17-02-23 - Obligatory reabsorption and secretion in the proximal convoluted tubule Flashcards
Learning outcomes
- Describe the modifications to the proximal tubule epithelia which maximise transport.
- Describe the transport processes in the Proximal Tubule enabling Na+, K+ and H2O reabsorption.
- Describe the transport processes in the Proximal Tubule enabling reabsorption of glucose and amino acids.
- Define the term “solvent drag”.
- Define the term transport maximum (Tm) for a substance.
- Define the concept of renal threshold and what happens when it is exceeded.
- Understand the concept of renal clearance and define the equation used to define it.
- Describe the clearance of inulin/creatinine, PAH and glucose and what these can be used to calculate.
- Understand what the eGFR is.
- Understand the concept of clearance ratios.
What is ultrafiltrate?
Where does the ultrafiltrate enter?
What is the composition of ultrafiltrate?
- Ultrafiltrate is plasma filtered across the glomerulus
- Ultrafiltrate enters the renal tubule at the proximal convoluted tubule (PCT)
- The ultrafiltrate mimics the ionic plasma composition of the renal blood without all the cellular components and large proteins e.g sodium and potassium will be typical of blood ECF values
What volume of blood plasma is filtered across the glomerulus every day?
What % is reabsorbed?
What volume of urine is produced every day?
What are 2 key functions of the kidney?
What is the formula for urinary excretion?
What part of the tubule has a high capacity for reabsorption?
- 180 L/day of blood plasma is filtered across the glomerulus everyday
- 99% is reabsorbed in the kidney tubule, with 1-1.5 litres of urine being produce
- 2 key function of the kidney is to reabsorb and secrete substances along with length of the tubule (e.g secrete ammonia and protons)
- Urinary excretion = Glomerular filtration – tubular reabsorption + tubule secretion
- The proximal tubule has a high capacity for reabsorption
What is the renal tubule lined by?
How does this vary along segments of the tubule?
- The renal tubule is lined by a single, continuous layer of epithelia
- The structure and properties of the epithelial cells change along the various segments of the tubule, with the structure usually linked to their function
What are structural features in the proximal tubule epithelia for?
What are 3 structural features in the proximal tubule epithelium that aid the large amount of reabsorption that occurs?
- The proximal tubule epithelium has structural features that aid the large amount of reabsorption that occurs
- 3 structural features in the proximal tubule epithelium that aid the large amount of reabsorption that occurs:
1) Apical microvilli, called the brush border membrane, which increase surface area for transporters
2) Also have deep infoldings of the basolateral membrane, again allowing expression of more transporters and pumps
3) Have a rich supply of mitochondria which will help with ATP production
What electrical properties do epithelial cells have?
Why is this important?
What are 2 mechanisms of transport across the tubular epithelium?
- Epithelial cells are polarised e.g. distinct apical and basolateral membranes
- This is important as it allows the apical and basolateral surfaces to express different transporters/channels, which can be used to transport substances against gradients
- 2 mechanisms of transport across the tubular epithelium:
1) Transcellular
* Requiring transporters/channels to cross through cell
2) Paracellular
Requires “leaky” tight junctions, to move between cells
Obligatory reabsorption in the proximal tubule.
Substances reabsorbed in the proximal tubule:
* 100% (3)
* 90% (2)
* 80% (2)
* 65% (3)
* 50% (1)
* 35% (1)
Where does regulated reabsorption/secretion occur?
- Obligatory reabsorption in the proximal tubule
- Substances reabsorbed in the proximal tubule:
- 100% (3) - glucose, amino acids, proteins
- 90% (2) - phosphate, sulphate
- 80% (2) - potassium, filtered bicarbonate
- 65% (3) - water and sodium, calcium
- 50% (1) - chloride
- 35% (1) - magnesium
- Regulated reabsorption/secretion occurs later in the tubule
Na+ reabsorption across the tubular epithelia.
How is Low Na+ maintained inside the cell?
Where does K+ from the Na+/K+ ATPase go?
What does this generate?
What does the transcellular route for Na+ require?
What else can be absorbed using this?
What does paracellular transport of Na+ require?
- Na+ reabsorption across the tubular epithelia
- Low Na+ inside cell is maintained by Na+/K+ ATPase on basolateral membrane which extrudes Na+ back into the blood
- K+ from the Na+/K+ ATPase leaks back to the blood in the interstitium of the capillaries
- This movement of K+ out of the cell generates an electrochemical gradient for Na+ to enter via apical membrane (consider electrical and/or chemical), where the inside of the cell is more negative compared to the intersitium and tubular fluid
- The transcellular fluid for Na+ requires channels / transporter (exchangers and co-transporters)
- Other molecules can be absorbed through the gradient generated
- Paracellular transport of Na+ requires “leaky” tight junctions
What are 3 transcellular movements of Na+ in the proximal tubule?
What is the paracellular movement of Na+ like in the proximal tubule?
- 3 transcellular movements of Na+ in the proximal tubule:
1) Apical Na+/H+ exchanger
2) Many apical cotransporters (details on later slides)
3) Exits towards blood via the basolateral Na+/K+ ATPase
- Paracellular movement of Na+ is through “leaky” tight junctions, with nearly 2/3 of the Na+ reabsorbed via transcellular pathways will leak back to lumen through paracellular pathway (Na+ can move both ways)
Does the water move through the proximal tubule by osmosis?
What are 2 transcellular movements of water through the proximal tubule?
What substance creates osmotic gradients?
What is the paracellular movement of water through the proximal tubule?
- Water will not move through cells in the proximal tubule by osmosis into the cell, it has to either have a channel or paracellular pathway that allows it to move
- 2 transcellular (dominant route) movements of water through the proximal tubule:
1) Apical water channels (Aquaporins)
2) High density in membrane - Whenever sodium moves, it will create on osmotic gradient for water to follow it
- Paracellular movement of water will occur due to osmotic gradient generated by e.g. transcellular Na+ reabsorption
How does glucose move across the apical membrane in the proximal tubule?
What channel does it use?
How does glucose move across the basolateral membrane of the proximal tubule?
- Glucose moves across the apical membrane in the proximal tubule by coupling the Na+ gradient generated by the Na+/K+ ATPase pump (secondary active transport)
- Glucose crosses the apical membrane via a Na+-glucose co-transporter (SGLT1 and SGLT2)
- Glucose is extruded across basolateral membrane by the GLUT1 uniporter (glucose can’t move back through the co-transporter)
How do amino acids cross the apical membrane in the proximal tubule?
How do amino acids cross the basolateral membrane in the proximal tubule?
- Amino acids cross the apical membrane via co-transport with either Na+ or H+ (remember Na+/H+ exchanger)
- Amino acids are extruded across basolateral membrane by uniporters (various)
Where does K+ enter epithelial cells in the proximal tubule?
How does it leave? Is there apical entry of K+?
Where is K+ predominantly transported?
- K+ enters basolateral surfaces of cells in the proximal tubule epithelium via the Na+/K+ ATPase pump
- K+ leaves the cells via basolateral leak K+ channels and Cl-/K+ co-transporter, allowing extrusion towards the blood
- There is no apical entry of K+
- The filtered K+ in the lumen is predominantly transported back to the blood via the paracellular pathway
How is paracellular transport of H2O enabled?
What is solvent drag?
What does this mean in terms of movement of ions?
What 6 substances can move via solvent drug?
- The osmotic gradient generated by ions being reabsorbed from the tubule lumen back to the blood (peritubular capillaries) enables paracellular transport of H2O
- Solvent drag is when this movement of H2O carries some of the solutes/ions with it
- It means that unusually, ions can move against their electrochemical gradient because they being “dragged” through the leaky tight junctions
- 6 substances that can move via solvent drug:
1) Na+
2) Cl-
3) Ca2+
4) (Mg2+)
5) K+
6) Urea
What is the transport maximum of a substance (Tm)?
Why is there a transport maximum for substances?
What will happen to the concentration of a substance if the transport maximum is reached?
What are 2 examples of substances that are fully absorbed/secreted?
- The transport maximum of a substance (Tm) is the maximal amount of a substance (in mg) which can be transported (reabsorbed or secreted) by tubular cells / min
- There is a transport maximum for substances because the rate of reabsorption can become saturated and will not transport any more, even if there is more that could move
- If the Tm of a substances is riches and the substance can’t move it will build up e.g. more will be excreted in urine or more will remain in blood (remember transport can be in both directions)
- 2 examples of substances that are fully absorbed/secreted
1) Glucose - 100% reabsorbed from filtrate, not secreted (in healthy person)
2) Para-aminohippuric acid (PAH) – not reabsorbed, fully secreted
What are 2 ways Tm can be reached?
How is glucose filtered?
What is the filtered load of glucose?
What is the Tm of glucose?
What is excess glucose in urine called?
- 2 ways Tm can be reached:
1) Large increase in GFR
2) Large increase in plasma concentration of substance - Glucose is freely filtered, so all glucose in plasma will appear in ultrafiltrate
- Filtered load of glucose is ~125 mg/min (GFR of 125 ml/min X [glucose]plasma of 1 mg/ml)
- The Tm for glucose is 375 mg/min
- Excess glucose in urine is called glucosuria
At what plasma concentration will small amounts of glucose start to appear in the urine?
What is this known as?
How does Tm for glucose vary among nephrons?
When will Tm for glucose be reached based on this?
- When [glucose]plasma exceeds 200mg/100ml, giving a filtered load of 250 mg/min, small amount of glucose starts to appear in urine
- This is known as the threshold, and occurs before Tm
- Not all nephrons have the same Tm for glucose
- Some will start to excrete glucose before others
- The Tm for glucose therefore reflects the point at which all nephrons have reached their maximum capacity to reabsorb glucose
When can plasma concentration of glucose exceeds Tm?
What is the Tm for 7 substances reabsorbed?
What is the Tm for 2 substances secreted?
- [glucose]plasma almost never exceeds Tm
- However, in uncontrolled diabetes mellitus, [glucose]plasma may exceed the Tm resulting in urinary glucose excretion
- Tm for substances reabsorbed:
1) Glucose 375 mg/ml
2) Phosphate 0.10 mg/ml
3) Sulphate 0.06 mg/ml
4) Amino acids 1.5 mg/ml
5) Urate 15 mg/ml
6) Lactate 75 mg/ml
7) Plasma protein 30 mg/ml - Tm for substances secreted:
1) Creatinine 16 mg/ml
2) PAH 80 mg/ml
What does renal clearance reflect the ability of?
What 3 renal processes determine and modify the composition of urine?
What is the formula for urinary excretion?
- Renal clearance reflects the ability of the kidneys to remove molecules from blood plasma by excreting them into the urine
- 3 renal processes determine and modify the composition of urine:
1) Glomerular filtration
2) Tubular reabsorption
3) Tubular secretion - Urinary excretion = Glomerular filtration – tubular reabsorption + tubule secretion
What 4 things can we quantify using renal clearance?
What is the formula for renal clearance (in picture)?
- 4 things can we quantify using renal clearance:
1) Renal blood flow
2) Glomerular filtration rate
3) Tubular reabsorption rate
4) Tubular secretion rate - Formula for renal clearance (in picture)
What 3 criteria a substance must meet to measure GFR using our clearance equation?
What is an example of a substance that meets these criteria?
What is it used for?
- 3 criteria a substance must meet to measure GFR using our clearance equation:
1) Freely filtered at the glomerulus
2) Neither secreted nor reabsorbed in the tubule
3) Not synthesised nor metabolised in the tubule - Inulin is a substance that meets these criteria, so for Inulin Cin = GFR (clearance = GFR)
- It is a plant fructose polymer
- It is an Ideal substance but has to be administered intravenously to ensure constant plasma concentrations
Renal clearance – measuring GFR example questions (in picture)
What might we use as an alternative to Inulin?
Why is this?
What is creatinine produced by?
Why does creatinine not complete fit the criteria for using clearance to calculate GFR?
- We may use creatinine as an alternative to Inulin
- This is because it would be less invasive to use an endogenous compound to avoid having to infuse inulin
- Creatinine is produced via metabolism of creatine phosphate in skeletal muscle
- Creatinine does not completely fit the criteria for using clearance to calculate GFR as it is not reabsorbed, but a little bit is secreted
- It fits the other 2 criteria:
1) Freely filtered at the glomerulus
2) Not synthesised nor metabolised in the tubule
What 2 equations can be used to estimate GFR?
What are 3 problems with estimated GFR (EGFR) of creatinine?
- 2 equations that can be used to estimate GFR:
1) Cockcroft-Gault Formula
2) MDRD Study Equation (MDRD: Modification of renal disease diet)
- 3 problems with estimated GFR (EGFR) of creatinine:
1) A small amount of creatinine is secreted in the proximal tubule, so more is excreted, therefore overestimates GFR
2) PCr will not reveal big changes in GFR e.g. if GFR declines
3) Compensation by other nephrons e.g 20% drop in GFR would cause an increase in PCr of 0.002 mg/ml
What needs to happen for a substance to be completely cleared from the plasma?
What formula will this give for Para-aminohippuric acid (PAH)?
What can this be corrected to (in picture)?
Why is PAH not a perfect example?
- For a substance to be completely cleared from the plasma, it must also be secreted into the tubule as only 20% of the renal blood flow is filtered (80% of the substance is still in circulation and needs to be secreted into tubule)
- Therefore, RPF x PPAH = V x UPAH (for Para-aminohippuric acid (PAH))
- This equation can be corrected to the formula in the picture
- PAH is not a perfect example as only about 9-% is cleared
Corrected RPF calculation example
What can we then do with the corrected RPF value we’ve calculated?
How is this done?
How can we calculate RPF from corrected RPF?
What problems are the same for both PAH and inulin?
- With the corrected RPF – it is possible to work backwards to calculate renal blood flow (RBF)
- RBF calculation using corrected RPF:
- RPF is the non-haematocrit fraction (55%) so is 650ml/min = 55%, then 1182ml/min = 100% of renal blood flow (RBF)
- GFR calculation using corrected RPF:
- RPF is what enters afferent arteriole and only 20% of that is then filtered, so GFR here would be: 650 ml/min / 5 = 130 ml/min for GFR
- PAH has same problems as inulin: needs intravenous admin & urine collection
What 2 things can the renal clearance equation be used for calculating? (Equation in picture)
- 2 things the renal clearance can equation be used for calculating (equation in picture):
1) GFR using inulin
2) RPF using PAH
What 4 criteria does glucose meet?
How would we calculate clearance for glucose and other substances that meet these criteria? (In picture)
- 4 criteria glucose meets:
1) Freely filtered at the glomerulus
2) Fully reabsorbed in the tubule
3) Not secreted in the tubule
4) Not synthesised nor metabolised in the tubule - How we would calculate clearance for glucose and other substances that meet these criteria (in picture)
What is the filtration fraction (FF)?
How do we get the values to put into this equation?
Example of FF equation in picture
- The filtration fraction (FF) is the fraction of the RPF which is filtered (FF = GFR/RPF)
- We can get the RPF value (via PAH clearance) and GFR value (via inulin clearance) to plug into the FF equation
- Example of FF equation in picture
What is the clearance ration of a substance?
What values do we need to calculate clearance ratio?
What does a clearance ratio =1, <1, >1 mean?
Example of clearance ratio calculation for phosphate in picture
- The clearance ratio of a substance is the net reabsorption/secretion of a substance
- If rates of GFR (clearance of inulin) and renal excretion of substance are known (Us and V) we can calculate the clearance ratio of a substance
- Clearance ratio = 1
- Substance neither secreted nor reabsorbed
- Clearance ratio > 1
- Substance is secreted
- Clearance ratio < 1
- Substance is reabsorbed
- Example of clearance ratio calculation for phosphate in picture
