Concentration And Dilution Of Urine Flashcards
What is plasma osmolarity?
290 mOsm/L(we use 300 for calculations)
What happens when plasma osmolarity changes?
If this changes then.
-kidneys can either retain or excrete water. This process is called osmoregulation
Water regulation is independent of solute excretion
- To osmoregulate the urine can be very dilute or concentrated
- Thus depends on if Antidiuretic (ADH) is high or low
How are hypo-hyper and isoosmot8c ruined defined?
Hyperosmotic- above 300 mOsm/L
Isosmot8c urine- 300 mOsmoles/L
Hyposmoles- below 300 mOsmoles/L
How does the kidney dilute or concentrate the urine?
Solute wastes- 600 mOsm /day must be excreted in urine
-If maximum urine concentration ability is 120p mOsm/L, then the minimum ursine volume that needs to be excreted is 600/1200mOSM/day= 0.5L/day
This is called the obligatory volume. Normal urine volume is about 1.5-2.5L/day
What are the 3 processes that contribute to corticopapillary osmotic gradient?
- Countercurrent multiplication
- Urea recycling
- Slow rates flow
How does the medullary interstitium get so concentrated?
-Descending LH is water permeable not solutes
—Thin and thick ascending LH are impermeable to water
-Thick ascending LH pumps NaCl actively into the interstitium
- Thin ascending is permeable to NaCl
- Movement is passive
Collecting duct: depends on ADH
What generates & maintenance in the medullary interstitium is due to…
- ATP dependent solute transport
- Increase in medullary osmolarity
- Slow tubular fluid flow through vasa recta
This occurs throughout the length of the loop
There is a 200 mOsm gradient difference between the tubule & the interstitium
What is the function of urea in the counter current multiplier?
-Na+, K+, CL- can establish,y up to 600 mOsm
How do we get to 1200-1400 mOsm?
Through urea. Urea is the end-product of muscle metabolism
What are the sites of of tubular urea transport?
In the presence of ADH
-As the H2O is reabsorbed from CD, urea concentration rises
- ADH unregulates UTA1 & UTA3 transporters
- Urea diffuses passively via transporters into the interstitium
- Some of its secreted into both thin Loops of Henle
This is urea cycling.
50% is absorbed in proximal tubule. Of the 50% in the lumen after recycling 20% is excreted
How does the counter current multiplier work?
Urea is necessary for increasing osmolarity of inner medulla
Contributes to-half the hyperosmotic gradient
Urea is passively reabsorbed from the tubule(MCD)
-Requires ADH
Low protein states- urea is a by product of protein metabolism infants below age 1
Both have limited ability to concentrate urine
How do we stop the gradient in hyperosmotic interstitial fluid from simply disappearing?
Blood flow rates. Prevent medullary hypertonicity
Vasa recta: counter current exchangers
Describe counter current exchange in vasa recta
-Passive- no energy required
Downstream-gains NaCl & loses water, so plasma osmolarity increases to 1200 mOsm at the tip of the medulla
Upstream- it gains water & salt so plasma restores to 300 mOsm it drains into systemic circulation
How is osmoreceptors coupled to ADH secretion?
ADH is synthesized in the hypothalamus & stored in posterior pituitary & released if plasma osmolarity is high or fluid loss> 10%
Why does the loop of henle deposits more salt than water in the interstitium so;
Medullary interstitium becomes hypertonic- potential for urine concentration
What 8s the effect of low ADH?
Collecting duct is impermeable to water
No urea Reabsorption
Medullary slightly hyperosmotic
What is the effect of high ADH?
Collecting duct permeable to water (AQP2)
Urea Reabsorption (MCD)
Medulla highly hyperosmotic
What is diabetes insipidus?
Due to lack of ADH or receptors unresponsive to ADH
Water conversation fails in Diabetes inspidus
What is central diabetes insipidus?
Lack ADH
- congenital lack of ADH production
- acquired e.g. head trauma
What is nephrogenic diabetes insipidus?
Unresponsive to ADH
-V2-receptor mutations;
AQP2 mutations
-acquired e.g. lithium therapy
What is the renal clearance of a substance?
The volume of plasma cleared of a substance is by kidneys per unit time
Clearance= COSM= (UOSM. X V)/POSM
UOSM= urine osmolarity
POSM= plasma osmolarity
V-urine flow rate
COSM= total clearance of solutes from a volume of plasma/min
What is the free water clearance of the kidneys?
CH2O: the volume of free water cleared from plasma, & excreted in urine per unit time
This volume of water excreted in urine does not contain any solutes. This portion is additional to obligatory urine volume that contains solutes
CH2O indicates the ability of the kidney to concentrate or dilute urine (water conservation or water lost in body)
It is calculated as:
CH2O= V- (UOSM X V/POSM)= COSM
