Lecture 18: Urine Concentration And Dilution

Question 1

Describe the regulation of extracellular osmolarity

Answer 1

• Major functions of the kidneys include the regulation of extracellular osmolality, including water loss and conservation.
• Regulation of extracellular osmolality depends on transport mechanisms already discussed in previous lectures and in the text chapter 28.

Question 2

Describe excretion of dilute urine

Answer 2

• When there is a large excess of water in the body:
• Kidneys can excrete as much as 20 liters per day with a concentration as low as 50 mOsm/L.
• • Kidneys continue to reabsorb solutes
• • Simultaneously fail to reabsorb large amounts of water (Can reabsorb more solutes than water, resulting in a dilute large amount of urine output)
• See slide 6

Question 3

Describe the ascending thick limb of henle

Answer 3

• Sodium, potassium, chloride are avidly reabsorbed.
• This segment is impermeable to water.
• Tubular fluid becomes more dilute as it flows up the ascending loop of Henle.
• Osmolarity is about 100 mOsm/L at the early distal tubular segment.
• Note that whether ADH is present or not does not matter at this point.

Question 4

Describe the late distal convoluted tubule

Answer 4

• Additional reabsorption of sodium chloride
• Impermeable to water in absence of ADH
• Osmolarity reaches 50 mOsm/L
• See Slide 8

Question 5

Describe the excretion of concentrated urine

Answer 5

• Kidney can produce a maximal urine concentration of 1200 to 1400 mOsm/L.
• Requirements for forming a concentrated urine:
• Presence of ADH
• High osmolarity of renal medullary interstitial tubule:
• • Establishes osmotic gradient necessary for water reabsorption to occur
• A normal 70-kg human must excrete about 600 mOsm of solute each day in order to get rid of waste products of metabolism and ions that are ingested.
• Maximal urine concentrating ability = 1200 mOsml/L
• 600 mOsm/day / 1200 mOsm/L = 0.5 L/day (Obligatory Urine Volume)
• See Slide 10

Question 6

Describe the proximal tubule

Answer 6

• Reabsorbs about 65% of filtered electrolytes
• Highly permeable to water
• Tubular osmolarity ≈ 300 mOsm/L

Question 7

Describe the descending loop of henle

Answer 7

• Highly permeable to water
• Less permeable to sodium chloride and urea
• Osmolarity of tubular fluid increases to ≈ 1200 mOsm/L when [ADH] is high.

Question 8

Describe the thin ascending loop of henle

Answer 8

• Impermeable to water
• Reabsorbs sodium chloride
• Tubular fluid becomes more dilute
• Urea also diffuses into the ascending limb
• Comes from urea absorbed into interstitium from collecting ducts

Question 9

Describe the thick ascending loop of henle

Answer 9

• Impermeable to water
• Large amounts of sodium chloride, potassium, and other ions are actively transported from tubule into medullary interstitium.
• Tubular fluid becomes dilute: 100 mOsm/L.

Question 10

Describe the Early and late distal tubule and cortical collecting tubule

Answer 10

• Early distal tubule:
• Similar to thick ascending loop of Henle
• Tubular fluid becomes more dilute:
• 50 mOsm/L.
• Late distal tubule and cortical collecting tubule:
• Osmolarity of fluid depends on ADH.
• Urea is not very present

Question 11

Describe the inner medullary collecting duct

Answer 11

Osmolarity of fluid depends on ADH and surrounding interstitium osmolarity

See Slide 16-23

Question 12

Describe the role of urea in concentrating urine

Answer 12

• Ascending loop of Henle and distal cortical collecting tubule are impermeable to urea.
• ↑[ADH] and cortical collecting tubule:
• Water is reabsorbed from cortical collecting tubule
• Urea is not very permeant here and becomes more concentrated in the tubule.
• ↑[ADH] and medullary collecting duct:
• More water is reabsorbed from medullary collecting duct, resulting in a higher concentration of urea.
• Higher concentration of urine results in diffusion of urea out of duct into interstitial fluid.
• Facilitated by UT-A1 and UT-A3 (ADH-activated) transporters
• Simultaneous movement of water and urea out of the inner medullary collecting ducts maintains a high concentration of urea in the tubular fluid and, eventually, in the urine, even though urea is being reabsorbed.
• See Slides 26-32

Question 13

Describe the osmoreceptor ADH-Feedback Mechanism

Answer 13

• Controls extracellular fluid sodium concentration and osmolaraity:
• ↑[extracellular fluid osmolarity] →
• Shrinking of osmoreceptor cells in ant. hypothalamus → acon potenals →
• Release of ADH →
• Increases water permeability in distal nephron segments
• ADH is formed in magnocellular neurons in:
• Supraoptic nuclei
• Paraventricular nuclei
• Osmoreceptor cells
• In the vicinity of the AV3V region
• • Anterior region of third ventricle
• See Slides 35-38