7. Renal transport mechanisms Flashcards
why filter 180L/day and reabsorb 99%?
- foreign substances are filtered into the tubule but not reabsorbed into the blood
- filtering ions and water into the tubule makes regulation simple
what plays the biggest role in the reabsorption capabilities of the proximal convoluted tubule?
Na/K ATPase in the basolateral membrane
all reabsorption is somehow linked to this transporter
how much of the filtrate is reabsorbed in the PCT
approx. 67%
where is the energy needed for antiport derived from?
usually Na/K ATPase
creates gradient needed for one molecule to assist in the transportation of another in the opposite direction
how is chloride reabsorbed
Passive paracellular movement
as filtrate is moving down the PCT, more water and sodium are being reabsorbed than chloride. by the later segment of the proximal tubule, concentration of chloride rises by ~20%. this creates a concentration gradient and allows for passive paracellular chloride movement
what determines paracellular movement of water
presence or the absence of tight junctions between cells
where are aquaporins present
transcellular route for water
AQP-1 (proximal tubule absorbs 65-70%)
AQP-2 (CD under the control of ADH)
net water permeability of the renal tubules is determined by what two factors
presence or absence of tight junctions (paracellular)
presence or absence of aquaporins (transcellular)
two types of sodium-coupled glucose transporters
SGLT-1 (3rd section of PCT; absorbs 10% of glucose)
SGLT-2 (1st and 2nd section of PCT; absorbs 90% of glucose)
ACTIVE transport
what is the transport maximum (Tm) of glucose
375mg/min = plasma glucose of about 200mg/dL
anything above this is not reabsorbed, but excreted in urine
where are the NK2Cl cotransporters located
thin and thick ascending loop
how does furosemide work
loop diuretic
inhibits NaK2Cl cotransporter in thick asacending loop
inhibits NaCl reabsorption by competing for the Cl- binding site on the carrier
decreased reabsorption of Na, K and Cl
diuresis
Distal tubule (DCT)
no K reabsorption
NaCl cotransport
relatively impermeable to water (early is late isn’t)
Thiazide diuretics
blocks NaCl symporter (reabsorption in cortical segment of DCT)
increases NaCl excretion
enhance Na Ca exchange (enhancing Ca reabsorption)
natriuresis and decreased BV and BP
useful in treating calcium-subtype of kidney stones and may be useful for treating osteoporosis
collecting ducts
impermeable to water without ADH
aldosterone “fine-tunes” Na reabsorption (targets principal (light) cells)
