Reabsorption Flashcards
(26 cards)
How is glucose reabsorbed across the membrane into the ICF?
Via carrier mediated transport systems
What substances are reabsorbed via carrier mediated transport systems?
Glucose
Amino acids
Organic acids
Sulphate and phosphate ions
What happens when Tm is suceeded?
The excess substrate enters the urine and is excreted
Tm is the maximum transport capacity - one saturated the remaining substances are excreted
What determines the transport capacity (Tm)?
Number of carriers
What is the renal plasma threshold for glucose?
10 mmol/L
If plasma glucose goes above 10mmol/l then it will be excreted in the urine (this is what occurs in diabetes)
What substances does the kidney not regulate their Tm?
And why is this of clinical significance?
Glucose and amino acids
They are regulated by insulin and other glucose counter regulatory hormones therefore if there is glucose in the urine (in diabetics) it is failure of insulin not the kidneys
What substances does the kidney regulate the Tm of?
Sulphate and phosphate ions
What other hormone regulates phosphate excretion ?
Parathyroid hormone
By what mechanism is Na ions reabsorbed?
Via active transport
What percentage of Na is reabsorbed per day?
99.5%
Where does majority of Na reabsorption occur?
Proximal tubule
Describe the reabsorption process of Na.
Na enters the cells through membrane proteins down its electrochemical graident
Na is pumped out the basolateral side of cell by the Na K ATPase
This active transport creates an electrochemical graident which allows Na to enter the cell from the lumen passively down a conc gradient
Where are the active Na pumps located?
Basolateral surfaces where there is high amounts of mitochondria
How does the reabsorption of Na draw water out of the tubules?
Reabsorption of Na establishes an electrochemical gradient which allows negative ions such as Cl to diffuse passively across the proximal tubular membrane down its conc gradient
This creates an osmotic force which drives H20 out of the tubules
Removal of water from proximal tubule = concentrated substances = ongoing concentration gradients = ongoing diffusion of other ions
Is the tubular membrane permeable to urea?
Only moderately permeable therefore only 50% is reabsorbed
What is responsible for the passive diffusion of impermeable ions across the membrane for reabsorption?
Na
Active transport establishes electrochemical gradient
How would decreased blood flow to the kidney result in dehydration?
Decreased blood flow = decreased 02
02 is required for the ATPase responsible fort the active transport for Na
If this gradient isnt established then Na and other ions such as Cl wont be reabsorbed and either will H20
= increased excretion of water = dehydration
Does a high or low Na fascilitate glucose transport?
High Na
Glucose crosses the membrane via sodium dependant glucose transporter so requires it for transport
Describe how glucose enters and leaves the cell for reabsorption.
Glucose enters the proximal tubule cell via Na dependant glucose transporter
It then leaves the cell into the ISF via GLUT transporter protein
what is the renal threshold?
plasma threshold at which saturation occurs
what is the Tm for amino acids?
it is unlimited in order for amino acids to be reabsorbed so we don’t waste it
what effect does PTH have on phosphate and calcium in the kidney and how?
PTH increases phosphate excretion in distal tubule and increases Ca reabsorption in ascending L.O.H, distal tubule and collecting duct
reduces the number of channel proteins
what enzyme is involved in bicarbonate reabsorption?
carbonic anhydrase
how much bicarbonate is reabsorbed by the kidney?
99.9%
