Urine concentration and Dilution Flashcards
How much urine can the kidneys excrete a day? What is the lowest osmolarity of the urine?
20 L/ 50 mOsm
What are the two portions of the nephron that are involved with the excretion of dilute urine?
Ascending thick limb of Henle and th eLAte distal convoluted tubule
Describe the ascending thick limb of Henle
- absorbs sodium, potassium, and chloride
- impermeable to water
- more dilute as it flows up the tubule
- ADH does not effect
Describe the late distal convoluted tubule
- reabsorb sodium chloride
- impermable to water in the absence of ADH
What is the maximum urine concentration that the kidney can produce?
1200-1400 mOsm
What are the two requirements for forming concentrated urine?
- presence of ADH
- high osmolarity of renal medullary interstitial tubule (establishes gradient)
Why is there an obligatory volume of excreted urine of 0.5 L a day?
Because the maximum osmolality of the urine is about 1200 and the average human must secrete 600 mOsm of solute a day to get rid of the waste
600/1200=0.5 L/day
to get rid of the basic metabolic wastes that would otherwise be problematic
Where in the kidney are most of the filtered electrolytes absorbed?
proximal tubule reabsorbs about 65%
Describe the proximal tubule
65% of electrolytes
highly permeable to water
300 mOsm
Describe the descending loop of Henle
-highly permeable to water
-less permeable to sodium chloride and urea
osmolarity increases to 1200 mOsm/L with high ADH
Describe the thin ascending loop of Henle
- impermeable to water
- reabsorbs sodium chloride
- tubular fluid is more dilute
- urea diffuses into the ascending limb
Describe the thick ascending loop of henle
- impermeable to water
- large amounts of sodium chloride, potassium, and other ions are transported in
- tubular fluid is dilute (100 mOsm)
Describe the early distal tubule
- similar to the thick ascending loop of henle
- fluid is more dilute
Describe the late distal tubule and cortical collecting tubule
osmolarity depends on ADH
urea is not permeant
Describe the inner medullary collecting duct
osmolarity of the fluid depends on the ADH and surrounding intersititium osmolarity
What are the portions of the nephron that are impermeable to urea?
Ascending thick limn of Henle and the distal cortical collecting tubule
What happens in the cortical collecting tubule if ADH is increased?
water is reabsorbed and urea is not very permeant here and becomes more concentrated
What happens in the medullary collecting duct if ADH is increased?
More water is reabsorbed form the medullary collecting duct
higher concentration of the urea
-diffusion of urea into the interstitial fluid
What are the ADH activated transporters?
UT-A1 and Ut-A3
What helps to maintain the high concentration of urea in the tubular fluid?
simultaneous movement of water and urea in and out of the inner medullary collecting duct
What happens to the urine concentration if the ADH levels are low?
diluted urine
What happens to the urine concentration if the ADH levels are high?
concentrated urine
What prevents the medullary hyperosmolarity from being dissipated?
Vasa recta
Describe the osmoreceptor-ADH feedback mechanism
- controls the extracellular fluid sodium concentration and osmolarity
- ADH is formed in the magnoceullluar neurons
- osmoreceptor cells
What happens if there is and increased extracellular fluid osmolarity?
There is a shrinking of the osmoreceptor cells in the anterior hypothalamus which leads to increased action potentials which causes the release of ADH
This increases water permeability in the distal nephron segment and the urine will be concentrated
Where is ADH formed in the magnocellular nephron?
supraoptic nuclei and the paraventricular nuclei
Where are the osmoreceptor cells?
In the vicinity of the AV3V region (anterior region of the third ventricle
Describe the osmoreceptor feedback mechanism
- water deficit
- increased extracellular osmolarity
- ADH secretion form the posterior pituitary
- increased in the plasma ADH
- Increased H2O permeability in the distal tubules and the collecting ducts
- increased H2O reabsorption
- decreased water is excreted
- Concentrated urine
Describe what happens to osmolarity and volume in isotonic volume depletion
osmolarity stays the same and volume decreases
Describe what happens to osmolarity and volume in isovolumetric osmotic increase
volume is the same but the osmolarity increases
What are the steps of the countercurrent multiplier system?
- concentration in the loop of henle is the same
- concentration is reduced because of the activation of an active ion pump pulling out solutes
- concentration in the descending limb reaches osmotic equilibrium because of osmosis
- hyperosmotic fluid flows into the ascending limb
- More ions are pumped out fo the ascending limb (dilutes more)
- Repeat steps 4-6 over and over again until we reach
- equilibrium
Which tubules are permeable to water?
descending
Which tubules are impermeable to water?
ascending
What is the overall role of ADH?
To make the tubules permeable to water by inserting aquaporins into the cellular membranes
ALSO inserts urea transporters
What is reabsorbed in the proximal tubule?
Water, NaCl and urea
what is reabsorbed in the descending loop of henle when ADH is high
water
What is reabsorbed in the ascending loop of Henle when AHD is high
NaCl
What is reabsorbed in the medullary area when ADH is high?
Water, NaCL, urea
CONSERVING WATER
What is reabsorbed in the proximal tubule when the ADH levels are low?
NACl and H2O
What is reabsorbed in the descending loop when the ADH levels are low?
H20
What is reabsorbed in the medullary region when the ADH levels are low?
NaCl, urine will be diluted
Where does the urea that is reabsorbed from the medullary region go?
Goes to the loop of henle and is recycled into the tubular fluid
What are the vasa recta?
Capillaries that parallel the loop of henle; water leaves the descending portion and inserts into the ascending limb
help to maintain the equilibrium of the interstitial concentration differences
True or false: the main purpose of the vasa recta is to supply blood to the nephron
False, they help to maintain the equilibrium of the concentration gradient
Where is most of the change in the concentration of the urine occurring when there is increased ADH?
In the collecting tubule in the medullary region and the urine
Does blocking the aldosterone system have an effect on the sodium intake and extracellular sodium concentrations relationships?
Nah
