Lecture 16 Tubular Reabsorption and Secretion Flashcards
where must a substance first pass during passive transport?
across the tubular epithelial membranes into interstitial fluid
what follows transport across the membranes and renal interstitial fluid?
transport through peritubular capillary membrane and back to the blood
what are the two roots water takes from the lumen to the interstitium?
transcellular (thru cell) and paracellular(between cells) via osmosis
specific water channels that transport across membrane
aquaporins
aquaporin 1 location
widespread, including renal tubules
aquaporin 2 locations
present in apical membranes of collecting tubule cells
controlled by ADH (secreted from posterior pituitary)
aquaporin 3 locations
present in basolateral membranes of collecting tubule cells
what is bulk flow?
flow between tubular cells and peritubular capillaries that occur as a result of hydrostatic and osmotic pressures
depends on starling
insertion of more aquaporins in membrane causes…
more water reabsorption
ATPase pumps
establish ion gradients across the nephron cell membranes
movement of Na
gradients that drive reabsorption or secretion of many other solutes
symport ATPase
aka cotransport
moves with Na+ gradient
antiport ATPase
solute moves opposite Na+ gradient
Ex. ATPase channels
ENaC channel
CFTR
K+ channels
Uniporters
ENaC pump
ATPase
found in apical membrane of nephron cells
closed by amiloride (drug)
open by hormones
where are CTFR channels open? what do they transport?
chloride
found in apical membranes of some parts of nephron
where are K+ channels located in kidney?
found in apical membranes of some parts of nephron
what drives uniporters in cell membranes
[gradient] of substance concerned
what type of transport occurs through channels or uniporters
facilitated transport
i.e. glucose transport
what type of transport is directly coupled to energy source
active transport
what type of transport is coupled indirectly to an energy source (i.e. ion gradient)
secondary active transport
primary active transporters ex.
Na+K+ ATPase
H+ ATPase
H+K+ ATPase
Calcium ATPase
examples of Secondary active transport in kidney
reabsorption of glucose or amino acids by renal tubule
where are sodium glucose cotransporters located?
brush border of proximal tubules
SGLT1
reabsorbs 10% of glucose in late proximal tubule
SGLT2
reabsorbs 90% of glucose in early proximal tubule
what substances are actively secreted into renal tubules?
creatine
para-aminohippuric acid
transport maximum
rate at which a solute can be transported
what limits transport maximum?
saturation of a specific transport system
why would some passively reabsorbed substances not have a transport maximum
- rate of diffusion is deterred by electrochemical gradient of substance
- permeability of membrane for the substance
- time fluid containing substance remains in tubule
2 factors rate of transfusion depends on
electrochemical gradient
time the substance is in the tubule (tubular flow rate)
solvent drag
passive water reabsorption by osmosis is coupled mainly to sedum reabsorption
osmotic movement of water that is carrying some solutes
which of the following (in L/day) represents the GFR in the average adult human?
A. 3
B. 125
C. 180
D. 360
180
what is the transport max for glucose?
375 mg/min
which of the following will result in a decrease of glomerular hydrostatic pressure?
a. arterial pressure
b. afferent arterioler resistance
c. efferent arterioler resistance
d. both B and C
E. all of the above
B. Afferent arterioler resistance
almost 90% of glucose reabsorption occurs in the early proximal tubule. Which of the following mechanism is responsible for moving glucose from the tubular lumen into cytoplasmic compartment?
a. primary active transport via glucose ATPase pump
b. secondary active transport via Na/glucose cotransporter
c. secondary active transport via Na/glucose antiporter
d. diffusion due to concentration differences between tubular fluid and cytoplasm
b. secondary active transport via Na/glucose cotransporter
proximal tubule composition
highly metabolic with large numbers of mitochondria
extensive brush border on luminal surface
extensive intercellular and basal channels on interstitial surfaces
what does the proximal tubule reabsorb?
65% of filtered Na, Cl, bicarbonate, and K
reabsorb all filtered glucose and amino acids
what does proximal tubule secrete?
organic acids, bases, and H ions into lumen
sodium reabsorption in first half of proximal tubule
occurs via co-transport with glucose, amino acid, and other solutes
sodium reabsorption in second half of proximal tubule
reabsorption is coupled with chloride ions
Sodium transport entry is via
antiport with H+
loop of henle
thin descending loop of hence and permeability
highly permeable to water and moderately permeable to most solutes (urea and sodium)
how is H + removed from the cell?
anti port with Na+
H+ ATPase
thin ascending segment of loop of hence and water permeability
impermeable to water
thick ascending loop of henle transport compositions
Na+K+ATPase pump in basolateral membranes
Na,K, Cl co transporter
Slight back leak of K into lumen
what is the electrical gradient of the cytoplasm of proximal tubule? tubular lumen?
-70mV, -3 mV
Luminal Na+ concentration
140 mOsm
Cytoplasmic Na+ concentration
30mOsm
describe H+ and bicarbonate ions in proximal tubule
H+ increases in lumen due to anti port transport with Na+
H+ combines with luminal bicarbonate to form carbonic acid
carbonic anhydrase in lumen splits carbonic acid into CO2 and Water
what happens after Co2 and Water enter the cell?
CO2 and water combine to form carbonic acid
carbonic acid dissociates to from bicarbonate ion and H+
bicarbonate ion diffuses out of cel inter intersistial space
Na+K+Atpase pump in basolateral membranes of loop of henle…
drive reabsorption of K+ into cell against concentration gradient
Na, K, Cl co transporter movement
1 Na, 2 Cl, and 1 K into cell
Leaking of K+ back into lumen causes
a positive charge of +8 mv
Mg++ and Ca++ diffusion through tubular lumen through paracellular space into interstitial fluid
permeability of the thick ascending segment of loop of henle
impermeable to water
site of action of powerful loop diuretics
diuretics that act on the loop of henle
furosemide
ethacrynic acid
butametanide
what does the first portion of the distal tubule form?
macula densa
function and permeability of distal tubule
impermeable to water and urea
reabsorbs most ions (diluting segments)
Na+Cl- cotransporter in luminal membrane
Na+K+ATPase pump in basolateral membrane
principle cells of the late distal tubule/corticle collecting tubule
reabsorption of Na+ and water from tubular lumen
secretion of K+ into tubular lumen
Use Na+K+ATPase pump
Primary site of K+ sparing diuretics
K+ sparing diuretics
spironolactone
eplerenon
amiloride
triameterene
intercalated cells of the late distal tubule
reabsorb K+ from tubular lumen
secrete H+ into tubular lumen (ea. H+ secretion reabsorbs bicarbonate ion, H+ from carbonic anhydrase, mediated by H+ ATPase)
intercalated cells (general)
reabsorb potassium and bicarbonate ions from lumen and secrete hydrogen ions
epithelial cells of medullary collecting duct
cuboidal, smooth surfaces with few mitochondria
permeability of medullary collecting duct
permeable to water (control by ADH)
permeable to urea (use urea transporters_
capable of secreting H+ against lg. [ ] gradient
aldosterone
source and site of action
from adrenal cortex, acts on principal cells of cortical collecting ducts
aldosterone
stimulus for secretion
increased levels of extracellular potassium and levels of angiotensin II
absence of aldosterone causes
addison’s disease, loss of sodium and accumulation potassium
function of aldosterone
increases sodium reabsorption and stimulates potassium secretion
stimulates Na+/K+ ATPase pump on basolateral side of cortical collecting tubule membrane
hyper secretion of aldosterone
conn’s syndrome
function of Angiotensin II
increased sodium and water reabsorption
returns blood pressure and extracellular volume to normal
effects of angiotensin II
stimulates aldosterone secretion
constricts efferent arterioles
directly stimulates Na reabsorption in proximal tubules, loops of Henle, distal tubules and collecting tubules
source of ADH
posterior pituitary
function of ADH
increases water reabsorption
effects of ADH
binds to V2 receptors in late distal tubules, collecting tubules, and collecting ducts
increases cAMP formation (stimulates movement of Aq-2 proteins to luminal side of cell membranes, form clusters)
source and function of Atrial natriuretic Peptide
source: cardiac atrial cells in response to distension
function: inhibits reabsorption of sodium and water
source and function of Parathyroid hormone
source: parathyroid glands
function: increases calcium reabsorption
renal clearance of a substance =
volume of plasma completely cleared of the substance by the kidneys per unit time
mathematical equation for clearance
Cs= (US x V)/Ps
Us = urine concentration V= urine flow Ps= plasma concentration Cs= clearance rate
GFR and rate of excretion
GFR = Us x V
SO GFR= Cs
