Lect 9: Tubular reabsorption & Secretion Flashcards Preview

Unit 7 - Repro & Renal Physiology > Lect 9: Tubular reabsorption & Secretion > Flashcards

Flashcards in Lect 9: Tubular reabsorption & Secretion Deck (35)
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What is reabsorption?

it is the movement of filtered solutes and water from the tubular fluid across the epithelial cell tubule into the peritubular capillaries and the circulation


What is secretion?

moving solutes from the circulation via the peritubular capillaries across the epithelial cell tubule into the tubular fluid.

Solutes can be secreted but water cannot


Two types of renal transport, what are they?

Transcellular and paracellular (solutes and water moving through cell junctions)


Transcellular Transport

can be actve or passive

depends on transporters facing the tubular fluid in the lumen and in the basolateral membrane facing the peritubular space and capillaries


What is the function of the Na/K ATPase?

it is found only on the basolateral side of all renal epithelial cells within the nephron where it maintains the Na gradient (Na high outside, K high inside...important for creating the inside negative membrane potential)


Secondary active transport can occur

in the same direction across the membrane (co-transport) or it could be transported in the opposite direction (counter-transport) depending on which solute has the greater gradient


Passive Transcellular transport

channels, uniporters


Paracellular transport

passive, and is driven by a transepithelial solute EC potential gradient; permeability property of the junctional resistance or leakiness between cells makes it tight o


Movement of water from the tubular lumen across the intracellular junctions to the peritubular space occurs by

osmosis (because it is following the increased osmolarity) and may entrain the movement of solute by a process of "solvent drag" which contributes to transtubular solute reabsorption or secretion


Tubular reabsorption of a solute can occur from

active transport (uptake) at the the lumen and passive transport at the basolateral membrane OR passive transport at the lumen and active transport at the basolateral membrane. The same is true for secretion


Transcellular reabsorption and secretion is inhibited by drugs

such as diuretics and circulating metabolites.


Which direction is reabsorption?

lumen to blood


Which direction is secretion?

blood to lumen


Renal Handling of Glucose: What is the equation for reabsorbed glucose?

Reabsorbed glucose = filtered glucose -- excreted glucose
Filtered glucose: Pglu x GFR -- Ugl x Vurine (rate of urine flow)
Excreted glucose is usually 0


Glucose Titration Curve

looks at the effect of raising glucose conc on the filtered load, reabsorption and the excretion of glucose


Increasing the plasma conc of glucose, what happens to the filtered load?

it goes up linearly.


At 5-10mM of glucose, how much of it is in the urine?

None of it. It is ALL REABSORBED but as we continue to increase the glucose conc, glucose begins to appear in the urine over 15mM....because we have saturated the proximal tubule to reabsorb the glucose.

--happens in diabetes mellitus


Glucose Clearance

C = U x V/ Pgl clearance is normally 0 but if you continue to incerase the plasma glucose conc then this formula becomes greater than 0.
==>Cgl/Cin will always be lower than inulin, less than 1, indicating reabsorption


Membrane Transport mediating transcellular glucose reabsorption on the lumenal side - active uptake with passive efflux

Lumenal membrane: Na-glucose cotransporter mediate the accumulation of glucose inside the cell via the inwardly directed Na-electrochemical potential gradient (SGLT2).


How is SGLT1 special?

the Na-glucose transport or coupling ratio is 1 to 2 to 1 resulting in net positive charge transfer with each glucose transported. Adding two Na concentrates glucose inside the cell to a much higher level


What about transporters on the basolateral membrane?

passive efflux mediated by facilitated diffusion


Phosphate Reabsorption

occurs in the proximal tubule and it increases with increasing filtered load. This reaches a maximum phosphate where the process of transcellular phosphate transport becomes saturated and the rate of phosphate reabsorption becomes maximal and constant.


Phosphate reabsorption similar to glucose

active uptake at the apical membrane and passive efflux at the basolateral membrane


Reabsorption of Amino Acids

active uptake at the apical membrane and passive efflux at the basolateral membrane


Secretion ...into the urine

Many blood borne solutes enter the fluid by transcellular secretion from the perutibular space to the lumen across the segments of the nephron


Renal Handling of PAH

....PAH is a secreted molecule
is an example of a solute which is secreted into the tubular fluid but not reabsorbed from the tubular fluid.

The amount of PAH excreted in the urine is a function of the amount filtered as well as the amount secreted by the tubule. Comtrast the renal handling of PAH with the renal handling of glucose, which are reabsorbed but not secreted


Formula for Secretion

secreted PAH = Excreted PAH -- Filtered PAH
Excretion > Filtered in secretion


PAH Clearance

clearance of PAH is much greater than the clearance of inulin.

At low plasma PAH conc the clearance of PAH equals RPF (renal plasma flow)


As plasma solute conc increases and approaches the tubular secretion maximum, the cellular transport processes mediating solute secretion become saturated and plasma solute conc begins to increase.

you will begin to see PAH in the renal vein.
==> At increasing plasma PAH conc where secretion becomes saturated, the secretory tubular maximum for PAH is achieved and PAH secretion becomes proressively smaller fraction of the amount of PAH in the urine and PAH filtration becomes a progressively larger fraction of the amount of PAH in the urine


How does the clearance of PAH equal the RPF?

At 100% of the filtered load, you have 500% secreted you have 600% remaining in the tubule and ultimately excreted in urine.