L8: urine concentration and dilution

Question 1

Urine compared to plasma

Answer 1

Urine is normally hyperosmotic compared to plasma. 400-1000mOsm/Kg H2O

Question 2

Where is water reabsorbed from in the nephron?

Answer 2

• PCT - obligatory with sodium reabsorption (70% of water)
• LoH (descending) - needs medullary gradient (5%)
• DCT (10%) - needs medullary gradient and ADH
• CDs - needs medullary gradient and ADH, but not a fixed volume of water reabsorbed, 23L can be either mainly excreted or mainly reabsorbed if ADH is present or not,

Question 3

Why might urine osmolality vary?

Answer 3

Low water intake means water needs to be conserved so concentrated urine is produced.
Excess water intake means more water needs to be excreted and dilute urine is produced.

Question 4

Role of collecting ducts and ADH

Answer 4

ADH is the major factor that can lead to production of a concentrated or dilute urine, final point of final dilution or concentration occurs in the collecting ducts.
ADH controls plasma osmolality and not volume.

Question 5

What happens if you are over-hydrated?

Answer 5

• osmolality of extracellular fluid decreases
• hyponatremia is a problem = low body sodium
• decreased ADH release so less water reabsorbed from DCT and CD
• large volume of a dilute urine produced - as low as 100mOsm/Kg H2O.

Question 6

What happens if you are dehydrated?

Answer 6

• osmolality of extracellular fluid increases
• ADH release increase, so aquaporin insertion occurs to facilitate water reabsorption from DCT and Cd
• small volume of a concentrated urine - as high as 1200mOsm/Kg H2O.

Question 7

What happens if you drink too much water?

Answer 7

• drinking too much water upsets the electrolyte balance in blood tissues and cells
• dilution of sodium or hyponatremia occurs in the extracellular fluid
• this will cause water to move into cells by osmosis from an area of low solute concentration ECF to one of a higher solute concentration ICF

Question 8

How does ADH affect urea?

Answer 8

Urea recycling can be made more effective in the presence of ADH. This is because when body water is low, ADH is released to facilitate water reabsorption from the CDs, and at the same time ADH increases CD permeability to urea. More urea is added to medullary interstitium, increasing the osmotic gradient, and so no more water can be pulled out of CDs by osmosis.

Question 9

What happens if maximal ADH secretion occurs?

Answer 9

Urine osmolality would be 1400mOsm/Kg H2O. Same as the intestitial osmolality of the deepest medulla or close to the papilla.
Urine volume would be 300-400ml a day.

Question 10

What happens if no ADH is secreted?

Answer 10

Urine osmolality would be 60mOsm/Kg H2O. Same as the filtrate osmolality before it gets to the collecting ducts. Urine volume would be 25L a day.

Question 11

What is central diabetes insipidus?

Answer 11

You cannot produce ADH. The bladder capacity is 400ml so frequent urination occurs.

Question 12

What drug can help central diabetes insipidus?

Answer 12

Desmopressin - synthetic version of ADH. Stop kidneys producing urine and losing the water.
Too much desmopressin or drinking too much fluid while taking it can cause water retention which cane result in headaches, dizziness, feeling bloated, hyponatremia and seizures.