urinary concentrating mechanisms

Question 1

tubular osmolarity of proximal tubule

Answer 1

300mOsm/l

Question 2

initial and final volume of fluid in proximal tubule

Answer 2

125ml/min

45ml/min

Question 3

osmolarity definition

Answer 3

concentration of a solution expressed as the total number of solute particles per litre

Question 4

tubular osmolarity in descending loop of Henle

Answer 4

1200 mOsm/L

Question 5

tubular osmolarity in ascending limb + why

Answer 5

less than 300mOsm/L

extraction of salt decreases osmolarity

Question 6

volume in loop of Henle + why

Answer 6

25ml/min due to loss of water

Question 7

daily water intake + source

Answer 7

25000ml

fluid intake and food metabolism

Question 8

daily loss of water + sources

Answer 8

1000ml- sweat, faeces, airways

remainder lost in urine

Question 9

what is the default urine type produced?

Answer 9

dilute urine

Question 10

diuresis definition

Answer 10

water loss

Question 11

what happens during diuresis

Answer 11

large volumes of dilute hypotonic urine produced

20 litres a day

osmotic potential of 50mOsm/L

Question 12

antidiuresis definition

Answer 12

water conservation

Question 13

what happens during antidiuresis?

Answer 13

small volume of concentrated, hypertonic urine

0.5L a day

12000mOsm/L

Question 14

important system in creating urine

Answer 14

countercurrent system

Question 15

explain counter current system

Answer 15

loop of Henle acts as a countercurrent multiplier, as the different limbs have different permeabilities

Question 16

explain descending limb movements

Answer 16

unregulated water permeability

water moves out of the tubule, leading to increased tubular osmolarity

Question 17

explain structure of ascending limb

Answer 17

composed of the thick and thin ascending limb- thin within medulla and then becomes thick in cortex

thick and thin refers to appearance of cells, not diameter

Question 18

explain permeability in thin ascending limb

Answer 18

high permeability of sodium- sodium reabsorbed - moves into interstitium

not permeable to water

Question 19

explain permeability in thick ascending limb

Answer 19

high permeability of sodium

not permeable to water

Question 20

explain stages of counter current multiplication

Answer 20

1. Thick ascending limb actively transports sodium into the interstitium
2. interstitial fluid then becomes hypertonic
3. water leaves the permeable descending limb , increasing the osmolarity to 1200mosm/l
4. sodium ions thus become more concentrated until they can passively diffuse out of the thin ascending limb, lowering the osmolarity

Question 21

gradient in cortico-papillary interstitium

Answer 21

greatest osmolarity near the bottom- least amount of water, greatest amount of sodium

Question 22

how is sodium absorbed by the thick ascending limb of the loop of Henle?

Answer 22

secondary active absorption

1. basolateral sodium potassium ATPase creates inward sodium gradient
2. one sodium, one potassium and two chlorides then move in via the apical NKCC channel down sodium’s electrochemical gradient
3. apically, potassium then leaves down its conc gradient via a potassium leak channel
4. chloride moves out basolaterally passively

Question 23

important molecule that increases hypertonicity

Answer 23

urea

Question 24

what percentage of inner medullary interstitial osmolarity is provided by urea?

Answer 24

50%

Question 25

explain movement of urea

Answer 25

1. urea leaves the inner medullary collecting duct via the urea transporter
2. increases the osmolarity of the interstitium so more water is reabsorbed
3. urea passively moves bak into the loop of Henle, know as urea recycling

Question 26

explain counter current exchange in the vasa recta

Answer 26

1. blood moves down descending limb, water leaves into the hypertonic interstitium
2. sodium and other solutes diffuse into the blood down their concentration gradient
3. ascending limb, water is drawn back into the vessels and the solutes move down their concentration gradient

Question 27

what is the net result of the counter current exchange in vasa recta?

Answer 27

water is excluded from interstitial fluid and salt is trapped in the interstitium

Question 28

how does water move out the descending limb?

Answer 28

via aquaporins

Question 29

different aquaporins + locations

Answer 29

AQP1- proximal tubule and descending limb cells

AQP2- apical membrane of collecting duct principal cells

AQP3/AQP4- basolateral membranes of collecting duct principals

Question 30

two different forms of expression of the channels explained

Answer 30

constitutively expressed- constant- AQP1, AQP3, AQP4

regulatively expressed- AQP2

Question 31

What regulates the expression of AQP2?

Answer 31

ADH, antidiuretic hormone, vasopressin

Question 32

where is ADH produced and secreted?

Answer 32

neuroendocrine cells of the neurohypophysis

in the supraoptic and paraventricular nuclei of the hypothalamus

Question 33

ADH function

Answer 33

causes the insertion of AQP II in the apical membrane of the principal cells of the collecting duct

increases the permeability of the collecting duct, resulting in more concentrated urine

Question 34

mechanism of ADH

Answer 34

1. binds to V2 receptor on basolateral principle cells
2. activates adenylate cyclase, producing cAMP and PKA
3. vesicles containing AQP II phosphorylated resulting in their fusion

Question 35

other functions of ADH

Answer 35

activation of NK2CC

activation or urea transporter in inner medullary collecting duct cells

slow blood flow through vasa recta