Transport Processes

Question 1

Balance of excretion, filtration, and reabsorption

Answer 1

Glomerulotubular balance: ability to adjust reabsorption rate to balance filtered load
Paracellular or transcellular transport
All water movement is passive in response to hydrostatic pressure or osmotic influences

Question 2

Reabsorption of water in proximal tubule

Answer 2

In proximal tubule net pressure is greater in tubule than in peritubular capillary, water is reabsorbed
Renal blood flow influences balances of forces
Tubular cells joined by tight junctions (paracellular)
Most reabsorption of sodium is transcellular across apical membrane down its electrochemical gradient
Reabsorption of sodium and other substances with it has an osmotic effect, which increases intracellular osmolarity and water moves into cells, and water and solutes move into plasma

Question 3

Solvent drag

Answer 3

Reabsorption of water, especially in paracellular, brings solutes with it

Question 4

Osmotic diuresis

Answer 4

Decreased ability to reabsorption solutes and water and thus increased urine volume

Question 5

The forces that determine reabsorption of fluid

Answer 5

Balance of hydrostatic and protein oncotic pressures favor reabsorption of water in proximal tubule
Reabsorption of solute creates osmotic gradient which favors the movement of water from the tubular fluid into intracellular space, largest contributor is reabsorption of sodium

Question 6

Reabsorption of sodium

Answer 6

Occurs by secondary symport with glucose (SGLT2), amino acids, and lactate, and by antiport with H+
Other substances are being transported agInst their concentration gradient
Energy provided by electrochemical gradient for sodium

Question 7

Sodium transport in second half of proximal tubule

Answer 7

Organic bases (anions) are secreted
Anion countertransport with chloride operates with Ana-H exchange
Acid and base combine, diffuse back into cell, dissociate, each recycling a Na and Cl brought into cell and then reabsorbed
Increases reabsorption of salt and thus water
Reabsorption of cations balanced by reabsorption of anions

Question 8

Chloride reabsorption

Answer 8

Increased chloride concentration in tubular fluid
Creates a concentration gradient that drive chloride reabsorption by paracellular route
Positive charge drives cations through paracellular route

Question 9

Water transport

Answer 9

Proximal tubule, 67%, passive
Loop of Henle, 15%, passive in descending loop only
Distal tubule, impermeable
Late distal tubule and collecting duct, 8-17%, passive, ADH increases, ANP decreases, and inhibits response to ADH

Question 10

NaCl transport in Nephron

Answer 10

Proximal: 67%, Na/H exchanger Na cotransport with aa and organic solutes Na/H/Cl anion exchange, Angiotensin II NE Epi and Dopamine
Loop of Henle: 25%, 1Na/2Cl/1K symport, aldosterone and angiotensin II
Distal tubule: 5%, NaCl symport (early), Aldosterone and Angiotensin II
Late distal tubule: 3%, Na channels, Aldosterone ANP BNP Angiotensin II

Question 11

Organic Anions

Answer 11

Primary: Na/K pump, Secondary: Na pump (symport)
Tertiary active transport of organic acids
Organic acids compete against penicillin for transport and thus excretion

Question 12

Thin descending limb

Answer 12

High water permeability
no active transport of solutes and permeability to solutes is low 
volume of tubular fluid decreases
Osmolarity of tubular fluid increases
Concentrating segment

Question 13

Thick ascending limb

Answer 13

Impermeable to water
Large quantity of solutes reabsorbed, solute concentration drops below plasma levels
1Na/2Cl/1K symport and Na/H antiport
More than half of potassium reabsorbed
Half of transport is paracellular
No change of fluid volume
Hyperosmotic interstitium
Diluting segment

Question 14

Ascending thin limb

Answer 14

Impermeable to water some solute reabsorbed passively

Osmolarity begins to drop from its peak

Question 15

Early distal tubule and JGA

Answer 15

Impermeable to water
Continues diluting tubular fluid
Active transport: Na/2Cl/K and Na/Cl
Target of diuretics
Detection of TL of NaCl by measuring transport rate
Sensor region for tubuloglomerular feedback (JGA and renin secretion)

Question 16

Late distal tubule and cortical collecting duct

Answer 16

Variable permeability to water
Major actions of aldosterone
Reabsorption NaCl and water isoosmotically
Secretion of K
Secretion of acid or bicarbonate if necessary
Negative lumen promotes Cl uptake
Aldosterone stimulates acid secretion during acidosis or bicarbonate if alkalosis

Question 17

Two types of cells in LDT and CCD

Answer 17

Principle:
Secretes K
Reabsorbed Na
Uses ADH
Isoosmotic reabsorption
Intercalated:
Pump H ions into tubular fluid or bicarbonate if necessary

Question 18

Glomerulartubular balance

Answer 18

Ability to adjust reabsorption rate to balance filtered load

Question 19

Aldosterone

Answer 19

DOES NOT directly change or control plasma sodium concentration
Increases blood volume, renal excretion of acid, and decreases plasma potassium

Question 20

Collecting duct depends on two factors

Answer 20

Presence of ADH

Pot load of solutes in interstitium

Question 21

Collecting duct

Answer 21

Critical regulation of water excretion
ADH regulates number of water channels (absence blocks water reabsorption)
V1: vasoconstriction
V2: aquaporin insertion for ADH

Question 22

Transport maximum processes

Answer 22

Carrier mediated
Typically active
Reabsorption = FL until FL = Tm, however there is a splay