Lecture 36 4/11/24

Question 1

What is the importance of osmolarity?

Answer 1

-cells of the body must be bathed with extracellular fluid with a relatively constant conc. of electrolytes
-must be precisely regulated to prevent cells from shrinking or swelling

Question 2

What regulates ECF osmolarity and NaCl conc.?

Answer 2

the amount of extracellular water

Question 3

What controls total body water?

Answer 3

-fluid intake
-renal water excretion

Question 4

What happens when body fluid osmolarity increases above normal?

Answer 4

-post. pituitary secretes more ADH
-ADH increases permeability of distal tubules and collecting ducts to water
-water reabsorption is increased and urine volume is decreased without altering the rate of solute excretion

Question 5

What is the maximum diluted urine the kidney can excrete?

Answer 5

20 L/day with a conc. of 50 mOsm/L

Question 6

How does urine become dilute as it passes through the nephron?

Answer 6

-in proximal tubules, fluid remains isosmotic
-in thick ascending LOH, fluid becomes diluted (not permeable to water)
-in distal and collecting tubules, fluid becomes more dilute IF there is an absence of ADH (only permeable to water when ADH is present)

Question 7

Why is it important that the kidney can form concentrated urine?

Answer 7

minimizes the fluid intake required to match what is lost via lungs/feces/skin/urine

Question 8

How do the kidneys respond to a water deficit in the body?

Answer 8

excreting solutes while increasing water reabsorption and decreasing urine volume

Question 9

What is obligatory urine loss?

Answer 9

the minimum volume of urine that must be excreted in order to get rid of metabolic waste and ingested electrolytes

Question 10

What is urine specific gravity?

Answer 10

measure of the weight of solutes in a given volume of urine, determined by number and size of the molecules

Question 11

When is it possible for urine specific gravity to falsely suggest a highly concentrated urine?

Answer 11

when there are significant amounts of large molecules in the urine, such as glucose, radiocontrast media, or some antibiotics

Question 12

What are the basic requirements for forming a concentrated urine?

Answer 12

-high level of ADH
-high osmolarity of renal medullary interstitial fluid

Question 13

Why is it important that the renal medullary interstitium is typically hyperosmotic?

Answer 13

when ADH levels are high, water will move through the tubular membrane via osmosis into the renal interstitium due to the conc. gradient

Question 14

What are the components of the countercurrent mechanism?

Answer 14

-countercurrent multiplier system in the nephron loops of juxtamedullary nephrons
-recycling of urea in the medullary collecting ducts
-countercurrent exchanger in the vasa recta

Question 15

What are the major factors that contribute to a high solute conc. in the renal medulla?

Answer 15

-active transport of Na+ and co-transport of K+, Cl-, and other ions out of the thick ascending LOH into the interstitium
-active transport of ions from the collecting ducts into the interstitium
-facilitated diffusion of urea from the inner medullary collecting ducts into the interstitium
-diffusion of only small amounts of water from the medullary tubules into the interstitium

Question 16

Why is it important that the descending limb of the LOH is permeable to water?

Answer 16

allows for the tubular fluid osmolarity to quickly become equal to the renal medullary osmolarity

Question 17

What is the countercurrent multiplier?

Answer 17

-repetitive reabsorption of NaCl by the thick ascending LOH
-continued inflow of new NaCl from the proximal tubule into the LOH

Question 18

Why does the early distal tubule further dilute the tubular fluid?

Answer 18

it actively transports NaCl out of the tubule while being relatively impermeable to water

Question 19

Why is it important that large amounts of water is reabsorbed into the renal cortex and not the medulla?

Answer 19

helps preserve the high medullary interstitial fluid osmolarity

Question 20

What are the characteristics of urea?

Answer 20

-contributes 40-50% of the interstitial osmolarity in the renal medullar interstitium
-passively reabsorbed from the tubule
-large amounts are passively reabsorbed when ADH is high

Question 21

What is the mechanism for reabsorption of urea into the renal medulla?

Answer 21

-little urea is reabsorbed in the ascending LOH and collecting tubules; segments are impermeable to urea
-with high ADH, water is reabsorbed rapidly from cortical collecting tubule; urea conc. increases rapidly
-more water reabsorption occurs in inner medullary collecting ducts, further concentrating urea
-high concentration of urea eventually causes urea to diffuse out of tubule and into renal interstitial fluid

Question 22

What are the characteristics of urea transporters?

Answer 22

-facilitate urea diffusion
-activated by ADH

Question 23

How does malnutrition impact urine concentration?

Answer 23

-malnutrition is associated with low urea concentration
-causes considerable impairment of urine-concentrating ability

Question 24

What determines the rate of urea excretion?

Answer 24

-concentration of urea in the plasma
-GFR
-renal tubular urea reabsorption

Question 25

What is urea recirculation a mechanism for?

Answer 25

forming a hyperosmotic renal medulla

Question 26

Why is it essential to concentrate urea before excretion when there is inadequate water in the body?

Answer 26

it allows for more water to be absorbed

Question 27

Why is it important that the renal medulla has a special blood flow system?

Answer 27

prevents dissipation of the solutes pumped into the renal medulla by the countercurrent multiplier system

Question 28

What are the special features of the renal medullary blood flow system?

Answer 28

-blood flow is slow/sluggish,; sufficient to supply the metabolic needs of the tissues while minimizing solute loss
-vasa recta serve as countercurrent exchangers, minimizing the washout of solutes from the medullary interstitium

Question 29

What are the characteristics of the vasa recta?

Answer 29

-highly permeable to solutes in the blood, except plasma proteins
-blood within the vasa recta becomes progressively more concentrated as it goes into the medulla due to solute entry and water loss
-blood conc. similar to that of the medullary interstitium
-blood becomes less concentrated as it goes back towards the cortex; solutes move out, water moves in

Question 30

Which disorders can impact urinary concentrating ability?

Answer 30

-inappropriate secretion of ADH
-inability of distal tubules/collecting tubules/collecting ducts to respond to ADH
-impairment of countercurrent mechanism