Tubular Transport I

Question 1

Are H+ and para-aminohippuric acid (PAH) both filtered at the glomerulus and secreted at the PCT?

Answer 1

YES

Question 2

For H+ and PAH, prior to the inflection or saturation point, are they being reabsorbed or secreted efficiently?

Answer 2

YES

Question 3

What happens to H+ and PAH after the saturation point?

Answer 3

the excretion rate and filtration rates become proportional.

Question 4

** What is clearance?

Answer 4

the rate at which the kidneys clear various substances from the plasma (ml/min). Same equation as GFR (when using inulin) :)
*Clearnace= concentration of urine ‘substance’ x flow of urine/ concentration of plasma ‘substance’

Question 5

** What substance is used to measure GFR?

Answer 5

INULIN, because the only way for it to be eliminated is via filtration. It cannot be reabsorbed or secreted.

Question 6

** What is a more clinically useful way to measure GFR?

Answer 6

• creatinine= by-product of muscle metabolism that is cleared from the body fluids almost entirely by glomerular filtration (it is partly secreted and not reabsorbed).
• plasma creatinine concentration= 1 mg/dL

Question 7

What happens to creatinine if you decrease GFR by 50% (aka you lose a kidney)?

Answer 7

It will increase in the blood to double the amount.

Question 8

What molecule is used to measure renal plasma flow?

Answer 8

PAH because 90% of it is cleared in one pass through the kidney (due to the ability for the kidney to both filter and secrete this molecule).
*Use same formula as inulin clearance (GFR). This comes out to be about 650 ml/min (after dividing by 0.9, because you’re correcting for the 10% that is not cleared), which is about equal to the renal plasma flow.

Question 9

How do you calculate TOTAL BLOOD FLOW through the kidneys using the renal PLASMA flow?

Answer 9

take the renal plasma flow/ (1- hematocrit)

* should be around 1200 ml/min

Question 10

Is there a Tm for inulin?

Answer 10

NO (duh because it is not reabsorbed).

Question 11

*** If Cx/C inulin = 1, is substance x being filtered, reabsorbed or secreted?

Answer 11

FILTERED only because it’s behaving just like inulin.

Question 12

*** If Cx/C inulin > 1, is substance x being filtered, reabsorbed or secreted?

Answer 12

FILTERED and SECRETED because this rate is greater than that of inulin and the only way to do that is to also secrete the substance (just like PAH).

Question 13

*** If Cx/C inulin

Answer 13

FILTERED AND REABSORBED (just like glucose).

Question 14

What are the 2 paths that water and electrolytes can be reabsorbed by the kidney?

Answer 14

1. paracellularly (between cells)

2. trancellularly (through the cell itself)

Question 15

What percent of kidney energy consumption does the kidney expend to move Na+?

Answer 15

80%

Question 16

What channels does water move through?

Answer 16

aquaporins

Question 17

What are the 4 types of Na+ entry into the tubular cells at the PCT?

Answer 17

1. Na+/H+ antiporter= major transporter
2. Na+ nutrient pump (how glucose uses Na+ entry energy to also enter).
3. Na+/phosphate pump
4. Na+ sulfate pump
   Note Cl- follows to maintain equal charge through its own channel.

Question 18

How does Na+ exit out of the tubular cells at the basolateral membrane?

Answer 18

• Na+/K+ ATPase= major transporter
• Na+/HCO3- cotransporter
  Note Cl- passively diffuses to maintain equal charge

Question 19

Is there a lot of movement through the cells of the descending loop of henle in the CORTICAL nephrons?

Answer 19

NO because there isn’t a large change in concentration gradient in the interstitium.
*Remember only the juxtamedullary nephrons are the only ones that can concentrate the urine.

Question 20

What happens to the filtrate as it moves through the descending loop of henle in the JUXTAMEDULLARY nephrons?

Answer 20

it will increase from 300 to 1200 mOsm/kg because water is permeable but solutes are not in this area of the loop of henle.

Question 21

What happens tot he osmolarity of the filtrate as it moves up the ascending limb of the loop of henle?

Answer 21

it decreases because this portion is impermeable to water but Na+, K+, and 2 Cl- are moved out via their cotransporter. Aka this is diluting the filtrate (making it hypoosmotic).
*Remember this is where loop diuretics (furosemide) work.

Question 22

*** Is the filtrate in the early DCT isotonic, hypotonic, or hypertonic with respect to the blood plasma?

Answer 22

HYPOOSMOTIC

Question 23

Are there more or less Na+ channels on the luminal side of the the tubular cells at the DCT?

Answer 23

very few but the ones that are there are Na+/Cl- cotransporters. Thus, there is little water reabsorbed here also.
*Remember this is what thiazide diuretics inhibit.

Question 24

** What specific cells are primarily involved in the transport of Na+, Cl-, and water in the distal tubule?

Answer 24

PRINCIPLE CELLS of the cortical collecting tubules and collecting duct.

• Remember this is mainly where ADH (increases transcription/translation and upregulation of aquaporins to increase water reabsorption), ALDOSTERONE (increases Na+ and thus water reabsorption), and ANP/BNP (decrease Na+ reabsorption) act.
• This is also the site remember for K+ sparing diuretics like spirinolactone (inhibits aldosterone), and amiloride (Na+ reabsorbing channel blocker).

Question 25

Is the electrochemical gradient in the collecting tubules/ducts greater or less than the PCT?

Answer 25

LESS (-20 mV compared to -70 mV).

Question 26

What controls the permeability of water in the principle cells of the collecting tubule/duct?

Answer 26

under hormonal control via ALDOSTERONE and ADH/vasporessin (both of which will act to reabsorb water).

Question 27

What percent of the filtered load do the distal parts of the nephron handle?

Answer 27

10-15% but highly regulated compared to the PCT (high capacity, but low regularity).

Question 28

Will small fluctuations in K+ levels cause drastic effects?

Answer 28

YES. The kidney thus regulates this very tightly.

Question 29

Does the kidney do a good job of sparing K+ when K+ levels fall?

Answer 29

NO. It has a hard time fixing a hypokalemia. Thus you must increase your intake of K+ through your diet. Also if you’re already hypokalemic, and now you are dehydrated, the body uses Aldosterone to increase Na+ reabsorption (and thus water), but at the expense of K+ secretion, thus worsening an already hypokalemic state.
* It does however do a good job at fixing a hyperkalemic state. :)

Question 30

What areas of the kidney regulate K+?

Answer 30

DCT and collecting tubules/ducts.
*Note most filtered K+ is reabsorbed at the PCT (via PARACELLULAR transport), but this happens naturally and is not regulated.

Question 31

** What happens to potassium at the ascending loop of henle?

Answer 31

K+ is slightly permeable to the luminal membrane (not as much as the basal side though), and because K+ concentration is higher inside the cell than out, it will slowly leak into the tubular lumen. This creates a positive potential (+6 mV) to drive the reabsorption of other cations (Ca++ and Mg++) including 10% of K+ into the INTERSTITIUM, PARACELLULARLY. Also K+ is reabsorbed transcellularly via the Na+/K+/2Cl- symporter, thus supplying the driving force of the luminal leakage of K+.
*Thus using a loop diuretic like furosemide, you will block the Na+/K+/2Cl- symporter, preventing the reabsorption of water, Ca++, Mg++, and K+.

Question 32

• What do the alpha-intercalated cells of the collecting duct do?

Answer 32

reabsorb K+ from the tubular filtrate via an active K+/H+ antiporter and also a separate H+ATPase that will secrete H+ into the filtrate, on the luminal membrane.

Question 33

** What will an acidosis do to K+ reabsorption at the collecting duct?

Answer 33

INCREASE K+ reabsorption, in order to pump out more H+

Question 34

** What will an alkalosis do to K+ reabsorption at the collecting duct?

Answer 34

DECREASE K+ reabsorption, in order to prevent more H+ secretion.

Question 35

What will principle cells do when K+ is high or normal?

Answer 35

secrete K+

Question 36

** Will dehydration increase or decrease K+ loss?

Answer 36

INCREASE K+ loss :(