What is the threshold for glucose reabsorption? 1. transport max for glucose 2. filtered load for glucose 3. GFR x plasma glucose

1. 375 mg/min 2. 125 mg/min 3. 123 mg/min x 1 mg/ml

Tubular reabsorption and secretion Flashcards by Tori Tyler

In order for a substance to be reabsorbed, where must it be transported?

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

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Describe aquaporins (1, 2, and 3)

1- widespread; includes the renal tubules
2.- present in the apical membranes of the collecting tubule cells
3-present in the basolateral membranes of the collecting duct tubes

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What is the function of ATPases in the nephron cell membrane?

-establish ionic gradients

drive reabsorption or secretion of many other solutes

transported via secondary active transport

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Define symport

moves WITH the NA+ gradient

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Define antiport

moves AGAINST the Na+ gradient

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Define the ENaC channel

found in the apical membrane of the nephron cells

closed by amiloride and opened by hormones

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What are the channels that are found int he apical membrane of the nephron cells?

CTFR (chloride)
potassium
ENaC

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What are the primary active transporters that are involved in the nephron?

Na/K ATPase
H+ ATPase
H+K+ ATPase
Calcium ATPase

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Describe secondary transport in the renal tubule

reabsorption of glucose or amino acids by the renal tubule

SGLT2
SGLT1

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Define SGLT2

reabsorbs 90% of the glucose in the early proximal tubule

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Define SGLT1

Reabsorbs 10% of glucose in the late proximal tubule

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What are the substances that are actively secreted into the renal tubules?

Creatinine

Para-aminohippuric acid

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Define the transport maximum

limit to the rate at which the solute can be transported

due to saturation of the specific transport system

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What is the threshold for glucose reabsorption?

transport max for glucose
filtered load for glucose
GFR x plasma glucose

375 mg/min
125 mg/min
123 mg/min x 1 mg/ml

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What are reasons that some passively reabsorbed substances do not have a transport maximum?

rate of diffusion is determined by the electrochemical gradient of the substance
permeability for the substance
time that the fluid containing the substance remains within the tubule

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What does the rate of transport depend on?

electrochemical gradient

2. time the substance is in the tubule

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Define solvent drag

osmotic movement of water can also carry other solutes

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Describe the proximal tubule in terms of the surfaces

highly metabolic with large numbers of mitochondria
extensive brush borders on luminal surfaces
extensive intracellular and basal channels on intestinal surfaces

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What does the proximal tubule resorb?

65% of filtered sodium, chloride, bicarbonate, and potassium

all filtered glucose and amino acids

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What does the proximal tubule secrete?

organic acids, bases, and hydrogen ions into the tubular lumen

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Describe sodium reabsorption in the first half of the proximal tubule

reabsorption is via co-transport along with glucose, amino acids, and other solutes

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Describe the sodium reabsorption in the second half of the proximal tubule

reabsorption is mainly with chloride ions

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Describe the overall sodium transport in the proximal tubule

Na entry
Na pumping out

most of the sodium entry is via anti port with H+

Na+ is pumped out of the cell via the Na+ K+ ATPase pump

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What is the electrical gradient in the cytoplasm and the tubular lumen respectively?

-70 mV and -3 mV

What is the concentration gradient of the cytoplasm and the tubular lumen?

30 mOsm cyto and 140 mOsm luminal

Describe the transport of H+ and bicarb in and out of the proximal tubule

- increased H+ in the lumen because of the anti port transport with the Na+ - H+ binds with the bicarb that is in the lumen and forms carbonic acid (H2CO3) - carbonic anhydrase splits into co2 and h20

Describe the transport of CO2 and H20 in the cell

Enter into the cell and combine to form carbonic acid dissociates to form bicarb and H+ and then the bicarb can diffuse out and the H+ can be removed via anti port and the H+ATPase

Describe the thin descending segment of the loop of henle

-highly permeable to water and moderately permeable to most solutes including urea and sodium reabsorbs 20% of filtered water

Describe the thin ascending segment of the loop of henle

impermeable to water

Describe the thick ascending segment of the loop of henle

- NaK ATPase pump in the basolateral membrane - sodium, potassium, and chloride co-transporter - slight back leak of K+ into the lumen

Describe the sodium potassium and chloride co-transporter

Moves 2 Na , 2 chloride, and 1 potassium into the cell

What does the movement of he K+ int the lumen cause Mg and Ca to do?

diffuse through the tubular lumen through the paracellular space into the interstitial fluid

describe the thick ascending segment of the loop of henle

impermeable to water site of action of powerful loop diuretics -furosemide, ethacrynic acid, bumetanide

Describe the distal tubule

- first portion forms the macula densa - next protein is highly convoluted and has characteristics that are similar to the thick ascending loop - reabsorbs most ions but is impermeable to water and urea (diluting segment)

What are the ions that are pumped in and out of the distal tubule, and how?

Na and Cl co-transporter in the luminal membrane Na k ATPase pump that is present in the basolateral membrane

What is the function of the principal cells in the late distal tubule?

- reabsorb sodium and water from the tubular lumen - secrete potassium into the tubular lumen - uses the NAK pump - primary site of K+ sparing diuretics

What are the 4 K+ sparing diuretics?

spironolactone eplerenone amiloride trimeterene

What is the function of the intercalated discs that are present in the late distal tubule?

- reabsorb K+ from the tubular lumen | - secrete H+ into the tubular lumen

Describe the medullary collecting duct

- epithelial cells are cuboidal - permeability is controlled by ADH - permeable to urea - capable of secreting H+ against a large concentration gradient

What is the source of aldosterone?

adrenal cortex

What is the function of aldosterone

increases sodium reabsorption and stimulates potassium secretion stimulates the Na K pump one the basolateral side of the cortical collecting tubule membrane

What is the site of action of aldosterone?

principal cells of the cortical collecting ducts

What is the stimulus for secretion of aldosterone?

- increased extracellular potassium | - increased levels of angiotensin 2

What occurs in the absence of aldosterone?

addison's disease | marked loss of sodium and accumulation of potassium

What occurs with hyper secretion of aldosterone?

Conns syndrome

What is the function of angiotensin 2?

- increased sodium and water reabsorption | - returns blood pressure and extracellular volume toward normal

What are the effects of angiotensin 2?

- stimulate aldosterone secretion - constricts efferent arterioles - directly stimulates sodium reabsorption in the proximal tubules, loops, distal tubules, and collecting tubules

What is the source of ADH

posterior pituitary

What is the function of ADH?

increases water reabsorption

What are the effects of ADH?

binds to the V2 receptors in the late distal tubules, collecting tubules, and collecting ducts increases the formation of cAMP

What does cAMp do?

stimulate the movement of aquaporin 2 proteins to the luminal side of the cell membranes

What is the source of ANP?

cardial atrial cells in response to dissension

What is the function of ANP?

inhibits reabsorption of sodium and water

What is the source of PTH?

parathyroid glands

What is the function of the PTh?

increases calcium reabsorption

What is the renal clearance of a substance?

The volume of plasma that is completely cleared of the substance by the kidneys per unit time

What is the equation for renal clearance

cs X ps = us x v cs+ clearance rate of the substance ps= plasma concentration of the substance us- urine concentration of the substance v= urine flow

Define inulin

polysaccharide | not produced in the body

What is the equation for a substance that is completely filtered but not reabsorbed or sercreted

GFRxPs=UsxV