9 – Urine Dilution and Concentration

Question 1

Extracellular fluid (ECF) is composed of:

Answer 1

-water
-solutes

Question 2

Changes in water component of ECF will alter:

Answer 2

-concentration of solutes (osmolarity) in the ECF
*changes in plasma osmolarity should be closely regulated=kidneys(reduce or increase water excretion)

Question 3

What are the 2 main points of how kidneys regulate water excretion?

Answer 3

1. Capable of producing urine with a wide range of osmolarity
2. Change the volume of urine, without altering the rate of solute excretion

Question 4

How do the kidneys regulate water excretion?

Answer 4

-ADH

Question 5

ADH:

Answer 5

-major player in regulation of urine concentration

Question 6

What is the mechanism of ADH when osmolarity of body fluids is INCREASED?

Answer 6

-ADH is released
>enhances water permeability of distal tubules and collecting tubules+ducts
*reduces water excretion

Question 7

What is the mechanism of ADH when osmolarity of body fluids is DECREASED?

Answer 7

-less ADH released
>less water is reabsorbed
*increases water excretion

Question 8

How is urine diluted?

Answer 8

-reabsorption of more solutes and less water in renal tubules

Question 9

Diluting urine in proximal tubule:

Answer 9

-equal proportion of solutes and water reabsorbed
*tubular fluid stays isosmotic

Question 10

Diluting urine in descending limb of loop of Henle:

Answer 10

-as tubule goes into medulla, more water is reabsorbed b/c medulla is hyperosmotic
*tubular fluid osmolarity increases (matches the surrounding environment)

Question 11

Diluting urine in ascending limb of loop of Henle:

Answer 11

-Na, Cl, and K are highly reabsorbed (especially in thick section)
-not permeable to water=water can’t follow the solutes
*tubular fluid is diluted (hypo-osmotic)

Question 12

Diluting urine in distal and collecting tubules:

Answer 12

-more solutes are reabsorbed
-under low ADH: impermeable to water
*tubular fluid is diluted even greater (50mOsm vs. 300mOsm plasma)

Question 13

What are ways we are constantly losing water?

Answer 13

-respiration
-evaporation
-perspiration
-urine
-feces
*If intake of water is limited=kidneys reduce water excretion

Question 14

What are the 2 main factors that affect the urine concentrating process?

Answer 14

1. ADH
2. Hyperosmolarity of the renal medulla interstitium (ex. animals living in desert can increase it a lot more)

Question 15

Why is the renal medulla hyperosmotic?

Answer 15

-countercurrent mechanism related to the arrangement of loop of Henle and the vasa recta
*this arrangement allows solutes to be trapped in the medullary interstitium

Question 16

How is the hyperosmolarity of the renal medulla established?

Answer 16

1. Thick segment of loop of Henle: active pumping of Na and co-transport of other ions (ex. Cl, K) and not being permeable to water
2. Descending limb: permeable to water so when osmolarity of the interstitium increases=water is pulled out of descending limb
   >Osmolarity in the descending tubule increases
3. Reaches ascending part and more ions are removed=increased osmolarity
   *cycle repeats and more salts keep being transported to interstitum

Question 17

What factors help with the maintenance of hyperosmotic medullary interstitum?

Answer 17

1. Transport of Na (active), Cl, and K from thick segment of loop of Henle into medullary interstitium
2. Transport of ions (active) from collecting ducts into medullary interstitium
3. Diffusion of urea from collecting ducts into medullary interstitium
4. Reduced transfer of water from collecting ducts into medullary interstitium

Question 18

Role of distal tubules and collecting ducts in concentrating urine:

Answer 18

-actively reabsorb NaCl form tubules (tubular fluid is further dilated)
-if ADH present: cortical collecting tubule is permeable to water
>since tubular fluid is highly diluted, lots of water is reabsorbed into peritubular capillaries in the cortical area
>some water also absorbed from medullary collecting duct

Question 19

What happens to the osmolarity of the tubular fluid when more ADH is released?

Answer 19

-becomes similar to the osmolarity of the medullary interstitum