Lecture 6 - Urine Conc and Dilution

Question 1

What is the osmolality of urine in comparison to plasma?

Answer 1

normally hyperosmotic

400-1000mOs/kg H₂O

Question 2

Concentrated urine is produced by reabsorbing water. Where is water reabsorbed from?

Answer 2

• PCT - reabsorbs 65% of what was filtered, obligatory with sodium reabsorption
• LoH (descending) - needs medullary gradient
• DCT - needs medullary gradient & ADH
• CDs - needs medullary gradient & ADH

Question 3

What are the fixed values for water reabsorption along the tubules?

Answer 3

• PCT - 70% reabsorbed - 126L/day
• descending LoH - 5% - 10L
• DCT - 10% - 20L

leaves 23L being excreted or reabsorbed with presence of ADH

Question 4

What happens if you drink too much water?

Answer 4

• body fluid becomes hypo-osmolar <285mOsm/Kg H₂O
• dilutes body fluids, kidney passes out more H₂O
• large volume of dilute urine created (100mosm/kg)
• urine becomes hypo-osmolar too

opposite if you drink too little water

Question 5

Why are the collecting ducts ‘fine tuners’?

Answer 5

• water reabsorption from here regulated by ADH, so this segment fine tunes electrolyte and water concs in urine

Question 6

What is a complication of over hydration?

Answer 6

hyponatremia

symptoms: nausea, vomiting, headache

electrolyte imbalance and tissue swelling = irregular heartbeat and allows fluid to enter lungs

swelling = pressure on brain and nerves, causes behaviours resembling alcohol intoxication

swelling of brain tissues = seizures, coma and death unless hypertonic saline solution administered

Question 7

When is ADH released?

Answer 7

• when osmolality of extracellular fluid is higher
• small volume of conc urine produced (high as 1200mosm/kg water)

Question 8

Where is ADH produced and when?

Answer 8

pituitary gland

in response to hypovolemia and plasma hyperosmolality

Question 9

How is urea recylcing made more effective in the presence of ADH?

Answer 9

ADH increased CD permeabilitiy to urea

more urea added to medullary interstitium

osmotic gradient increased so more water pulled out via osmosis

Question 10

Describe the filtrate (iso/hyper/hypo) at the end of the PCT and why.

Answer 10

isotonic (285mOsm/Kg H₂O)

similar water and solute reabsorption into peritubular capillaries

Question 11

Describe the filtrate (iso/hyper/hypo) at the end of the descending LoH and why.

Answer 11

hypertonic (1200mosm/kg)

lots of water reabsorbed into vasa recta, no solute reabsorption

Question 12

Describe the filtrate (iso/hyper/hypo) at the end of the ascending LoH and why.

Answer 12

hypotonic (90mOsm/kg)

lots of solute into interstitium and some into vasa recta, no water reabsorption

Question 13

Describe the filtrate (iso/hyper/hypo) at the end of the DCT and CD and why when producing dilute urine.

Answer 13

hypotonic (65-70mOsm)

some solute reabsorption but no ADH release, no aquaporin insertion, so no water reabsorption

Question 14

Describe the filtrate (iso/hyper/hypo) at the end of the DCT and CD and why when producing conc urine.

Answer 14

hypertonic (1200mOsm)

ADH release, aquaporin insertion, water reabsorption

relies on corticomedullary solute gradient to increasingly reabsorb more water

Question 15

What is the urine volume with maximal ADH produced?

Answer 15

300-400ml a day

contains very conc toxins

Question 16

What is normal urine osmolality?

Answer 16

400mOsm/Kg H₂O

Question 17

What happens when you have no ADH?

Answer 17

central diabetes insipidus

25/L urine produced a day

bladder capacity is 400ml so need to urinate frequently and you also drink a lot due to thirst

Question 18

What is desmopresin; how does it work and when is it used?

Answer 18

• patient with central diabetes insipidus, bed wetting children, elderly in care homes (nocturnal enuresis)
• synthetic version of ADH
• stops kidneys producing urine
  
  • too much of it or too much fluid intake whilst on it = water retention = headaches, dizziness, feeling bloated, HYPONATREMIA and so seizures

Question 19

What are the five things involved in urine conc?

Answer 19

• loop of henle - creates corticomedullary hyperosmotic interstitial gradient
• urea recycling - creates corticomedullary hyperosmotic interstitial gradient
• vasa recta - stops salty gradient from being destroyed
• adh - fine tunes
• collecting ducts - fine tunes