Lecture 18: Urine Concentration And Dilution Flashcards Preview

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Flashcards in Lecture 18: Urine Concentration And Dilution Deck (13)
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Describe the regulation of extracellular osmolarity

* 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.


Describe excretion of dilute urine

* 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


Describe the ascending thick limb of henle

* 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.


Describe the late distal convoluted tubule

* Additional reabsorption of sodium chloride
* Impermeable to water in absence of ADH
* Osmolarity reaches 50 mOsm/L

- See Slide 8


Describe the excretion of concentrated urine

* 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


Describe the proximal tubule

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


Describe the descending loop of henle

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


Describe the thin ascending loop of henle

* 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


Describe the thick ascending loop of henle

* 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.


Describe the Early and late distal tubule and cortical collecting tubule

* 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


Describe the inner medullary collecting duct

Osmolarity of fluid depends on ADH and surrounding interstitium osmolarity

See Slide 16-23


Describe the role of urea in concentrating urine

* 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


Describe the osmoreceptor ADH-Feedback Mechanism

* 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