Tubular reabsorption and secretion Flashcards
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
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
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
Define symport
moves WITH the NA+ gradient
Define antiport
moves AGAINST the Na+ gradient
Define the ENaC channel
found in the apical membrane of the nephron cells
closed by amiloride and opened by hormones
What are the channels that are found int he apical membrane of the nephron cells?
CTFR (chloride)
potassium
ENaC
What are the primary active transporters that are involved in the nephron?
Na/K ATPase
H+ ATPase
H+K+ ATPase
Calcium ATPase
Describe secondary transport in the renal tubule
reabsorption of glucose or amino acids by the renal tubule
SGLT2
SGLT1
Define SGLT2
reabsorbs 90% of the glucose in the early proximal tubule
Define SGLT1
Reabsorbs 10% of glucose in the late proximal tubule
What are the substances that are actively secreted into the renal tubules?
Creatinine
Para-aminohippuric acid
Define the transport maximum
limit to the rate at which the solute can be transported
due to saturation of the specific transport system
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
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
What does the rate of transport depend on?
- electrochemical gradient
2. time the substance is in the tubule
Define solvent drag
osmotic movement of water can also carry other solutes
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
What does the proximal tubule resorb?
65% of filtered sodium, chloride, bicarbonate, and potassium
all filtered glucose and amino acids
What does the proximal tubule secrete?
organic acids, bases, and hydrogen ions into the tubular lumen
Describe sodium reabsorption in the first half of the proximal tubule
reabsorption is via co-transport along with glucose, amino acids, and other solutes
Describe the sodium reabsorption in the second half of the proximal tubule
reabsorption is mainly with chloride ions
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
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
