Lecture 34 4/8/24

Question 1

How must a substance be transported in order to be reabsorbed?

Answer 1

-across the tubular epithelial membranes into the interstitial fluid
-through the peritubular capillary membrane back into the blood

Question 2

How can water and solutes be transported?

Answer 2

-through the cell membranes (transcellular)
-between the cell junctions (paracellular)

Question 3

What mediates ultrafiltration?

Answer 3

hydrostatic and colloid osmotic forces

Question 4

How does primary active transport differ from secondary active transport?

Answer 4

-primary is coupled directly to an energy source
-secondary is coupled indirectly to an energy source

Question 5

Which active transporters are present in the kidneys?

Answer 5

-sodium/potassium ATPase
-hydrogen ATPase
-hydrogen/potassium ATPase
-calcium ATPase

Question 6

What are the characteristics of the sodium glucose co-transporters (SGLT1 and SGLT2)?

Answer 6

-located on brush border of proximal tubular cells
-carry glucose into the cell cytoplasm against the concentration gradient
-90% of filtered glucose reabsorbed by SGLT2; 10% by SGLT1

Question 7

What are the characteristics of GLUT1 and GLUT2 in the kidney?

Answer 7

-on the basolateral side of the membrane
-allow glucose to diffuse out of cells and into interstitial spaces

Question 8

What is pinocytosis?

Answer 8

active transport mechanism allowing for the reabsorption of small peptides

Question 9

What is the transport maximum?

Answer 9

limit to the rate at which a solute can be transported

Question 10

What causes the existence of a transport maximum?

Answer 10

saturation of the transport systems involved

Question 11

How does renal tubular transport maximum impact glucose reabsorption?

Answer 11

with large increases in GFR and/or plasma glucose concentration, excess glucose cannot be reabsorbed above the transport maximum; excess glucose is passed into the urine

Question 12

What is the threshold for glucose?

Answer 12

the point at which glucose begins to appear in the urine before the transport maximum is reached

Question 13

Why is there a difference between threshold and transport maximum?

Answer 13

not all nephrons have the same transport maximum, so some of the nephrons will max out and excrete glucose before others

Question 14

When is transport maximum reached?

Answer 14

once all nephrons have reached their maximum capacity to reabsorb glucose

Question 15

Which factors can impact rate of transport and lead to some substances NOT having a transport maximum?

Answer 15

-electrochemical gradient for diffusion
-permeability of the membrane for the substance
-time that the fluid containing the substance remains in the tubule

Question 16

What is gradient-time transport?

Answer 16

transport in which the rate of transport depends on the electrochemical gradient and the time the substance is in the tubule (depends on tubular flow rate)

Question 17

What does the rate of “backleak” depend on?

Answer 17

-permeability of tight junctions
-interstitial physical forces

Question 18

What is an example of gradient-time transport?

Answer 18

sodium reabsorption in the proximal tubule

Question 19

How does the concentration of sodium in the proximal tubule impact its reabsorption rate?

Answer 19

higher conc. of sodium leads to higher reabsorption rate

Question 20

How does flow rate of the tubular fluid in the proximal tubule impact sodium reabsorption?

Answer 20

slower flow rate of tubular fluid allows for a greater percentage of sodium to be reabsorbed

Question 21

Which part of the renal tubule is especially permeable to water?

Answer 21

proximal tubule

Question 22

What are the characteristics of proximal tubule permeability?

Answer 22

-high permeability to water due to aquaporins and tight junctions
-permeability to most ions that is less than that to water, but still significant

Question 23

What is solvent drag?

Answer 23

movement of solutes across tight junctions due to them being carried by water

Question 24

Why are the more distal parts of the nephron less permeable to water?

Answer 24

-tight junctions become less permeable
-epithelial cells have a greatly decreased membrane surface area

Question 25

Which hormone greatly increases water permeability of the distal and collecting tubules?

Answer 25

ADH

Question 26

Which hormone can increase the transport maximum of sodium?

Answer 26

aldosterone

Question 27

How does water permeability change throughout the kidney?

Answer 27

-high in proximal tubule -medium in the descending loop of Henle -low in the thin and thick ascending LOH -variable in distal tubule, collecting tubule, and collecting ducts

Question 28

How does tight junction permeability and surface area change throughout the nephron?

Answer 28

both decrease as you move through the nephron

Question 29

What creates the electrochemical gradient that allows for chloride resorption?

Answer 29

-transport of positively charged sodium ions out of the tubular lumen, which leaves the lumen neg. charged -transport of water out of the tubular lumen, which concentrates the chloride ions

Question 30

Which region of the nephron has the most passive reabsorption of urea?

Answer 30

inner medullary collecting duct

Question 31

What allows for the proximal tubule to have a high capacity for reabsorption?

Answer 31

-large numbers of mitochondria -extensive brush border -lots of intercellular and basal channels -lots of protein carriers

Question 32

What are the characteristics of early proximal tubule reabsorption?

Answer 32

-sodium is reabsorbed via co-transport along with glucose, amino acids, and other solutes -sodium is also reabsorbed via counter-transport that secretes substances like H+ ions

Question 33

What are the characteristics of late proximal tubule reabsorption?

Answer 33

-very little glucose and amino acids remain to be absorbed -sodium is now primarily reabsorbed with chloride ions

Question 34

Why does sodium concentration remain relatively constant in the proximal tubule?

Answer 34

water permeability of the proximal tubules is so great that water reabsorption keeps pace with sodium reabsorption

Question 35

Why do substances like glucose, AAs, and bicarb. exhibit marked decreases in concentration along the length of the proximal tubule?

Answer 35

these substances are reabsorbed more avidly than water

Question 36

Which substances become more concentrated down the length of the proximal tubule?

Answer 36

those that are not readily reabsorbed, such as creatinine

Question 37

Which substances are secreted by the proximal tubule?

Answer 37

-bile salts -oxalate -urate -catecholamines -toxins and drugs

Question 38

What are the general characteristics of the thin descending and ascending LOH segments?

Answer 38

-thin epithelial membranes -no brush borders -few mitochondria -minimal levels of metabolic activity

Question 39

What are the general characteristics of the thick ascending LOH segment?

Answer 39

-thick epithelial cells -high metabolic activity -capable of reabsorption of sodium, chloride, and potassium

Question 40

What is the function of the thin descending LOH loop?

Answer 40

allow simple diffusion of substances, mainly water, through its walls

Question 41

Why is the ascending LOH loop important for concentrating urine?

Answer 41

it is virtually impermeable to water

Question 42

What is the most important component of reabsorption in the thick ascending LOH?

Answer 42

sodium/potassium ATPase pump

Question 43

What are the characteristics of NKCC2?

Answer 43

-1-sodium, 2-chloride, 1-potassium transporter -uses the potential energy of downhill sodium diffusion to drive the reabsorption of potassium into the cell -found in thick ascending loop

Question 44

Which other mechanisms of absorption are used in the thick ascending loop?

Answer 44

-sodium/hydrogen counter-transporter -paracellular ion reabsorption

Question 45

Why does the tubular fluid in the ascending limb become very dilute?

Answer 45

ascending limb is impermeable to water while allowing for lots of solute reabsorption

Question 46

What is the first portion of the distal tubule?

Answer 46

macula densa

Question 47

What is the purpose of the macula densa?

Answer 47

provide feedback control of the GFR and blood flow in the same nephron it is found in

Question 48

What are the characteristics of the distal tubule segment that comes after the macula densa?

Answer 48

-highly convoluted -avidly reabsorbs ions -virtually impermeable to water and urea -"diluting segment" because it dilutes the tubular fluid

Question 49

What is the target of thiazide diuretics?

Answer 49

sodium/chloride co-transporters in the early distal tubule

Question 50

Which cell types are found in the late distal tubule?

Answer 50

-principle cells -intercalated cells

Question 51

What are the functions of principal cells?

Answer 51

-reabsorb sodium and water -secrete potassium

Question 52

What are the functions of type A intercalated cells?

Answer 52

-reabsorb potassium -secrete hydrogen

Question 53

What are the functional characteristics of the late distal tubule?

Answer 53

-impermeable to urea -reabsorb sodium -sodium reabsorption and potassium secretion rates controlled by aldosterone -secrete potassium

Question 54

Why is water reabsorption in the late distal tubule referred to as facultative water reabsorption?

Answer 54

water is only reabsorbed as needed by the body, based on signals from hormones

Question 55

What are the characteristics of aldosterone?

Answer 55

-increases cell permeability to sodium ions -increases sodium reabsorption and potassium secretion by increasing number of Na/K pumps -indirectly increases reabsorption of water and chloride -stimulates secretion of hydrogen

Question 56

What are the characteristics of ADH?

Answer 56

-causes water retention and reduces urine output -causes aquaporins to be inserted into cell membranes to increase water reabsorption

Question 57

What are the characteristics of atrial natriuretic peptide?

Answer 57

-triggers natriuresis/urinary excretion of sodium -inhibits release of ADH and aldosterone

Question 58

What is the role of aldosterone antagonists?

Answer 58

compete with aldosterone for binding sites to inhibit its effects

Question 59

What is the role of sodium channel blockers?

Answer 59

inhibit entry of sodium into sodium channels

Question 60

What are the steps of potassium secretion?

Answer 60

-potassium enters cell via Na/K pump, which maintains a high intracellular K+ conc. -potassium diffuses down its concentration gradient to enter the luminal fluid

Question 61

Which transporters are used by type A intercalated cells to secrete hydrogen ions?

Answer 61

-hydrogen ATPase transporter -hydrogen-potassium ATPase transporter

Question 62

How is hydrogen generated in type A intercalated cells?

Answer 62

carbonic anhydrase acts on water and carbon dioxide within the cell to form carbonic acid, which dissociates into hydrogen and bicarb.

Question 63

Why is it important that type A intercalated cells have an active hydrogen ATPase?

Answer 63

can move hydrogen against the conc. gradient

Question 64

What are the functions of type B intercalated cells?

Answer 64

-reduce alkalosis -reabsorb H+ -secrete HCO3- -secrete K+

Question 65

What is the characteristics of the medullary collecting duct?

Answer 65

-final site for urine processing -water permeability controlled by ADH -permeable to urea -involved in acid/base regulation

Question 66

What determines whether a solute becomes concentrated?

Answer 66

the relative degree of reabsorption of the solute compared to the reabsorption of water