forming HCOOH which can cross back into the cell. Once the Cl is in the cell, it leaves via channels and cotransporters to move into the interstitial space.
How does organic anion secretion occur in the PCT?
Many organic anions (eg. PAH) are actively secreted in the proximal tubule by an organic anion transporter in exchange for alpha-ketoglutarate which maintains its gradient.
