Lecture 18: Urine [ ]

Question 1

How much water can be excreted by the kidneys per day when there is a large excess of water in the body?

Answer 1

Kidneys can excrete as much as 20 L /day w/ a [ ] as low as 50 mOsm/L

Question 2

What is the maximum urine [ ] that the kidneys can produce?

Answer 2

1200-1400 mOsm/L

Question 3

What is required by the kidney tubules for the formation of a concentrated urine?

Answer 3

1. Presence of ADH
2. High osmolarity of renal medullary interstitial tubule:
   Establishes osmotic gradient necessary for water reabsorption to occur

Question 4

Explain why there is an obligatory volume of excreted urine of 0.5 L/day?

Answer 4

There is about 600 mOsm of solute needed to be excreted each day to rid body of waste products.

Max urine [ ]= 1200 mOsm/L

600/1200= 0.5 L/day

this is the obligatory urine volume.

Question 5

Where in the kidney tubule are most of filtered electrolytes reabsorbed ?

Answer 5

Proximal tubule:

Reabsorbs ~65% of filtered electrolytes

Question 6

Reabsorbs 65% electrolytes,

Highly permeable to water

–300 mOsm/L

Answer 6

Proximal tubule

Question 7

Highly permeable to water

Less permeable to NaCl and urea

–1200 mOsm/L

Answer 7

Descending loop of henle

Question 8

Impermeable to water

Reabsorbs NaCl

Urea diffusion (absorbed interstitium from collecting ducts)

Answer 8

Thin ascending loop of henle

Question 9

Impermeable to water

LARGE amt.s NaCl, K+, and other ions are ACTIVELY transported from tubule to interstitium.

–100 mOsm/L

Answer 9

Thick ascending loop of henle

Question 10

Similar to thick ascending loop of henle

Becomes more dilute

Answer 10

Early distal tubule

Question 11

Osmolarity of fluid DEPENDS on ADH

Urea is not very permeable

Answer 11

Late distal tubule
And
Cortical collecting tubule

Question 12

Osmolarity of fluid depends on ADH and surrounding interstitium osmolarity

Answer 12

Inner medullary collecting duct

Question 13

Describe how the counter current mechanism concentrates solute in the interstitium of a kidneys medulla and the importance of this action

Answer 13

The loop of henle must come to equilibrium in mOsm for routine urine formation.

Begins w/ 300 mOsm of filtrate in descending loop, then Na+ ions are pump in ascending loop. This creates a gradient that the two sides of the loop must balance in the middle. This is multiplied several times until the gradient is maximum at 1200 at the junction btw descending and ascending loop of henle.

Question 14

Be familiar w/ the permeability properties of various parts of the tubule to urea

Answer 14

Ascending loop of henle and distal cortical collecting tubule are impermeable to urea.

Urea not very permeable in cortical collecting tubule-becomes more concentrated in the tubule.

Medullary collecting duct CAN diffuse urea into interstitial fluid d/t higher [ ] of urine.

Question 15

Explain how a high [ ] of urea can be maintained in the collecting ducts even though urea is being reabsorbed

Answer 15

It remains a high [ ] of urea b/c of the interstitial gradient that is found around the collecting duct.

Question 16

What is the role of the osmoreceptor-ADH feedback mechanism?

Answer 16

Osmoreceptor-ADH feedback mechanism controls extracellular fluid [Na+] and (as a result) osmolarity

When there’s inc. [Na+]->osmorecptors in ant. Hypothalamus send AP to release ADH-> this inc. water reabsorption from distal nephron to dilute blood.

Question 17

Where specifically is ADH formed?

Answer 17

Magnocellular neurons

Question 18

What is the function of the osmoreceptor cells

Answer 18

Located in the vicinity of the AV3V region.

Osmoreceptor cells sense when there is a high extracellular fluid osmolarity