Topic 8 A: Basic Principles Tubular Reabsorption & Secretion Flashcards
Excretion = equation?
Filtration–Reabsorption + Secretion
Filtration: occurs where?
Glomerulus
Reabsorption & Secretion occurs where?
Proximal tubule; loop of Henle;
distal tubule; collecting tubule
Filtration Rate = equation?
GFR x Plasma concentration
Glucose :(g/day) % of filtered load reabsorbed
100 %
Bicarb: mEq % of filtered load reabsorbed
99.9 %
Sodium: % of filtered load reabsorbed
99.4%
Chloride: % of filtered load reabsorbed
99.1%
Potassium: % of filtered load reabsorbed
87.8%
Urea: % of filtered load reabsorbed
50%
Creatinine: % of filtered load reabsorbed
0%
Kidneys has independent control over exertion rate by changing what?
appropriate reabsorption rate
Several distinct movements for water and solutes (4)
Transcellular path
Paracellular path
From interior of cell into tubular interstitial space
From interstitial space into peritubular capillary
Transcellular path
From tubular lumen into tubular cells
Paracellular path
From tubular lumen into tubular interstitial space
From interstitial space into peritubular capillary
Driven by capillary filtration forces [bulk flow]–net movement into the capillaries
Primary active transport mechanisms of tubular reabsorption (4)
Na-K ATPase (basal lateral membrane)
Hydrogen ATPase
H-K ATPase
Ca ATPase
Secondary active transport: Co-transport mechanisms of tubular reabsorption (2)
Sodium-glucose
Sodium-amino acids
Secondary active transport: Counter-transport mechanisms of tubular reabsorption
Sodium-hydrogen
Pinocytosis mechanisms of tubular reabsorption
(requires energy)
Proteins–once in cell broken down to component amino acids and amino acids reabsorbed
Passive mechanisms of tubular reabsorption (2)
Osmotic movement of water
Bulk flow into peritubular capillaries
Na reabsorption:
Na-K ATPase on basolateral sides of tubular epithelial cells
Creates membrane potential of what?
-70 mV
Na reabsorption
Na reabsorption:
Brush board of proximal tubule luminal membrane creates what?
huge surface area for diffusion (20x increase)
part of Na reabsorption
Sodium Rabsorption - Sodium reabsorption also enhanced by carrier proteins through luminal membrane
Co-transport & counter-transport proteins
Glucose Reabsorption: Co-transport mechanism so tied to what?
tied to sodium gradient from tubular lumen to interior of tubular cells
So efficient that usually removes all filtered glucose
KNOW
Glucose Reabsorption: Two luminal transporters?
SGLT2 and SGLT1
SGLT2 what is reabsorbed and where?
90% glucose reabsorbed via SGLT2 in early part of proximal tubule
SGLT1 what is reabsorbed and where?
10% reabsorbed in later part of proximal tubule
Bulk flow moves what?
glucose from interstitial spaces into the peritubular capillaries
Amino Acid Reabsorption: Co-transport mechanism tied to what gradient??!?!?
sodium gradient from tubular lumen to interior of tubular cells
So efficient that usually removes all filtered amino acids
⬆️RBF does what to Glucose Reabsorption?
⬆️Glucose Reabsorption
Amino Acid Reabsorption:
Luminal co-transporter system pumps the amino acids where???
Amino acids diffuse where??
–into the cells
–out of the cells into the interstitial spaces
Bulk flow moves the amino acids from where into where?
interstitial spaces into the peritubular capillaries
Hydrogen Secretion
Counter-transport mechanism tied to what?
sodium gradient from tubular lumen to interior of tubular cells
Hydrogen Secretion
Sodium-hydrogen exchanger is located where?
in brush boarder of the luminal membrane
Hydrogen Secretion: can move LRG amounts of H+ but can only do it across a small what?
gradient
Transport max
Max amount of solute that can be reabsorbed (transport max transport)
Transport maximum: Occurs when?
Occurs when tubular load (amount of solute delivered to tubule exceeds transport capacity of carrier proteins
Glucose transport max mg/min?
on that line graph
375 mg/min
Glucose filtered load = equation (normal amount)
GFR x [Glu] = 125 mls/min x 1 mg/ml = 125 mg/min
concentration where glucose first appears in urine?
on line graph thing
Threshold concentration (approx. 250 mg/dL)
Why is Threshold concentration is less than transport max?
Less than T max because each individual nephron is different–chart represents action of both kidneys so Tmax reached when ALL nephrons have reached their max
Glucose Transport Max
375 mg/min
Amino Acids transport max
1.5 mM/min
Plasma protein transport max
30 mg/min
Two excretion rates?
- -Before secretion Tmax is reached the amount excreted is sum of amount filtered and amount secreted (steepest slope of excretion curve)
- -After secretion Tmax is reached rate of excretion parallels filtration rate (slope of excretion curve matches slope of filtration curve)
Two excretion rates: steepest slope of excretion curve
Before secretion Tmax is reached the amount excreted is sum of amount filtered and amount secreted
Two excretion rates: slope of excretion curve matches slope of filtration curve
After secretion Tmax is reached rate of excretion parallels filtration rate
Change in GFR would change the slope of what line on the transport max secretion graph?
Filtered line
Creatinine Transport Max (mg/min)
16 mg/min
actively secreted
Rate of transport depends on: what 3 things ?
- Electrochemical gradient for solute
- Membrane permeability for solute
- Time fluid containing solute remains in tubule
- Transport rate inversely related tubular flow rate
⬆️GFR ⬆️ Tubular Flow - does what to Na reabsorption?
⬇️Na reabsorption
Sodium transport max? in Proximal Tubule
does not show a transport maximum even though it is actively reabsorbed
Proximal Tubule: Significant amount of transported sodium leaks back into the tubular lumen affected by what? (2)
- Permeability of tight junctions between cells
- Forces controlling bulk flow of water & solute into peritubular capillaries
Na reabsorption:
As plasma concentration of sodium increases, what happens in the Proximal Tubule and toreabsorption?
sodium concentration in proximal tubule increases and sodium reabsorption increases
A decrease in tubular flow rate will also increase sodium reabsorption
Na reabsorption: Proximal Tubule: A decrease in tubular flow rate will_____ sodium reabsorption?
increase
Na reabsorption: Distal Tubule: Sodium reabsorption shows classic what?
tubular max transport
Na reabsorption: Distal Tubule: Capacity of Na-K ATPase does not exceed rate of what?
net sodium reabsorption
Minimal back leak of sodium into tubular lumen
Tighter (less permeable tight junctions) coupled transport of much smaller amount of sodium
Na reabsorption: Distal Tubule: Minimal back leak of Na into where? and why?
Minimal back leak of sodium into tubular lumen
Tighter (less permeable tight junctions) coupled transport of much smaller amount of sodium
Na reabsorption in Distal Tubule: what increases the Threshhold max level?
Aldosterone
Passive Reabsorption of water driven by what?
Driven by osmotic differences created by movement of solute (mainly sodium) from tubular lumen to the tubular interstitial spaces
Passive Reabsorption of water affected by what?
Affected by cellular permeability (cell membranes and tight junctions)
Increased permeability
Passive Reabsorption of water Increased permeability means what for reabsorption and excretion?
means increased reabsorption and decreased water excretion
Passive Reabsorption of water in proximal tubule?
Highly permeable
Rapid movement so overall solute gradient across cell is minimal
Solvent drag–water carries significant amount of sodium, chloride, potassium, calcium, magnesium because of high permeability
Passive Reabsorption of water Loop of Henle (ascending loop) permeability?
Low permeability
Little movement of water even though there is a large osmotic gradient
Passive Reabsorption of water Distal tubule / Collecting tubules / Collecting ducts: permeability?
Variable permeability
Cellular permeability depends on presence of antidiuretic hormone (ADH)
Permeability directly related to [ADH]
Changing water permeability only affects amount of water reabsorbed not the amount of solute due to low solute permeability
⬆️ADH = does what to permeability of H2O
⬆️H2O Urine permeability (reabsortion)
Passive Reabsorption: Chloride & Urea
Sodium diffusion into cells creates electrical
gradient that pulls what ions into the cell?
negative chloride ions into the cell
Passive Reabsorption: Chloride & Urea
Movement of water into cells concentrates
urea creating??
concentration gradient into cell but urea not nearly as permeable as water
What % of filtered urea is reabsorbed ?
Only 50% of filtered urea is reabsorbed
