tubular reaborption

Question 1

name the functions and locations of aquaporin-1, aquaporin-2 and aquaporin-3

Answer 1

Aquaporin-1: wide spread and on renal tubules

Aquaporin-2: present in apical membranes of collecting tubule cells. Controlled by ADH

Aquaporin-3: present in basolateral membranes of collecting tubule cells.

Question 2

What is the ENaC channel? Where is it found? How is it closed and opened?

Answer 2

Sodium and Calcium channel. Found on the apical membrane of nephron cells
Closed by drug amiloride
Opened by a number hormones

Question 3

What two mechanisms are used to transport Na+ through tubular epithelial cells?

Answer 3

Cell moves Na+ by ATPase and Na+ also moves towards the cell because of the -70mV charge

Question 4

glucose transporters, an example of secondary active transportinthe kidneys, are responcible for how much glucose reabsorption? What kinds are there? How do they work?

Answer 4

There’s SGLT1 and SGLT2.
SGLT1 reabsorbs 10% of glucose
SGLT2 reabsorbs 90% of glucose

One exchanges glucose for Na+ on the tubular lumen and on the interstitial fluid side the GLUT transporter is run on energy from the ATPase

Question 5

What substances are actively secreted into the renal tubules?

Answer 5

Creating and para-aminohippuric

Question 6

What is transport maximum? What is glucose’s transport maximum for glucose?

Answer 6

The transport maximum is the limit of the rate a solute can be transported. The transport maximum is determined by the saturation of a specific transport system.
Max for glucose reabsorption is 375 mg/min
Filtered load for glucose is 125 mg/min
GFR x plasma glucose is 125 ml/min x 1 mg/ml

Anything that exceed the maximum will fail to be reabsorbed and be excreted in the urine.

Question 7

Why do some passively reabsorbed substances lack a transport maximum?

Answer 7

The rate of diffusion is determined by the electrochemical gradient of the substance

The membrane is permeable to the substance

How long fluid containing the substance is in the tubule effects whether or not there is a transport max. (Which is dependent on the tubular flow rate)

Question 8

Transport characteristics of the proximal tubule and what molecules/ions and the direction they are transported in?

Answer 8

Highly metabolic and a lot of mitochondria
Has extensive brush borders on the luminal surface
Extensive intercellular and basal channels on interstitial surfaces

Reabsorbed 65% of filtered sodium, chloride, bicarbonate and potassium
All glucose and amino acids

Secretes: organic acids, bases and hydrogen ions

Question 9

Loop of henle

Answer 9

Has thin and thick segment, ascending and descending

Thin descending is high permeable to water, ~20% of filtered water
Thin ascending is impermeable to water
Thick ascending is also impermeable to water, Na+K+ATPase in basolateral membranes. Sodium-potassium-chloride transporter: 1:2:1. K+ leaks back in which helps move Mg++ and Ca++ out the tubule to the interstitial fluid

Question 10

What are the loop diuretics?

Answer 10

Furosemide
Ethacrynic acid
Bumetanide

Works on the Na-Cl-K cotransporters

Question 11

How does the distal tubule work?

Answer 11

Part forms the macula densa
Reabsorbs most ions, impermeable to water and urea
Luminal membrane: Na+Cl- co-transporter - inhibited by thiazide (increases urine output)
basolateral membrane: Na+K+ ATPase pump

Question 12

What are principal and intercalated cells and where do you find them inthe renal system?

Answer 12

Principal cells reabsorbed Na+ and water fro mthe tube. Secrete K+ into the tube. -sodium-pot pump.
Site of K+ sparing diuretics:
(Spironolactone, eplerenone,) aldostrone agonist (amiloride, trimeterene) Na+ channel blockers

Intercalated cells: reabsorbs K+ and H+ from the tube. by H+ ATPase, protons generated by carbonic anhydrase, and bicarbonate absorbtion.

Found on late distal tubule

Question 13

Medullary collecting duct

Answer 13

Cuboidal epithelil cells have smooth surfaces and little mitochondria
Water permeability is controlled by ADH
Urea transporter allow permeability to urea
Secretes H+ against large conc

Question 14

Aldosterone

Answer 14

Source: adrenal cortex
Function: increases sodium reabsorption and stimulates potassium secretion
-sodium=pot pump on basolateral side
Acts on the principal cells of the cortical collecting ducts

Question 15

What stimulates the secretion of aldosterone, what happens in the absense of and hypersecretion of aldostrone?

Answer 15

Stimulus: increased extracellular potassium and increased levels of angiotensin II

Absense: addison’s disease, causes marked loss of sodium and accumulation of potassium

Hypersecretion: conn’s syndrome

Question 16

Angiotensin II

Answer 16

Works to increase sodium and water reabsorption
Returns BP and extracellular volume to normal

It effects aldostrone secretion, it constricts efferent arterioles (to increase blood pressure) and directly stimulates sodium reabsorption in proximal tubules, loop of henle, distal tubules and collecting tubules

angiotensin IIbinds to it’s receptors and causes intracellular activation of receptors

Question 17

ADH

Answer 17

Posterior pituitary releases ADH
ADH - Antidiuretic hormone 
It increases water reabsorption 
It does this by binding to V2 receptors in the late distal tubules, collecting tubules and collecting ducts. 
Increases the formation of cAMP 
-conserves water

Question 18

Atrial Natriuretic Peptide

Parathyroid hormone

Answer 18

Source: cardiac atrial cells in responce to distention (stretch receptors)
Function: inhibits reabsorption of sodium and water

Source: parathyroid glands
Function: increases Calcium reabsorption

Question 19

What is renal clearance? How do you calculate it?

Answer 19

It’s the volume of plasma that is completely cleared of the substance by the kidneys per unit time.

Clearance rate of substance X plasma conc of substance = urine conc of substance X urine flow

Cs = (Us X V)Ps

Substance filtered but not reabsorbed or secreted :
GFR = (Us X V)/Ps = Cs