Lecture 6: Water transport in the kidney II

Question 1

What is the countercurrent system in the kidney? what is its function

Answer 1

he process of using energy to generate an osmotic gradient that enables you to reabsorb water from the tubular fluid and produce concentrated urine.

Question 2

where does the countercurrent mechanism occur in the kidney

Answer 2

loop of Henle

Question 3

describe the anatomy of the loop of Henle

Answer 3

• descending - permeable to water, impermeable to ions
• ascending - impermeable to water, highly permeable to sodium
• establishes osmotic gradient; allows varying urine concentration
• basolateral surface faces interstitium
• apical surface faces lumen
• surrounded by peritubular cappillaries aka. vasa recta
• reabsorption/secretion to and from blood via interstitium

Question 4

why is the ascending loop of henle impermeable to water

Answer 4

due to it having no aquaporin channels

Question 5

describe the anatomy and physiological features of the descending loop of Henle

Answer 5

• filtrate that enters has osmolarity of ~300 mOsm/L
• squamous epithelial cells have aquaporins on either surface
• water moves via osmosis into the interstitium
• osmolarity can reach 1200 mOsm/L at bottom of loop

Question 6

what does countercurrent multiplication refer to

Answer 6

process of creating a concentration gradient along the loop of Henle, it uses ATP

Question 7

describe the anatomy and physiological features of the thin ascending loop of Henle

Answer 7

• no aquaporins but sodium and chloride channels instead
• move from lumen to interstitium along CG
• osmolarity can decrease to ~600 mOsm/L at top of thin loop

Question 8

describe the anatomy and physiological features of the thick ascending loop of Henle

Answer 8

• cuboidal epithelium
• Na/K/Cl cotransporter
• Na+ K+ and 2Cl- moved from lumen into cells using Na+ concentration gradient
• Na/K ATPase working to generate energy
• K+ and Cl- move into interstitium from cell down their concentration gradients
• osmolarity can decrease to ~325 mOsm/L at top

Question 9

why “countercurrent” “multiplication”

Answer 9

countercurrent - because descending and ascending limb go in opposite directions
multiplication - when we reabsorb ions in the ascending limb and make the medulla salty, by not reabsorbing water, that drives water to be reabsorbed passively in the descending limb

use energy to reabsorb ions in ascending limb - active transport
water reabsorption is passive

Question 10

what are the basic requirement for forming concentrated urine?

Answer 10

1. a high level of antidiuretic hormone
2. a high osmolarity of the renal medullary interstitial fluid which provides the osmotic gradient necessary for water reabsorption to occur

Question 11

how does the renal medullary interstitial fluid remain hyperosmotic

Answer 11

due to the countercurrent mechanism

Question 12

using sodium transporters, the loop of henle can establish a difference of ____ mOsm/L between the loop and the interstitium

Answer 12

200

Question 13

what is the term reffered to that allows water to move from the descending limb into the interstium due to an increase in sodium concentration

Answer 13

osmotic pressure to drive water out of the descending limb

Question 14

what is the purpose of water movement out of the descending limb? what is the consequence of this?

Answer 14

to make the concentration of sodium ions equal in the interstitum and descending loop of henle

causes filtrate to become more concentrated/sodium concentration increases in the descending loop of Henle because youre removing water

filtrate keeps moving along, same process occurs making it more and more concentrated until we reach maximmum concentration in the medulla - 1200 mOsm/L

Question 15

NaCl transport generates only a —- gradient across any portion of the ascending limb, but countercurrent exchange can multiply this single effect to produce a —- gradient between cortex and papilla

Answer 15

~200-mOsm
900-mOsm

Question 16

What is a single cycle in the countercurrent mechanism made up of

Answer 16

“single effect” - generates a 200 mOsm gradient by NaCl transport from lumen of ascending limb to the interstitium and water equilibrating this change from descending loop
“axial shift” - movment of tubule fluid along loop of Henle with an instantenous equlibration between the lumen of descending limb and the interstitum

Question 17

what is antidiuretic hormone

Answer 17

• peptide hormone prevents excessive urine production by reabsorbing water from the kidneys
• it allows the body to control the amount of fluid retention

Question 18

What triggers ADH (aka vasopressin) production

Answer 18

• osmoreceptors in anterior hypothalamus, baroreceptors in CV system; stimulated by angiotensin II
• causes smooth muscle cells in arteries to constrict

Question 19

How does ADH reach the kidneys from the hypothalamus

Answer 19

• produced in anterior hypothalamus
• stored in posterior pituitary
• when needed, released into blood, travels to kidneys
• binds to AVPR2 receptors on basolateral membrane of principal cells along collecting ducts of nephrons
• AVPR2 signals adenyl cyclase to convert ATP to cAMP –> cell produces aquaporins, opens existing aquaporins in apical membrane, osmosis pulls water from lumen of ducts into interstitium, reabsorbed into circuclation

Question 20

what is the main function of ADH

Answer 20

• increase water permabiltiy of distal tubule, collecting duct and collecting duct epithelia
• conserve water
• triggered by decrease in water excretion

Question 21

explain the process of osmoreceptors firing

Answer 21

• Osmoreceptros, cell body located where blood brain barrier is leaky – sample plasma going past
• Specialised – shape and structure is resposive to changes in osmolarity, changes in cell shape triggers change in activity
• Increased osmolarity – more osmoles in plasma, water leaves osmoreceptor, change in shape, causes it to fire, depolarise, adh released, to plasma to kidney

Question 22

Why is it crucial to have adequate amounts of ADH in the body

Answer 22

low - can get central and nephrogenic diabetes insipidus
high - hyponatremia

Question 23

central diabetes insipidus

Answer 23

• decreased release of ADH from neurohypophysis
• can be due to impaired synthesis in the hypothalamus, impaired transport to the neurohypophysis, or impaired storage and secretion from the neurohypophysis

Question 24

nephrogenic diabetes insipidus

Answer 24

• plasma ADH levels can be normal but response of the collecting duct to the hormone is depressed

Question 25

diabetes insipidus in general refers to what?

Answer 25

a condition where synthesis, secretion or action of ADH is impaired resulting in polyuria - production of large amounts of dilute urine

Question 26

how is central diabetes insipidus treated

Answer 26

administration of exogenous ADH

Question 27

hypernatremia

Answer 27

too much sodium in the blood

Question 28

how is hypernatremia related to ADH

Answer 28

plasma osmolarity high, osmoreceptors sense this, release of ADH to kidneys

Question 29

hyponatremia

Answer 29

when the concentration of sodium in your blood is abnormally low

Question 30

give a possible reason for hyponatremia

Answer 30

excessive fluid intake - polydipsia
- degree of hyponatremia depends on the level of fluid intake relative to renal water excretion

Question 31

what is maximal diuresis in a human

Answer 31

18-20 L/d

Question 32

what happens if fluid intake exceeds maximal excretory capacity?

Answer 32

a reduction in plasma Na+ concentration must occur

Question 33

How would you distinguish between diabetes milletus and central/nephrogenic insipidus?

Answer 33

urine glucose levels