Tubular Transport Flashcards
What kinds of molecules will be reabsorbed out of the lumen and back into the blood?
Valuable molecules like glucose and amino acids
What kind of molecules will be secreted from the blood into the lumen?
Waste products, metabolites, toxins
How do you calculate the Filtered Load?
FL=(GFR)(Px)
What value is this:
“Amount of material in glomerular filtrate”
Filtered load
How do you calculate Excretion rate?
ER= (Ux)(V)
What is this value:
“Amount of material lost in urine”
Excretion rate
What is a transport rate?
The amount of material added to or removed from glomerular filtrate
How do you calculate the transport rate of a substance?
Tx=FL-ER
Remember FL=(GFR)(Px)
ER=(Ux)(V)
What does it mean if your transport rate is positive/.
It means that some of the material was reabsorbed
Tx=FL-ER
(Amount excreted was less than the amount filtered)
What does it mean if your transport rate is negative?
It means that some of that material was secreted/
Tx=FL-ER
The amount excreted was more than the amount filtered
What does it mean if your transport rate is 0?
It means that the substance is a GFR marker molecule and is neither secreted nor reabsorbed!
(Creatinine, inulin)
Where is the luminal/apical membrane?
It faces the lumen of the nephron (touches the pee)
Where is the basolateral membrane?
Against the capillary
B for blood side
What are the 2 barriers that a substance must cross when it is reabsorbed transcellularly?
- Apical membrane
2. Basolateral membrane
What is paracellular reabsorption?
The substance sneaks from the pee into the blood between two “leaky” tubular epithelial cells.
Does NOT have to cross the apical membrane or the basolateral membrane
What is transcellular reabsorption?
It means the substance has to go through a tubular epithelial cell to get from the pee to the blood
(Involves transporters, pores etc)
How is glucose brought back into the blood?
- Secondary active transport gets it across the apical membrane (comes in with Na+)
- Facilitated diffusion (carrier protein) moves it across the basolateral membrane
- The sodium potassium ATPase keeps working in the background using primary active transport to pump Na+ out of the cell so the glucose can flow in with it in step 1
How much sodium is reabsorbed in the proximal tubule, and is it active or passive transport?
2/3 of the filtered Na+
Active
How much of the filtered water is reabsorbed in the proximal tubule and is it active or passive transport?
2/3 of the filtered water is reabsorbed
Passive transport; solute-linked
How much sodium and water is reabsorbed in the loop of henle and is it active or passive?
Na: 25%, active
Water: 15%, passive; solute linked
How much sodium and water is reabsorbed in the distal and collecting tubule?
Na: 8%, active, controlled by aldosterone
Water: 20%, passive. Not solute-linked. Controlled by ADH.
Where does the bulk of water reabsorption happen?
Proximal tubule (2/3)
In the proximal tubule, how does sodium get across the basolateral membrane and into the blood?
Sodium potassium ATPase transports it, and water passively follows it via diffusion
What is the primary source driving fluid movement into the peritubular capillaries?
The oncotic pressure of the blood in the peritubular capillaries
(ηpc)
In the proximal tubule, is the reabsorption of water and electrolytes considered to be isosmotic?
Yes
The osmotic gradients are very slight
What is the NHE and where is it?
Sodium-hydrogen exchanger
In the proximal convoluted tubule on the apical side
What effect does angiotensin have on the proximal tubule?
It stimulates Sodium and H+ exchange across the apical membrane via the NHE transporter.
(Enhances sodium recovery and enhances proton secretion into the urine)
What effect does SNS activity have on the proximal convoluted tubule?
It enhances Na+ reabsorption
What effect does parathyroid hormone have on the proximal convoluted tubule?
It inhibits Na+ phosphate cotransport
(Inhibits Na+ recovery and increases urinary excretion of phosphate)
(Which makes sense since Parathyroid hormone increases Calcium, and if calcium goes up, phosphate goes down)
In Bowman’s space, what is the ratio of solute concentration in the tubular fluid to the plasma for freely filtered substances?
1:1
They are equal
What happens to the ratio of inulin in the tubular fluid to plasma (TF/P ratio) across the length of the proximal convoluted tubule?
TF/P for inulin goes up! Not because more of it is going IN to the PCT, but because so much water is reabsorbed, so it’s concentration increases
Across the length of the PCT, what happens to osmolarity?
Does not change
If TF/P for a substance is 1, then what does that mean?
Reabsorption has been exactly proportional to the reabsorption of water
If TF/P for a substance is less than 1, what does that mean?
Reabsorption of the substance was greater than the reabsorption of water
TF
____
P
Think about it…P got bigger and made it less than 1
If TF/P is more than 1 for a substance, what does that mean?
Reabsorption of the substance was less than the reabsorption of water, OR that there has been net secretion of the substance
TF
____
P
TF is bigger
What part of the nephron has the most Transport Maximums (TM’s)
The proximal convoluted tubule
What is Tm?
The maximum rate (mg/min) of a solute than can be transported.
Below Tm, all of the filtered load is reabsorbed. Any portion of the load above Tm will be excreted
What does threshold mean regarding glucose reabsorption?
It is the plasma concentration where glucose first appears in the urine
(Tm gets exceeded)
Is the threshold plasma concentration of glucose dependent on GFR?
Yes
If GFR is increased, what happens to the threshold (plasma concentration where glucose first appears in the urine)?
It decreases
(The higher your GFR, the fewer cookies you can eat before glucose spills out into your urine. The Tm of the transporters gets overwhelmed faster)
If GFR decreases, what happens to the threshold for plasma concentration in the urine?
It increases
You can eat more cookies before glucose spills out into the urine.
Transporters don’t reach their Tm as fast if GFR is lower
Is glucose the only substance with a Tm (Transport maximum)?
No, there are others
Sugars, amino acids, lactate, ketone bodies, phosphate ions, vitamins, etc
What happens to mannitol in the proximal tubule?
It is NOT reabsorbed at all, so ALL of it stays in the PCT and it prevents osmotic water flow out of the lumen. It also causes back-diffusion of Na+ into the lumen
Causes water and electrolyte loss in the urine (diuresis)
What is meant by osmotic diuresis?
The concentration of a substance in the urine (glucose, mannitol, urea, etc) prevents the reabsorption of water and electrolytes, so you lose a lot of water and electrolytes in your urine
(Polyuria, which causes polydipsia, …gee this sounds like diabetes….)
Why are diabetics thirsty and peeing all the time?
Because in the PCT, there is so much glucose in the urine that it overwhelms the glucose reabsorption transporters, so it stays in the urine and then causes osmotic diuresis by preventing reabsorption of water
What can you give to a patient if you want to induce an osmotic diuresis?
Inject them with mannitol
A non reabsorbed carbohydrate
Which route is used more for tubular secretion:
Paracellular or transcellular
Transcellular
What are the 2 main transport systems for tubular secretion?
One for organic cations
One for organic anions
VERY NON SPECIFIC**
What kinds of compounds are secreted into the lumen?
Metabolites, waste products, foreign chemicals, things “tagged” by the liver as trash
Organic anions are secreted via ________________ transport
Tertiary active transport
Not primary, not secondary….
PAH is an organic anion. (That gets completely secreted into the lumen)
How does it get into the lumen from the capillary?
- PAH is taken up into the cell for an α-ketoglutarate (done by an OAT…organic anion transporter) *set up by the Na gradient established by Na-K-ATPase which is why this is tertiary active transport
- PAH leaves leaves the cell on the apical side via a PAH-anion antiporter
What kind of transporters are involved in organic anion secretion?
OAT-1 and OAT-3 transporters located in the basolateral membranes of PCT cells
Can OAT-1 and OAT-3 transporters become saturated and have Tm’s?
Yes
Along the length of the proximal tubule, where does the most secretion happen?
Late proximal tubule
