Lecture 17: Urine Concentration and Dilution Flashcards
How much H2O can be excreted by the kidneys per day when there is a large excess of water in the body?
20 L/day with a concentration as low as 50 mOsm/L (very dilute urine)
What is the maximum urine concentration that the kidneys can produce?
1200-1400 mOsm/L
What is required by the kidney tubules for the formation of a concentrated urine?
Presence of ADH, high osmolarity of renal medullary interstitial tissue
Why is the obligatory urine volume 0.5 L/day?
Normal human must excrete 600 mOsm solute daily to get rid of waste products; maximal urine concentrating ability = 1200 mOsm/L: 600/1200 = 0.5
Where in the kidney tubule are most are most of the filtered electrolytes reabsorbed?
Proximal tubule (reabsorbes 65% filtered electrolytes and highly permeable to water)
Descending loop of Henle
“Concentrating segment;” highly permeable to H2O, less permeable to NaCl and urea; increased osmolarity of tubular fluid (1200 mOsm/L)
Thin ascending loop of Henle
“Diluting segment;” Impermeable to H2O, reabsorbs NaCl; tubular fluid more dilute (urea also diffuses into ascending limb)
Thick ascending loop of Henle
Impermeable to water; large amounts of NaCl, K+, etc actively transported from tubule to medullary interstitium; tubular fluid dilute (100 mOsm/L)
Early distal tubule
Similar to thick ascending loop of Henle with more dilute tubular fluid (50 mOsm/L)
Late distal tubule
Osmolarity of fluid depends on ADH (ADH present = H2O reabsorbed, ADH absent = H2O excreted as dilute urine); urea not very permeant
Inner medullary collecting duct
Osmolarity of fluid depends on ADH and surrounding interstitium osmolarity
Which areas of the kidney tubule are impermeable to urea?
Ascending loop of Henle, distal cortical collecting tubule
What effect does increased ADH have on cortical collecting tubule?
Water is reabsorbed from cortical collecting tubule, urea not very permanent here and becomes more concentrated in tubule
What effect does increased ADH have on medullary collecting duct?
More water reabsorbed from medullary collecting duct, resulting in a higher concentration of urea; urea diffused out of duct into interstitial fluid via UT-A1 and UT-A3
How is a high concentration of urine maintained in collecting ducts even though it is being reabsorbed?
Movement of water, urea out of inner medullary collecting ducts maintains high [urea] in tubular fluid and eventually in urine, even though urea is reabsorbed
