Tubular Reabsorption & Secretion Flashcards

1
Q

steps of solute/water reabsorption

A

tubule –> interstitium –> capillaries

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2
Q

is reabsorption the same across the entire nephron

A

no - spatial; different components get reabsorbed at different points along the tubule

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3
Q

types of transport

A

paracellular (between cells)
transcellular (through cells)

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4
Q

what does it mean that transporter mediated transport is saturable?

A

reabsorption is dependent on the number of available transporters - will reach a maximum rate when all transporters occupied

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5
Q

is filtration saturable

A

NO - filtration only depends on GFR and plasma concentration

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6
Q

what does it mean if there is no solute detected in the urine

A

filtration of solute = reabsorption of solute

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7
Q

does reabsorption increase with filtration

A

yes - reabsorption and filtration will increase at the same rate until plasma concentration of the solute gets too high and all transporters are occupied (causes filtration to continue increasing while reabsorption plateaus)

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8
Q

what does reabsorption depend on

A

plasma concentration
GFR
Tm

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9
Q

what is transport maximum (Tm)

A

the maximum rate of reabsorption (mass of solute that can be transported per minute)

appears as a constant Y value (horizontal line)

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10
Q

what is renal threshold

A

the plasma concentration of a substance at which it begins to appear in urine (when reabsorption < filtration)

appears as a constant X value (vertical line)

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11
Q

how does decreasing GFR affect Tm and renal threshold

A
  • NO EFFECT on Tm
  • increases renal threshold (more time available to clear transporters)
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12
Q

what are the starling forces entering peritubular capillaries

A

efferent arterioles: low hydrostatic and high oncotic –> favors reabsorption

peritubular cap: high hydrostatic pressure of interstitium and oncotic pressure of capillaries; low hydrostatic pressure of capillaries and oncotic pressure of interstitium

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13
Q

what transport mechanisms are in the proximal convoluted tubule

A

Na/K ATPase
Na/Glu cotransporter
Na/H antiporter
Na/PO4 cotransporter
Paracellular Cl, Ca, Mg
Urea transporters
Aquaporins

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14
Q

what is the osmolarity of filtrate going through the PCT

A

isotonic - equal amounts of solute and water reabsorbed

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15
Q

what transport mechanisms are in the descending loop of henle

A

Aquaporins
Urea transporters - secretes urea INTO filtrate from interstitium

IMPERMEABLE to NaCl

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16
Q

what is the osmolarity of the filtrate going through the descending loop

A

osmolarity INCREASES down descending loop

urea concentration INCREASES down descending loop

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17
Q

what transport mechanisms are in the ascending loop of henle

A

Na/K ATPase
NKCC (Na/K/2Cl) cotransporter
Paracellular Ca, Mg

IMPERMEABLE to water

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18
Q

what is the osmolarity of the filtrate as it goes through the ascending loop

A

osmolarity DECREASES up ascending loop

19
Q

what transport mechanisms are in the distal convoluted tubule

A

Na/K ATPase
TRPV5 (Ca from lumen –> cytosol)
NCX1 (Ca reabsorption)

Early DCT:
- NCC (Na/Cl cotransporter)
- ROMK (K excretion)

Late DCT:
- ENaC (Na transporter)
- ROMK (K excretion)
- some NCC

20
Q

what is the osmolarity of the filtrate going through the DCT

A

hypo-osmotic filtrate enters but DCT is load sensitive

if filtrate Na concentration is HIGH - will increase reabsorption
if filtrate Na concentration is LOW - will decrease reabsorption

21
Q

what transport mechanisms are used in the collecting ducts

A

Na/K ATPase
ENaC
ROMK
Urea transporters (ADH dependent) - reabsorbs out of filtrate ONLY when ADH is present

22
Q

is reabsorption rate constant regardless of changes in GFR

A

NO - reabsorption rate changes with GFR to prevent excess losses

dependent on starling forces in peritubular capillaries

23
Q

antidiuretic hormone action

A

ADH responds to high NaCl or low BP

produced in hypothalamus and secreted by pituitary

causes translocation of aquaporins and urea transporters to CD membrane

effect: increases water and urea reabsorption

24
Q

how does ADH secretion vary based on hydration state

A

dehydrated: increase ADH
hydrated: decrease ADH

25
how does urea contribute to medullary hyperosmolarity
urea concentration in the filtrate increases from PCT --> DCT when ADH is present: urea gets reabsorbed which decreases osmolarity of the tubule, leading to increased water reabsorption
26
what is countercurrent multiplication
creates and maintains medullary interstitial osmotic gradient descending LoH: interstitium is hyperosmotic --> pulls water out of filtrate --> causes filtrate osmolarity to increase down descending loop ascending LoH: hyperosmotic filtrate causes NaCl reabsorption into interstitium --> decreases filtrate osmolarity as it enters DCT effect of differences in Na and water concentration get multiplied as fluid moves counter currently
27
RAAS signal
low NaCl and low BP/ECFV sensed by baroreceptors, macula densa cells, and sympathetic nervous system
28
RAAS mechanism of action
sensors signal to JG cells --> increase renin secretion renin cleaves angiotensinogen --> ANG I ACE cleaves ANG I --> ANG II ANG II --> binds to AT1 receptors on targets
29
angiotensin II targets
- efferent > afferent arterioles - Na/K ATPase - NCC transporter - ROMK transporter - adrenal gland
30
angiotensin II effects
- vasoconstriction (dec. RBF/GFR/P(GC)/NFP) - increased Na reabsorption - increased aldosterone secretion
31
aldosterone targets
- NCC - ENaC - ROMK
32
aldosterone effects
- increase Na reabsorption - increase K excretion
33
what is the main nephron region targeted by aldosterone
collecting ducts
34
what are the different cell types in collecting ducts and why are there different types?
principal cells, intercalated cells different types allow for separate regulation of Na reabsorption and K excretion / acid/base balance
35
principal cells
Na and K regulation
36
intercalated cells
H+ secretion and acid/base balance
37
where is the main nephron region of PO4 reabsorption
proximal convoluted tubule Na/PO4 cotransporter (driven by Na/K ATPase)
38
what stimulates PO4 reabsorption
calcitriol low dietary PO4
39
what inhibits PO4 reabsorption
PTH FGF-23 high dietary PO4
40
where is the main site of Ca reabsorption
proximal convoluted tubule paracellular transport - regulated by CaSR/claudins and driven by electrical gradients
41
what stimulates Ca reabsorption
PTH calcitriol hypocalcemia thiazide diuretics
42
what inhibits Ca reabsorption
hypercalcemia furosemide
43
where is the main site of Mg reabsorption
thick ascending LoH paracellular - regulated by CaSR/claudins and driven by electrochemical gradients from NKCC2