Reabsorption/Secretion in the Proximal Tubule Flashcards
T or F. The proximal tubule (PT) is the site of mass reabsoprtion of glomerular filtrate?
T
What is important for regulation of ECF volume?
reabsorbing 2/3 of golmerular ultra filtrate
How do you calculate the rate of flow into loop of Henle?
GFR - Reabsorption + secretion
What are the major solutes that contribute to isotonic reabsoprtion in the PT?
Na, Cl, and Bicarb
T or F. Na reabsorption occurs only at the PT.
F. it occurs all along the nephron
What are 3 components found at sites of active Na reabsorption?
- tight junctions
- luminal membrane Na channel
- NaKATPase ion pump (found on basolateral side of cell)
What doe the NaKATPase result in?
It’s the driving force for Na Absorption
- decreased IC [Na]
- decreased in membrane potential
What is used to drive reabsorption of other solutes from the lumen of PT?
the potential energy generated from passive diffusion of Na down its electrochemical gradient
What drives Cl transport in PT?
rapid Na absorption resulting in luminal fluid being more negative than ISF
What allows paracellular space transport of solutes?
a leaky epithelium
What happens in the straight tubule of the PT?
Cl absorption is reduced and bicarb is rapidly absorbed
Where are the different types of aquaporins found? AQP 1, 4/5, 2
1 –> luminal membrane
4/5 –> basolateral membrane
2 –> distal tubule
What drives water reabsorption in PT?
osmotic gradient facilitated by leaky epithelium w/ high hydraulic conductivity (high Kf value)
What does reabsorption mean?
capillary uptake of fluid from ISF
T or F. Reabsorption from PT is iso-osmotic but selective.
T
How is HCO3 absorbed?
indirectly w/ a H+/Na+ exchanger
- proton secretion
- H+ reacts w/ bicarb to form CO2 and H2O
- CO2 absorbed and converted to Bicarb via CA and HCO3- reabsorbed into blood
How is glucose reabsorbed?
via a Na-glucose co-transporter, secondary active transport.
What is the Tm of glucose?
370-390, point at which all the glucose transporters are transported
What is the threshold of glucose?
point at which glucose is first seen in the blood. It’s when reabsoprtion doesn’t match filtration so glucose is excreted
What are some causes of glucosuria?
thirst and nocturia, pregnancy, DM, renal glucosuria (mutation in glucose transporter)
What are the transporters for glucose called?
- SGLT1 – found in int and kidney
2. SGLT2 – found in kidney
How are amino acids reabsorbed and how much are?
Na co-transporter, only 0.5-2% excreted
When is protein excretion high?
in MS, hemoglobinemia, myoglobinemia
How are organic acids reabsorbed?
coupled to Na electrochemical gradient
Does phosphate has a high or a low threshold when it comes to reabsorption? What does that mean?
low –> partially excreted continuously in urine
What regulates the Tm of phosphate?
hormones –> PTH decreases Tm so promotes phosphate excretion
What allows Cl to be passively reabsorbed?
- concentration gradient created by water reabsorption
2. electrochemical grandient created by Na reabsorption
Why is only 60% of Cl reabsorbed in PT?
b/c of active transport of HCO3-
If there is an increase in urine flow what effect will that have on urea clearance?
it will increase it
how much urea is reabsorbed?
50%
What will freely filtered but not reabsorbed substances do?
increase osmolarity and cause diuresis
What is the clinical significance of only freely filtered substances?
reduce intracranial and intraocular pressure, promote excretion of toxins, edema
What is an example of a freely filtered substance only?
mannitol
What is mannitol?
monosaccharide that has no transporters –> reduces water reabsorption and increase excretion
