Tubular Transport of NaCl and H2O

Question 1

For water and NaCl

a) amount filtered of each
b) percent of filtered amount that is reabsorbed

overall, how significant is regulation for reabsorption

Answer 1

Water

a) 190L
b) > 86% reabsorbed

NaCl

a) 1500 g
b) > 98% reabsorbed

most filtered water + salt is reabsorbed so small fraction under homeostatic control –> regulation only affects 14% water, < 2% NaCl not auto reabsorbed

Question 2

Movement of

1) Na+
2) Cl-
3) H2O (2 routes)

Answer 2

1) Na+ flows down gradient passively then pumped out other side (serosal)
2) Cl- travel through tight junction following Na+ (paracellular)
3) Osmotic pressure of Na+, Cl-, causes water to follow via either aquaporis on apical/basolateral or through tight junctions in proximal tubule (paracellular or transcellular)

Question 3

Movement of glucose and other active metabolites

Answer 3

Filtered

Cotransporter using energy from Na+ downhill gradient coupled to flow of glucose against gradient (2ndary active transport)

Glucose then passively diffued across

Question 4

All energy fro ion and H2O movement is derived from ___

Answer 4

Na+/K+ ATPase

Question 5

Epithelial transport mechanism of Prox Tubule

1) What gets transported and how?
2) max ____ rate for each substance because?
3) tubular fluid is ___

Answer 5

1) Na+ transferred from lumen of epith cell to serosa (basolateral) via Na+/K+ ATPase
2) Na+ passive from lumen into cell
3) Cl- follow Na+ via tight junction
4) water follows due to osmosis from Na+ and Cl-
5) also glucose + other metab (AA and HCO3-) using Na+ gradient

_____

2) reabsorption rate because discrete # of transporter

____

ISOTONIC FLUID

Question 6

EPITHELIAL TRANSPORT MECHANISM FOR LOOP OF HENLE!

1) what gets transferred and how
2) tubular fluid becomes ___ and interstitium becomes ___

Answer 6

1) NaCl reabsorption in ascending loop via Na+/K+/2cl- cotransporter on lumen side and Na+/K+ ATPase on serosal side

ASCENDING = IMPERMEABLE TO H2O
2) NaCl reabsorbed in descending loop and water reabsorbed due to osmosis and incr permeability but GREATER NACL REABSORB COMPARED TO WATER

____

TUBULAR FLUID = hypotonic

interstitium = hypertonic (more NaCl reabsorbed)

Question 7

Mechanism of epithelial transport in distal tubule and collecting ducts

1) what is transported and how
2) how is this segment different from other loop of henle and proximal tubule

Answer 7

same exactly as prox tubule ==> Na+, Cl-, and Water reabsorbed

____

2) differs because reabsorption is varied by circulating hormones and there are tight junctions between individual cells so most reabsorption occurs TRANSCELLULARLY

Aldosterone and ADH incr NaCl and H2O reabsorption

Question 8

What is a common target of loop diuretics

Answer 8

Na+/K+/2Cl- channel in loop of henle

Question 9

Effect of aldosterone in fine tuning distal tubule/collecting duct

Answer 9

Creates more Na+ pumps and channels in DT/CD –> incr Na+ reabsorption

incr ATP synthesis for the pumps

Question 10

Mechanism of how aldosterone incr Na+ pumps

Answer 10

1) Aldosterone enters cell and hits receptor
2) aldosterone + receptor –> nucleus to turn on genes and make more Na+ pumps on BASOLATERAL surface and channels on APICAL surface–> INCR NA+ REABSORBED

also vesicles already making pumps/channels that can be integrated more quickly than making new channels/pumps

HOURS!!!!

Question 11

Mechanism of ADH/vasopressin on channels in cell

Answer 11

1) ADH + ADH receptor cause pre-formed vesicles with aquaporins to enter into apical (lumenal) membrane TO INCR REABSORPTION OF H2O
2) interstitium is hypertonic because 25% of Na+ reabsorption in ascending limb (IMPERMEABLE TO WATER) drives water gradient

RAPID AND SHORT

Question 12

Amount of H2O reabsorpton in

1) proximal tubule
2) descending lop
3) ascending lop
4) fine tuning (obligatory vs. hormones)

Answer 12

1) 65%
2) 15%
3) 0%
4) 6% obligator

14% hormones

Question 13

Amount of NaCl reabsorpton in

1) proximal tubule
2) descending lop
3) ascending lop
4) fine tuning (obligatory vs. hormones)

Answer 13

1) 65%
2) 0%
3) 25%
4) 8% obligatory

2% hormones

Question 14

Unique permeability traits of

1) proximal tubule
2) descending loop
3) ascending loop

Answer 14

1) isotonic, also glucose, AA, bicarb reabsorbed
2) impermeable to NaCl
3) impermeable to H2O

Question 15

proximal tubule and loop of henle are responsible for the ___, ____ reabsorption of salt and H2O

distal tubule and collecting duct responsible for the ____ in maintaining homeostaiss and a degree of ___

Answer 15

obligatory, unregulated

regulatory control of reabsorption; fine tuning

Question 16

where do adh/vasopressin and aldosterone act?

Answer 16

distal tubule/collecting duct

Question 17

STARLING EQUATION FOR flow of sodium from renal interstitium to peritubular capillaries

when does it apply?

what is the main driving force for flow?

Answer 17

after H2O and salt reabsorbed from nephron –> must reach capillaries

Fic = K’ (Pint + πcap – Pcap – πint)

colloid oncotic pressure (πcap) due to extraction of H2O from plasma upstream from capillary through glomerular filtration

Question 18

Define variables in Starling equation and give values for each

Answer 18

F​ic​= flow from interstitium to capillary = ΔP/R, 1/R = K’

P​int=​ pressure in interstitium from all the H​2O​in that space=7mmHg

P​cap​= pressure in the capillary resisting the pressure in the interstitium = 11 mmHg

π​cap​= osmotic capillary pressure drawing fluid into the capillary = 35 mmHg

π​int​= osmotic pressure in interstitium keeping fluid in the interstitium = 6 mmHg

Net filtration pressure (NFP) for reabsorbtion = 25 mmHg

Question 19

Starling force is entirely ___ generated flow because of albumin in blood leftover

Answer 19

osmotically

capillary very permeable to Na+

Question 20

What is the effect of incr and decr tubular flow on water and sodium excretion and reabsorption

Answer 20

1) incr tubular flow, decr reabsorption, incr excretion of substances (ex = diuretics)

flow too fast in tubule so less chance to interact with transporters for reabsorption

so more substance escape reabsorption

2)decr tubular flow, incr reabsorption, decr excretion of substances

Question 21

what is purpose of glomerulotubular balance and tubuloglomerular feedback

Answer 21

maintaining constant delivery of water and NaCl to distal tubule and collecting duct despite changes in GFR

Question 22

Define glomerulotubular balance

Answer 22

ability of obligatory reabsorption in proximal tubule to compensate for changes in filtered load

ex:proximal tubule reabsorption readjusts to filtration changes so FIXED PROPORTION OF FILTERED WATER AND NaCl ALWAYS REABSORBED = 65%

Question 23

xDefine tubuloglomerular feedback

Answer 23

regulates GFR in each nephron in response to changes in NaCl concentration by special epithelial cells = macula densa

contact afferent arteriole–> cause arteriole to constrict or dilate

Question 24

Describe macula densa cells

Answer 24

in contact with afferent arteriole –> cause constrict or dilate

start of distal tubule –> monitor status of obligatory reabsorption just before fluid enters fine tuning segments

Question 25

what is the effect of incr GFR on tubular fluid flow

mechanism of tubuloglomerular feedback

Answer 25

1) initial incr GFR, incr in tubular fluid flow in ascending limb
2) decr % NaCl reabsorption in loop –> compensation by glomerulotubular balance in prox tubule (incr [NaCl] in lumen)

uncompensated part causes fluid to move faster in loop of henle

3) decr [NaCl] reabsorbed in ascending limb so NaCl incr in lumen of ascending and transported to macula densa
4) when fluid leaves loop, contacts macula densa –> senses incr in NaCl –> incr signaling to JGA –> causes afferent arteriole to contract
5) incr secretion of AGII and adenosine to constrict afferent arteriole
6) decr Pgc to return GFR back to normal

Question 27

Causes of water intoxication

Answer 27

impaired GFR

using ecstasy

(ingest large volume of water overwhelming excretory abiility)

Question 29

primary energetic event

Answer 29

Na+ pump out across basolateral via Na/K ATPase

Question 30

Why is there a transport maximum for a given substance

glucosuria is a diagnostic indicator of ___

Answer 30

because fixed number of transporters in prox tubule

diabetes mellitus

Question 31

Unique features of distal tubule

Answer 31

1) tight junctions = TIGHT so majority reabs through transcellular with selective transporter
2) rate of transport varied by circulating hormones

Question 32

Action of ADH

Answer 32

1) phosphorylation cascade to initate synthesis of aquaporins
2) release of premade aquaporins from small vesicles into apical membrane

Question 33

main driving force for reabsorption fo fluid is

why?

Answer 33

osmotic pressure of capillary plasma = 35 = colloid osmotic pressure of plasma

plasma has significant amount of water extracted upstream due to glomerular filtration

Question 34

Effect of diuretics on tubular flow and excreteion

Answer 34

Diuretics incr urine output by decr water reabsorption

incr tubular flow rate past transporters

incr excretion rate of Na, K, Cl indirectly

Question 35

What happens if imperfect autoregulation resulted in small 1 mm incr in Pgc at glomerulus

Answer 35

incr NFP from 6 –> 7

incr GFR by 17%

incr delivery of tubular fluid to fine tuning segments and majority escape reabsorption to be excreted

Question 36

what happens if decr GFR and constant reabsorption

Answer 36

Pgc decr by 1 mm

complete obligatory reabsorption of filtered load

no excretion