The Urinary System - Part 2 (week 10)

Question 1

state the daily intake of NaCl (in grams)

Answer 1

8.5 g (from food)

Question 2

state the daily sources of NaCl loss and the values for each one (3 different sources + values)

Answer 2

1. sweat - 0.25 g
2. feces - 0.25 g
3. urine - 8 g

Question 3

explain primary active Na+ reabsorption (3 steps)

Answer 3

1. Na+ reabsorption is driven by primary active transport throughout the tubule into the interstisutal fluid
2. achieved by Na+/K+ ATPase pumps in the basolateral membranes
3. Na+ active transport out of the cell keeps intracellular conc^ of Na+ low compared to the tubular lumen, so Na+ moves ‘downhill’ out of the lumen into the tubular epithelial cells

Question 4

the mechanism for the downhill Na+ movement across the luminal membrane into the cell varies from…

Answer 4

segment to segment depending on which channels and/or carriers are present in their luminal membranes

Question 5

state the differences between the basolateral Na+ reabsorption and the luminal Na+ reabsorption

Answer 5

1. the basolateral step is the same in all Na+ reabsorbing segments - Na+/K+ ATPase pumps
2. the luminal entry step for Na+ in the cortical collecting duct occurs primarily via Na+ channels

Question 6

what process does water get reabsorbed by

Answer 6

coupling of water reabsorption to Na+ reabsorption (via osmosis)

Question 7

state the 4 steps of ‘coupling water reabsorption to Na+ reabsorption’

Answer 7

1. Na+ transported from tubular lumen to interstitial fluid across epithelial cells
2. the removal of salts from the tubular lumen lowers the local osmolarity of the tubular fluid adjacent to the cell (i.e. water conc^ inc^). at the same time, the appearance of solutes in the interstitial fluid just outside the cell increases it’s local osmolarity
3. difference in water conc^ between lumen and interstitial fluid causes net diffusion of water from lumen across tubular cells PM’s and/or tight junctions into the interstitial fluid
4. from there, water, Na+, and everything else dissolved in the interstitial fluid moves together by bulk flow into the peritubular capillaries

Question 8

movement of water across the tubular epithelium can only occur if what?

Answer 8

if the tubular epithelium is permeable to water

Question 9

state 3 facts about proximal tubule water permeability

Answer 9

1. varies between segments and depends on aquiporin availability
2. proximal tubule very permeable to water so reabsorbs water molecules as rapidly as Na+
3. as a result, proximal tubule reabsorbs large amounts of H2O and Na+ in the same amounts

Question 10

what part of the nephron’s water permeability is under physiological control

Answer 10

last part of the tubules, cortical and medullary collecting ducts can vary due to physiological control

Question 11

state the function of vasopressin

Answer 11

vasopressin stimulates the insertion of a particular group of aquiporins into the luminal membrane (aquiporins made by collecting duct cells)

Question 12

explain the process vasopressin plays in the reabsorption of water (4 points)

Answer 12

1. enters interstitial fluid + binds to receptors on basolateral membrane causing internal production of 2nd messenger cAMP
2. cAMP activates Protein Kinase A (PKA) which phosphorylates proteins which inc^ fusion of vesicles containing AQP2 with the luminal membrane
3. inc^ rate of water reabsorption at cytosol
4. water then diffuses through AQP3 and AQP4 on the basolateral membrane into the interstitial fluid and into the blood

Question 13

are basolateral aquiporins vasopressin dependent?

Answer 13

basolateral aquiporins are not vasopressin dependent

Question 14

state what it is meant by the key term - water diuresis

Answer 14

a large volume of water remaining in the tubule to be excreted as urine resulting from low vasopressin concentrate

Question 15

state what it is meant by the key term - diuresis

Answer 15

simply means large urine flow as a result of any cause

Question 16

state the difference between water diuresis and osmotic diuresis

Answer 16

in water diuresis, there is an increase in urine flow, but not an increase in solute excretion. in all other cases of diuresis, termed osmotic diuresis, the increased urine flow is the result of an increase in solute excretion

Question 17

state what it is meant by the key term - obligatory water loss

Answer 17

the volume of urine is known as the obligatory urine loss (the loss of this minimal volume of urine contributes to dehydration when water intake is zero)

Question 18

the ability of the kidney to produce…

Answer 18

the ability of the kidney to produce hyper osmotic urine is a major determinant of the ability to survive with limited water intake

Question 19

explain how the ‘countercurrent multiplier system’ make the medullary interstitial fluid become hyper-osmotic (5 points)

Answer 19

1. the countercurrent anatomy of the loop of Henle of juxtamedullary nephrons
2. reabsorption of NaCl in the ascending loop of Henle
3. impermeability of H2O in the ascending limbs
4. trapping of urea in the medullar
5. hairpin loops of vasa recta to minimise washout of the hyper-osmotic medullar

Question 20

because the proximal tubule always reabsorbs Na+ and H2O in the same proportions…

Answer 20

the fluid entering the descending loop of Henle from the proximal tubule has the SAME OSMOLARITY as the plasma

Question 21

explain the role of the ascending loop of Henle (5 points)

Answer 21

1. entire length, Na+ and Cl- reabsorbed from lumen into medullary interstitial fluid
2. upper (thick) portion, achieved by transport and active co-transport
3. transporters not present in lower (thin) portion so occurs via simple diffusion
4. ascending limb relatively impermeable to water so little follows the salts
5. fluid in medulla becomes hyper-osmotic compared to fluid in ascending limb as only salts reabsorbed

Question 22

explain the role of the descending loop of Henle (3 points)

Answer 22

1. doesn’t reabsorb Na+ and Cl- and is highly permeable to H2O
2. net diffusion of water occurs at descending limb into concentrated medullary interstitial fluid until osmolarities equilibrate
3. the interstitial hyper-osmolarity is maintained during the equilibrium as ascending limb continues to pump Na+ and Cl- out to maintain concentration gradient

Question 23

explain the role of the DCT (2 points)

Answer 23

1. actively transports Na+ and Cl- out of tubule but is relatively impermeable to water
2. this hypo-osmotic fluid then enters the cortical collecting ducts

Question 24

where does vasopressin not influence water reabsorption

Answer 24

vasopressin only influences water reabsorption in the tubules parts of the medullary and cortical collecting ducts. does not affect permeability in tubules prior.

Question 25

when plasma vasopressin concentration is low, both cortical and medullary collecting ducts are relatively impermeable to water. as a result…

Answer 25

large volumes of hypo-osmotic urine is excreted, thereby eliminating excess water from the body

Question 26

what is the function of the vasa recta?

Answer 26

form hairpin loops that run parallel to the loop of Henle and the medullary collecting ducts. function to minimise excessive loss of solutes from the interstitium

Question 27

what maintains the steady countercurrent gradient set up by the loop of Henle

Answer 27

salt and water being reabsorbed from the loop of Henle and collecting ducts are carried away by bulk flow