Unit 4: Renal Physiology: PART TWO: Reabsorption and Secretion Flashcards
What is the Glomerular Filtration rate?
180 L/ day = 125 mL/min
What are the two ways to get ride of stuff through the kidney?
- Filtration–> and do NOT reabsorb it
2. Secretion (from blood in peritubular capillaries into where filtered stuff went too)
What is the amount of a substance filtered across the glomerular called?
the filtered load
What is occurring during filtration?
an interstitial like fluid is filtered across the glomerular cap into Bowman’s space
What does the Filtered Load equal?
= GFR x plamsa conc. of substance X x % unbound in plasma
What does the Filtration Fraction equal?
= GFR / RPF
the fraction of renal plasma flow that is filtered across the glomerular capillaries
- normally is 20% (.2)
What is the fraction of renal plasma flow that is filtered across the glomerular capillaries? What is it normally?
Filtered Fraction
normally is .2 (20%)
What does the excretion rate equal?
urine flow x conc. of substance X in urine
What does Reabsorption or secretion rate = ?
= Filtered Load - excretion rate
What is the fluid in Bowman’s Space and lumen of a nephron called?
tubular fluid
What sort of things are reabsorbed?
- water
- Na+
- Cl-
- HCO3-
- glucose
- AAs
- urea
- Ca++
- Mg++
- phosphate
- lactate
- citrate
What is the mechanism for reabsorption?
transporters in membrane of tubular epithelial cells
What is the MOST important function of the kidney? Why?
reabsorption of Na+–> b/c it is linked to ALL other reabsorption and virtually drives all resabsorption in the kidney!!!!*
What is the major cation in the ECF?
Na+ (plasma and interstitial fluid)
What does the amount of Na+ in ECF influence?
influences ECF volume–> influencing plasma volume, blood volume, and BP
How much of sodium is reabsorped?
output = intake
What happens if we increase the intake of Na+?
volume expansion
What substances are SECRETED from peritubular blood into the tubular fluid via the tubular epithelium?
organic acids, organic bases, K+
Secretion–> is an additional mechanism fro excreting substance into the urine
What refers to the amount of substance excreted per unit of time?
Excretion
What is excretion the net result of?
Excretion = filtration - reabsorption + secretion
What can we compare excretion rate to in order to determine if a substance has been reabsorbed or secreted?
compare to Filtrated Load
What is the equation for Excretion Rate? What is it for Filtered Load?
Excretion rate = urine flow x urine conc. of substance X
Filtered Load = GFR x plasma conc. of substance X
What if the filtered load is greater than the excretion rate? What substance is this an example of?
= net reabsorption
Ex: Na+
What if the filtered load is less than the excretion rate? What substance is this an example of?
= net secretion
Ex: PAH
What are the two ways needed to get glucose reabsorbed and then transported into peritubular blood?
in early Proximal Tubule by Carrier-Mediated mechanisms (along with Na+)
- “SGLT”–> Secondary Active Transport (from tubular fluid into cell)
- GLUT 1 and GLUT 2–> Facilitate Diffusion (from cell into peritubular blood)
What type of transporter is used for glucose absorption, to get it from the tubular fluid into the tubular cell? What is moving uphill vs downhill? What other pump is necessary for this to occur?
Na+ glucose cotransporter (symporter) called “SGLT” (Sodium Glucose linked Transporter)
2 Na+ and 1 glucose
- glucose moves uphill
- Na+ moves downhill
Secondary Active Transport–>Uses:
Na+/K+ pump on peritubular capillary side keeps Na+ low in ECF
How is glucose transported from the tubular cell into the peritubular capillary?
via Facilitated Diffusion
- moving DOWN conc. gradient
- no energy needed
- GLUT 1 and GLUT 2 (= insulin independent)
What is the filtered load for Glucose? What happens as plasma conc. of glucose increases?
glucose is freely filtered
Filtered Load = GFR x Plasma conc. of glucose
filtered load increases linearly
At what plasma concentration is all glucose reabsorbed?
less than 200 mg/dl
all glucose can be reabsorbed
At what plasma concentration does “splay” occur with glucose reabsorption? What is “splay”?
greater than 200 mg/dl, but less than 350 mg/dl
splay = bending of reabsorption curve–> a threshold where not all glucose is reabsorbed
reabsorption is approaching saturation
What are two reasons for “splay” to occur for glucose titration curve?
- Low affinity of Na+ glucose cotransporter (glucose detaches and is excreted; few remaining binding sites near Tm)
- Heterogeneity of nephrons (some have higher Tm, some have lower; some glucose excreted before average Tm is reached)
At what plasma glucose concentration is Tm reached? What occurs here on the titration curve?
greater than 350 mg/dl
glucose carries fully saturated and reabsorption of glucose flattens out
At what plasma glucose conc. does excretion of glucose occur? When does excretion follow a linear increase as there is an increase in filtered load?
beings when it is greater than 200 mg/dl (hit threshold)
greater than 350 mg/dl
What is the term for glucose in the urine? What three things could this be due to?
Glucosuria; Due to:
- High plasma glucose (>200mg/dl); diabetes
- Pregnancy (increased GFR which increases filtered load–> may spill some glucose into urine)
- Congenital abnormalities of Na+-glucose cotransporter
Is urea freely filtered across the glomerular capillaries? How is it secreted or reabsorbed?
yes; and it is transported in most segments of the nephron
secreted or reabsorbed by diffusion (simple or facilitated)
How is the secretion or reabsorption rate of Urea determined?
concentration gradient and permeability
What happens to urea concentration in the tubular fluid as water is reabsorbed from the tubule? What does this create?
urea conc. increases–> creates a conc. gradient driving reabsorption of Urea
T/F. Urea does not follow water reabsorption
False— urea does follow water reabsorption
Where is 50% of Urea reabsorbed? And by what type of transport?
50% reabsorbed in Proximal Tubule via simple diffusion
Where is urea secreted in the nephron? Is more or less of urea secreted than what was previous reabsorbed in the Proximal Tubule?
in the thin descending limb–> due to high conc. in interstitial fluid in inner medulla
MORE secreted than previously reabsorbed in Proximal tubule
What parts of the nephron are impermeable to urea? What occurs here under the influence of ADH?
- Thick ascending limb
- distal tubule
- cortical collecting duct
- outer medullary collecting duct
ADH–> will stimulate water reabsorption here and therefore Urea is left behind (increase urea conc. in tubule fluid)
What is the total filtered load for Urea? Of that, what is reabsorbed? What is excreted?
110% filtered load
- 70% reabsorbed
- 40% excreted
After urea has traveled through the Loop of Henle, where is is reabsorbed? By what transporter?
Inner Medullary collecting duct–> by Urea Transporter 1
What activates the Urea Transporter 1? Where is this located? And what is it doing?
ADH activates it; in inner medullary collecting ducts
Facilitated diffusion from tubular lumen–> ISF
70% of filtered load reabsorbed –the other 40% is excreted
Can urea to toxic?
yes, at high levels
What does the Urea recycling that occurs in the nephron cause? And therefore what does it help with?
causes buildup of high urea concentration in inner medulla–> creating osmotic gradient at Loop of Henle so water can be reabsorbed
What substance is an example of secretion and is used to measure renal plasma flow?
PAH (para-aminohippuric acid)
RPF = to clearance of PAH
Of the filtered load for PAH, what percentage is filterable and what isn’t and why?
10% filterable
90% not filterable b/c bound to plasma proteins
Where is PAH secreted in the nephron? What happens when Tm is reached?
Proximal Tubules in peritubular membranes by transporters
Tm is reach and secretion is maxed out
What happens to excretion of PAH as unbound plasma conc. of PAH increases?
excretion rises sharply and BOTH filtration and secretion are increased
Once Tm is reached for PAH, what happens to excretion and filtered load?
rise in excretion parallels an increase in filtered load
What does excretion equal for PAH?
= sum of filtration + secretion (as non is reabsorbed)
What inhibits the secretion of PAH by transporters in peritubular membranes of Proximal Tubular cells?
probenecid
What happens once the PAH transporters have hit maximum secretion? What then occurs with an increase in filtration?
filtration and excretion are parallel
and increase filtration results in an increase excretion
What are the two forms that many substances secreted by the Proximal Tubule exist in?
(many substances are weak acids and weak bases)
- charged and uncharged
- relative amounts depend on pH
What are examples of weak acids?
What are examples of weak bases?
PAH and salicylic acid (aspirin)
quinine, morphine
When it comes to charges, what is the acid form of a weak acid? What is the conjugate base form?
How does the charge effect reabsorption?
acid form = HA (uncharged)
- can be pulled back
conjugate base form = A- (charged)
- can NOT be pulled back and is secreted
At a low pH, what form of a weak acid will predominate?
the HA (acid form) predominates–> therefore can reabsorb easier
At a high pH, what form of a weak acid will predominate?
the A- (conjugate base) form predominates–> can excrete more readily
If we want to excrete an acid, will we want to increase or decrease the pH?
increase the pH or urine –> therefore it will be in the A- form and cannot be reabsorbed
What is the base form and the conjugate acid form for a weak base?
base form = B (uncharged)
- pull back easier
conjugate acid form = BH+ (charged)
- cannot be pulled back and is excreted
At a high pH, what form of a weak base predominates?
the B form (base form) –> can reabsorb easier
At a low pH, what form of a weak base predominates?
BH+ form (conjugate acid)–> therefore can excrete
If we want to excrete a base, what do we want to pH of urine to be, high or low?
want pH to be low–> therefore will be in conjugate acid form (BH+)–> making it charged and therefore cannot be reabsorbed and will be secreted
What substances can diffuse across the tubular cells and therefore can be reabsorbed?*
the UNcharged substances
charged forms are excreted
At a high pH what are we excreting, weak acid or weak base? What about at a low pH?
high pH–> excretion of weak acid
low pH–> excretion of weak base
What are the two forms of Aspirin and which has a charge? Which can be reabsorbed and which cannot?
salicylic acid = HA form
- can be reabsorbed
salicylate = A- form
- cannot be reabsorbed
If someone has an aspirin overdose, what do we want to do to maximize the loss of it?
put it in charged form–> so want to put it at a high pH, therefore it would be in the A- form (salicylate)
