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SP1 Physiology/Microanatomy 2 > Lecture 34 4/8/24 > Flashcards

Lecture 34 4/8/24 Flashcards

1
Q

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

A

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

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2
Q

How can water and solutes be transported?

A

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

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3
Q

What mediates ultrafiltration?

A

hydrostatic and colloid osmotic forces

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4
Q

How does primary active transport differ from secondary active transport?

A

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

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5
Q

Which active transporters are present in the kidneys?

A

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

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6
Q

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

A

-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

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7
Q

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

A

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

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8
Q

What is pinocytosis?

A

active transport mechanism allowing for the reabsorption of small peptides

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9
Q

What is the transport maximum?

A

limit to the rate at which a solute can be transported

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10
Q

What causes the existence of a transport maximum?

A

saturation of the transport systems involved

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11
Q

How does renal tubular transport maximum impact glucose reabsorption?

A

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

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12
Q

What is the threshold for glucose?

A

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

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13
Q

Why is there a difference between threshold and transport maximum?

A

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

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14
Q

When is transport maximum reached?

A

once all nephrons have reached their maximum capacity to reabsorb glucose

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15
Q

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

A

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

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16
Q

What is gradient-time transport?

A

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)

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17
Q

What does the rate of “backleak” depend on?

A

-permeability of tight junctions
-interstitial physical forces

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18
Q

What is an example of gradient-time transport?

A

sodium reabsorption in the proximal tubule

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19
Q

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

A

higher conc. of sodium leads to higher reabsorption rate

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20
Q

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

A

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

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21
Q

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

A

proximal tubule

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22
Q

What are the characteristics of proximal tubule permeability?

A

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

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23
Q

What is solvent drag?

A

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

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24
Q

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

A

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

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25
Q

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

A

ADH

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26
Q

Which hormone can increase the transport maximum of sodium?

A

aldosterone

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27
Q

How does water permeability change throughout the kidney?

A

-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

28
Q

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

A

both decrease as you move through the nephron

29
Q

What creates the electrochemical gradient that allows for chloride resorption?

A

-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

30
Q

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

A

inner medullary collecting duct

31
Q

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

A

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

32
Q

What are the characteristics of early proximal tubule reabsorption?

A

-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

33
Q

What are the characteristics of late proximal tubule reabsorption?

A

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

34
Q

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

A

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

35
Q

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

A

these substances are reabsorbed more avidly than water

36
Q

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

A

those that are not readily reabsorbed, such as creatinine

37
Q

Which substances are secreted by the proximal tubule?

A

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

38
Q

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

A

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

39
Q

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

A

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

40
Q

What is the function of the thin descending LOH loop?

A

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

41
Q

Why is the ascending LOH loop important for concentrating urine?

A

it is virtually impermeable to water

42
Q

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

A

sodium/potassium ATPase pump

43
Q

What are the characteristics of NKCC2?

A

-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

44
Q

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

A

-sodium/hydrogen counter-transporter
-paracellular ion reabsorption

45
Q

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

A

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

46
Q

What is the first portion of the distal tubule?

A

macula densa

47
Q

What is the purpose of the macula densa?

A

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

48
Q

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

A

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

49
Q

What is the target of thiazide diuretics?

A

sodium/chloride co-transporters in the early distal tubule

50
Q

Which cell types are found in the late distal tubule?

A

-principle cells
-intercalated cells

51
Q

What are the functions of principal cells?

A

-reabsorb sodium and water
-secrete potassium

52
Q

What are the functions of type A intercalated cells?

A

-reabsorb potassium
-secrete hydrogen

53
Q

What are the functional characteristics of the late distal tubule?

A

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

54
Q

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

A

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

55
Q

What are the characteristics of aldosterone?

A

-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

56
Q

What are the characteristics of ADH?

A

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

57
Q

What are the characteristics of atrial natriuretic peptide?

A

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

58
Q

What is the role of aldosterone antagonists?

A

compete with aldosterone for binding sites to inhibit its effects

59
Q

What is the role of sodium channel blockers?

A

inhibit entry of sodium into sodium channels

60
Q

What are the steps of potassium secretion?

A

-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

61
Q

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

A

-hydrogen ATPase transporter
-hydrogen-potassium ATPase transporter

62
Q

How is hydrogen generated in type A intercalated cells?

A

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

63
Q

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

A

can move hydrogen against the conc. gradient

64
Q

What are the functions of type B intercalated cells?

A

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

65
Q

What is the characteristics of the medullary collecting duct?

A

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

66
Q

What determines whether a solute becomes concentrated?

A

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

SP1 Physiology/Microanatomy 2 (50 decks)

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